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3.2.2 Propagation of electromagnetic waves
Among electromagnetic fields in general, generated by electric charges and their movement, it is customary to attribute to radiation that part of alternating electromagnetic fields that is capable of propagating the farthest from its sources - moving charges, fading most slowly with distance. Such radiation is called electromagnetic waves.
Electromagnetic waves can propagate in almost all media. In a vacuum (a space free of matter and bodies that absorb or emit electromagnetic waves), electromagnetic waves propagate without attenuation over arbitrarily large distances, but in some cases they propagate quite well in a space filled with matter (although somewhat changing their behavior).
To measure distances, electromagnetic waves are used in almost all ranges indicated in Table. 3.1, except for ultraviolet radiation in the optical range, short radio waves and ionizing radiation.
When measuring distances with electromagnetic waves, both range and accuracy are strongly affected by propagation conditions. This is understood as a whole complex of factors: the properties of the waves themselves, the nature of the underlying surface, the time of day, the meteorological conditions of the atmosphere, etc.
Light waves and VHF bands propagate almost rectilinearly.
Diffraction centimeter waves used in radio rangefinders and VHF systems is so small that it does not envelop the Earth's surface. Such an envelope exists to a small extent only due to refraction .
(Diffraction - this is the phenomenon of deviation from the laws of geometric optics during the propagation of waves. In particular, this is a deviation from the straightness of the propagation of a light beam. Refraction or refraction - this is a change in the direction of propagation of electromagnetic radiation that occurs at the interface between two media transparent to these waves or in the thickness of a medium with continuously changing properties).
The maximum range of VHF systems is limited by line of sight
. Line-of-sight limits on the physical surface of the Earth depend on the height of the antennas and the terrain. If we take into account only the curvature of the spherical Earth (without relief) and neglect refraction, then the limiting line-of-sight distance between two points is determined by the heights of the points 
and 
in the following way:
where is expressed in kilometers and heights are in meters.
When taking into account the refractive curvature of the trajectory (for normal refraction), the coefficient 3.57 in equation (3.29) is replaced by 4.12 for radio waves, and 3.83 for optical waves, i.e. refraction increases the line-of-sight distance by about 15% for radio waves, and by 7% for optical waves.
In the event that, for example, the rangefinder and reflector antennas are mounted on an ordinary wooden tripod, i.e. 
, then the line-of-sight distance calculated by formula (3.29) will be 
. If the antennas are raised to a height 
, then the line-of-sight distance will be 
.
For optical waves, in addition to line of sight, the presence of optical visibility (transparency) .
The propagation of long and medium radio waves has specific features. The most significant feature is the reflection from the upper, highly ionized layers of the atmosphere, located at altitudes of more than 60 km.
This leads to the fact that not only a direct wave propagating along the surface of the Earth (surface wave) can reach the receiving point, but also a wave reflected from the ionosphere, the so-called sky wave (Fig. 3.11). In the zone where the surface and spatial waves meet, their interference occurs, due to which the surface wave transmitting the useful signal receives amplitude and phase distortions, and if the receiving equipment is located in such a zone, then measurements can be very difficult, and often impossible.
A sky wave reflected from the ionosphere can propagate to much greater distances than a surface wave, for which the shape of the Earth with its relief creates obstacles. Due to diffraction, these obstacles can be enveloped by a surface wave, and the range of its propagation depends on the absorbing properties of the earth's surface. For a sky wave, its partial absorption by the ionosphere and the earth's surface is also observed during multiple reflection from the ionospheric layers. Absorption by the earth's surface depends on the wavelength, its polarization and the electrical characteristics of a particular underlying surface.
The property of long-range propagation of a sky wave with multiple reflections from the ionosphere is successfully used in radio communications, broadcasting, and long-range radio navigation. However for radio geodetic purposes sky wave use impossible, since the geometry of its passage is not subject to strict consideration. Therefore, for accurate measurements, only the surface wave should be used .
Based on the above, for the purposes of geodetic measurements, only optical and VHF waves are suitable .
Geodetic rangefinders of the optical wavelength range are mainly used to measure distances up to 10 km.
Geodetic radio range finders are used to measure distances of the order of several tens of kilometers.
However, at present, almost all manufacturers of geodetic rangefinders have stopped producing radio rangefinders, and have focused their efforts on light rangefinders or electronic total stations, of which the light rangefinder is an integral part. This situation is explained by the fact that in the practice of geodetic work, technologies provided by global satellite navigation systems have become widespread, thanks to which it has become possible to determine the coordinates of points on the earth's surface with high accuracy. But it was precisely for this task that radio rangefinders were designed. The distance between the points measured using radio rangefinders was then used to calculate the coordinates of the determined point. The use of GNSS receivers makes it possible to eliminate the intermediate operation of measuring the distance between points, and immediately obtain the coordinates of the point being determined.
Electromagnetic waves are the propagation of electromagnetic fields in space and time.
As noted above, the existence of electromagnetic waves was theoretically predicted by the great English physicist J. Maxwell in 1864. He analyzed all the laws of electrodynamics known by that time and made an attempt to apply them to time-varying electric and magnetic fields. He introduced the concept of a vortex electric field into physics and proposed a new interpretation of the law of electromagnetic induction discovered by Faraday in 1831: any change in the magnetic field generates a vortex electric field in the surrounding space, the lines of force of which are closed.
He put forward a hypothesis about the existence of the reverse process: a time-varying electric field generates a magnetic field in the surrounding space. Maxwell was the first to describe the dynamics of a new form of matter - an electromagnetic field, and derived a system of equations (Maxwell's equations) that connects the characteristics of an electromagnetic field with its sources - electric charges and currents. Mutual transformations of electric and magnetic fields occur in an electromagnetic wave. Fig. 2 a, b illustrate the mutual transformation of electric and magnetic fields.
Figure 2 - Mutual transformation of electric and magnetic fields: a) The law of electromagnetic induction in Maxwell's interpretation; b) Maxwell's hypothesis. A changing electric field generates a magnetic field
The division of the electromagnetic field into electric and magnetic depends on the choice of reference system. Indeed, there is only an electric field around charges resting in one frame of reference; however, the same charges will move relative to another frame of reference and generate in this frame of reference, in addition to electric, also a magnetic field. Thus, Maxwell's theory linked together electrical and magnetic phenomena.
If an alternating electric or magnetic field is excited with the help of oscillating charges, then a sequence of mutual transformations of electric and magnetic fields occurs in the surrounding space, propagating from point to point. Both these fields are vortex, and the vectors and are located in mutually perpendicular planes. The process of propagation of the electromagnetic field is schematically shown in Fig.3. This process, which is periodic in time and space, is an electromagnetic wave.
Figure 3 - The process of electromagnetic field propagation
This hypothesis was only a theoretical assumption that did not have experimental confirmation, however, on its basis, Maxwell managed to write down a consistent system of equations describing the mutual transformations of electric and magnetic fields, i.e., a system of equations for the electromagnetic field.
So, a number of important conclusions follow from Maxwell's theory - the main properties of electromagnetic waves.
There are electromagnetic waves, i.e. electromagnetic field propagating in space and time.
In nature, electrical and magnetic phenomena act as two sides of a single process.
Electromagnetic waves are emitted by oscillating charges. The presence of acceleration is the main condition for the radiation of electromagnetic waves, i.e.
- - any change in the magnetic field creates a vortex electric field in the surrounding space (Fig. 2a).
- - any change in the electric field excites a vortex magnetic field in the surrounding space, the induction lines of which are located in a plane perpendicular to the lines of the alternating electric field, and cover them (Fig. 2b).
The lines of induction of the emerging magnetic field form the "right screw" with the vector. Electromagnetic waves are transverse - vectors and are perpendicular to each other and lie in a plane perpendicular to the direction of wave propagation (Fig. 4).
Figure 4 - Transverse electromagnetic waves
Periodic changes in the electric field (strength vector E) generate a changing magnetic field (induction vector B), which in turn generates a changing electric field. Oscillations of the vectors E and B occur in mutually perpendicular planes and perpendicular to the wave propagation line (velocity vector) and coincide in phase at any point. The lines of force of the electric and magnetic fields in an electromagnetic wave are closed. Such fields are called vortex.
Electromagnetic waves propagate in matter with a finite speed, and this once again confirmed the validity of the short-range theory.
Maxwell's conclusion about the finite propagation velocity of electromagnetic waves was in conflict with the long-range theory adopted at that time, in which the propagation velocity of electric and magnetic fields was assumed to be infinitely large. Therefore, Maxwell's theory is called the short-range theory.
Such waves can propagate not only in gases, liquids and solid media, but also in vacuum.
Speed of electromagnetic waves in vacuum с=300000 km/s. The speed of propagation of electromagnetic waves in vacuum is one of the fundamental physical constants.
The propagation of an electromagnetic wave in a dielectric is a continuous absorption and re-emission of electromagnetic energy by electrons and ions of a substance that perform forced oscillations in an alternating electric field of the wave. In this case, the wave velocity decreases in the dielectric.
Electromagnetic waves carry energy. When waves propagate, a flow of electromagnetic energy arises. If we single out an area S (Fig. 4) oriented perpendicular to the direction of wave propagation, then in a short time Dt, an energy DWem will flow through the area equal to
DWem \u003d (we + wm) xSDt.
When moving from one medium to another, the frequency of the wave does not change.
Electromagnetic waves can be absorbed by matter. This is due to the resonant absorption of energy by charged particles of matter. If the natural frequency of oscillations of the particles of the dielectric differs greatly from the frequency of the electromagnetic wave, absorption occurs weakly, and the medium becomes transparent to the electromagnetic wave.
Getting to the interface between two media, part of the wave is reflected, and part passes into another medium, being refracted. If the second medium is a metal, then the wave that has passed into the second medium quickly decays, and most of the energy (especially for low-frequency oscillations) is reflected into the first medium (metals are opaque to electromagnetic waves).
Propagating in media, electromagnetic waves, like any other waves, can experience refraction and reflection at the interface between media, dispersion, absorption, interference; when propagating in inhomogeneous media, wave diffraction, wave scattering, and other phenomena are observed.
It follows from Maxwell's theory that electromagnetic waves must exert pressure on an absorbing or reflecting body. The pressure of electromagnetic radiation is explained by the fact that under the influence of the electric field of the wave, weak currents arise in the substance, that is, the ordered movement of charged particles. These currents are affected by the Ampère force from the side of the magnetic field of the wave, directed into the thickness of the substance. This force creates the resulting pressure. Usually the pressure of electromagnetic radiation is negligible. So, for example, the pressure of solar radiation coming to the Earth on an absolutely absorbing surface is approximately 5 μPa.
The first experiments to determine the radiation pressure on reflecting and absorbing bodies, which confirmed the conclusion of Maxwell's theory, were carried out by the outstanding physicist of Moscow University P.N. Lebedev in 1900. The discovery of such a small effect required from him extraordinary ingenuity and skill in setting up and conducting an experiment. In 1900 he succeeded in measuring light pressure on solids, and in 1910 on gases. The main part of the P.I. Lebedev, to measure the pressure of light, were light disks 5 mm in diameter, suspended on an elastic thread (Fig. 5) inside an evacuated vessel.
Figure 5 - Experiment P.I. Lebedev
The disks were made of various metals and could be changed during experiments. Light from a strong electric arc was directed onto the disks. As a result of the action of light on the disks, the thread twisted and the disks deflected. The results of the experiments of P.I. Lebedev were fully consistent with Maxwell's electromagnetic theory and were of great importance for its approval.
The existence of electromagnetic wave pressure allows us to conclude that a mechanical impulse is inherent in an electromagnetic field. This relationship between the mass and energy of an electromagnetic field in a unit volume is a universal law of nature. According to the special theory of relativity, it is true for any bodies, regardless of their nature and internal structure.
Since the pressure of the light wave is very small, it does not play a significant role in the phenomena that we encounter in everyday life. But in cosmic and microscopic systems opposite in scale, the role of this effect sharply increases. Thus, the gravitational attraction of the outer layers of the matter of each star to the center is balanced by a force, a significant contribution to which is made by the pressure of light coming from the depths of the star outward. In the microcosm, the pressure of light is manifested, for example, in the phenomenon of light recoil of the atom. It is experienced by an excited atom when it emits light.
Light pressure plays a significant role in astrophysical phenomena, in particular, in the formation of comet tails, stars, etc. The light pressure reaches a significant value in the places where the radiation of powerful quantum light generators (lasers) is focused. Thus, the pressure of focused laser radiation on the surface of a thin metal plate can lead to its breakdown, that is, to the appearance of a hole in the plate. Thus, the electromagnetic field has all the features of material bodies - energy, finite propagation velocity, momentum, mass. This suggests that the electromagnetic field is one of the forms of existence of matter.
The discovery of electromagnetic waves is a remarkable example of the interaction between experiment and theory. It shows how physics has combined seemingly completely dissimilar properties - electricity and magnetism - revealing in them different aspects of the same physical phenomenon - electromagnetic interaction. Today it is one of the four known fundamental physical interactions, which also include the strong and weak nuclear interactions and gravity. The theory of electroweak interaction has already been constructed, which describes electromagnetic and weak nuclear forces from a unified standpoint. There is also the next unifying theory - quantum chromodynamics - which covers the electroweak and strong interactions, but its accuracy is somewhat lower. describe all Fundamental interactions from a unified position have not yet been achieved, although intensive research is being carried out in this direction within the framework of such areas of physics as string theory and quantum gravity.
Electromagnetic waves were theoretically predicted by the great English physicist James Clark Maxwell (probably for the first time in 1862 in his work "On Physical Lines of Force", although a detailed description of the theory appeared in 1867). He diligently and with great respect tried to translate into strict mathematical language Michael Faraday's slightly naive pictures describing electrical and magnetic phenomena, as well as the results of other scientists. Having ordered all electrical and magnetic phenomena in the same way, Maxwell discovered a number of contradictions and a lack of symmetry. According to Faraday's law, alternating magnetic fields generate electric fields. But it was not known whether alternating electric fields generate magnetic fields. Maxwell managed to get rid of the contradiction and restore the symmetry of the electric and magnetic fields by introducing an additional term into the equations, which described the appearance of a magnetic field when the electric field changed. By that time, thanks to Oersted's experiments, it was already known that direct current creates a constant magnetic field around the conductor. The new term described another source of the magnetic field, but it could be thought of as some kind of imaginary electric current, which Maxwell called bias current to distinguish from ordinary current in conductors and electrolytes - conduction current. As a result, it turned out that alternating magnetic fields generate electric fields, and alternating electric fields generate magnetic ones. And then Maxwell realized that in such a combination, oscillating electric and magnetic fields can break away from the conductors that generate them and move through vacuum with a certain, but very high speed. He calculated this speed, and it turned out to be about three hundred thousand kilometers per second.
Shocked by the result, Maxwell writes to William Thomson (Lord Kelvin, who, in particular, introduced the absolute temperature scale): “The speed of transverse wave oscillations in our hypothetical medium, calculated from the electromagnetic experiments of Kohlrausch and Weber, coincides so exactly with the speed of light, calculated from optical experiments of Fizeau that we can hardly refuse the conclusion that light consists of transverse vibrations of the same medium, which is the cause of electrical and magnetic phenomena". And further in the letter: “I received my equations while living in the provinces and not suspecting the closeness of the speed of propagation of magnetic effects found by me to the speed of light, so I think that I have every reason to consider the magnetic and luminous media as one and the same medium. ..."
Maxwell's equations go far beyond the scope of a school physics course, but they are so beautiful and concise that they should be placed in a conspicuous place in the physics classroom, because most of the natural phenomena that are significant to humans can be described with just a few lines of these equations. This is how information is compressed when previously dissimilar facts are combined. Here is one of the types of Maxwell's equations in differential representation. Admire.
I would like to emphasize that a discouraging consequence was obtained from Maxwell's calculations: the oscillations of the electric and magnetic fields are transverse (which he himself emphasized all the time). And transverse vibrations propagate only in solids, but not in liquids and gases. By that time, it was reliably measured that the speed of transverse vibrations in solids (simply the speed of sound) is the higher, the, roughly speaking, the harder the medium (the greater the Young's modulus and the lower the density) and can reach several kilometers per second. The speed of the transverse electromagnetic wave was almost a hundred thousand times higher than the speed of sound in solids. And it should be noted that the stiffness characteristic is included in the equation for the speed of sound in a solid under the root. It turned out that the medium through which electromagnetic waves (and light) pass has monstrous characteristics of elasticity. An extremely difficult question arose: “How can other bodies move through such a solid medium and do not feel it?” The hypothetical medium was called - ether, attributing to it at the same time strange and, generally speaking, mutually exclusive properties - enormous elasticity and extraordinary lightness.
Maxwell's work caused shock among contemporary scientists. Faraday himself wrote with surprise: "At first I was even frightened when I saw such a mathematical force applied to the question, but then I was surprised to see that the question withstands it so well." Despite the fact that Maxwell's views overturned all the ideas known at that time about the propagation of transverse waves and about waves in general, far-sighted scientists understood that the coincidence of the speed of light and electromagnetic waves is a fundamental result, which says that it is here that the main breakthrough awaits physics.
Unfortunately, Maxwell died early and did not live to see reliable experimental confirmation of his calculations. International scientific opinion changed as a result of the experiments of Heinrich Hertz, who 20 years later (1886–89) demonstrated the generation and reception of electromagnetic waves in a series of experiments. Hertz not only obtained the correct result in the quiet of the laboratory, but passionately and uncompromisingly defended Maxwell's views. Moreover, he did not limit himself to experimental proof of the existence of electromagnetic waves, but also investigated their basic properties (reflection from mirrors, refraction in prisms, diffraction, interference, etc.), showing the complete identity of electromagnetic waves with light.
It is curious that seven years before Hertz, in 1879, the English physicist David Edward Hughes (Hughes - D. E. Hughes) also demonstrated to other major scientists (among them was also the brilliant physicist and mathematician Georg-Gabriel Stokes) the effect of propagation of electromagnetic waves in the air. As a result of discussions, scientists came to the conclusion that they see the phenomenon of Faraday's electromagnetic induction. Hughes was upset, did not believe himself, and published the results only in 1899, when the Maxwell-Hertz theory became generally accepted. This example shows that in science persistent dissemination and propaganda of the results obtained is often no less important than the scientific result itself.
Heinrich Hertz summed up the results of his experiments in the following way: "The described experiments, as, at least, it seems to me, eliminate doubts about the identity of light, thermal radiation and electrodynamic wave motion."
Chapter 1
MAIN PARAMETERS OF ELECTROMAGNETIC WAVES
What is an electromagnetic wave, it is easy to imagine the following example. If you throw a pebble on the surface of the water, then waves diverging in circles are formed on the surface. They move from the source of their occurrence (perturbation) with a certain speed of propagation. For electromagnetic waves, disturbances are electric and magnetic fields moving in space. A time-varying electromagnetic field necessarily causes an alternating magnetic field, and vice versa. These fields are interconnected.
The main source of the spectrum of electromagnetic waves is the Sun star. Part of the spectrum of electromagnetic waves sees the human eye. This spectrum lies within 380...780 nm (Fig. 1.1). In the visible spectrum, the eye perceives light differently. Electromagnetic oscillations with different wavelengths cause the sensation of light with different colors.
Part of the spectrum of electromagnetic waves is used for the purposes of radio and television broadcasting and communications. The source of electromagnetic waves is a wire (antenna) in which electric charges fluctuate. The process of formation of fields, which began near the wire, gradually, point by point, captures the entire space. The higher the frequency of the alternating current passing through the wire and generating an electric or magnetic field, the more intense the radio waves of a given length created by the wire.
Electromagnetic waves have the following main characteristics.
1. Wavelength lv, - the shortest distance between two points in space, at which the phase of a harmonic electromagnetic wave changes by 360 °. A phase is a state (stage) of a periodic process (Fig. 1.2).
In terrestrial television broadcasting, meter (MB) and decimeter waves (UHF) are used, in satellite - centimeter waves (CM). As the frequency range of the CM is filled, the range of millimeter waves (Ka-band) will be mastered.
2. Wave oscillation period T- the time during which one complete change in the field strength occurs, i.e., the time during which the point of the radio wave, which has some fixed phase, travels a path equal to the wavelength lb.
3. Frequency of oscillations of the electromagnetic field F(number of field oscillations per second) is determined by the formula
The unit of frequency is hertz (Hz) - the frequency at which one oscillation occurs per second. In satellite broadcasting, one has to deal with very high frequencies of electromagnetic oscillations measured in gigahertz.
For satellite direct television broadcasting (SNTV) along the line Space - Earth, the C-band low range and part of the Ku range (10.7 ... 12.75 GGi) are used. The upper part of these ranges is used to transmit information over the Earth-Space line (Table 1.1).
4. Velocity of wave propagation With - speed of successive propagation of a wave from an energy source (antenna).
The speed of propagation of radio waves in free space (vacuum) is constant and equal to the speed of light C= 300,000 km/s. Despite such a high speed, an electromagnetic wave travels along the Earth-Space-Earth line in 0.24 s. On the ground, radio and television transmissions can be received almost instantaneously at any point. When propagating in real space, for example, in air, the speed of a radio wave depends on the properties of the medium, it is usually less With on the value of the refractive index of the medium.
The frequency of electromagnetic waves F, the speed of their propagation C and the wavelength l are related by the relation
lv=C/F, and since F=1/T , then lv=C*T.
Substituting the value of the speed С= 300,000 km/s into the last formula, we get
lv(m)=3*10^8/F(m/s*1/Hz)
For high frequencies, the wavelength of the electromagnetic oscillation can be determined by the formula lv (m) = 300 / F (MHz) Knowing the wavelength of the electromagnetic oscillation, the frequency is determined by the formula F (MHz) = 300 / lv (m)
5. Polarization of radio waves. The electric and magnetic components of the electromagnetic field are respectively characterized by the vectors E and H which show the value of the field strengths and their direction. Polarization is the orientation of the electric field vector E waves relative to the surface of the earth (Fig. 1.2).
The type of polarization of radio waves is determined by the orientation (position) of the transmitting antenna relative to the earth's surface. Both terrestrial and satellite television use linear polarization, i.e. horizontal H and vertical V (Fig. 1.3).
Radio waves with a horizontal electric field vector are called horizontally polarized, and with a vertical - vertically polarized. The polarization plane of the last waves is vertical, and the vector H(see Fig. 1.2) is in the horizontal plane.
If the transmitting antenna is mounted horizontally above the earth's surface, then the electrical field lines will also be horizontal. In this case, the field will induce the greatest electromotive force (EMF) in the horizontal
Fig 1.4. Circular polarization of radio waves:
LZ- left; RZ- right
umbrella-mounted receiving antenna. Therefore, when H polarization of radio waves, the receiving antenna must be oriented horizontally. In this case, there will theoretically be no reception of radio waves on a vertically located antenna, since the EMF induced in the antenna is zero. Conversely, with the vertical position of the transmitting antenna, the receiving antenna must also be placed vertically, which will allow you to get the highest EMF in it.
In television broadcasting from artificial Earth satellites (AES), in addition to linear polarizations, circular polarization is widely used. This is due, oddly enough, to the tightness of the air, since there are a large number of communication satellites and satellites for direct (direct) television broadcasting in orbits.
Often in tables of satellite parameters they give an abbreviation for the type of circular polarization - L and R. Circular polarization of radio waves creates, for example, a conical spiral on the feed of the transmitting antenna. Depending on the direction of winding of the spiral, circular polarization is left or right (Fig. 1.4).
Accordingly, a polarizer must be installed in the irradiator of the terrestrial satellite television antenna, which responds to the circular polarization of radio waves emitted by the transmitting satellite antenna.
Let us consider the issues of modulation of high-frequency oscillations and their spectrum during transmission from a satellite. It is advisable to do this in comparison with terrestrial broadcasting systems.
The separation between the image and audio carrier frequencies is 6.5 MHz, the rest of the lower sideband (to the left of the image carrier) is 1.25 MHz, and the audio channel width is 0.5 MHz
(Fig. 1.5). With this in mind, the total width of the television channel is assumed to be 8.0 MHz (according to the D and K standards adopted in the CIS countries).
The transmitting television station has two transmitters. One of them transmits electrical image signals, and the other - sound, respectively, at different carrier frequencies. A change in some parameter of a carrier high-frequency oscillation (power, frequency, phase, etc.) under the influence of low-frequency oscillations is called modulation. Two main types of modulation are used: amplitude (AM) and frequency (FM). In television, image signals are transmitted from AM, and sound from FM. After modulation, electrical oscillations are amplified in power, then they enter the transmitting antenna and are radiated by it into space (ether) in the form of radio waves.
8 terrestrial television broadcasting, for a number of reasons, it is impossible to use FM for transmitting image signals. There are much more places on the air on SM, and such an opportunity exists. As a result, the satellite channel (transponder) occupies a frequency band of 27 MHz.
Advantages of frequency modulation of a subcarrier signal:
less sensitivity to interference and noise compared to AM, low sensitivity to the nonlinearity of the dynamic characteristics of signal transmission channels, as well as stability of transmission over long distances. These characteristics are explained by the constancy of the signal level in the transmission channels, the possibility of frequency correction of predistortion, which favorably affects the signal-to-noise ratio, due to which the FM can significantly reduce the transmitter power when transmitting information over the same distance. For example, terrestrial broadcasting systems use 5 times more powerful transmitters to transmit image signals on the same television station than to transmit audio signals.
Electromagnetic radiation exists exactly as long as our Universe lives. It has played a key role in the evolution of life on Earth. In fact, this is a perturbation of the state of the electromagnetic field propagating in space.
Characteristics of electromagnetic radiation
Any electromagnetic wave is described using three characteristics.
1. Frequency.
2. Polarization.
Polarization- one of the main wave attributes. Describes the transverse anisotropy of electromagnetic waves. Radiation is considered polarized when all wave oscillations occur in the same plane.
This phenomenon is actively used in practice. For example, in the cinema when showing 3D films.
With the help of polarization, IMAX glasses separate the image, which is intended for different eyes. 
Frequency is the number of wave crests that pass by the observer (in this case, the detector) in one second. Measured in hertz.
Wavelength- a specific distance between the nearest points of electromagnetic radiation, oscillations of which occur in one phase.
Electromagnetic radiation can propagate in almost any medium: from dense matter to vacuum.
The speed of propagation in vacuum is 300 thousand km per second.
An interesting video about the nature and properties of EM waves, see the video below:
Types of electromagnetic waves
All electromagnetic radiation is divided by frequency. 
1. Radio waves. There are short, ultra-short, extra-long, long, medium.
The length of radio waves ranges from 10 km to 1 mm, and from 30 kHz to 300 GHz.
Their sources can be both human activities and various natural atmospheric phenomena.
2. . The wavelength lies within 1mm - 780nm, and can reach up to 429 THz. Infrared radiation is also called thermal radiation. The basis of all life on our planet.
3. Visible light. Length 400 - 760/780nm. Accordingly, it fluctuates between 790-385 THz. This includes the entire spectrum of radiation that can be seen by the human eye.
4. . The wavelength is shorter than in infrared radiation.
It can reach up to 10 nm. such waves is very large - about 3x10 ^ 16 Hz.
5. X-rays. waves 6x10 ^ 19 Hz, and the length is about 10 nm - 5 pm.
6. Gamma waves. This includes any radiation, which is greater than in x-rays, and the length is less. The source of such electromagnetic waves are cosmic, nuclear processes.
Scope of application
Somewhere since the end of the 19th century, all human progress has been associated with the practical application of electromagnetic waves.
The first thing worth mentioning is radio communication. She made it possible for people to communicate, even if they were far from each other.
Satellite broadcasting, telecommunications are a further development of primitive radio communications.
It is these technologies that have shaped the information image of modern society.
Sources of electromagnetic radiation should be considered as large industrial facilities, as well as various power lines.
Electromagnetic waves are actively used in military affairs (radar, complex electrical devices). Also, medicine has not done without their use. Infrared radiation can be used to treat many diseases.
X-rays help identify damage to a person's internal tissues.
With the help of lasers, a number of operations are carried out that require jewelry precision. 
The importance of electromagnetic radiation in the practical life of a person is difficult to overestimate.
Soviet video about the electromagnetic field:
Possible negative impact on humans
Despite their usefulness, strong sources of electromagnetic radiation can cause the following symptoms:
Fatigue;
Headache;
Nausea.
Excessive exposure to certain types of waves cause damage to internal organs, the central nervous system, and the brain. Changes in the human psyche are possible.
An interesting video about the effect of EM waves on a person:
To avoid such consequences, almost all countries of the world have standards governing electromagnetic safety. Each type of radiation has its own regulatory documents (hygienic standards, radiation safety standards). The effect of electromagnetic waves on humans is not fully understood, therefore WHO recommends minimizing their impact.
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