6.1 Radio Wave Overview

In wireless telecommunications, there is no direct connection between sources and receivers of signals in the form of electrical or optical cables. Radio communications, broadcasting and television as the medium for the propagation of electromagnetic waves usually use the atmosphere of the Earth. The main parameters of the communication system are largely determined by the characteristics of the propagation of electromagnetic waves. Consider them in more detail.

It is known from the physics course that a conductor through which a constant electric current flows creates a constant magnetic field in the surrounding space. In general, almost any segment of a conductor with alternating current flowing through it is a source of alternating electromagnetic field. A feature of the alternating electromagnetic field is its ability to propagate in the surrounding space.

In free space, electromagnetic collars propagate rectilinearly and uniformly, that is, with a constant speed equal to the speed of light (c = 3 • 10 ⁸ m / s). The propagation of electromagnetic waves in a non-free space is significantly affected by the environment. In particular, the propagation of radio waves in the Earth’s conditions depends on many factors: terrain relief, climatic conditions, time of day and year, and, first of all, on the wavelength of this range.

Electromagnetic waves located in the frequency range from 10 to 10 ¹³ Hz are used in radio engineering and are called radio waves. The international classification of radio wave bands is given in table 6.1. The wavelength λ of electromagnetic coli *** is related to the frequency f of this colo *** and the speed from the propagation of electromagnetic waves in free space by the relation

λ = c / f . (6.1)

Table 6.1

A simplified mechanism for the formation of an electromagnetic field can be represented as follows. An alternating current flowing through a conductor in accordance with the law of electromagnetic induction will excite an alternating magnetic field in the space surrounding the dipole. The changing magnetic field, in turn, generates an alternating electric field in the surrounding space. In the process of mutual transformation of a changing magnetic field into an electric one, and of an alternating electric field into a magnetic one, a single electromagnetic field is formed. The phenomenon of excitation in space of an electromagnetic field by an alternating current flowing in a conductor is called electromagnetic radiation.

In the general case, any segment of a conductor through which alternating current flows creates an electromagnetic field in the surrounding space. These phenomena are related by the principle of duality: in any segment of a conductor located in an electromagnetic field, a variable electromotive force (EMF) is induced. The magnitude of the EMF induced in the conductor depends both on the energy of the electromagnetic field, and on the configuration of the conductor and the ratio of its dimensions and the wavelength of the electromagnetic columns ***.

To estimate the energy characteristics of electromagnetic waves, the power flux density passing through a unit of area perpendicular to the direction of wave propagation is used. If we assume that the radiation source is point (in practice, this means that the size of the radiation source is negligible compared to the wavelength of the radiated colossus ***), then we can assume that the electromagnetic wave will be uniformly radiated in all directions. At a distance R from the radiation source, the power flux density P generated by a point source is the same and is determined by the expression

P = P _T / (4πR2) , (6.2)

where P _T is the power of the radiation source.

The range of the communication system is determined by the transmitter power and receiver sensitivity. The strengths of the electric and magnetic components of the electromagnetic field generated by this source are determined by the radiation power of the source P _T and the distance R from the source. So, to estimate the electric field strength Е _T during the propagation of radio waves in free space, you can use an approximate relationship:

(6.3)

Radio waves - electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have a frequency of 3 kHz to 300 GHz, and the corresponding wavelength from 100 kilometers to 1 millimeter. Like all other electromagnetic waves, radio waves travel at the speed of light. Natural sources of radio waves are lightning and astronomical objects. Artificial radio waves are used for fixed and mobile radio communications, broadcasting, radar and other navigation systems, communication satellites, computer networks and other uncountable applications. Different frequencies of radio waves propagate differently in the atmosphere of the Earth: long waves can cover part of the Earth very consistently, shorter waves can bounce off the ionosphere and spread around the world, and with even shorter wavelengths, they are bent or reflected very little and propagate within a straight line. visibility.

Research history

About radio waves for the first time in his works in 1868, James Maxwell told ^[1] . He proposed an equation that describes light and radio waves as waves of electromagnetism. In 1887, Heinrich Hertz experimentally confirmed Maxwell’s theory, receiving radio waves in his laboratory several tens of centimeters long ^[2] .

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Radio frequency bands and radio wavelengths

Radio frequencies - frequencies or frequency bands in the range of 3 kHz - 3000 GHz, to which conditional names are assigned. This range corresponds to the frequency of the alternating current electrical signals for generating and detecting radio waves. Since most of the range lies beyond the boundaries of the waves that can be produced by mechanical vibration, radio frequencies usually refer to electromagnetic collisions.

The RF Law “On Communications” establishes the following notions related to radio frequencies:

• radio frequency spectrum - a set of radio frequencies within the limits established by the International Telecommunication Union, which can be used for the operation of radio-electronic means or high-frequency devices;
• radio frequency - the frequency of electromagnetic collars ***, established to denote a single component of the radio frequency spectrum;
• allocation of radio frequency bands - determination of the purpose of radio frequency bands through entries in the Table of distribution of radio frequency bands between the radio services of the Russian Federation, on the basis of which permission is given to use a specific radio frequency band, and the conditions for such use are established.

The use of bands for radio services is governed by the Radio Regulations of the Russian Federation and international agreements.

According to the regulations of the International Telecommunication Union, radio waves are divided into ranges from 0.3 * 10 ^N Hz to 3 * 10 ^N Hz, where N is the range number. Russian GOST 24375-80 almost completely repeats this classification.

ITU Identification	Wavelengths	Name of the waves	Frequency range	Frequency name	Photon energy, eV,	Application
ELF	100 Mm - 10 Mm	Decamegameters	3-30 Hz	Extremely Low (ELF)	12.4 feV - 124 feV	Submarine communications, geophysical surveys
SLF	10 Mm - 1 Mm	Megametrovye	30—300 Hz	Ultra Low (VLF)	124 feV - 1.24 peV	Submarine communications, geophysical surveys
ULF	1000 km - 100 km	GC	300–3000 Hz	Infra-low (INCH)	1.24 peV - 12.4 peV
Vlf	100 km - 10 km	Miriametrovye	3-30 kHz	Very low (VLF)	12.4 peV - 124 peV	Communication with submarines
Lf	10 km - 1 km	Kilometers	30-300 kHz	Low (LF)	124 peV - 1.24 neV	Radio broadcasting, radio communication
Mf	1000 m - 100 m	Hectometric	300-3000 kHz	Medium (MF)	1.24 neV - 12.4 neV	Radio broadcasting, radio communication
HF	100 m - 10 m	Decameter	3-30 MHz	High (HF)	12.4 neV - 124 neV	Broadcasting, radio, radio
VHF	10 m - 1 m	Meter waves	30–300 MHz	Very high (VHF)	124 neV - 1.24 μeV	Television, radio broadcasting, radio communication, walkie-talkies
Uhf	1000 mm - 100 mm	Decimeter	300-3000 MHz	Ultra High (UHF)	1.24 μeV - 12.4 μeV	Television, radio, Mobile phones, walkie-talkies, microwave ovens, satellite navigation.
SHF	100 mm - 10 mm	Centimeter	3-30 GHz	Ultra high (microwave)	12.4 μeV - 124 μeV	Radiolocation, Internet, satellite TV, radio communication, Wireless computer networks.
EHF	10 mm - 1 mm	Millimeter	30-300 GHz	Extremely high (EHF)	124 μeV - 1.24 meV	Radio astronomy, high-speed radio-relay communication, meteorological radars, medicine
Thf	1 mm - 0.1 mm	Decimillimeter	300-3000 GHz	Hyperhigher, longwave region of infrared radiation	1.24 meV - 12.4 meV	The experimental “terahertz camera” recording the image in the long-wave IR (which is emitted by warm-blooded organisms, but, unlike the shorter-wave IR, is not delayed by dielectric materials).

Classification GOST 24375-80 is not widely used and in some cases it conflicts with national standards (GOST) in the field of radio electronics. In practice, the low frequency range refers to the sound range, and the high frequency range refers to the entire radio frequency range above 30 kHz, including microwave frequency (above 300 MHz). The traditional designations of the radio frequency bands in the West took shape during the Second World War. Currently, they are enshrined in the US IEEE standard, as well as the international standard ITU.

Examples of dedicated radio bands

Title	Frequency band	Wavelengths	Photon energy, eV,
Medium wave range (MW)	530—1610 kHz	565.65-186.21 m	2.19-6.66 neV
Short wave range	5.9—26.1 MHz	50.8–11.49 m	24.4—107.9 neV
Civic range	26.965-27.405 MHz	11.118–10.940 m	111.5–113.3 neV
TV channels: from 1 to 5	48–100 MHz	6.25-3.00 m	198.5—413.6 neV
TV channels: from 6 to 12	174-230 MHz	1.72-1.30 m	719.6—951.2 neV
TV channels: from 21 to 39	470–622 MHz	6.38–4.82 dm	1.94–2.57 μeV
Ultrashort wave range (UKW)	62-108 MHz (except 76-90 MHz in Japan)	1m	256.42—446.65 neV (except 314.31—372.21 neV)
ISM range
Military frequency bands	29.50-31.75 MHz
Civil aviation frequency bands
Sea and river ranges

Radio frequency ranges in civil radio communications

In Russia, for civilian radio communications, three frequency ranges are distinguished:

Some ranges of civil aviation

Some radar ranges