What radio waves propagate well in water. School encyclopedia

Radio frequency range and its use for radio communications

2.1 Radio propagation basics

Radio communications ensure the transmission of information over a distance using electromagnetic waves (radio waves).

Radio waves– these are electromagnetic oscillations propagating in space at the speed of light (300,000 km/sec). By the way, light also belongs to electromagnetic waves, which determines their very similar properties (reflection, refraction, attenuation, etc.).

Radio waves carry energy emitted by a generator through space electromagnetic vibrations. And they are born when there is change electric field, for example, when an alternating electric current passes through a conductor or when sparks jump through space, i.e. a series of rapidly successive current pulses.

Rice. 2.1 Structure of an electromagnetic wave.

Electromagnetic radiation is characterized by frequency, wavelength and power of transferred energy. The frequency of electromagnetic waves shows how many times per second the direction of the electric current changes in the emitter and, therefore, how many times per second the magnitude of the electric and magnetic fields changes at each point in space.

Frequency is measured in hertz (Hz), a unit named after the great German scientist Heinrich Rudolf Hertz. 1Hz is one vibration per second, 1 MegaHertz (MHz) is a million vibrations per second. Knowing that the speed of electromagnetic waves is equal to the speed of light, we can determine the distance between points in space where the electric (or magnetic) field is in the same phase. This distance is called the wavelength.

The wavelength (in meters) is calculated using the formula:

, or approximately

where f is frequency electromagnetic radiation in MHz.

It is clear from the formula that, for example, a frequency of 1 MHz corresponds to a wavelength of about 300 m. With increasing frequency, the wavelength decreases, and with decreasing frequency, it increases.

Electromagnetic waves travel freely through air or outer space (vacuum). But if a metal wire, antenna or any other conducting body meets on the path of the wave, then they give up their energy to it, thereby causing an alternating electric current in this conductor. But not all the wave energy is absorbed by the conductor; part of it is reflected from the surface. By the way, this is the basis for the use of electromagnetic waves in radar.

Another useful property of electromagnetic waves (as well as any other waves) is their ability to bend around bodies in their path. But this is only possible when the dimensions of the body are smaller than the wavelength or comparable to it. For example, in order to detect an aircraft, the length of the locator radio wave must be less than its geometric dimensions (less than 10 m). If the body is longer than the wavelength, it can reflect it. But it may not reflect - remember “Stealth”.

The energy carried by electromagnetic waves depends on the power of the generator (emitter) and the distance to it, i.e. the energy flow per unit area is directly proportional to the radiation power and inversely proportional to the square of the distance to the emitter. This means that the communication range depends on the power of the transmitter, but to a much greater extent on the distance to it.

For example, the energy flow of electromagnetic radiation from the Sun onto the Earth's surface reaches 1 kilowatt per square meter, and the energy flow of a medium-wave broadcast radio station is only thousandths and even millionths of a watt per square meter.

2.2 Radio frequency spectrum allocation

Radio waves (radio frequencies) used in radio engineering occupy a spectrum from 10,000 m (30 kHz) to 0.1 mm (3,000 GHz). This is only part of the vast spectrum of electromagnetic waves. Radio waves (in decreasing length) are followed by thermal or infrared rays. After them there is a narrow section of waves visible light, then - the spectrum of ultraviolet, x-rays and gamma rays - all these are electromagnetic vibrations of the same nature, differing only in wavelength and, therefore, frequency.

Although the entire spectrum is divided into regions, the boundaries between them are tentatively outlined. The regions follow one another continuously, transition into one another, and in some cases overlap.

But these ranges are very extensive and, in turn, are divided into sections that include the so-called broadcasting and television ranges, ranges for land and aviation, space and sea communications, for data transmission and medicine, for radar and radio navigation, etc. Each radio service is allocated its own section of the spectrum or fixed frequencies. In reality, for radio communication purposes, oscillations in the frequency range from 10 kHz to 100 GHz are used. The use of a particular frequency range for communication depends on many factors, in particular on the propagation conditions of radio waves of different ranges, the required communication range, the feasibility of transmitter power values ​​in the selected frequency range, etc.

According to international agreements, the entire spectrum of radio waves used in radio communications is divided into ranges (Table 1):

Table 1

No.	Range name			Range limits
	Waves	Obsolete terms	Frequencies	Radio waves	Frequencies
1	DKMGMVDecaMega Meter		Extremely low frequencies (ELF)	100.000-10.000km	3-30 Hz
2	MGMVMegameter		Ultra-low frequencies (ELF)	10.000-1.000 km	30-3.000Hz
3	GCMMVHectakilometer		Infra-low frequencies (ILF)	1.000-100 km	0.3-3 kHz
4	MRMVMmyriameter	ADD	Very Low Frequency (VLF) VLF	100-10 km	3-30kHz
5	KMVKilometer	Far East	Low frequencies (LF) LF	10-1 km	30-300kHz
6	GCMVHectameter	NE	Mid frequencies (MF) VF	1000-100m	0.3-3 MHz
7	DKMVDecameter	HF	High Frequency (HF) HF	100-10m	3-30 MHz
8	MVMeter	VHF	Very High Frequency (VHF) VHF	10-1m	30-300 MHz
9	DCMVDecimeter	VHF	Ultra high frequencies (UHF) UHF	10-1 dm	0.3-3 GHz
10	SMVCentimeter	VHF	Ultra high frequencies (microwave) SHF	10-1 cm	3-30 GHz
11	MMVMillimeter	VHF	Extremely high frequencies (EHF) EHF	10-1 mm	30-300 GHz
12	DCMMVDecimillie- meter Submilli- meter	SUMMV	Hyper high frequencies (HHF)	1-0.1 mm	0.3-3 THz
13	Light			< 0,1 мм	> 3 THz

Rice. 2.2 Example of spectrum allocation between different services.

Radio waves are emitted through an antenna into space and propagate as energy electromagnetic field. And although the nature of radio waves is the same, their ability to propagate strongly depends on the wavelength.

The earth is a conductor of electricity for radio waves (albeit not a very good one). Passing over the surface of the earth, radio waves gradually weaken. This is due to the fact that electromagnetic waves excite electric currents in the surface of the earth, which consumes part of the energy. Those. energy is absorbed by the earth, and the more, the shorter the wavelength (higher the frequency).

In addition, the wave energy weakens also because the radiation propagates in all directions of space and, therefore, the further the receiver is from the transmitter, the less energy falls per unit area and the less it gets into the antenna.

Transmissions from long-wave broadcast stations can be received at distances of up to several thousand kilometers, and the signal level decreases smoothly, without jumps. Medium wave stations can be heard within a range of thousands of kilometers. As for short waves, their energy decreases sharply with distance from the transmitter. This explains the fact that at the dawn of the development of radio, waves from 1 to 30 km were mainly used for communication. Waves shorter than 100 meters were generally considered unsuitable for long-distance communications.

However, further studies of short and ultrashort waves showed that they quickly attenuate when they travel near the Earth's surface. When the radiation is directed upward, short waves return back.

Back in 1902, the English mathematician Oliver Heaviside and the American electrical engineer Arthur Edwin Kennelly almost simultaneously predicted that there is an ionized layer of air above the Earth - a natural mirror that reflects electromagnetic waves. This layer was named ionosphere.

The Earth's ionosphere should have made it possible to increase the range of propagation of radio waves to distances exceeding line of sight. This assumption was experimentally proven in 1923. RF pulses were transmitted vertically upward and the returning signals were received. Measuring the time between sending and receiving pulses made it possible to determine the height and number of reflection layers.

2.3 Influence of the atmosphere on the propagation of radio waves

The nature of the propagation of radio waves depends on the wavelength, the curvature of the Earth, the soil, the composition of the atmosphere, the time of day and year, the state of the ionosphere, the Earth's magnetic field, and meteorological conditions.

Let us consider the structure of the atmosphere, which has a significant influence on the propagation of radio waves. Depending on the time of day and year, the moisture content and air density change.

Air surrounding earth's surface, forms an atmosphere whose height is approximately 1000-2000 km. The composition of the earth's atmosphere is heterogeneous.

Rice. 2.3 Structure of the atmosphere.

The layers of the atmosphere up to a height of approximately 100-130 km are homogeneous in composition. These layers contain air containing (by volume) 78% nitrogen and 21% oxygen. The lower layer of the atmosphere 10-15 km thick (Fig. 2.3) is called troposphere. This layer contains water vapor, the content of which fluctuates sharply with changes in meteorological conditions.

The troposphere gradually turns into stratosphere. The boundary is the height at which the temperature stops falling.

At altitudes of approximately 60 km and above above the Earth, under the influence of solar and cosmic rays, air ionization occurs in the atmosphere: some of the atoms disintegrate into free electrons And ions. In the upper layers of the atmosphere, ionization is insignificant, since the gas is very rarefied (there are a small number of molecules per unit volume). As the sun's rays penetrate into denser layers of the atmosphere, the degree of ionization increases. As it approaches the Earth, the energy of the sun's rays decreases, and the degree of ionization decreases again. In addition, in the lower layers of the atmosphere, due to the high density, negative charges cannot exist for a long time; the process of restoration of neutral molecules occurs.

Ionization in a rarefied atmosphere at altitudes of 60-80 km from the Earth and above persists for a long time. At these altitudes, the atmosphere is very rarefied, the density of free electrons and ions is so low that collisions, and hence the restoration of neutral atoms, occur relatively rarely.

The upper layer of the atmosphere is called the ionosphere. Ionized air has a significant effect on the propagation of radio waves.

During the day, four regular layers or ionization maxima are formed - layers D, E, F 1 and F 2. The F 2 layer has the greatest ionization (the largest number of free electrons per unit volume).

After sunset, ionizing radiation drops sharply. Neutral molecules and atoms are reduced, which leads to a decrease in the degree of ionization. At night the layers disappear completely D And F 2, layer ionization E decreases significantly, and the layer F 2 retains ionization with some attenuation.

Rice. 2.4 Dependence of radio wave propagation on frequency and time of day.

The height of the ionosphere layers changes all the time depending on the intensity of the sun's rays. During the day, the height of the ionized layers is less, at night it is greater. In summer at our latitudes, the electron concentration of ionized layers is greater than in winter (with the exception of the layer F 2). The degree of ionization also depends on the level solar activity, determined by the number of sunspots. The period of solar activity is approximately 11 years.

In polar latitudes, irregular ionization processes associated with so-called ionospheric disturbances are observed.

There are several paths along which a radio wave arrives at the receiving antenna. As already noted, radio waves propagating over the surface of the earth and bending around it due to the phenomenon of diffraction are called surface or ground waves (direction 1, Fig. 2.5). Waves propagating in directions 2 and 3 are called spatial. They are divided into ionospheric and tropospheric. The latter are observed only in the VHF range. Ionospheric are called waves reflected or scattered by the ionosphere, tropospheric– waves reflected or scattered by inhomogeneous layers or “grains” of the troposphere.

Rice. 2.5 Paths of radio wave propagation.

Surface wave the base of its front touches the Earth, as shown in Fig. 2.6. This wave, with a point source, always has vertical polarization, since the horizontal component of the wave is absorbed by the Earth. At a sufficient distance from the source, expressed in wavelengths, any segment of the wave front is a plane wave.

The Earth's surface absorbs part of the energy of surface waves propagating along it, since the Earth has active resistance.

Rice. 2.6 Propagation of surface waves.

The shorter the wave, i.e. the higher the frequency, the greater the current induced in the Earth and the greater the losses. Losses in the Earth decrease with increasing soil conductivity, since the waves penetrate into the Earth less, the higher the soil conductivity. Dielectric losses also occur in the Earth, which also increase with wave shortening.

For frequencies above 1 MHz, surface wave is actually highly attenuated due to absorption by the Earth and is therefore not used except in local coverage. At television frequencies, the attenuation is so large that the surface wave can be used at distances of no more than 1-2 km from the transmitter.

Communication over long distances is carried out mainly by spatial waves.

To obtain refraction, that is, the return of a wave to the Earth, the wave must be emitted at a certain angle relative to the Earth's surface. The greatest angle of radiation at which a radio wave of a given frequency returns to the earth is called critical angle for a given ionized layer (Fig. 2.7).

Rice. 2.7 Influence of the radiation angle on the passage of a spatial wave.

Each ionized layer has its own critical frequency And critical angle.

In Fig. 2.7 shows a beam that is easily refracted by a layer E, since the beam enters at an angle below the critical angle of this layer. Beam 3 passes through the area E, but returns to Earth as a layer F 2 because it enters at an angle below the critical layer angle F 2. Beam 4 also passes through the layer E. It goes into the layer F 2 at its critical angle and returns to Earth. Ray 5 passes both areas and is lost in space.

All rays shown in Fig. 2.7, refer to the same frequency. If a lower frequency is used, larger critical angles are required for both regions; conversely, if the frequency increases, both regions have smaller critical angles. If you continue to increase the frequency, there will come a point when the wave propagating from the transmitter parallel to the Earth will exceed the critical angle for any area. This state occurs at a frequency of about 30 MHz. Above this frequency, space wave communication becomes unreliable.

So, each critical frequency has its own critical angle, and, conversely, each critical angle has its own critical frequency. Consequently, any spatial wave whose frequency is equal to or lower than the critical one will return to Earth at a certain distance from the transmitter.

In Fig. 2.7 ray 2 falls on layer E at a critical angle. Note where the reflected wave hits the Earth (the signal is lost beyond a critical angle); the spatial wave, having reached the ionized layer, is reflected from it and returns to Earth at a great distance from the transmitter. At a certain distance from the transmitter, depending on the transmitter power and wavelength, it is possible to receive a surface wave. From the place where the surface wave reception ends, the silence zone and it ends where the reflected spatial wave appears. The silence zones do not have a sharp boundary.

Rice. 2.8 Reception zones of surface and spatial waves.

As the frequency increases, the value dead zone increases due to a decrease in the critical angle. To communicate with a correspondent at a certain distance from the transmitter at certain times of the day and seasons, there is maximum permissible frequency, which can be used for space wave communication. Each ionospheric region has its own maximum permissible frequency for communication.

Short and, especially, ultrashort waves in the ionosphere lose an insignificant part of their energy. The higher the frequency, the shorter the distance electrons travel during their oscillations, as a result of which the number of their collisions with molecules decreases, i.e., the loss of wave energy decreases.

In lower ionized layers, losses are greater, since high blood pressure indicates a higher gas density, and with a higher gas density the probability of particle collisions increases.

Long waves are reflected from the lower layers of the ionosphere, which have the lowest concentration of electrons, at any elevation angle, including those close to 90°. Soil of average moisture is almost a conductor for long waves, so they are well reflected from the Earth. Multiple reflections from the ionosphere and the Earth explain the long-range propagation of long waves.

Long wave propagation does not depend on the time of year and meteorological conditions, on the period of solar activity and on ionospheric disturbances. When reflected from the ionosphere, long waves undergo great absorption. This is why high power transmitters are needed to communicate over long distances.

Medium waves are noticeably absorbed in the ionosphere and soil of poor and medium conductivity. During the day, only a surface wave is observed, since the sky wave (longer than 300 m) is almost completely absorbed in the ionosphere. For total internal reflection, medium waves must travel some distance in the lower layers of the ionosphere, which have, although a low concentration of electrons, but a significant air density.

At night, with the disappearance of the D layer, absorption in the ionosphere decreases, as a result of which it is possible to maintain communications using sky waves at distances of 1500-2000 km with a transmitter power of about 1 kW. Communication conditions in winter are slightly better than in summer.

The advantage of medium waves is that they are not affected by ionospheric disturbances.

According to international agreement, distress signals (SOS signals) are transmitted on waves with a length of about 600 m.

The positive side of spatial wave communication on short and medium waves is the possibility of long-distance communication with low transmitter power. But spatial wave communication has and significant shortcomings.

Firstly, instability of communication due to changes in the height of the ionized layers of the atmosphere during the day and year. To maintain contact with the same point per day, you have to change the wavelength 2-3 times. Often, due to changes in the state of the atmosphere, communication is completely disrupted for some time.

Secondly, the presence of a silence zone.

Waves shorter than 25 m They are classified as "daytime waves" because they travel well during the day. “Night waves” include waves longer than 40 m. These waves travel well at night.

The conditions for the propagation of short radio waves are determined by the state of the ionized layer Fg. The electron concentration of this layer is often disrupted due to the unevenness of solar radiation, causing ionospheric disturbances and magnetic storms. As a result, the energy of short radio waves is significantly absorbed, which degrades radio communication, even sometimes making it completely impossible. Ionospheric disturbances are observed especially often at latitudes close to the poles. Therefore, shortwave communication there is unreliable.

Most notable ionospheric disturbances have their own periodicity: they repeat through 27 days(time of revolution of the Sun around its axis).

In the short wave range, the influence of industrial, atmospheric and mutual interference is strongly affected.

Optimal communication frequencies on short waves are selected on the basis of radio forecasts, which are divided into long-term And short-term. Long-term forecasts indicate the expected average state of the ionosphere over a certain period of time (month, season, year or more), while short-term forecasts are compiled for a day, five days and characterize possible deviations of the ionosphere from its average state. Forecasts are compiled in the form of graphs as a result of processing systematic observations of the ionosphere, solar activity and the state of terrestrial magnetism.

Ultrashort waves(VHF) are not reflected from the ionosphere, they pass through it freely, i.e. these waves do not have a spatial ionospheric wave. The surface ultrashort wave, on which radio communication is possible, has two significant drawbacks: firstly, the surface wave does not bend around the earth's surface and large obstacles and, secondly, it is strongly absorbed in the soil.

Ultrashort waves are widely used where a short radio range is required (communication is usually limited to line of sight). In this case, communication is carried out by a spatial tropospheric wave. It usually consists of two components: direct beam and the beam reflected from the Earth (Fig. 2.9).

Rice. 2.9 Direct and reflected rays of a spatial wave.

If the antennas are close enough, both beams will usually reach the receiving antenna, but their intensities will be different. The beam reflected from the Earth is weaker due to losses occurring during reflection from the Earth. A direct beam has almost the same attenuation as a wave in free space. In the receiving antenna, the total signal is equal to the vector sum of these two components.

The receiving and transmitting antennas are usually the same height, so the path length of the reflected beam is slightly different from the direct beam. The reflected wave has a phase shift of 180°. Thus, neglecting losses in the Earth during reflection, if two beams travel the same distance, their vector sum is zero, as a result there will be no signal at the receiving antenna.

In reality, the reflected beam travels a slightly greater distance, therefore the phase difference in the receiving antenna will be about 180°. The phase difference is defined by the path difference in wavelength ratios rather than in linear units. In other words, the overall signal received under these conditions depends mainly on the frequency used. For example, if the operating wavelength is 360 m and the path difference is 2 m, the phase shift will differ from 180° by only 2°. As a result, there is an almost complete absence of signal in the receiving antenna. If the wavelength is 4 m, the same path difference of 2 m will cause a phase difference of 180°, completely canceling out the 180° phase shift of reflection. In this case, the signal doubles in voltage.

It follows from this that at low frequencies the use of spatial waves is not of interest for communication. It is only at high frequencies, where the path difference is commensurate with the wavelength used, that sky wave is widely used.

The range of VHF transmitters increases significantly when aircraft communicate in the air and with the Earth.

TO advantages of VHF The possibility of using small antennas should be considered. In addition, a large number of radio stations can simultaneously operate in the VHF range without mutual interference. In the wave range from 10 to 1 m, it is possible to place more simultaneously operating stations than in the short, medium and long wave ranges combined.

Relay lines operating on VHF have become widespread. Between two communication points located at a great distance, several VHF transceivers are installed, located within line of sight from one another. Intermediate stations operate automatically. The organization of relay lines allows you to increase the communication range on VHF and implement multi-channel communication (conduct several telephone and telegraph transmissions simultaneously).

Currently, much attention is being paid to the use of the VHF range for long-distance radio communications.

The most widely used are communication lines operating in the range of 20-80 MHz and using the phenomena of ionospheric scattering. It was believed that radio communication through the ionosphere is possible only at frequencies below 30 MHz (wavelength more than 10 m), and since this range is fully loaded and a further increase in the number of channels in it is impossible, the interest in the scattered propagation of radio waves is quite understandable.

This phenomenon lies in the fact that some of the energy of ultra-high frequency radiation is scattered by inhomogeneities existing in the ionosphere. These inhomogeneities are created by air currents of layers with different temperatures and humidity, wandering charged particles, ionization products of meteorite tails and other still poorly understood sources. Since the troposphere is always inhomogeneous, scattered refraction of radio waves exists systematically.

Scattered propagation of radio waves is similar to the scattering of spotlight light on a dark night. The more powerful the light beam, the more diffused light it produces.

When studying long-distance ultrashort waves, the phenomenon of a sharp short-term increase in the audibility of signals was noticed. Such bursts of a random nature last from a few milliseconds to several seconds. However, in practice they are observed throughout the day with interruptions that rarely exceed a few seconds. The appearance of moments of increased audibility is explained mainly by the reflection of radio waves from the ionized layers of meteorites burning at an altitude of about 100 km. The diameter of these meteorites does not exceed a few millimeters, and their traces stretch for several kilometers.

From meteorite trails Radio waves with a frequency of 50-30 MHz (6-10 m) are well reflected.

Every day, several billion of these meteorites fly into the earth's atmosphere, leaving behind ionized traces with a high density of air ionization. This makes it possible to obtain reliable operation of long-distance radio links when using relatively low power transmitters. An integral part of stations on such lines is auxiliary direct-printing equipment equipped with a memory element.

Since each meteorite trail lasts only a few seconds, transmission is carried out automatically in short bursts.

Currently, communications and television broadcasts via artificial Earth satellites are widely used.

Thus, according to the mechanism of radio wave propagation, radio communication lines can be classified into lines using:

the process of radio waves propagating along the earth's surface and bending around it (the so-called earthly or surface waves);

the process of propagation of radio waves within the line of sight ( straight waves);

reflection of radio waves from the ionosphere ( ionospheric waves);

the process of radio wave propagation in the troposphere ( tropospheric waves);

reflection of radio waves from meteor trails;

reflection or relay from artificial satellites Earth;

reflection from artificially created formations of gas plasma or artificially created conducting surfaces.

2.4 Features of the propagation of radio waves of various ranges

The conditions for the propagation of radio waves in the space between the transmitter and the correspondents' radio receiver are influenced by the finite conductivity of the earth's surface and the properties of the environment above the earth. This influence is different for different wave ranges (frequencies).

Myriameter And kilometer waves (ADD And Far East) can propagate both as terrestrial and ionospheric ones. The presence of an earth wave propagating over hundreds and even thousands of kilometers is explained by the fact that the field strength of these waves decreases with distance quite slowly, since the absorption of their energy by the earth or water surface is small. The longer the wave and the better the conductivity of the soil, the longer distances radio communication is provided.

Sandy, dry soils and rocks absorb electromagnetic energy to a large extent. When propagating due to the phenomenon of diffraction, they bend around the convex earth's surface and obstacles encountered along the way: forests, mountains, hills, etc. Starting from a distance of 300-400 km from the transmitter, an ionospheric wave appears, reflected from the lower region of the ionosphere (from layer D or E). During the day, due to the presence of the D layer, the absorption of electromagnetic energy becomes more significant. At night, with the disappearance of this layer, the communication range increases. Thus, the passage of long waves at night is generally better than during the day. Global communications on the LW and LW are carried out by waves propagating in a spherical waveguide formed by the ionosphere and the earth's surface.

Advantages of the VLF and LW bands:

radio waves of the VLF and DV ranges have the property of penetrating into the water column and also propagating in some soil structures;

due to waves propagating in the Earth’s spherical waveguide, communication is provided over thousands of kilometers;

the communication range depends little on ionospheric disturbances;

good diffraction properties of radio waves in these ranges make it possible to provide communications over hundreds and even thousands of kilometers using ground waves;

the constancy of the radio link parameters ensures a stable signal level at the receiving point.

FlawsSDV-,DV,- range:

Effective radiation of waves in the considered range sections can be achieved only with the help of very bulky antenna devices, the dimensions of which are commensurate with the wavelength. The construction and restoration of antenna devices of this size in a limited time (for military purposes) is difficult;

since the dimensions of actual antennas are smaller than the wavelength, compensation for their reduced efficiency is achieved by increasing the transmitter power to hundreds or more kW;

the creation of resonant systems in this range and at significant powers determines the large size of the output stages: transmitters, the difficulty of quickly tuning to another frequency;

To supply power to radio stations in the VLF and LW bands) large power plant capacities are required;

a significant disadvantage of the VLF and LW bands is their small frequency capacity;

a fairly high level of industrial and atmospheric interference;

dependence of the signal level at the receiving point on the time of day.

Region practical application radio waves VLF-, DV-range:

communication with underwater objects;

global backbone and underground communications;

radio beacons, as well as communications in long-range aviation and the Navy.

Hectometer waves(NE) can propagate by surface and spatial waves. Moreover, their communication range with a surface wave is shorter (does not exceed 1000-1500 km), since their energy is absorbed by the soil more than that of long waves. Waves reaching the ionosphere are intensively absorbed by the layer D when it exists, but is well layered E.

For medium waves, the communication range is very dependent from time of day. During the day the medium waves are so strong are absorbed in the lower layers of the ionosphere, that the sky wave is practically absent. Night layer D and the bottom of the layer E disappear, so the absorption of medium waves decreases; and spatial waves begin to play a major role. Thus, an important feature of medium waves is that during the day, communication on them is supported by a surface wave, and at night - by both surface and spatial waves simultaneously.

Advantages of the CB range:

at night in summer and during most of the day in winter, the communication range provided by the ionospheric wave reaches thousands of kilometers;

medium-wave antenna devices turn out to be quite effective and have acceptable dimensions even for mobile radio communications;

the frequency capacity of this range is greater than that of the VLF and LW bands;

good diffraction properties of radio waves in this range;

transmitter powers are lower than those in the VLF and LW bands;

low dependence on ionospheric disturbances and magnetic storms.

Disadvantages of the CB range:

the congestion of the CB band with powerful broadcast radio stations creates difficulties in its widespread use;

the limited frequency capacity of the range makes it difficult to maneuver frequencies;

the communication range on the NE during the daytime in summer is always limited, since it is only possible by ground wave;

sufficiently high transmitter powers;

it is difficult to use highly efficient antenna devices, the complexity of construction and restoration in a short time;

a fairly high level of mutual and atmospheric interference.

Area of ​​practical application of microwave radio waves; Medium-wave radio stations are most often used in Arctic regions, as backup in cases of loss of widely used short-wave radio communications due to ionospheric and magnetic disturbances, as well as in long-range aviation and the Navy.

Decameter waves (K.B.) occupy a special position. They can propagate both as ground waves and as ionospheric waves. Ground waves, with relatively low transmitter powers characteristic of mobile radio stations, propagate over distances not exceeding several tens of kilometers, since they experience significant absorption in the ground, which increases with increasing frequency.

Due to single or multiple reflections from the ionosphere, under favorable conditions, ionospheric waves can propagate over long distances. Their main property is that they are weakly absorbed by the lower regions of the ionosphere (layers D And E) and are well reflected by its upper regions (mainly the layer F2 . located at an altitude of 300-500 km above the earth). This makes it possible to use relatively low-power radio stations for direct communication over an unlimited range of distances.

A significant decrease in the quality of KB radio communications by ionospheric waves occurs due to signal fading. The nature of fading mainly comes down to the interference of several rays arriving at the receiving location, the phase of which is constantly changing due to changes in the state of the ionosphere.

The reasons for the arrival of several rays at the signal reception location may be:

irradiation of the ionosphere at angles at which the rays undergoing

different numbers of reflections from the ionosphere and the Earth converge at the receiving point;

the phenomenon of double refraction under the influence of the Earth’s magnetic field, due to which two rays (ordinary and extraordinary), reflected from different layers of the ionosphere, reach the same receiving point;

heterogeneity of the ionosphere, leading to diffuse reflection of waves from its various regions, i.e. to the reflection of beams of many elementary rays.

Fading can also occur due to polarization fluctuations of waves when reflected from the ionosphere, leading to a change in the ratio of the vertical and horizontal components of the electric field at the reception site. Polarization fadings are observed much less frequently than interference ones and account for 10-15% of their total number.

The signal level at reception points as a result of fading can vary over a wide range - tens and even hundreds of times. The time interval between deep fading is a random variable and can vary from tenths of a second to several seconds, and sometimes more, and the transition from high to low level can occur either smoothly or very abruptly. Fast level changes often overlap slow ones.

The conditions for the passage of short waves through the ionosphere vary from year to year, which is associated with almost periodic changes in solar activity, i.e. with a change in the number and area of ​​sunspots (Wolf number), which are sources of radiation that ionizes the atmosphere. The recurrence period of maximum solar activity is 11.3±4 years. During years of maximum solar activity, the maximum applicable frequencies (MUFs) increase and the operating frequency ranges expand.

In Fig. Figure 2.10 shows a typical family of daily MUF and Least Applicable Frequency (LOF) graphs for a radiated power of 1 kW.

Rice. 2.10 Progress of the MUF and NFC curves.

This family of daily charts corresponds to specific geographic areas. It follows from this that the applicable frequency range for communication over a given distance may be very small. It is necessary to take into account that ionospheric forecasts may have an error, therefore, when choosing maximum communication frequencies, they try not to exceed the line of the so-called optimal operating frequency (ORF), which is 20-30% below the MUF line. Of course, this further reduces the working width of the range section. The decrease in signal level when approaching the maximum applicable frequency is explained by the variability of ionospheric parameters.

Due to the fact that the state of the ionosphere changes, communication with sky waves requires the correct choice of frequencies during the day:

During the DAY they use frequencies of 12-30 MHz,

MORNING and EVENING 8-12 MHz, NIGHT 3-8 MHz.

It is also clear from the graphs that as the length of the radio communication line decreases, the range of applicable frequencies decreases (for distances up to 500 km at night it can be only 1-2 MHz).

Radio communication conditions for long lines turn out to be more favorable than for short ones, since there are fewer of them, and the range of suitable frequencies for them is much wider.

Ionospheric and magnetic storms can have a significant impact on the state of KB radio communications (especially in polar regions), i.e. disturbances of the ionosphere and the Earth's magnetic field under the influence of streams of charged particles emitted by the Sun. These flows often destroy the main reflective ionospheric layer F2 in the region of high geomagnetic latitudes. Magnetic storms can occur not only in the polar regions, but throughout the entire globe. Ionospheric disturbances have periodicity and are associated with the time of revolution of the Sun around its axis, which is equal to 27 days.

Short waves are characterized by the presence of silence zones (dead zones). The silence zone (Fig. 2.8) occurs during radio communication over long distances in areas to which the surface wave does not reach due to its attenuation, and the sky wave is reflected from the ionosphere over a greater distance. This occurs when using highly directional antennas when emitting at small angles to the horizon.

Advantages of the HF range:

ionospheric waves can propagate over long distances due to single or multiple reflections from the ionosphere under favorable conditions. They are weakly absorbed by the lower regions of the ionosphere (layers D and E) and well reflected by the upper ones (mainly by the F2 layer);

the ability to use relatively low-power radio stations for direct communication over an unlimited range of distances;

the frequency capacity of the HF range is significantly greater than that of the VLF, LW, and SV bands, which allows for simultaneous operation large number radio stations;

antenna devices used in the decameter wave range have acceptable dimensions (even for installation on moving objects) and can have clearly defined directional properties. They have a short deployment time, are cheap and can be easily repaired if damaged.

Disadvantages of the HF range:

radio communication by ionospheric waves can be carried out if the frequencies used are below the maximum values ​​(MUF), determined for each length of the radio communication line by the degree of ionization of the reflecting layers;

communication is possible only if the powers of the transmitters and the gains of the antennas used, with the absorption of energy in the ionosphere, provide the necessary strength of the electromagnetic field at the receiving point. This condition limits the lower limit of applicable frequencies (ULF);

insufficient frequency capacity to use broadband operating modes and frequency maneuvers;

a huge number of simultaneously operating radio stations with a long communication range creates a high level of mutual interference;

the long communication range makes it easy for the enemy to use deliberate interference;

the presence of silent zones when ensuring communication over long distances;

a significant decrease in the quality of KB radio communications by ionospheric waves due to fading of signals arising due to the variability of the structure of the reflecting layers of the ionosphere, its constant disturbance and multipath wave propagation.

Field of practical application of HF radio waves

KB radios find the widest practical application for communication with remote subscribers.

Meter waves (VHF) include a number of sections of the frequency range that have enormous frequency capacity.

Naturally, these areas differ significantly from each other in the properties of radio wave propagation. VHF energy is strongly absorbed by the Earth (in general, proportional to the square of the frequency), so the ground wave attenuates quite quickly. VHF is not characterized by regular reflection from the ionosphere; therefore, communication is calculated using ground waves and waves propagating in free space. Sky waves shorter than 6-7 m (43-50 MHz), as a rule, pass through the ionosphere without being reflected from it.

VHF propagation occurs in a straight line, the maximum range is limited by the line of sight range. It can be determined by the formula:

where Dmax – line of sight range, km;

h1 – height of the transmitting antenna, m;

h2 – height of the receiving antenna, m.

However, due to refraction (refraction), the propagation of radio waves is bent. In this case, the more accurate coefficient in the range formula will be not 3.57, but 4.1-4.5. From this formula it follows that to increase the communication range on VHF it is necessary to raise the antennas of the transmitter and receiver higher.

An increase in transmitter power does not lead to a proportional increase in communication range, so low-power radio stations are used in this range. When communicating due to tropospheric and ionospheric scattering, transmitters of significant power are required.

At first glance, the communication range of ground waves on VHF should be very short. However, it should be taken into account that as the frequency increases, the efficiency of antenna devices increases, thereby compensating for energy losses in the Earth.

The communication range of ground waves depends on the wavelength. The greatest range is achieved on meter waves, especially on waves adjacent to the HF range.

Meter waves have the property diffraction, i.e. ability to bend around uneven terrain. The increase in communication range at meter waves is facilitated by the phenomenon of tropospheric refraction, i.e. the phenomenon of refraction in the troposphere, which ensures communication on closed routes.

In the meter wavelength range, long-distance propagation of radio waves is often observed, which is due to a number of reasons. Long-range propagation can occur when sporadic ionized clouds form ( sporadic layer Fs). It is known that this layer can appear at any time of the year and day, but for our hemisphere - mainly in late spring and early summer during the daytime. A feature of these clouds is a very high ion concentration, sometimes sufficient to reflect waves of the entire VHF range. In this case, the zone of location of radiation sources relative to the receiving points is most often at a distance of 2000-2500 km, and sometimes closer. The intensity of signals reflected from the Fs layer can be very high even at very low source powers.

Another reason for the long-distance propagation of meter waves during the years of maximum solar activity may be the regular F2 layer. This distribution manifests itself in the winter months during the illuminated time of the reflection points, i.e. when the absorption of wave energy in the lower regions of the ionosphere is minimal. The communication range can reach global scales.

Long-distance propagation of meter waves can also occur during high-altitude nuclear explosions. In this case, in addition to the lower region of increased ionization, an upper one appears (at the level of the Fs layer). Meter waves penetrate through the lower region, experience some absorption, are reflected from the upper region and return to Earth. The distances covered in this case range from 100 to 2500 km. Field strength reflected nykh waves depends on frequency: the lowest frequencies undergo the greatest absorption in the lower ionization region, and the highest ones experience incomplete reflection from the upper region.

The interface between KB and meter waves occurs at a wavelength of 10 m (30 MHz). The propagation properties of radio waves cannot change abruptly, i.e. there must be a region or section of frequencies that is transitional. Such a section of the frequency range is the section 20-30 MHz. During years of minimum solar activity (as well as at night, regardless of the phase of activity), these frequencies are practically unsuitable for long-distance communication by ionospheric waves and their use is extremely limited. At the same time, under the specified conditions, the propagation properties of waves in this section become very close to the properties of meter waves. It is no coincidence that this frequency range is used in the interests of radio communications focused on meter waves.

Advantages of the VHF range:

the small dimensions of the antennas make it possible to realize pronounced directional radiation, compensating for the rapid attenuation of radio wave energy;

propagation conditions generally do not depend on the time of day and year, as well as solar activity;

the limited communication range makes it possible to repeatedly use the same frequencies on surface areas, the distance between the boundaries of which is not less than the sum of the range of radio stations with the same frequencies;

lower level of unintentional (natural and artificial origin) and intentional interference due to highly directional antennas and og limited communication range;

huge frequency capacity, allowing the use of noise-resistant broadband signals for a large number of simultaneously operating stations;

when using broadband signals for radio communications, the frequency instability of the radio line δf=10 -4 is sufficient;

the ability of VHF to penetrate the ionosphere without significant energy losses has made it possible to carry out space radio communications over distances measured in millions of kilometers;

high quality radio channel;

due to very low energy losses in free space, the communication range between aircraft equipped with relatively low-power radio stations can reach several hundred kilometers;

property of long-range propagation of meter waves;

low power of transmitters and small dependence of communication range on power.

Disadvantages of the VHF range:

short range of radio communication by ground wave, practically limited by line of sight;

when using highly directional antennas, it is difficult to work with several correspondents;

When using antennas with a circular direction, the communication range, reconnaissance immunity, and noise immunity are reduced.

Field of practical application of VHF radio waves Range used simultaneously a large number radio stations, especially since the range of mutual interference between them is usually small. The propagation properties of ground waves ensure the widespread use of ultrashort waves for communication at the tactical control level, including between various types of moving objects. Communication over interplanetary distances.

Taking into account the advantages and disadvantages of each range, we can conclude that the most acceptable ranges for low-power radio stations are the decameter (KB) and meter (VHF) wave ranges.

2.5 The influence of nuclear explosions on the state of radio communications

In nuclear explosions, instantaneous gamma radiation interacts with atoms environment, creates a stream of fast electrons flying at high speed mainly in the radial direction from the center of the explosion, and positive ions remaining almost in place. Thus, a separation of positive and negative charges occurs in space for some time, which leads to the emergence of electric and magnetic fields. Due to their short duration, these fields are usually called electromagnetic pulse (AMY) nuclear explosion. The duration of its existence is approximately 150-200 milliseconds.

Electromagnetic pulse (fifth damaging factor of a nuclear explosion) in the absence of special protective measures, can damage control and communication equipment and disrupt the operation of electrical devices connected to long external lines.

Communication, signaling and control systems are most susceptible to the effects of the electromagnetic pulse of a nuclear explosion. As a result of the impact of EMR from a ground or airborne nuclear explosion, electrical voltage is induced on the antennas of radio stations, under the influence of which breakdown of insulation, transformers, melting of wires, failure of spark gaps, damage to electronic tubes, semiconductor devices, capacitors, resistances, etc. can occur. .

It has been established that when equipment is exposed to EMR, the greatest voltage is induced on the input circuits. In relation to transistors, the following dependence is observed: the higher the gain of the transistor, the lower its electrical strength.

Radio equipment has a DC voltage strength of no more than 2-4 kV. Considering that the electromagnetic pulse of a nuclear explosion is short-lived, the ultimate electrical strength of equipment without protective equipment can be considered higher - approximately 8-10 kV.

In table 1 shows the approximate distances (in km) at which voltages exceeding 10 and 50 kV, dangerous for equipment, are induced in the antennas of radio stations at the time of a nuclear explosion.

Table 1

At greater distances, the impact of EMR is similar to the impact of a not very distant lightning discharge and does not cause damage to equipment.

The impact of electromagnetic pulses on radio equipment is sharply reduced if special protective measures are used.

The most effective way to protect electronic equipment located in buildings is the use of electrically conductive (metal) screens, which significantly reduce the voltage levels induced on internal wires and cables. Protective means similar to lightning protection means are used: arresters with drainage and locking coils, fuse-links, decoupling devices, circuits for automatically disconnecting equipment from the line.

A good protective measure is also reliable grounding of the equipment at one point. It is also effective to implement radio engineering devices block by block, with protection for each block and the entire device as a whole. This makes it possible to quickly replace a failed unit with a backup one (in the most critical equipment, units are duplicated with automatic switching if the main ones are damaged). In some cases, selenium elements and stabilizers can be used to protect against EMI.

In addition, can be applied protective entry devices, which are various relay or electronic devices that respond to excess voltage in the circuit. When a voltage pulse induced in the line by an electromagnetic pulse arrives, they cut off the power from the device or simply break the operating circuits.

When choosing protective devices, it should be taken into account that the impact of EMR is characterized by mass character, that is, the simultaneous activation of protective devices in all circuits located in the explosion area. Therefore, the protection circuits used must automatically restore the functionality of the circuits immediately after the termination of the electromagnetic pulse.

The resistance of equipment to the effects of voltage arising in lines during a nuclear explosion largely depends on the correct operation of the line and careful monitoring of the serviceability of protective equipment.

TO important operating requirements This includes periodic and timely checking of the electrical strength of the insulation of the line and input circuits of the equipment, timely identification and elimination of wire grounding problems, monitoring the serviceability of arresters, fuse-links, etc.

High altitude nuclear explosion accompanied by the formation of areas of increased ionization. In explosions at altitudes up to approximately 20 km, the ionized region is limited first by the size of the luminous region, and then by the explosion cloud. At altitudes of 20-60 km, the size of the ionized region is slightly larger than the size of the explosion cloud, especially at the upper limit of this altitude range.

During nuclear explosions at high altitudes, two areas of increased ionization appear in the atmosphere.

First area is formed in the area of ​​the explosion due to the ionized substance of the ammunition and the ionization of the air by the shock wave. The dimensions of this area in the horizontal direction reach tens and hundreds of meters.

Second area increased ionization occurs below the center of the explosion in the layers of the atmosphere at altitudes of 60-90 km as a result of absorption of penetrating radiation by air. The distances at which penetrating radiation produces ionization in the horizontal direction are hundreds and even thousands of kilometers.

Areas of increased ionization that occur during a high-altitude nuclear explosion absorb radio waves and change the direction of their propagation, which leads to significant disruption of the operation of radio equipment. In this case, interruptions in radio communication occur, and in some cases it is completely disrupted.

The nature of the damaging effect of the electromagnetic pulse of high-altitude nuclear explosions is basically similar to the nature of the damaging effect of EMR from ground and air explosions.

Measures to protect against the damaging effects of electromagnetic pulses from high-altitude explosions are the same as against EMP from ground and air explosions.

2.5.1 Protection from ionizing and electromagnetic radiation

high-altitude nuclear explosions (HEA)

RS interference can occur as a result of explosions of nuclear weapons, accompanied by the emission of powerful electromagnetic pulses of short duration (10-8 sec) and changes in the electrical properties of the atmosphere.

EMP (radio flash) occurs:

Firstly , as a result of the asymmetric expansion of the cloud of electrical discharges formed under the influence ionizing radiation explosions;

Secondly , due to the rapid expansion of a highly conductive gas (plasma) formed from the explosion products.

After an explosion in space, a fireball is created, which is a highly ionized sphere. This sphere rapidly expands (at a speed of about 100-120 km/h) above the earth's surface, transforming into a sphere of false configuration, the thickness of the sphere reaches 16-20 km. The electron concentration in the sphere can reach up to 105-106 electrons/cm3, i.e. 100-1000 times higher than the normal electron concentration in the ionospheric layer D.

High-altitude nuclear explosions (HAE) at altitudes greater than 30 km significantly affect the electrical characteristics of the atmosphere over large areas over a long period of time, and, therefore, have a strong influence on the propagation of radio waves.

In addition, the powerful electromagnetic pulse that occurs during IJV induces high voltages (up to 10,000-50,000 V) and currents of up to several thousand amperes in wired communication lines.

The power of the EMR is so great that its energy is sufficient to penetrate into the thickness of the earth up to 30 m and induce an EMF within a radius of 50-200 km from the epicenter of the explosion.

However, the main impact of INVs is that the huge amount of energy released by the explosion, as well as intense fluxes of neutrons, X-rays, ultraviolet and gamma rays lead to the formation of highly ionized areas in the atmosphere and an increase in the electron density in the ionosphere, which in turn leads to to the absorption of radio waves and disruption of the stability of the control system.

2.5.2 Characteristic signs of IJV

An IJV in or near a given area is accompanied by an immediate cessation of reception of distant stations in the HF wavelength range.

At the moment the connection is stopped, a short click is observed in the phones, and then only the receiver’s own noise and weak crackling sounds such as thunder discharges are heard.

A few minutes after communication on HF ceases, interference from distant stations in the meter wavelength range on VHF increases sharply.

The radar range and coordinate measurement accuracy are reduced.

The basis of the protection of electronic means is the correct use of the frequency range and all the factors that arise as a result of the use of INV

2.5.3 Basic definitions:

reflected radio wave (reflected wave ) – a radio wave propagating after reflection from the interface between two media or from inhomogeneities of the medium;

direct radio wave (straight wave ) – a radio wave propagating directly from sources to the receiving location;

earth radio wave (earth wave ) – a radio wave propagating near the earth’s surface and including a direct wave, a wave reflected from the earth, and a surface wave;

ionospheric radio wave (sky wave ) – a radio wave propagating as a result of reflection from the ionosphere or scattering on it;

radio wave absorption (absorption ) – a decrease in the energy of a radio wave due to its partial conversion into thermal energy as a result of interaction with the environment;

multipath propagation of radio waves (multipath propagation ) – propagation of radio waves from the transmitting to the receiving antenna along several trajectories;

effective layer reflection height (effective altitude ) is the hypothetical height of the reflection of a radio wave from the ionized layer, depending on the distribution of electron concentration over the height and length of the radio wave, determined through the time between transmission and reception of the reflected ionospheric wave during vertical sounding under the assumption that the speed of propagation of the radio wave along the entire path is equal to the speed of light in vacuum;

ionospheric jump (leap ) – the trajectory of radio wave propagation from one point on the Earth’s surface to another, the passage along which is accompanied by one reflection from the ionosphere;

maximum applicable frequency (MUHR) – the highest frequency of radio emission at which there is ionospheric propagation of radio waves between given points at a given time under certain conditions, this is the frequency that is still reflected from the ionosphere;

optimal operating frequency (ORCH) – frequency of radio emission below the IF, at which stable radio communication can be carried out under certain geophysical conditions. As a rule, the ORF is 15% lower than the MUF;

vertical ionospheric sounding (vertical sounding ) – ionospheric sounding using radio signals emitted vertically upward relative to the Earth’s surface, provided that the points of emission and reception are combined;

ionospheric disturbance – a disturbance in the distribution of ionization in the layers of the atmosphere, which usually exceeds changes in the average ionization characteristics for given geographical conditions;

ionospheric storm – prolonged ionospheric disturbance of high intensity.

Factors affecting the propagation of radio waves

The medium of propagation of radio waves can be either a natural path or an artificial one. The natural route is the earth's surface, atmosphere or outer space. Such an environment cannot be controlled, which is very important for organizing radio communications. The propagation paths of radio waves along natural paths have the form:

(FIGURE 12).

Radio waves (1) propagate in the immediate vicinity of the Earth are called terrestrial waves. The most noticeable influence on the propagation of radio waves in the atmosphere is exerted by the troposphere and ionosphere. The propagation of tropospheric waves (2) in the troposphere occurs due to scattering and reflection from inhomogeneities in the troposphere; radio waves (3) propagate by reflection from the ionosphere, or scattering in it is called ionospheric. Radio waves 4.5 are used for radio links Earth-to-space, space-to-space and do not have a special name. In free space, a radio wave has a transverse structure, i.e. the interconnected electrical and magnetic fields perpendicular to each other and to the direction of propagation. In Fig. 13, vector E characterizes at some point in time the direction of the electric field of the wave, vector H-magnetic field, vector P is the direction of propagation of the em wave. The location of vector E in space characterizes the polarization of the radio wave. Depending on the change in the direction of the vector, polarization can be linear, circular, or elliptical. With linear polarization, the vector E remains parallel to itself during propagation, periodically changing in magnitude and direction. The mathematical law of vector change, provided that in a rectangular coordinate system it changes in a plane passing through the Z axis, can be written: Ez=Emcos(?t-kz) (1) or in complex form: Ez=Em*(e**j)*cos(?t-kz) (2), where?=2πƒ-κcircular frequency, k=2π/λ – spatial frequency or wave coefficient. In the general case, the quantity k has the meaning of a vector and characterizes the direction of wave propagation. The law of change in vector H is written in a similar way due to the fact that only under this condition is the propagation of radio waves possible. In the case of propagation of a linearly polarized wave near an interface between two media, a distinction is made between vertical polarization if the vector E lies in the plane of incidence of the wave and horizontal polarization if the vector E is parallel to the interface. The concept of polarization is relative; in the general case, a wave is considered to be polarized arbitrarily relative to the interface. In this case, the vector E is decomposed into two components, one of which will correspond to the vertical polarization, and the second to the horizontal. With circular polarization, the vector E, while remaining constant in magnitude, rotates in such a way that its end describes a circle. With elliptical polarization, the vector E changes in time in direction and magnitude so that its end describes an ellipse.

The polarization of radio waves is determined by the type of transmitting antenna and physical properties environment in which radio waves propagate. Only in outer space do radio waves propagate as if in free space. Otherwise, the propagation condition is determined by the electrical properties of the Earth and atmosphere, as well as by the terrain. The earth's surface has a significant influence on the propagation of terrestrial radio waves. Her elementary properties characterized mainly by two parameters: dielectric constant? and conductivity?. Is the earth's surface uniform in depth characterized by constant parameters? And? in the entire range of radio waves longer than meter ones. On dm and shorter waves? decreasing, huh? increases with increasing frequency. Highest value? And? have liquid media, and dry soil, ice, snow, vegetation have relatively small values? And?. Therefore, depending on the frequency of radio waves, the properties of the earth's surface change. For example, for cm wavelengths, seawater is considered a dielectric, and wet soil can be considered a dielectric for meter and shorter waves. The parameters ε and γ determine the degree of absorption of radio wave energy when propagating over the earth's surface; quantitative energy losses are described by the absorption coefficient α≈6πγ/√(ε). (3)

Physical losses are caused by the transition of radio wave energy into thermal energy of the movement of molecules of the propagation medium. When a radio wave propagates in sea water and moist soil at low frequencies, the absorption coefficient increases with increasing frequency; at high frequencies it ceases to change, as is the case in a dielectric. If e.m. When a wave falls on the smooth surface of the Earth, it is partially reflected from the interface between the media and partially passes into the depths of the second medium. Therefore, in the atmosphere there are incident and reflected waves, and in the second medium there is a refracted wave. When waves are reflected, its polarization can change, and the refracted part of the wave is absorbed by the medium. The reflection of radio waves from a smooth flat surface obeys the law geometric optics. If the surface of the earth is not flat, then radio waves are reflected in various directions, including in the opposite direction. The scattered signal may have, in addition to a component of the same polarization as the incident wave, a component of orthogonal polarization. The surface is considered smooth if the maximum height of the unevenness hн satisfies the condition: hнλ/(8cosφ) (4). , γwhere? is the angle of incidence of the radio wave. For a VHF line, in which communication is carried out only at a line-of-sight distance, raising the antennas above the ground allows you to increase the length of communication. For SW and DV, an increase in the length of radio links is ensured by diffraction of radio waves, i.e. avoiding obstacles encountered on their way. The influence of the troposphere on the propagation of radio waves, as well as in the case of the propagation of terrestrial radio waves, is mainly determined by the nature of the change in the dielectric constant and conductivity of the medium, which in turn depend on physical and chemical properties gases entering the troposphere. The relative gas composition of the troposphere remains constant throughout the entire height, only the content of water vapor changes, which depends on meteorological conditions and decreases with height. When a radio wave of cm or shorter wavelengths propagates in the troposphere, it loses energy due to absorption by water droplets and scattering in them. When radio waves pass through each drop of water, polarization currents are induced, which cause energy losses. In this case, each drop re-radiates em. waves, evenly in all directions, which leads to dissipation of radio wave power. Mm waves experience additional absorption in water vapor and oxygen molecules. When radio waves are distributed in the troposphere, curvatures of the wave trajectory are observed, and the degree of curvature and direction of the wave depend on the state of the troposphere. Is this phenomenon of trajectory curvature called refraction explained by a change in dielectric constant? and the refractive index of the troposphere with height. Let us imagine the troposphere in the form of thin spherical layers with constant refractive indices in the layer and different in different layers. When a radio wave passes through the boundaries of the layers, it will be refracted. If the refractive index decreases with height, then the refractive angle increases, i.e. dn/dh 0, then negative tropospheric refraction takes place and the trajectories of radio waves are curved upward from the ground. With positive tropospheric refraction, there are 3 special cases: 1) normal refraction 2) critical refraction 3) super-refraction Normal tropospheric refraction occurs in the normal troposphere, the parameters of which (P, t, humidity height) correspond to a certain average value. The distribution trajectory of radio waves is curved towards the earth's surface, which leads to an increase in the range of the radio link. The degree of deflection of radio waves depends on the wavelength and the state of the troposphere. Under some conditions, the curvature is such that the radio wave travels parallel to the ground at a constant height. This type of refraction is called critical. When the refractive index sharply decreases with height, the radio wave is completely internally reflected from the troposphere, and it returns to the ground. This phenomenon is called superrefraction and is observed in the VHF range.

Figure 16

When the region of superrefraction occupies a significant distance above the earth's surface, VHF can propagate over very long distances. In this case, the radio wave propagates through a sequential alternation of two phenomena: refraction in the troposphere and reflection from the ground. This phenomenon is called the propagation of radio waves under tropospheric waveguide conditions. Such waveguide propagation is possible for cm and dm waves. The height of tropospheric waveguides can reach several tens of meters. In the troposphere, other conditions are created that ensure long-distance propagation of radio waves. At altitudes of 1-3 km, inversion layers are observed, i.e. layers with a sharp change in refractive index that can reflect radio waves. The thickness of the inversion layer can vary from several meters to one hundred meters. In this case, the reflection coefficient is sufficient only for the shallowest rays with a small layer thickness compared to the wavelength; it follows that sufficient intensity of reflections is observed at meter waves. Long waves are reflected weaker. When reflected from high inversion layers, radio waves can travel a distance of up to 200-400 km. However, this phenomenon, like the tropospheric waveguide for creating a regularly operating radio link, is limited by the irregularity of its manifestation. More realistic is the use of long-range tropospheric propagation for VHF scattering on tropospheric inhomogeneities. Tropospheric heterogeneities are areas in which the dielectric constant differs from the average value for the surrounding troposphere. Inhomogeneities create secondary radiation, which is multi-beam in nature. The maximum of re-emission is oriented towards the initial propagation of the wave and only a certain part towards it. The length of the radio link in the case of tropospheric scatter reaches 300-500 km. Such radio links are currently widely used where intermediate relay stations cannot be installed (above the straits, in northern and sparsely populated areas). These radio links provide good reliability for the transmission of telephone and telegraph messages. The influence of the ionosphere on the propagation of radio waves is determined by two main factors - the presence of inhomogeneities and the relatively high concentration of electrons. Ionospheric irregularities are certain areas in which the electron density differs from the average value at a given altitude. The sizes of heterogeneities can range from several meters to several kilometers. In region D, small heterogeneities up to tens of meters in size predominate, in layer E up to 200-300 m, and in layer F up to several kilometers. Although the inhomogeneities of the ionosphere are constantly changing, they are nevertheless used in radio communications at meter waves at a distance of 1-2 thousand km. The presence of electrons and ions in the ionosphere determines the value of the dielectric constant, on which the attenuation of ionospheric waves depends. The dielectric constant of an ionized gas is always 2 (5), where f is the operating frequency, Ne is the electron density. From formula (5) it is clear that at a certain value of the electron density, the dielectric constant can become equal to 0. The frequency f 0 at which ε = 0 is called the natural frequency of the ionized gas. In this case, formula (5) looks like:
(6). At f (7). From formula (7) it is clear that each frequency has its own phase velocity. This speed is > the speed of light in free space. Thus, wave dispersion manifests itself during the simultaneous propagation of several monochromatic waves of different frequencies, which almost always occurs. The spectral component of a radio signal in a dispersive medium propagates at different phase velocities, which leads to signal distortion. The group velocity is the speed of propagation of the maximum of the signal envelope. For an ionized gas, the group velocity Ugr of wave propagation in a dispersive medium is determined by the expression:
(8). Γgroup and phase velocities are related by the relation: Ugr*Uph=s 2 (9) Thus. In ionized gas, radio signals travel at speeds slower than the speed of light. It is obvious that when propagating in the ionosphere, broadband signals, which include short pulses, will experience the greatest distortion.

Pulse 1, after passing through the ionosphere, takes on form 2. When propagating through the ionosphere, pulses with a duration of several seconds undergo distortion due to dispersion. And long-term telegraph pulses are practically not distorted due to dispersion. When a radio wave propagates through the ionosphere, its trajectory is curved; at a certain dielectric constant, electron density, angle of incidence of the wave, and its operating frequency, the radio signal can be reflected from the ionosphere. In this case, the angle of incidence Θ must be equal to or exceed a certain critical angle Θcr. Reflection of radio waves is also possible during normal incidence on the ionosphere, and it occurs at the altitude where the operating frequency is equal to the natural frequency of the ionized gas. The higher the electron density, the higher the reflection condition is satisfied for higher frequencies. The maximum frequency at which a radio wave is reflected in the case of a vertical incidence on the ionosphere is called the critical frequency f CR. If the operating frequency is greater than the critical one, then during normal incidence on the ionosphere, reflection does not occur and the wave goes into outer space. During solar flares Ionospheric magnetic storms occur, deteriorating VHF and HF communications. That. The parameters of the troposphere and ionosphere fluctuate over time. This leads to random changes in the amplitude and phase of the radio signal and causes distortion. The fluctuation in signal amplitude is called fading.

Medium wave (MV) propagation

SW have =100-1000 m and can be propagated by both ground and ionospheric waves. Terrestrial radio waves (RF) of the SW range experience significant absorption in the semiconducting surface of the Earth, which limits their propagation to a distance of 500-700 km. Ionospheric radio waves of the NE range can spread over much greater distances, but this occurs at night. During the day, SW propagation occurs almost exclusively by ground waves, because the ionospheric wave is absorbed in the D layer and quickly attenuates. At night, the D layer disappears and SWs propagate by reflection from the E layer of the ionosphere. That. in the SW range at some distance from the transmitter, the simultaneous arrival of ground and ionospheric waves (IW) is possible.

Due to the fact that the length of the IW path changes according to a random law, when the electron density of the ionosphere changes, the phase difference of the waves arriving at a certain receiving point B changes. If the phase difference between the earth and IW = 0, then the signal is maximum, and if = 180 o, then it is minimal. Such a change in field strength, i.e. signal is called near field fading.

Another type of fading is also possible, the so-called far-field fading. It occurs when an IW arrives at a certain point C (Fig. 18) through one (curve 3) and two (curve 2) reflections from the ionosphere. A change in the phase difference between these two waves also leads to fluctuations in electrical voltage. fields. The shorter the , the deeper and more frequent the fading. Average duration fading in the CB range varies from 1s to 10 seconds.

Deep fading in the CB range greatly complicates the reception of information transmitted over a radio link. To combat fading on the transmitting side of the radio line, special antennas are used, in which the maximum radiation is pressed to the earth's surface. In this case, the near-fading zone moves away from the transmitter, and long-range fading will not occur at all, because a wave arriving through two reflections will be greatly weakened. In radio receiving devices, automatic gain control (AGC) is used to combat fading, which ensures that the output signal level is maintained constant regardless of the signal. input voltage fluctuations. Reducing the level of ionization in the winter months makes it possible to increase the length of radio links in the NE range in winter.

CBs find a variety of applications for building radio communications over relatively short distances (up to 1000 km). There are radio broadcasting stations on the NE. In on-board devices, SVs are used for radio communications and radio navigation.

Short Wave Propagation (SW)

HF includes RF with  = (10-100) m. They can propagate both in the form of ground waves (GW) and ionospheric waves (IW). Due to strong absorption in the earth. surface and poor diffraction conditions, the terrestrial RF HF range extends to distances of up to 100 km. Over the sea, pollutants experience less absorption, so the range of HF radio communications increases to several hundred km. If the transmitting and receiving antennas are raised above the earth's surface, the absorption of pollutants decreases, and the range of the radio link will reach up to 1000 km. This occurs, for example, in radio communications between aircraft or between an aircraft and the ground. The propagation of HF by an ionospheric wave occurs through multiple successive reflections from the F layer of the ionosphere and the earth's surface. HFs do not experience noticeable absorption when crossing layers E and D, which ensures the possibility of their propagation over arbitrarily large distances. This requires radio transmitters of relatively low power, which is a valuable feature of the HF range. Another feature of this range is the possibility of creating directed radioactive radiation, which makes it possible to reduce radiation along the earth's surface and, consequently, reduce energy losses.

To communicate with an ionospheric wave in the HF range, two conditions must be met: 1.) the waves must be reflected from the ionosphere (I); 2) they should not be strongly absorbed in the I layers.

These conditions primarily influence the choice of operating frequencies.

To reflect a wave, it is necessary that the electron density of the electron be sufficient. The operating frequency f  at which the waves will be reflected from the ionosphere at a given electron density N E and angle of incidence  0 is equal to:

(10)

From this condition, the maximum applicable frequency (MUF) is selected, which is the upper limit of the operating range. The lower limit of the operating range is determined by the degree of absorption of HF in the I.. In the HF range, a decrease in absorption occurs with increasing frequency. The lowest applicable frequency (LOF) is determined from the condition of obtaining at a certain point in space an EM field strength sufficient for reception at a given transmitter power. The electron density of radiation changes throughout the day, so during the day the operating wave range is 10-25 m, at night 35-100 m. The need to correctly select the wavelength complicates the organization of radio communications.

HF radio lines are characterized by another feature - the presence of a so-called silent zone. A silence zone (ZZ) is a ring-shaped area around the transmitter, within which radio reception is impossible. The presence of EM is explained by the fact that earth's radio waves 1 quickly attenuate, and IW 2 arrive at a certain point on the earth's surface at a considerable distance from the radio transmitter, because For IWs falling at small angles on the I., the reflection condition (10) is not satisfied and they (Fig. 19) go into outer space. The limits of the silent zone depend on the operating wavelength and the level of electron density. During the day, when communicating on waves of 10-25m, the coverage reaches 1000 km, and at night, when communicating on waves of 35-100m, the width of the coverage decreases to several hundred km. As the transmitter power increases, the magnitude also decreases.

When HF propagates, just as in the mid-wave range, the phenomenon of random changes in signal level over time is observed, i.e. fading. There are fast and slow fading.

FIGURE 20

Fast fading is a consequence of multipath propagation of radio waves (Fig. 20a). First of all, the cause of fading is the arrival at the receiving point of radio waves that have undergone one or two reflections from the I. Radio waves 2 and 3 travel different paths, so their phases are not the same. In addition, a change in electron density leads to a change in the path length of each wave. Such changes occur continuously, so fluctuations in electrical voltage. fields in the HF range are frequent and deep. Fading of radio signals is also caused by scattering of radio waves on inhomogeneities of the radiation (Fig. 20b) and interference of scattered waves. The IV on the HF range under the influence of the earth's magnetic field breaks up into two components - ordinary and extraordinary, propagating with different phase velocities (Fig. 20c). The interference of the components of a magnetically split wave also leads to fading. When reflected from the light, a rotation of the plane of polarization of the wave is also observed. If the receiving antenna receives waves of the same polarization, then random changes in the polarization of the radio wave will lead to fluctuations in the level of the incoming signal. All of the above reasons for signal fading usually operate simultaneously. A change in the absorption of radioactive substances in the I. also causes fading; in terms of the time of manifestation, they are slower.

To combat fading, directional antennas are used; they organize the reception of radio waves at spaced apart antennas, because a spacing of about 10 ensures reliable reception. Antenna polarization diversity is effective, i.e. radio reception with two antennas having mutually perpendicular polarization. Under favorable propagation conditions, HFs can bend around Earth one or more times.

Then at the receiving point, in addition to the main signal, with some delay (0.1s), the same signal may appear. This phenomenon, called radio echo, degrades the quality of radio signal reception. HF have found a wide and very diverse application, primarily in the organization of long-distance communications for radio navigation and radio broadcasting, for radar purposes for over-the-horizon detection of objects.

VHF propagation

VHF includes a relatively large wave range =10-0.001m. The VHF range is divided into subbands of meter (MV), decimeter (CM), centimeter (CM) and millimeter (MM) waves. Each of the sub-bands has its own propagation characteristics, however, there are general principles characteristic of the entire VHF range. The conditions for VHF propagation are determined primarily by the properties of the path. VHFs weakly diffract around the convex surface of the Earth and large irregularities on it and for this reason propagate over distances only slightly exceeding the line-of-sight range. In order to increase the range of VHF communications and reduce the influence of irregularities surrounding the antenna, radio lines tend to be raised as high as possible above the earth's surface. The range of the radio link, taking into account atmospheric refraction, is determined by the formula

, (11)

where h 1, h 2 is the height of the antennas in meters, D is the range of the radio link in km. If the length of the VHF radio link is much less than the maximum possible line-of-sight range, then the sphericity of the Earth and refraction in the troposphere do not affect the propagation of radio waves. Such radio links are characterized by greater stability and constancy of the radio signal level over time, if the transmitter and receiver are stationary. If at least one of the subscribers of the VHF radio line is a moving object, then the level of the radio signal changes over time due to a change in the observation angle when the object moves and the ruggedness (?) of the radiation zone of the stationary transmitting antenna.

If the length of the VHF radio link exceeds the limits of line of sight, then the quality of its operation is affected by the sphericity of the Earth, the phenomenon of refraction, as well as weather conditions. The sphericity of the Earth has a noticeable weakening effect, and tropospheric refraction mostly improves reception conditions. With normal tropospheric refraction, the length of the VHF radio link can exceed the limits of line of sight by 15. FOR terrestrial radio links with low-lying antennas, the maximum VHF propagation range does not exceed several km. With antennas raised to a height of about 20-25m, the maximum range is 40-60 km. For aircraft flying at medium altitudes, it is equal to 300-400 km. When VHF propagates over rough terrain, obstacles weaken the signals if they block the line of sight between the antennas of the receiving and transmitting devices.

At the same time, on VHF routes in mountainous conditions, an improvement in the propagation of radioactive substances is observed. For example, on routes 100-150 km long passing through mountains 1-2 km high, the phenomenon reinforcement by obstacle. This phenomenon lies in the fact that the intensity of the EM field of a radio wave at some distance beyond an obstacle turns out to be greater than in the case of propagation without an obstacle. This is explained by the fact that the top of the mountain serves as a natural passive repeater.

The field exciting the top of the mountain consists of direct wave 1 and reflected wave 2. The waves diffract at the sharp top and propagate to the area behind the mountain. Waves 3 and 4 arrive at the location of the receiving antenna A2, the sum of which significantly exceeds the signal level at this point in space when the radio waves propagate without obstacles. The phenomenon of obstacle amplification is economically beneficial and allows you to organize a radio link in the mountains without a relay station.

VHF propagation over long distances (up to 200-1000 km) is possible by scattering on irregularities in the troposphere, which act as secondary emitters. The field created near the earth's surface is the result of the interference of fields re-emitted by a large number of inhomogeneities. Sm and dm waves are well scattered on irregularities in the troposphere. range-nov. Meter-wave waves are re-emitted by irregularities in the ionosphere.

The maximum length of a radio line using ionospheric waves of the meter range reaches 2000-2300 km. Such radio communications have a great advantage over short-wave communication lines in the possibility of round-the-clock operation on the same frequency without noticeable communication disruptions.

Ultra-long-range communication on VHF can be based on the use of the phenomenon of super-refraction in the troposphere. If the superrefractive region occupies a significant volume above the earth's surface, then VHF propagation over long distances is ensured under the conditions of the so-called tropospheric waveguide. This connection has disadvantages: 1) reception of radio waves is possible if the receiver and transmitter are located within the waveguide; 2) the irregular appearance of waveguides cannot provide stable communication over long distances.

The phenomenon of superrefraction also has a negative side. It can cause mutual interference created by stations operating in the cm-range, as well as interference with radar stations for detecting airborne objects.

VHF is widely used on space radio links, divided into Earth-to-space and space-to-space types. Interplanetary plasma has a weak absorbing or scattering effect on radio waves. On the Earth-to-space link, signal attenuation is crucial due to the large length of the path and absorption in the Earth’s atmosphere. For space systems Optimal connections are waves with a length of 3 to 10 cm.

In modern radio communication lines, VHF occupies a special place, because have a number of advantages compared to RVs of other ranges:

1. The VHF band occupies a very wide spectrum of frequencies, which allows you to place in it a large number of radio equipment operating simultaneously without mutual interference, as well as maneuver their operating wavelength.

2. In the VHF range it is possible to create broadband radio links, such as television lines or broadband FM radio links.

3. The use of VHF makes it relatively easy to carry out highly directional radiation and reception of radio waves using relatively small antennas.

4.VHF radio reception is less susceptible to atmospheric and industrial interference.

5. Limiting the range of VHF propagation ensures relative secrecy of information transmission.

MV and UHF are used for transmitting TV images, for radio communication between aircraft and with ground points. Sm waves are suitable for communication lines for a wide range of purposes, and mm waves are also used for the same communication.

In most cases, the receiving and transmitting antennas, or at least one of them, are located at such distances from the earth's surface that it is necessary to take into account its influence on the propagation of radio waves. In this case, the electric field at the receiving site can be represented as a combination of a primary field corresponding to the field of a vibrator in an unlimited homogeneous medium in the absence of the earth's surface, and a secondary field due to the general influence of the Earth on the processes of radio wave propagation.

To determine the magnitude of the electric field strength, it is first necessary to know the electrical parameters - the dielectric constant and conductivity of various types of the earth's surface. In table Table 2.1 shows the values ​​of the electrical parameters of the most typical types of the earth's surface in a wide range of waves. These values ​​were determined experimentally by the absorption and reflection of radio waves by various surfaces. It is characteristic that for the earth's surface, uniform in depth, in the entire range of radio waves longer than meter, the parameters ε and γ do not depend on the operating frequency, but at decimeter and shorter waves ε decreases, and γ increases with increasing frequency.

Most (71%) of the globe is water. The electrical properties of water depend on the degree of its salinity: with increasing salinity, the specific electrical conductivity γ increases (at waves longer than 3 cm).

Conditionally consider the maritime and fresh water, although the salt content in the water of different seas is not the same. Fresh water also contains various impurities. Therefore, in Table. 2.1 indicates the limits of possible changes in the value of γ.

The electrical properties of soil depend on its structure, degree of moisture, homogeneity, and temperature. With increasing humidity, the electrical conductivity of the soil increases.

The earth's surface is heterogeneous in depth. It can usually be thought of as a structure consisting of an upper layer that is no more than a few meters thick and a lower layer that extends to infinity. Ratio dielectric constants and the conductivities of the layers may be different. So, if the upper layer is more humid, and the soil below is dry, then the values ​​of ε and γ in the upper layer are greater than in the lower; when the upper layer freezes, its parameters ε and γ may become smaller than in the lower layer.

Vegetation, snow, ice covering the soil can be considered as semi-conducting layers lying on the surface of the soil.

Let us estimate the ratio of the density of conduction currents and displacement currents in various types earth's surface. Using formula (1.38) and the parameters ε and γ indicated in table. 2.1, we see that for sea water equality of the density of conduction currents and displacement currents occurs at a wavelength

Therefore, for radio waves in the centimeter range sea ​​water can be considered a dielectric.

For wet soil, the condition 60γλ / ε = 1 is satisfied on the wave

Wet soil can be considered as a dielectric for meter and shorter waves.

Thus, for centimeter waves, all types of the earth's surface have properties close to those of an ideal dielectric.

The absorption coefficients α and phase velocity β during the propagation of radio waves in sea water and wet soil at low frequencies, as can be seen from formula (1.57), increase with increasing frequency. At high frequencies, these quantities, according to equations (1.54) and (1.56), cease to change with increasing frequency, as is the case in an ideal dielectric. Graphs of the frequency dependence of α and υ f are presented in Fig. 2.1 and 2.2.

The graphs show that the absorption of radio waves in seawater is significantly higher than the absorption of radio waves in moist soil.

If Maxwell had not predicted the existence of radio waves, and Hertz had not discovered them in practice, our reality would have been completely different. We could not quickly exchange information using radio and mobile phones, explore distant planets and stars using radio telescopes, observe airplanes, ships and other objects using radars.

How do radio waves help us with this?

Radio wave sources

The sources of radio waves in nature are lightning - giant electrical spark discharges in the atmosphere, the current strength of which can reach 300 thousand amperes and the voltage can reach a billion volts. We see lightning during a thunderstorm. By the way, they arise not only on Earth. Lightning flashes have been detected on Venus, Saturn, Jupiter, Uranus and other planets.

Almost all cosmic bodies (stars, planets, asteroids, comets, etc.) are also natural sources of radio waves.

In radio broadcasting, radar, communication satellites, fixed and mobile communications, and various navigation systems, radio waves obtained artificially are used. The source of such waves are high-frequency generators of electromagnetic vibrations, the energy of which is transmitted into space using transmitting antennas.

Properties of radio waves

Radio waves are electromagnetic waves whose frequency ranges from 3 kHz to 300 GHz and length from 100 km to 1 mm, respectively. When spreading in the environment, they obey certain laws. When moving from one medium to another, reflection and refraction are observed. The phenomena of diffraction and interference are also inherent in them.

Diffraction, or bending, occurs if there are obstacles in the path of radio waves that are smaller than the wavelength of the radio wave. If their sizes turn out to be larger, then radio waves are reflected from them. Obstacles can be of artificial (structures) or natural (trees, clouds) origin.

Radio waves are also reflected from the earth's surface. Moreover, the surface of the ocean reflects them about 50% stronger than the land.

If the obstacle is a conductor of electric current, then the radio waves give some part of their energy to it, and an electric current is created in the conductor. Part of the energy is spent on exciting electric currents on the Earth's surface. In addition, radio waves radiate from the antenna in circles in different directions, like waves from a pebble thrown into water. For this reason, radio waves lose energy and attenuate over time. And the farther the radio wave receiver is from the source, the weaker the signal that reaches it.

Interference, or superposition, causes radio waves to strengthen or weaken each other.

Radio waves travel in space at a speed equal to the speed of light (by the way, light is also an electromagnetic wave).

Like any electromagnetic waves, radio waves are characterized by wavelength and frequency. Frequency is related to wavelength as follows:

f = c/ λ ,

Where f – wave frequency;

λ - wavelength;

c - speed of light.

As you can see, the longer the wavelength, the lower its frequency.

Radio waves are divided into the following ranges: ultra-long, long, medium, short, ultra-short, millimeter and decimmillimeter waves.

Radio propagation

Radio waves of different lengths do not travel equally in space.

Ultra long waves(wavelengths of 10 km or more) easily bend around large obstacles near the Earth's surface and are very weakly absorbed by it, so they lose less energy than other radio waves. Consequently, they also fade much more slowly. Therefore, in space such waves propagate over distances of up to several thousand kilometers. The depth of their penetration into the environment is very great, and they are used for communication with submarines located at great depths, as well as for various studies in geology, archeology and engineering. The ability of ultra-long waves to easily circle the Earth makes it possible to study the Earth's atmosphere with their help.

Long, or kilometer, waves(from 1 km to 10 km, frequency 300 kHz - 30 kHz) are also subject to diffraction, and therefore can propagate over distances of up to 2,000 km.

Average, or hectometer, waves(from 100 m to 1 km, frequency 3000 kHz - 300 kHz) they bend around obstacles on the Earth’s surface worse, are absorbed more strongly, and therefore attenuate much faster. They extend over distances of up to 1,000 km.

Short waves behave differently. If we tune a car radio in a city to a short radio wave and start moving, then as we move away from the city, the reception of the radio signal will get worse, and at a distance of about 250 km it will stop completely. However, after some time the radio broadcast will resume. Why is this happening?

The thing is that short-range radio waves (from 10 m to 100 m, frequency 30 MHz - 3 MHz) at the surface of the Earth attenuate very quickly. However, waves leaving at a large angle to the horizon are reflected from the upper layer of the atmosphere - the ionosphere, and return back, leaving behind hundreds of kilometers of a “dead zone”. These waves are then reflected from the earth's surface and again directed to the ionosphere. Repeatedly reflected, they are able to circle the globe several times. The shorter the wave, the greater the angle of reflection from the ionosphere. But at night the ionosphere loses its reflectivity, so communication on short waves is worse in the dark.

A ultrashort waves(meter, decimeter, centimeter wavelengths shorter than 10 m) cannot be reflected from the ionosphere. Spreading in a straight line, they penetrate it and go higher. This property is used to determine the coordinates of air objects: airplanes, flocks of birds, the level and density of clouds, etc. But ultrashort waves also cannot bend around the earth’s surface. Due to the fact that they propagate within line of sight, they are used for radio communications at a distance of 150 - 300 km.

In their properties, ultrashort waves are close to light waves. But light waves can be collected into a beam and directed to the desired location. This is how a spotlight and a flashlight work. The same applies to ultrashort waves. They are collected with special antenna mirrors and a narrow beam is sent in the desired direction, which is especially important, for example, in radar or satellite communications.

Millimeter waves(from 1 cm to 1 mm), the shortest waves in the radio range, similar to ultrashort waves. They also spread in a straight line. But a serious obstacle for them is precipitation, fog, and clouds. In addition to radio astronomy and high-speed radio relay communications, they have found application in microwave technology used in medicine and in everyday life.

Submillimeter, or decimmillimeter waves (from 1 mm to 0.1 mm), according to the international classification, also belong to radio waves. IN natural conditions they almost don't exist. They occupy a negligibly small share of the solar spectrum energy. They do not reach the Earth's surface, as they are absorbed by water vapor and oxygen molecules in the atmosphere. Created by artificial sources, they are used in space communications, to study the atmospheres of the Earth and other planets. The high degree of safety of these waves for the human body allows them to be used in medicine for scanning organs.

Submillimeter waves are called “waves of the future.” It is quite possible that they will give scientists the opportunity to study the structure of the molecules of substances in a completely new way, and in the future, perhaps, they will even allow them to control molecular processes.

As you can see, each radio wave range is used where the features of its propagation are used to maximum benefit.

Communication with submarines when they are submerged is a fairly serious technical challenge. The main problem is that electromagnetic waves with frequencies used in traditional radio communications are greatly attenuated when passing through a thick layer of conductive material, which is salt water.

In most cases, the simplest solution is sufficient: float to the very surface of the water and raise the antenna above the water. But this solution is not enough for a nuclear submarine. These ships were developed during cold war and could remain submerged for several weeks or even months. But, nevertheless, they had to quickly launch ballistic missiles in the event of a nuclear war.

Being at periscope depth, the boat can raise that same periscope and use the antennas installed on it for radio communication. The problem is that such a periscope, hung with antennas, will give the boat away perfectly, since it can be detected by a variety of enemy radars. It is interesting that they try to make the periscopes of modern boats in their surface part inconspicuous (using technology, so to speak, “Stealth”). Moreover, they try to minimize the time the periscope is present above the water: for example, the periscope can rise, perform a very fast scan of the horizon, transmit short messages via satellite using a special type of signal, and immediately hide back under the water.

Communication with submarines underwater is carried out in the following ways:

Acoustic transmission

Sound can travel far enough in water that underwater speakers and hydrophones can be used for communication. In any case, the navies of both the USSR and the USA installed acoustic equipment on seabed areas that were frequently visited by submarines, and connected them with submarine cables to land-based communication stations.

One-way communication in a submerged position is possible through the use of explosions. A series of explosions that follow at certain intervals propagate through the underwater sound channel and are received by hydroacoustics.

Radio communications in the very low frequency range

Very low range radio waves (VLF, VLF, 3-30 kHz) can penetrate seawater to depths of up to 20 meters. This means that a submarine located at shallow depths can use this range for communication. Even a submarine located much deeper can use a buoy with an antenna on a long cable. The buoy can be located at a depth of several meters and, due to its small size, is not detected by enemy sonars. One of the first VLF transmitters, “Goliath,” was built in Germany in 1943, transported to the USSR after the war, restored in the Nizhny Novgorod region in 1949-1952 and is still in use today.

Extremely low frequency radio waves (ELF, up to 3 kHz) easily pass through the Earth and sea water. Building an ELF transmitter is an extremely difficult task due to the enormous wavelength. The Soviet ZEUS system operates at a frequency of 82 Hz (wavelength - 3658.5 km), the American "Seafarer" (English navigator) - 76 Hz (wavelength - 3947.4 km). The wavelength in these transmitters is comparable to the radius of the Earth. It is obvious that the construction of a half-wavelength dipole antenna (with a length of ≈ 2000 km) is unrealistic on this moment task.

Instead, you should find a region of the Earth with a sufficiently low specific conductivity and drive 2 huge electrodes into it at a distance of about 60 km from each other. Since the Earth's conductivity in the area of ​​the electrodes is quite low, the electric current between the electrodes will penetrate deep into the Earth's interior, using them as part of a huge antenna. Due to the extremely high technical complexity of such an antenna, only the USSR and the USA had ELF transmitters.

Satellites

If the submarine is on the surface, it can use the normal radio range, like other seagoing vessels. This does not mean using the usual shortwave band: most often it is communication with a military communications satellite. In the United States, such a communications system is called the Submarine Satellite Information Exchange Sub-System (SSIXS), part of the Navy Ultra High Frequency Satellite Communications System (UHF SATCOM). ).

Auxiliary submarines

In the 1970s, the USSR developed a project for modifying Project 629 submarines to use them as signal repeaters and ensure communication between ships from anywhere in the world with the Navy command. Three submarines were modified under the project.

Aircraft

Being at a shallow depth, the boat can receive low-frequency radio waves (for example, “short waves”) - they penetrate to a certain depth under the surface of the water. In this case, in general, radio waves with lower frequencies penetrate somewhat deeper under the surface of the water. This is how it is possible to receive messages from airplanes.

Stealth

Communication sessions, especially when the boat surfaces, violate its secrecy, exposing it to the risk of detection and attack. Therefore, various measures are being taken to increase the secrecy of the boat, both technical and organizational. Thus, boats use transmitters to transmit short pulses in which all the necessary information is compressed. Also, transmission can be carried out by a pop-up and sub-pop-up buoy. The buoy can be left by the boat at a specific location for data transmission, which starts when the boat itself has already left the area.