What is radio waves light emission. Theory of radio waves: educational program. Radio spectrum allocation

The dielectric constant of the ionized gas is less than unity and depends on the vibration frequency. Media in which the speed of propagation of radio waves depends on frequency are called dispersive media. In dispersive media, phase and group velocities of radio wave propagation are distinguished. The speed that characterizes the speed of movement of the wave front is called phase. The phase velocity is determined by the formula (1.45) or (for media, approaching in their properties to a dielectric) (1.55). Therefore, for an ionized gas without taking into account losses according to expression (4.8)

Consequently, each frequency has its own phase speed, and this speed is greater than the speed of light.

In order to transmit a signal, it is necessary to create some disturbance - the beginning of the transmission of sinusoidal oscillations, a break or an impulse, that is, to transmit a certain group of waves (Fig. 4.8).

In a nondispersive medium, a group of waves is transmitted undistorted. In a dispersive medium, each of the frequencies of the pulse spectrum is transmitted at a different speed, and the pulse as a whole is transmitted at a different speed. To determine the group speed of the games of wave propagation in a dispersive medium, one should use the formula known from the course "Electrodynamics":

After calculating the differential of the denominator

equation (4.36) is simplified:

Comparison of formulas (4.35) and (4.37) shows the relationship between the phase and group velocities of wave propagation in an ionized gas:

υ gr υ Ф \u003d с 2. (4.38)

Thus, in an ionized gas, the signal propagates at a speed less than the speed of light.

When the operating frequency approaches the natural frequency of the ionized gas (ω → ω 0), the group velocity decreases (υ gr → 0), and the phase velocity sharply increases (υ f → ∞). In fact, due to the loss of wave energy in a real ionized gas, the phase velocity reaches a large finite value.

To transmit a pulse, it is necessary to transmit a certain frequency band, the width of which is inversely proportional to the pulse duration. Each of the groups of pulse harmonics propagates with its own group velocity. If the pulse is not very short and its spectrum is not wide, then the difference in the group velocities of individual groups of pulse harmonics is small and it can be assumed that the entire pulse propagates at a speed corresponding to the group velocity of the carrier frequency. Short pulses contain a wide spectrum of frequencies and are distorted when passing through the ionosphere. The nature of the distortion of a rectangular pulse is shown in Fig. 4.9.

A group of high harmonics propagates with a large group velocity and creates an impulse - a precursor (see Fig. 4.9, part a-b). The main part of the energy - the "body" of the pulse (see Fig. 4.9, part b-c) propagates at a speed corresponding to the carrier frequency. A group of low harmonics propagates with a lower group velocity and creates a retarded pulse (see Fig. 4.9, part c-d), the pulse itself turns out to be "blurred". Distortions are severe when the pulse is short and the carrier frequency is close to the natural frequency of the ionized gas. When propagating through the ionosphere, dispersion distortions undergo pulses with a duration of several microseconds. Long telegraph pulses are practically not distorted due to dispersion.

WHAT ARE RADIO WAVES

Radio waves are electromagnetic vibrations that propagate through space at the speed of light (300,000 km / sec). By the way, light is also electromagnetic waves with properties similar to radio waves (reflection, refraction, attenuation, etc.).

Radio waves carry energy emitted by an electromagnetic oscillator through space. And they are born when the electric field changes, for example, when an alternating electric current passes through a conductor or when sparks slip through space, i.e. a series of rapidly following one after another current pulses.

Electromagnetic radiation is characterized by frequency, wavelength and power of the transmitted 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. The frequency is measured in hertz (Hz) - units named after the great German scientist Heinrich Rudolf Hertz. 1 Hz is one oscillation per second, 1 megahertz (MHz) is a million oscillations per second. Knowing that the speed of movement of electromagnetic waves is equal to the speed of light, it is possible to 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 roughly,
where f is the frequency of electromagnetic radiation in MHz.

The formula shows that, for example, a frequency of 1 MHz corresponds to a wavelength of approx. 300 m. With increasing frequency, the wavelength decreases, with decreasing - guess yourself. In the following, we will make sure that the wavelength directly affects the length of the radio communication antenna.

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

Another useful property of electromagnetic waves is their ability to bend around some obstacles on their way. But this is possible only when the dimensions of the object are less than the wavelength, or comparable to it. For example, to detect an airplane, the length of the radar 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. Think of the military stealth technology that develops geometries, absorbing materials and coatings to reduce the visibility of objects to locators.

The energy carried by electromagnetic waves depends on the power of the generator (emitter) and the distance to it. Scientifically, it sounds like this: the energy flux 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.

SPECTRUM DISTRIBUTION

Radio waves used in radio engineering cover an area or more scientifically - the 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 visible light waves, then - the spectrum of ultraviolet, X-ray and gamma rays - all these are electromagnetic oscillations of the same nature, differing only in wavelength and, therefore, in frequency.

Although the entire spectrum is divided into regions, the boundaries between them are outlined conventionally. Regions follow continuously one after another, pass one into another, and in some cases overlap.

By international agreements, the entire spectrum of radio waves used in radio communication is divided into ranges:

Range frequencies	Frequency range name	Name wave range	Wavelength
	Very low frequencies (VLF)	Myriameter
	Low frequencies (LF)	Kilometer
300-3000 kHz	Mid frequencies (midrange)	Hectometric
	Treble (HF)	Decameter
	Very high frequencies (VHF)	Meter
300-3000 MHz	Ultra High Frequency (UHF)	Decimeter
	Ultra-high frequencies (microwave)	Centimeter
	Extreme high frequencies (EHF)	Millimeter
300-3000 GHz	Hyper-high frequencies (HHF)	Decimillimeter

Allocation of spectrum between different services.

This breakdown is quite confusing, which is why many services use their own "internal" terminology. Usually, the following names are used to designate the bands allocated for land mobile communications:

	Frequency range	Explanations
		Due to the nature of distribution, it is mainly used for long-distance communications.
	25.6-30.1 MHz	The civilian range in which individuals can communicate. In different countries, this section is allocated from 40 to 80 fixed frequencies (channels).
		Range of mobile terrestrial communications. It is not clear why, but in the Russian language there was no term that defines this range.
	136-174 MHz	The most common range of mobile terrestrial communications.
	400-512 MHz	Range of mobile terrestrial communications. Sometimes this section is not allocated to a separate range, but they say VHF, implying a frequency band from 136 to 512 MHz.
	806-825 and 851-870 MHz	Traditional "American" range; widely used by mobile communications in the United States. We have not received much distribution.

The official names of the frequency bands should not be confused with the names of the areas allocated to various services. It should be noted that the world's major manufacturers of equipment for mobile terrestrial communications produce models designed to work within these areas.

In the future, we will talk about the properties of radio waves in relation to their use in land mobile radio communications.

HOW RADIO WAVES SPREAD

Radio waves are radiated through the antenna into space and propagated as energy in an electromagnetic field. Although the nature of radio waves is the same, their propagation ability is highly dependent on wavelength.

For radio waves, earth is a conductor of electricity (although 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, for which part of the energy is spent. Those. energy is absorbed by the earth, and the more, the shorter the wavelength (higher frequency).

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

Long-wave broadcasting stations can be received at a distance of several thousand kilometers, and the signal level decreases smoothly, without jumps. Medium wave stations can be heard within a thousand kilometers. As for short waves, their energy sharply decreases with distance from the transmitter. This explains the fact that at the dawn of radio development, 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 have shown that they quickly decay when they travel near the Earth's surface. When the radiation is directed upward, short waves come back.

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

The ionosphere of the Earth was supposed to allow increasing the range of propagation of radio waves at distances exceeding line of sight. This assumption was experimentally proven in 1923. RF pulses were transmitted vertically upward and returned signals were received. Measurements of the time between sending and receiving pulses made it possible to determine the height and number of reflection layers.

Long and short wave propagation.

After being reflected from the ionosphere, short waves return to the Earth, leaving hundreds of kilometers of the "dead zone" under them. Having traveled to the ionosphere and back, the wave does not "calm down", but is reflected from the Earth's surface and again rushes to the ionosphere, where it is again reflected, etc. Thus, repeatedly reflecting, a radio wave can go around the globe several times.

It was found that the reflection height depends primarily on the wavelength. The shorter the wave, the higher its reflection occurs and, therefore, the larger the "dead zone". This dependence is valid only for the short-wavelength part of the spectrum (up to about 25-30 MHz). For shorter wavelengths, the ionosphere is transparent. Waves penetrate her and go into outer space.

The figure shows that the reflection depends not only on the frequency, but also on the time of day. This is due to the fact that the ionosphere is ionized by solar radiation and gradually loses its reflectivity with the onset of darkness. The degree of ionization also depends on solar activity, which changes throughout the year and from year to year in a seven-year cycle.

Reflection layers of the ionosphere and propagation of short waves depending on frequency and time of day.

VHF radio waves are more similar in properties to light rays. They practically do not reflect from the ionosphere, very slightly bend around the earth's surface and spread within the line of sight. Therefore, the range of action of ultrashort waves is short. But this has a definite advantage for radio communications. Since in the VHF range the waves propagate within the line of sight, it is possible to place radio stations at a distance of 150-200 km from each other without mutual influence. And this allows neighboring stations to use the same frequency many times.

Propagation of short and ultrashort waves.

The properties of radio waves in the DTSV and 800 MHz ranges are even closer to light rays and therefore have another interesting and important property. Let's remember how a flashlight works. Light from a light bulb located at the focus of the reflector is collected in a narrow beam of rays that can be sent in any direction. Roughly the same can be done with high frequency radio waves. You can collect them with antenna mirrors and send them in narrow beams. It is impossible to build such an antenna for low-frequency waves, since its dimensions would be too large (the diameter of the mirror should be much larger than the wavelength).

The possibility of directed wave emission improves the efficiency of the communication system. This is due to the fact that a narrow beam provides less energy dissipation in side directions, which allows the use of less powerful transmitters to achieve a given communication range. Directional radiation creates less interference with other communication systems that are not in line with the beam.

Receiving radio waves can also take advantage of directional radiation. For example, many are familiar with parabolic satellite dishes that focus the radiation from a satellite transmitter to a point where a receiving sensor is installed. The use of directional receiving antennas in radio astronomy has made many fundamental scientific discoveries possible. The ability to focus high-frequency radio waves has ensured their widespread use in radar, radio relay communications, satellite broadcasting, wireless data transmission, etc.

Parabolic directional satellite dish (photo from ru.wikipedia.org).

It should be noted that with decreasing wavelength, the attenuation and energy absorption in the atmosphere increase. In particular, the propagation of waves shorter than 1 cm begins to be influenced by such phenomena as fog, rain, clouds, which can become a serious obstacle limiting the communication range.

We have found that radio waves have different propagation properties depending on wavelength and that each portion of the radio spectrum is used where its benefits are best exploited.

Historically, radio waves were discovered when light phenomena were already fairly well studied, and when Maxwell's theory already existed, which described light waves as elastic waves in the ether, propagating in it with a characteristic speed c... When it was found that the speed of radio waves coincides with this speed [F2], then, to joy, they decided that light and radio waves have the same physical nature, differing only in their frequency ranges. Until now, textbooks and reference books feature the "scale of electromagnetic waves", which covers all conceivable frequencies - from zero to infinity. This state of affairs is all the more surprising since direct indications of the fundamentally different nature of light and radio waves have long been known.

The main difference between the two is that light is a quantum transfer of energy, while radio waves are wave. Note that we are talking about the physical essence of these phenomena, and not about their mathematical description. Mathematically, both light and radio waves can be described both in terms of waves and in terms of quanta: paper will endure everything. Physically, there is a big difference. When a radio wave is emitted, propagated and received, charged particles can move at the wave frequency. As long as the generator works, continuously driving charges along the emitting antenna, charged particles flutter in the surrounding space just as long. In the case of light, there is no movement of charged particles at the light frequency. Where do they come from if the energy transfer mechanism is completely different? By the way, even an electron with its low mass, being free, could not vibrate at light frequencies due to its inert properties. At first, it was believed that electrons bound in atoms were capable of this - for example, according to J.J. Thomson's model of the atom. But this model was abandoned in favor of the Rutherford-Bohr model ... the concept of photons was formed ... which, according to the resolution of the First Solvay Congress, are emitted and absorbed by atoms instantly... From this it followed that in atoms there are no oscillations of charged particles that set the photon frequency. You see where the orthodox themselves have come: in the case of radio waves, oscillations of charged particles are present, but in the case of light, they are not. But they continued to attribute the same physical nature to radio waves and light. To make it more mysterious!

But this difference between the presence or absence of oscillations of charged particles is due not just to the difference in frequency ranges - in this case, it is a matter of principle [D10]. The navigator, whose work we outlined above ( 3.4 ), serves only quantum transfers of energy, namely, transfers of quanta of excitation energy from atom to atom, but certainly not from electron to electron. Because an object capable of acquiring and giving off excitation energy must have an appropriate structural organization, which provides an internal degree of freedom that allows the very possibility of excitation energy. And a free electron, which is an elementary particle, does not have such an internal degree of freedom. Therefore, an electron cannot acquire a quantum of excitation energy and, accordingly, cannot give it up.

What has been said is enough to realize that light and radio waves are fundamentally different physical phenomena. We will return to the question of the nature of radio waves below ( 5.3 ), but now note the following. When comparing the traditional concepts of light as flying photons, and our concepts of it as a chain of quantum transitions of excitation energy from atom to atom, their fundamental difference is striking. In the traditional approach, light "spat out" by matter has a self-sufficient existence independent of matter: a photon is supposedly capable of flying long light years in interstellar space until it hits an atom that will absorb it. In our approach, light does not exist in isolation from matter, because light energy is localized only on atoms, and, during quantum transfers from one atom to another, it does not move along the space separating the atoms. And now, since the academicians have enlisted the photon in the four fundamental, absolutely stable particles, the academicians, in defense of the idea of \u200b\u200bthe existence of photons independent of matter, have a touching thought experiment. Suppose, they say, a powerful flash of light was generated from us ten light years away, after which the emitter was immediately dismantled ... and we barely managed to build the receiver by the end of the tenth year - but we still received the light signal. Where, they say, was the light energy all these ten years, when the emitter was no longer there, and the receiver was not yet? We answer: light energy was transferred from atom to atom in interstellar space, moving towards the receiver under construction. “Then,” the academics exclaim solemnly, “the limiting intensity of the transmitted light would be determined by the concentration of atoms over which it was“ thrown ”! The lower this concentration, the worse the light would be transmitted! But this is not so: in laboratories we pass laser intensities through an ultrahigh vacuum! " Yes, it works in laboratories. But it turns out because here the volumes with an ultrahigh vacuum are small: for sender atoms located at the entrance window of a vacuum chamber, the Navigator successfully finds recipient atoms at its exit window or on a target inside it. Here, the "laser intensity" is passed through a short section of ultra-high vacuum as if this section does not exist at all. But if a section with an ultra-high vacuum had a sufficiently large extent, then everything would have happened differently. It seems logical to us that the Navigator has a certain maximum radius of space scanning in search of a recipient atom. If, upon reaching this limiting radius, the recipient atom is not detected, then the scan ends (and, possibly, its new cycle begins immediately). Then, with a sufficiently large length of a section of the path of light through a high vacuum, it is precisely a low concentration of matter that should serve as a limiter of the light transmission capacity of this section.

And, in fact, there is evidence that this is how everything happens in outer space. Why, for example, is the "solar constant" constant? the power of solar radiation per unit area at the radius of the Earth's orbit? Indeed, even in the years of the active Sun, with increased sunspot formation and a corresponding increased energy output to the outside, the named power practically does not change [R5]. They usually try to explain this phenomenon of stabilization of the solar radiation power by some mechanism of automatic regulation inherent in the Sun. It is difficult to believe in such a mechanism, looking at video footage of the Sun's surface: this surface boils and spews out monstrous prominences. Energy is bursting out, but something is holding it back. And it seems to us a plausible version that “ the flow of electromagnetic energy coming from the Sun is stabilized by the limited carrying capacity of a highly rarefied space environment"[K5]. If the concentration of atoms in interplanetary space was an order of magnitude greater, the Sun would have burned us. Here, look: when a large comet passed between the Sun and the Earth and “gazed” strongly enough, its tail, directed from the Sun, formed a cross section with an increased concentration of matter. Through this section, the Sun baked the Earth more than usual, which caused a surge in climatic anomalies and natural disasters. It seems that the fame that comes from the depths of centuries about comets, as the harbingers of misfortune and cataclysms, is based not on superstition, but on real cause-and-effect relationships.

But this story is, so to speak, the affairs of bygone days. Is there anything more modern, from the cutting edge of science and technology? But how! This is a cautionary tale about how shamefully the idea of \u200b\u200bhitting space objects with laser beams failed. After all, they made samples of gas-dynamic combat lasers that burn through armor and knock down cruise missiles. True, they do this near the surface of the Earth, in a standard atmosphere. If we proceed from the concept of flying photons, then in space these lasers should cope with combat missions even better. But no. It is only in films and computer games fabricated on the theme of "Star Wars" that spaceships are blown apart by laser beams. But in reality, it turns out that the laser beam, which burns through the air through the armor, in space barely copes with the ridiculous task of disabling the light-sensitive elements of the spy satellite. Remember, dear reader, there was a period when the US Strategic Defense Initiative (SDI) was the central topic in the media? They talked, talked about this initiative, and then suddenly - once! - and everything was instantly quiet. And later on central television, in the program "Vremya", there was a short story: at the demonstration tests of the space combat laser, the model of the warhead that fell under its beam was really blown to shreds - but this is because the brave American warriors prudently installed an explosive device in it, and at the right moment pressed the button. To be honest, they did not succeed: something prevented the combat photons from flying in the cosmic vacuum as dashingly as near the surface of the Earth. By the way, the question of why combat lasers did not live up to expectations in space was raised in specialized forums on the Internet. And, you know, this question was taken seriously! A crowd of lawyers began to answer this question, inventing the reasons for the resulting failure. Here, for example, one of their notions: the warhead in flight, you see, rotates, so the laser spot moves along its surface, so the laser does not "take" it. Well, just a bad luck: they riveted a strategic defense laser, put it into space ... and everything collapsed to hell! No one at the forefront of science and technology could have foreseen that the warhead would rotate in flight!

Physics textbooks contain abstruse formulas on the topic of the radio wave range, which are sometimes not fully understood even by people with special education and work experience. In the article, we will try to figure out the essence without resorting to difficulties. The first to discover radio waves was Nikola Tesla. In his time, where there was no high-tech equipment, Tesla did not fully understand what this phenomenon was, which he later called ether. An alternating current conductor is the origin of a radio wave.

Sources of radio waves

Natural sources of radio waves include astronomical objects and lightning. An artificial radiator of radio waves is an electric conductor with an alternating electric current moving inside. The vibrational energy of a high-frequency generator is distributed into the surrounding space through a radio antenna. The first working source of radio waves was Popov's radio transmitter-radio receiver. In this device, the function was performed by a high-voltage storage connected to an antenna - a Hertz vibrator. Artificially created radio waves are used for stationary and mobile radar, radio broadcasting, radio communications, communication satellites, navigation and computer systems.

Radio wave range

The waves used in radio communication are in the frequency range 30 kHz - 3000 GHz. Based on the wavelength and frequency of the wave, propagation features, the radio wave range is subdivided into 10 sub-bands:

SDV - extra long.
DV - long.
SV - medium.
KV - short.
VHF - ultrashort.
MV - meter.
UHF - decimeter.
CMB - centimeter.
MMV - millimeter.
SMMV - submillimeter

Radio frequency range

The radio wave spectrum is conventionally divided into sections. Depending on the frequency and length, radio waves are subdivided into 12 sub-bands. The frequency range of radio waves is related to the frequency of the alternating current of the signal. radio waves in the international radio regulations are represented by 12 names:

With an increase in the frequency of a radio wave, its length decreases, with a decrease in the frequency of a radio wave, it increases. Propagation depending on its length is the most important property of a radio wave.

The propagation of radio waves 300 MHz - 300 GHz is called ultra-high microwave frequencies due to their rather high frequency. Even the sub-bands are very extensive, so they, in turn, are divided into gaps, which include certain television and radio broadcasting bands, for maritime and space communications, land and aviation, for radar and radio navigation, for the transmission of medical data, and so on. Despite the fact that the entire range of radio waves is divided into regions, the indicated boundaries between them are conditional. The plots follow each other continuously, passing one into another, and sometimes overlap.

Features of radio wave propagation

The propagation of radio waves is the transfer of energy by an alternating electromagnetic field from one area of \u200b\u200bspace to another. In a vacuum, radio waves propagate from When radio waves are exposed to the environment, the propagation of radio waves can be difficult. This manifests itself in signal distortion, change in the direction of propagation, deceleration of phase and group velocities.

Each type of wave is applied in a different way. Longer ones can better avoid obstacles. This means that the range of radio waves can propagate along the plane of the earth and water. The use of long waves is widespread in submarines and marine vessels, which allows you to be in touch anywhere in the sea. The receivers of all beacons and rescue stations are tuned at six hundred meters with a frequency of five hundred kilohertz.

The propagation of radio waves in different bands depends on their frequency. The shorter the length and the higher the frequency, the straighter the wave path will be. Accordingly, the lower its frequency and the longer its length, the more capable it is to bend around obstacles. Each range of radio wavelengths has its own characteristics of propagation, however, at the border of neighboring ranges, a sharp change in distinctive features is not observed.

Propagation characteristic

Ultra-long and long waves go around the surface of the planet, spreading surface rays for thousands of kilometers.

Medium waves are subject to stronger absorption, therefore, they are able to cover a distance of only 500-1500 kilometers. When the ionosphere is densified in this range, a signal can be transmitted by a spatial beam, which provides communication over several thousand kilometers.

Short waves propagate only at short distances due to the absorption of their energy by the surface of the planet. Spatial ones are capable of repeatedly reflecting from the earth's surface and ionosphere, to overcome long distances, carrying out the transfer of information.

Ultrashort ones are capable of transmitting large amounts of information. Radio waves in this range penetrate through the ionosphere into space, so they are practically unsuitable for terrestrial communication. Surface waves of these ranges are emitted in a straight line, without bending around the planet's surface.

Transmission of huge amounts of information is possible in optical bands. Most often, the third optical waveband is used for communication. In the Earth's atmosphere, they are subject to attenuation, so in reality they transmit a signal over a distance of up to 5 km. But the use of such communication systems eliminates the need to obtain permits from telecommunication inspections.

Modulation principle

In order to transmit information, the radio wave must be modulated with a signal. The transmitter emits modulated radio waves, that is, modified. Short, medium and long waves are amplitude modulated, so they are referred to as AM. Before modulation, the carrier wave travels with constant amplitude. Amplitude modulation for transmission changes its amplitude, corresponding to the signal voltage. The amplitude of a radio wave changes in direct proportion to the signal voltage. Ultrashort waves are frequency modulated, which is why they are referred to as FM. imposes an additional frequency that carries information. To transmit a signal over a distance, it must be modulated with a higher frequency signal. To receive a signal, you need to separate it from the subcarrier wave. Frequency modulation creates less interference, but the radio station is forced to broadcast on VHF.

Factors affecting the quality and efficiency of radio waves

The quality and efficiency of radio reception is influenced by the method of directional radiation. An example would be a satellite dish that directs radiation to the location of an installed receiving sensor. This method allowed significant progress in the field of radio astronomy and made many discoveries in science. He discovered the possibilities of creating satellite broadcasting, wirelessly and much more. It turned out that radio waves are capable of emitting from the Sun, many planets outside our solar system, as well as cosmic nebulae and some stars. It is assumed that there are objects outside our galaxy that have powerful radio emissions.

The range of radio waves, the propagation of radio waves is influenced not only by solar radiation, but also by meteorological conditions. So, meter waves, in fact, do not depend on meteorological conditions. And the range of propagation of centimeter strongly depends on meteorological conditions. It occurs due to the fact that in the aquatic environment during rain or with a high level of humidity in the air, short waves are scattered or absorbed.

Their quality is also influenced by obstacles on the way. At such moments, the signal fading occurs, while audibility is significantly impaired or disappears altogether for a few moments or more. An example would be the reaction of the TV to an airplane flying when the image flickers and white stripes appear. This is due to the fact that the wave is reflected from the aircraft and passes by the TV antenna. Such phenomena with televisions and radio transmitters more often occur in cities, since the range of radio waves is reflected on buildings, high-rise towers, increasing the wave path.

Radio frequency range and its use for radio communication

2.1 Basics of radio propagation

Radio communication provides the transmission of information over a distance using electromagnetic waves (radio waves).

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

Figure: 2.1 Structure of an electromagnetic wave.

The frequency is measured in hertz (Hz) - units named after the great German scientist Heinrich Rudolf Hertz. 1Hz is one oscillation per second, 1 MegaHertz (MHz) is one million oscillations per second. Knowing that the speed of movement of electromagnetic waves is equal to the speed of light, it is possible to determine the distance between points in space where the electric (or magnetic) field is in the same phase. This distance is called the wavelength.

Wavelength (in meters) is calculated using the formula:

, or about

where f is the frequency of electromagnetic radiation in MHz.

It can be seen from the formula that, for example, a frequency of 1 MHz corresponds to a wavelength of about 300 m.With an increase in frequency, the wavelength decreases, with a decrease, it increases.

Electromagnetic waves freely pass 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 it their energy, thereby causing an alternating electric current in this conductor. But not all of the wave energy is absorbed by the conductor; some of it is reflected from the surface. By the way, the use of electromagnetic waves in radar is based on this.

Another useful property of electromagnetic waves (as well as any other waves) is their ability to bend around bodies on their way. But this is possible only when the body size is less than the wavelength, or comparable to it. For example, to detect an airplane, the length of the radar radio wave must be less than its geometric dimensions (less than 10m). 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 flux per unit area is directly proportional to the radiation power and inversely proportional to the square of the distance to the radiator. 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 to the Earth's surface reaches 1 kilowatt per square meter, while the energy flow of a medium-wave broadcasting radio station is only thousandths and even millionths of a watt per square meter.

2.2 Allocation of radio spectrum

Radio waves (radio frequencies) used in radio engineering cover 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 heat or infrared rays. After them there is a narrow section of visible light waves, then - the spectrum of ultraviolet, X-ray and gamma rays - all these are electromagnetic oscillations of the same nature, differing only in wavelength and, therefore, in frequency.

But these ranges are very extensive and, in turn, are divided into sections, which include the so-called broadcasting and television bands, ranges for terrestrial and aviation, space and maritime communications, for data transmission and medicine, for radar and radio navigation, etc. Each radio service is allocated its own section of the range 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 interval 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 the transmitter power values \u200b\u200bin the selected frequency interval, etc.

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

Table 1

Item No.	Range name			Range boundaries
Item No.	Waves	Obsolete terms	Frequencies	Radio waves	Frequencies
1	DKMGMVDecaMega Meters		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	GCMMVHect-kilometer		Infra-low frequencies (LF)	1.000-100 km	0.3-3 kHz
4	MRMV	ADV	Very Low Frequency (VLF) VLF	100-10 km	3-30kHz
5	KMVKilometer	DV	Low frequencies (LF) LF	10-1 km	30-300kHz
6	GCMVHectameter	SV	Mid frequencies (MF) VF	1000-100m	0.3-3 MHz
7	DKMVDecameter	Kv	Treble (HF) HF	100-10m	3-30 MHz
8	MVMeter	VHF	Very high frequency (VHF) VHF	10-1m	30-300 MHz
9	DCMV	VHF	Ultra High Frequency (UHF) UHF	10-1 dm	0.3-3 GHz
10	SMVS centimeter	VHF	Super high frequency (microwave) SHF	10-1 cm	3-30 GHz
11	MMVMillimeter	VHF	Extreme High Frequency (EHF) EHF	10-1 mm	30-300 GHz
12	DCMMVDetsimilli- meter Submillie- meter	SUM	Hyper-high frequencies (HHF)	1-0.1 mm	0.3-3 THz
13	Light			< 0,1 мм	\u003e 3 THz

Figure: 2.2 An example of spectrum allocation between different services.

However, further studies of short and ultrashort waves have shown that they quickly decay when they travel near the Earth's surface. When the radiation is directed upward, short waves come back.

The ionosphere of the Earth was supposed to allow increasing the range of propagation of radio waves at distances exceeding line of sight. Experimentally this assumption was proved in 1923. RF pulses were transmitted vertically upward and returned signals were received. Measurements of 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 radio wave propagation

The nature of the propagation of radio waves depends on the wavelength, curvature of the Earth, soil, atmospheric composition, 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 effect on the propagation of radio waves. Moisture content and air density change depending on the time of day and year.

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

Figure: 2.3 The structure of the atmosphere.

Layers of the atmosphere up to about 100-130 km high 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 changing meteorological conditions.

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

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

Ionization in a rarefied atmosphere at altitudes of 60-80 km from the Earth and higher 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, are relatively rare.

The upper 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 highest ionization (the largest number of free electrons per unit volume).

After sunset, ionizing radiation drops sharply. The restoration of neutral molecules and atoms occurs, which leads to a decrease in the degree of ionization. Layers disappear completely at night D and F 2, layer ionization E decreases significantly, and the layer F 2 retains ionization with some attenuation.

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

The height of the layers of the ionosphere 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 higher. In summer at our latitudes, the electron concentration of ionized layers is higher than in winter (with the exception of the layer F 2). The degree of ionization also depends on the level of solar activity, determined by the number of sunspots. The period of solar activity is approximately 11 years.

Irregular ionization processes associated with so-called ionospheric disturbances are observed at polar latitudes.

There are several paths that the radio wave takes to reach the receiving antenna. As already noted, radio waves propagating above the earth's surface and enveloping it due to the phenomenon of diffraction are called surface or earth 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 waves are called reflected or scattered by the ionosphere, tropospheric - waves reflected or scattered by inhomogeneous layers or "grains" of the troposphere.

Figure: 2.5 Ways of propagation of radio waves.

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

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

Figure: 2.6 Propagation of surface waves.

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

For frequencies above 1 MHz, the surface wave is in fact highly attenuated due to absorption by the Earth and is therefore not used except in the local coverage area. At television frequencies, the attenuation is so great 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 space waves.

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

Figure: 2.7 Influence of the angle of radiation on the passage of the sky wave.

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

In fig. 2.7 shows a ray that is easily refracted by a layer Esince the ray enters at an angle below the critical angle of this layer. Beam 3 passes the area Ebut returns to Earth in a layer F 2 because it enters at an angle below the critical angle of the layer F 2. Beam 4 also passes through layer E... It enters the layer F 2 at its critical angle and returns to Earth. Beam 5 passes through both areas and is lost in space.

All rays shown in Fig. 2.7 refer to one 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, then there will come a moment when the wave propagating from the transmitter parallel to the Earth will exceed the critical angle for any region. This condition occurs at a frequency of about 30 MHz. Above this frequency, skywave communication becomes unreliable.

So, each critical frequency has its own critical angle, and vice versa, each critical angle has its own critical frequency. Consequently, any sky wave, the frequency of which 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 (when the critical angle is exceeded, the signal is lost); The space wave, having reached the ionized layer, is reflected from it and returns to the Earth at a great distance from the transmitter. At some distance from the transmitter, depending on the transmitter power and wavelength, it is possible to receive a surface wave. From where the reception of the surface wave ends, silence zone and it ends where the reflected spatial wave appears. The zone of silence does not have a sharp border.

Figure: 2.8 Reception areas of surface and spatial waves.

As the frequency increases, the quantity 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 frequencywhich can be used for skywave communication. Each ionospheric region has its own maximum allowable frequency for communication.

Short and, moreover, ultrashort waves in the ionosphere lose an insignificant part of their energy. The higher the frequency, the less path the electrons travel during their oscillations, as a result of which the number of their collisions with molecules decreases, i.e., the wave energy losses decrease.

In lower ionized layers, the losses are greater, since an increased pressure indicates a higher gas density, and with a higher gas density, the probability of collision of particles increases.

Long waves are reflected from the lower layers of the ionosphere, which have the lowest electron concentration, at any elevation angles, including those close to 90 °. Medium moisture soil is almost a conductor for long waves, so they reflect well 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 large absorption. This is why high power transmitters are necessary for long distance communication.

Medium waves are appreciably absorbed in the ionosphere and soil of poor and medium conductivity. During the day, only a surface wave is observed, since a space wave (longer than 300 m) is almost completely absorbed in the ionosphere. For complete internal reflection, the average waves must travel a certain path in the lower layers of the ionosphere, which, although they have a low concentration of electrons, have a significant air density.

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

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

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

The positive side of skywave communication at short and medium waves is the possibility of long-distance communication with low transmitter power. But space wave linkhas and significant disadvantages.

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

Secondly, the presence of a zone of silence.

Waves shorter than 25 m are referred to as "daytime waves" as they spread well during the day. "Night waves" include waves longer than 40 m. These waves propagate 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 disturbed due to irregularity of solar radiation, which causes ionospheric disturbances and magnetic storms. As a result, the energy of short radio waves is significantly absorbed, which degrades radio communication, even sometimes makes it completely impossible. Ionospheric disturbances are especially often observed at latitudes close to the poles. Therefore, shortwave communication there is unreliable.

Most notable ionospheric disturbances have their own periodicity: they are repeated after 27 days (time of rotation of the Sun around its axis).

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

Optimal communication frequencies on short waves are selected based on 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 made for a day, five days and characterize possible deviations of the ionosphere from its average state. Forecasts are made 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 freely pass through it, i.e. these waves do not have a spatial ionospheric wave. The surface ultrashort wave, on which radio communication is possible, has two significant disadvantages: firstly, the surface wave does not go 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 range of a radio station 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: a direct ray and a ray reflected from the Earth (Fig. 2.9).

Figure: 2.9 Direct and reflected rays of the sky wave.

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

The receiving and transmitting antennas are usually of the same height, so the reflected beam path length is slightly different from the direct beam. The reflected wave is 180 ° out of phase. Thus, neglecting the losses in the Earth during reflection, if two beams have traveled the same distance, their vector sum is zero, as a result, there will be no signal in 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 determined by the path difference in terms of wavelength, not in linear units. In other words, the total 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 a signal in the receiving antenna. If the wavelength is 4 m, the same 2 m path difference will cause a 180 ° phase difference, fully compensating for the 180 ° phase shift in reflection. In this case, the signal is doubled in voltage.

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

The range of VHF transmitters is significantly increased when aircraft are in the air and with the ground.

TO advantages of VHF should include the possibility of using small antennas. In addition, a large number of radio stations can operate simultaneously in the VHF band without mutual interference. More stations can be deployed simultaneously in the 10 to 1 m wavelength range than in the short, medium and long wavelengths combined.

VHF relay lines are widely used. Between two communication points located at a great distance, several VHF transceivers are installed, located within the line of sight of one another. Intermediate stations work automatically. The organization of relay lines allows you to increase the communication range on VHF and to carry out multichannel communication (conduct several telephone and telegraph transmissions simultaneously).

Much attention is now paid to the use of the VHF band for long-distance radio communication.

The most widely used 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 scattered propagation of radio waves is understandable.

This phenomenon consists in the fact that some of the energy of ultrahigh-frequency radiation is scattered by irregularities 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 studied sources. Since the troposphere is always inhomogeneous, scattered refraction of radio waves exists systematically.

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

When studying distant spread of 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 several milliseconds to several seconds. However, in practice, they are observed during the day with interruptions that rarely exceed a few seconds. The appearance of moments of increased audibility is mainly due to the reflection of radio waves from 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 tracks stretch for several kilometers.

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

Several billion of these meteorites fly into the earth's atmosphere every day, leaving behind ionized trails with a high density of air ionization. This makes it possible to obtain reliable operation of long-distance radio links when using transmitters of relatively low power. An integral part of the 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 automatic in short bursts.

Currently, communication and television transmissions through 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 propagation of radio waves along the earth's surface with 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 propagation of radio waves in the troposphere ( tropospheric waves);

reflection of radio waves from meteor trails;

reflection or retransmission from artificial earth satellites;

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

2.4 Features of the propagation of radio waves of various bands

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

Myriameter and kilometer waves (ADV and DV) can propagate both terrestrial and ionospheric. 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 rather slowly with distance, 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 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, obstacles encountered in 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 the D or E layer). 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 in VLF and LW are carried out by waves propagating in a spherical waveguide formed by the ionosphere and the earth's surface.

Advantage of SDV-, DV- band:

vLF and LW radio waves have the property of penetrating the water column, and also spreading in some soil structures;

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

communication range depends little on ionospheric disturbances;

good diffraction properties of radio waves in these ranges make it possible to provide communication for hundreds and even thousands of kilometers with an earth wave;

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

disadvantagesSDV-, DV, - ranges:

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

since the dimensions of the actually manufactured antennas are less than the wavelength, then compensation for their reduced efficiency is achieved by increasing the power of the transmitters to hundreds or more kW;

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

for power supply of VLF- and DV-band radio stations), large power plants are required;

a significant disadvantage of the VLF and LW ranges is their low frequency capacity;

a sufficiently high level of industrial and atmospheric interference;

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

The area of \u200b\u200bpractical application of VLF-, DV-band radio waves:

communication with underwater objects;

global backbone and underground communications;

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

Hectometer waves (SV) can be propagated by surface and space waves. Moreover, the range of communication with a surface wave is shorter for them (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 intensely absorbed by the layer Dwhen it exists, but well discharged in a layer E.

For medium waves, the communication range is very dependent from time of day. During the day, the middle waves are so strong 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 space waves begin to play a major role. Thus, an important feature of medium waves is that during the day communication on them is maintained by a surface wave, and at night - by both surface and spatial waves simultaneously.

Benefits of the CB band:

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 are 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 ranges;

good diffraction properties of radio waves in this range;

the power of the transmitters is less than that of the VLF and LW bands;

low dependence on ionospheric disturbances and magnetic storms.

Disadvantages of the CB range:

the congestion of the MW band with powerful broadcasting radio stations creates difficulties in widespread use;

limited frequency bandwidth makes it difficult to maneuver frequencies;

the communication range on the NE in the daytime in summer is always limited, since it is possible only by an earth 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 sufficiently high level of mutual and atmospheric interference.

The area of \u200b\u200bpractical application of CB radio waves; Medium-wave radio stations are most often used in the Arctic regions, as a 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 (KB) occupy a special position. They can propagate both terrestrial and ionospheric waves. With relatively low transmitter powers typical of mobile radio stations, ground waves propagate over distances not exceeding several tens of kilometers, since they experience significant absorption in the ground, which increases with increasing frequency.

Ionospheric waves due to single or multiple reflections from the ionosphere under favorable conditions 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 by the layer F2 ... located at an altitude of 300-500 km above the ground). This makes it possible to use relatively low-power radio stations for direct communication over an infinitely wide range of distances.

A significant decrease in the quality of HF radio communication by ionospheric waves occurs due to signal fading. The nature of fading is mainly reduced to the interference of several rays arriving at the receiving site, the phase of which is constantly changing due to a change in the state of the ionosphere.

The reasons for the arrival of several beams at the place of receiving signals can be:

irradiation of the ionosphere at angles at which the rays undergoing

different number of reflections from the ionosphere and the Earth, converge at the point of reception;

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

inhomogeneity 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 receiving point. Polarization fading is observed much less frequently than interference fading and accounts for 10-15% of their total number.

As a result of fading, the signal level at the receiving points can vary over a wide range - tens and even hundreds of times. The time interval between deep fading is a random value and can vary from tenths of a second to several seconds, and sometimes more, and the transition from a high to a low level can be both smooth and very abrupt. Fast level changes often overlap with slow ones.

The conditions for the passage of short waves through the ionosphere vary from year to year, which is associated with an almost periodic change in solar activity, i.e. with a change in the number and area of \u200b\u200bsunspots (Wolf number), which are sources of radiation that ionize the atmosphere. The recurrence period of the maximum solar activity is 11.3 ± 4 years. During the years of maximum solar activity, the maximum usable frequencies (MUF) increase, and the areas of operating frequency ranges expand.

In fig. 2.10 shows a typical family of daily MUF and least usable frequencies (LUF) plots for a radiated power of 1 kW.

Figure: 2.10 The course of the MUF and NUF 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 should be borne in mind that ionospheric forecasts may have an error, therefore, when choosing the maximum communication frequencies, they try not to exceed the line of the so-called optimal operating frequency (OPF), passing below the MUF line by 20-30%. It goes without saying that the working width of the range is further reduced from this. The decrease in the signal level when approaching the maximum usable frequency is explained by the variability of the parameters of the ionosphere.

Due to the fact that the state of the ionosphere changes, communication by an ionospheric wave requires the correct choice of frequencies during the day:

DAY using frequencies 12-30 MHz,

IN THE MORNING AND EVENING 8-12 MHz, AT NIGHT 3-8 MHz.

The graphs also show that with a decrease in the length of the radio communication line, the range of applicable frequencies decreases (for distances of up to 500 km at night, it can be only 1-2 MHz).

The conditions of radio communication for long lines are 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 effect on the state of HF radio communication (especially in the polar regions), i.e. perturbations of the ionosphere and the Earth's magnetic field under the influence of charged particle streams erupted by the Sun. These streams often destroy the main reflecting ionospheric layer F2 in the region of high geomagnetic latitudes. Magnetic storms can manifest themselves not only in the polar regions, but throughout the entire globe. Ionospheric disturbances have a 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 zones of silence (dead zones). The zone of silence (Fig. 2.8) occurs during radio communication over long distances in areas where the surface wave does not reach due to its attenuation, and the space wave is reflected from the ionosphere at a greater distance. This occurs when using narrow beam antennas when radiating at small angles to the horizon.

Advantages of the HF band:

ionospheric waves can travel long distances through single or multiple reflections from the ionosphere under favorable conditions. They are weakly absorbed by the lower regions of the ionosphere (D and E layers) and are 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 infinitely wide range of distances;

the frequency capacity of the HF band is much greater than that of the VLF, DV, and MW bands, which makes it possible to operate a large number of radio stations simultaneously;

antenna devices used in the range of decameter waves have acceptable (even for installation on moving objects) dimensions and can have pronounced directional properties. They have short deployment times, are cheap, and are easily recoverable from damage.

Disadvantages of the HF band:

radio communication by ionospheric waves can be carried out if the frequencies used are below the maximum values \u200b\u200b(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 power of the transmitters and the gains of the antennas used, with the energy absorption in the ionosphere, provide the necessary strength of the electromagnetic field at the point of reception. This condition limits the lower limit of usable frequencies (LUF);

insufficient frequency capacity for using broadband operating modes and frequency maneuvering;

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

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

the presence of zones of silence when providing communication over long distances;

a significant decrease in the quality of HF radio communication 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 propagation of waves.

Practical application of HF radio waves

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

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

Naturally, these areas differ significantly from each other in the properties of radio wave propagation. The energy of VHF is strongly absorbed by the Earth (in the general case, proportional to the square of the frequency), so the Earth wave attenuates rather quickly. For VHF, regular reflection from the ionosphere is unusual, therefore, the communication is calculated on the use of an earth wave and a wave propagating in free space. Space waves shorter than 6-7 m (43-50 MHz), as a rule, pass through the ionosphere without being reflected from it.

VHF propagation is straightforward, the maximum range is limited by the line of sight. It can be determined by the formula:

where Dmax is the line-of-sight range, km;

h1 is the height of the transmitting antenna, m;

h2 - receiving antenna height, m.

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

An increase in the transmitter power does not lead to a proportional increase in the communication range, therefore, low-power radio stations are used in this range. Communication due to tropospheric and ionospheric scattering requires high power transmitters.

At first glance, the range of communication by ground waves on VHF should be very small. However, it should be borne in mind that with an increase in frequency, the efficiency of antenna devices increases, due to which energy losses in the Earth are compensated.

The communication range by terrestrial waves depends on the wavelength. The longest range is achieved at meter waves, especially at waves adjacent to the HF band.

Meter waves have the property diffraction, i.e. the property to bend around uneven terrain. The increase in the communication range on 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 range of meter waves, long-range propagation of radio waves is often observed, which is due to a number of reasons. Long-range propagation can occur with the formation of sporadic ionized clouds ( sporadic layer Fs). It is known that this layer can appear at any time of the year or day, but for our hemisphere - mainly in late spring and early summer in the daytime. A feature of these clouds is a very high ionic concentration, sometimes sufficient to reflect waves of the entire VHF range. In this case, the zone of the 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 propagation manifests itself in the winter months at the illuminated time of the reflection points, i.e. when the absorption of wave energy in the lower regions of the ionosphere is minimal. In this case, 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 the lower region, experiencing some absorption, are reflected from the upper one and return to the Earth. The distances covered in this case are in the range from 100 to 2500 km. Field strength reflected those 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 passes 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 a section of 20-30 MHz. In the 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 indicated conditions, the properties of wave propagation in this area become very close to the properties of meter waves. It is no coincidence that this section of frequencies is used in the interests of radio communication, oriented to meter waves.

Advantages of the VHF band:

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

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

limited communication range allows multiple use of 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) and intentional interference due to narrow directional antennas and oglimited communication range;

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

when using broadband signals for radio communication, the frequency instability of the radio link is sufficient δf \u003d 10 -4;

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

high quality radio channel;

due to the 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 a small dependence of the communication range on power.

Disadvantages of the VHF range:

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

when using narrowly directed antennas, it is difficult to work with several correspondents;

when using antennas with a circular directivity, the communication range, intelligence protection, and noise immunity are reduced.

The scope of practical application of VHF-Dianazon radio waves The range is used simultaneously by a large number of radio stations, especially since the range of mutual interference between them is, as a rule, small. The properties of the propagation of earth waves provide a wide application of ultrashort waves for communication in the tactical control link, including between various types of mobile objects. Communication over interplanetary distances.

Considering the advantages and disadvantages of each band, we can conclude that the most acceptable ranges for low-power radio stations are the decameter (KB) and meter (VHF) wavelengths.

2.5 Influence of nuclear explosions on the state of radio communications

In nuclear explosions, instantaneous gamma radiation, interacting with the atoms of the 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 that remain practically in place. Thus, in space for some time, there is a separation of positive and negative charges, 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 (the fifth damaging factor of a nuclear explosion) in the absence of special protection measures, it can damage the control and communication equipment, disrupt the operation of electrical devices connected to long external lines.

Communication, signaling and control systems are most susceptible to the effect of an electromagnetic pulse from a nuclear explosion. As a result of the impact of the EMP of a ground or air nuclear explosion on the antennas of radio stations, an electric voltage is induced in them, under the influence of which a breakdown of insulation, transformers, melting of wires, failure of arresters, damage to electronic lamps, semiconductor devices, capacitors, resistances, etc. ...

It has been established that when EMP is applied to the equipment, the highest voltage is induced on the input circuits. With regard to transistors, the following dependence is observed: the higher the gain of the transistor, the lower its dielectric strength.

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

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

Table 1

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

The effect of an electromagnetic pulse on radio equipment is sharply reduced in the case of applying special protection measures.

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

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

In addition, can be applied protective entrance devices, which are various relay or electronic devices that react to overvoltage in the circuit. When a voltage pulse arrives, induced in the line by an electromagnetic pulse, they turn off the power from the device or simply break the working circuits.

When choosing protective devices, it should be borne in mind that the impact of EMP is characterized by massiveness, that is, the simultaneous actuation of protective equipment in all circuits caught in the explosion area. Therefore, the applied protection schemes should automatically restore the operation of the circuits immediately after the termination of the electromagnetic pulse.

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

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

High altitude nuclear explosion accompanied by the formation of regions of increased ionization. In explosions at altitudes up to about 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 dimensions of the ionized region are somewhat larger than the dimensions of the explosion cloud, especially at the upper boundary of this altitude range.

In nuclear explosions at high altitudes, two regions of increased ionization appear in the atmosphere.

First area is formed in the area of \u200b\u200bthe 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 atmosphere at altitudes of 60-90 km as a result of absorption of penetrating radiation by air. The distances at which penetrating radiation produce ionization in the horizontal direction are hundreds and even thousands of kilometers.

Areas of increased ionization arising from a high-altitude nuclear explosion absorb radio waves and change the direction of their propagation, which leads to a significant disruption in 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 the EMP of ground and air explosions.

The measures of protection against the damaging effect of the electromagnetic pulse of high-altitude explosions are the same as against the EMP of ground and air explosions.

2.5.1 Protection against ionizing and electromagnetic radiation

high-altitude nuclear explosions (HNE)

Interference with RS 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 asymmetric expansion of the cloud of electrical discharges formed under the influence of ionizing radiation from 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 concentration of electrons in a sphere can reach 105-106 electrons / cm3, i.e., 100-1000 times higher than the normal concentration of electrons in the ionospheric layer D.

High-altitude nuclear explosions (HNE) at altitudes above 30 km significantly affect the electrical characteristics of the atmosphere for a long time in large areas, and, therefore, have a strong effect on the propagation of radio waves.

In addition, a powerful electromagnetic pulse arising during IYE induces high voltages (up to 10,000-50,000 V) and currents up to several thousand amperes in wire communication lines.

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

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

2.5.2 Characteristic signs of IJV

EYE in a given area or near it is accompanied by an instant cessation of reception of distant stations in the HF wave range.

At the moment of termination of communication, a short click is observed in the phones, and then only the receiver's own noises and weak crackling such as thunderbolts are heard.

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

The range of the radar and the accuracy of coordinate measurement are reduced.

The protection of electronic devices is based on the correct use of the frequency range and all the factors that arise as a result of the use of IYA

2.5.3 Basic definitions:

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

direct radio wave (straight wave ) - a radio wave propagating directly from sources to the place of reception;

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

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

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

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

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

maximum usable frequency (MUF) - 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;

optimum operating frequency (ORCH) - the frequency of radio emission below the IF, at which stable radio communication can be carried out in certain geophysical conditions. Typically, the ORF is 15% lower than the MUF;

vertical ionospheric sounding (vertical sounding ) - ionospheric sounding by means of radio signals emitted vertically upward relative to the Earth's surface, provided that the points of emission and reception are aligned;

ionospheric disturbance - violation in the distribution of ionization in the atmosphere, which usually exceeds the change in the average ionization characteristics for given geographic conditions;

ionospheric storm - long-term ionospheric disturbance of high intensity.

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