Radiocommunication principles and examples. Basics of radio communications and television

The English scientist James Maxwell on the basis of the study of experimental work Faraday for electricity expressed a hypothesis about the existence of special waves in nature capable of spreading in vacuo. These waves Maxwell called electromagnetic waves. According to Maxwell's ideas: with any change electric field A vortex magnetic field occurs and, on the contrary, with any change of the magnetic field there is a vortex electric field. Once, the process of mutual generation of magnetic and electric fields should continuously continue and capture all new and new areas in the surrounding space (Fig. 42). The process of the interaction of electrical and magnetic fields occurs in mutually perpendicular planes. The alternating electric field generates a vortex magnetic field, the alternating magnetic field generates a vortex electric field.

Electrical and magnetic fields may exist not only in substance, but also in vacuo. Therefore, the propagation of electromagnetic waves in vacuum should be possible.

The condition for the occurrence of electromagnetic waves is an accelerated movement electrical charges. So, the change in the magnetic field occurs

When the current changes in the conductor, and the current change occurs when the charge rate changes, i.e., when moving with acceleration. The speed of propagation of electromagnetic waves in vacuo, according to Maxwell, should be approximately equal to 300,000 km / s.

For the first time, an electromagnetic wave of physicist Heinrich Hertz received electromagnetic waves, while using the high-frequency spark arrester (hertz vibrator). Hertz experimentally determined the speed of electromagnetic waves. It coincided with the theoretical determination of the speed of waves Maxwell. The simplest electromagnetic waves are waves in which electric and magnetic field Make synchronous harmonic oscillations.

Of course, electromagnetic waves have all the main properties of the waves.

They are subject to the law of reflection of the waves: the angle of the fall is equal to the angle of reflection. When moving from one medium to another, the refractive effect of the waves is refracted: the ratio of the sinus of the angle of falling to the sinus of the refraction angle is the value constant for two data of the media and equal to the ratio of the velocity of electromagnetic waves in the first medium to the speed of electromagnetic waves in the second environment and is called the refractive index of the second media relative to the first.

The phenomenon of diffraction of electromagnetic waves, i.e. the deviation of the direction of their propagation from the straightforward, is observed at the region of the barrier or when passing through the hole. Electromagnetic waves are capable of interference. The interference is the ability of coherent waves to imposition, as a result of which the waves in some places are enhanced by each other, and in other places it is quenched. (Coherent waves are waves, the same frequency and phase of oscillation.) Electromagnetic waves have a dispersion, i.e. when the refractive index of the medium for electromagnetic waves depends on their frequency. Experiments with transmitting electromagnetic waves through a system of two lattices show that these waves are transverse.

When the electromagnetic wave is propagated, the vectors of the e and magnetic induction in perpendicular to the direction of propagation of the wave and mutually perpendicular to each other (Fig. 43).

The possibility of practical use of electromagnetic waves to establish communication without wires was demonstrated on May 7, 1895. Russian physicist A. Popov. This day is considered the day of the radio. To carry out radio communications, it is necessary to ensure the possibility of radiation of electromagnetic waves. If the electromagnetic waves occur in the contour from the coil and the condenser, the variable magnetic field turns to the coil, and the alternating electric field is concentrated between the condenser plates. This circuit is called closed (Fig. 44, a).

The closed oscillating circuit practically does not radiate electromagnetic waves into the surrounding space. If the contour consists of a coil and two plates of a flat capacitor, then these plates are deployed at the large angle, the more freely empty the electromagnetic field in the surrounding space (Fig. 44, b). The limit case of the disclosed oscillatory circuit is the removal of the plates on the opposite ends of the coil. Such a system is called an open oscillatory circuit (Fig. 44, B). In reality, the contour consists of a coil and a long wire - antenna.

The energy of the radiated (with the help of a generator of unsuccessful oscillations) of electromagnetic oscillations with the same amplitude of current fluctuations in the antenna is proportional to the fourth degree of the frequency of oscillations. At frequencies in tens, hundreds and even thousands of hertz, the intensity of electromagnetic oscillations is negligible. Therefore, electromagnetic waves are used to carry out radio and television communication with a frequency of several hundred thousand hertz to hundreds of megahertz.

When transmitting radio speech, music and other sound signals Apply various types of modulation of high-frequency (carrier) oscillations. The essence of modulation is that high-frequency oscillations produced by the generator are changed by the law of low frequency. This is one of the principles of radio transmission. Another principle is the reverse process - detection. When the radio received from the received antenna receiver, the modulated signal must be filtered by sound low-frequency oscillations.

Using radio waves, a transmission is transmitted to a distance of not only audio signals, but also images of objects. A large role in modern sea fleet, aviation and cosmonautics plays radar. The radar base is the property of reflection of waves from conductive bodies. (From the surface of the dielectric, electromagnetic waves are poorly reflected, and from the surface of the metals is almost completely.)

Electromagnetic waves I.

their properties. Radiocommunication Principles I.

examples of their practical

use

Answer Plan

1. Determination. 2. Condition of occurrence. 3. Sources of electromagnetic waves. 4.Clocked oscillating circuit. 5. Module and detection.

The English scientist James Maxwell on the basis of the study of experimental work Faraday for electricity expressed a hypothesis about the existence of special waves in nature capable of spreading in vacuo.

These waves Maxwell called Electromagnetic waves.According to Maxwell's ideas: with any change of the electric field, a vortex magnetic field occurs and, on the contrary, with any change in the magnetic field, a vortex electric field occurs.Once the process of mutual generation of magnetic and electric fields began to continuously continue and capture all new and new areas in the surrounding space (Fig. 31). The intercession of the interaction of electrical and magnetic fields occurs in mutually perpendicular planes. The alternating electric field generates a vortex magnetic field, the alternating magnetic field generates a vortex electric field.

Electrical and magnetic fields may exist not only in substance, but also in vacuo. Therefore, the propagation of electromagnetic waves in vacuum should be possible.

Condition of occurrenceelectromagnetic waves is an accelerated movement of electrical charges. Thus, the change in the magnetic field occurs when the current changes in the conductor, and the current change occurs when the charge rate changes, i.e., when moving them with acceleration. The speed of propagation of electromagnetic waves in vacuo for Maxwell calculations should be approximately equal to 300 000 km / s.

For the first time, the electromagnetic waves of the physicist Heinrich Hertz received electromagnetic waves, using the high-frequency spark arrester (hertz vibrator). Hertz experimentally determined the speed of electromagnetic waves. It coincided with the theoretical determination of the speed of waves Maxwell. The simplest electromagnetic waves are waves in which electrical and magnetic fields make synchronous harmonic oscillations.

Of course, electromagnetic waves have all the main properties of the waves.

They obey reflection lawwaves:

the angle of the fall is equal to the angle of reflection.When moving from one environment to another, refracted and obey refractibility lawwaves: the ratio of the sinus angle of the fall to the sinus of the refractive angle is the value constant for two data of the media and equal to the ratio of the speed of electromagnetic waves in the first medium to the speed of electromagnetic waves in the second environmentand called refractive indexthe second medium is relatively first.

I
the impairment of the diffraction of electromagnetic waves, i.e., the deviation of the direction of their propagation from the straightforward, is observed at the region of the barrier or when passing through the hole. Electromagnetic waves are capable of interference.Interference is the ability of coherent waves to imposition, as a result of which the waves in some places are enhanced by each other, and in other places, the skills. (Coherent waves are waves, the same in frequency and phase of oscillations.) Electromagnetic waves possess dispersioni.e. when the refractive index for electromagnetic waves depends on their frequency. Experiments with transmitting electromagnetic waves through a system of two lattices show that these waves are transverse.

When distributing electromagnetic waves, tension vectors E.and magnetic induction into perpendicular to the direction of propagation of the wave and mutually perpendicular to each other (Fig. 32).

The possibility of practical use of electromagnetic waves to establish communication without wires was demonstrated by 7th 1895. Russian physicist A. Popov. This day is considered the day of the radio. To carry out radio communications, it is necessary to ensure the possibility of radiation of electromagnetic waves. If the electromagnetic waves occur in the contour from the coil and the condenser, the variable magnetic field turns out to be associated with the coil, and the alternating electric field is-used between the condenser plates. This circuit is called closed(Fig. 33, a). The closed oscillating circuit practically does not radiate electromagnetic waves into the surrounding space. If the outline consists of a coil and two plates of a flat capacitor, then these plates are deployed at the large angle, the more freely exits the electromagnetic field into the surrounding space (Fig. 33, b).The limit case of the disclosed oscillatory circuit is the removal of the plates on the opposite ends of the coil. Such a system is called Open oscillatory contour(Fig. 33, c). In reality, the contour consists of a coil and long wire -antenna.

The energy of the radiated (with the help of a generator of unsuccessful oscillations) of electromagnetic oscillations with the same amplitude of current fluctuations in the antenna is proportional to the fourth degree of the frequency of oscillations. At frequencies in tens, hundreds and even thousands of hertz, the intensity of electromagnetic oscillations is negligible. Therefore, electromagnetic waves are used to carry out radio and television communication with a frequency of several hundred thousand hertz to hundreds of megahertz.

When transmitting radio speech, music and other sound signals using various types of modulation of high-frequency (carrier) oscillations. The essence of modulationit is that high-frequency oscillations produced by the generator are changed by the law of low frequency. This is one of the principles of radio transmission. Another principle is the reverse process - Detection.When the radio received from the received antenna receiver, the modulated signal must be filtered by sound low-frequency oscillations.

Using radio filters, a transmission is transmitted to a distance of not only sound signals, but also images of the subject. A large role in modern sea fleet, aviation and cosmonautics plays radar. The radar base is the property of reflection of waves from conductive bodies. (From the surface of the dielectric, electromagnetic waves are poorly reflected, and from the surface of the metals is almost completely.)

The possibility of practical use of electromagnetic waves to establish communication without wires was demonstrated on May 7, 1895. The famous Russian physicist Alexander Stepanovich Popov (1859-1906). This day is considered the day of the radio.

Receiver A. S. Popova consisted of antenna 1, coherer 2, electromagnetic relay 3, electrical bell 4 and DC source 5 (Fig. 245). Electromagnetic waves caused forced current fluctuations and voltage in the antenna. The alternating voltage from the antenna was supplied to two electrodes, which were located in a glass tube filled with metal sawdust. This tube is coherer. Consistent with the coherer included electromagnetic relay and a DC source.

Due to bad contacts between sawdust, the coherer resistance is usually large, so the electric current in the chain is small and the relay ring circuit does not closer. Under the action of alternating voltage of high frequency in the coherer, electrical discharges arise between individual sawdusts, particles of sawdust are sintered and its resistance decreases 100-200 times. The power of the current in the coil of the electromagnetic relay increases, and the relay includes an electric bell. This is how the antenna electromagnetic wave is recorded.

Punching a hammer of the Cogerrian call shake sawdust and returned it to the initial stateThe receiver was again ready for the registration of electromagnetic waves.

Open oscillating circuit.

To carry out radio communications, it is necessary to ensure the possibility of radiation of electromagnetic waves. If electromagnetic oscillations occur in the contour from the coil and the condenser, the variable magnetic field turns to be associated with the coil, and the alternating electric field is concentrated in the space between the plates of the condenser (Fig. 246, a). This circuit is called closed. The closed oscillating circuit practically does not radiate electromagnetic waves into the surrounding space.

If the outline consists of a coil and two plates of a flat capacitor, not parallel to each other, then the plates are deployed at a large angle,

moreover, the electromagnetic field is released into the surrounding space (Fig. 246, b).

The limit case of the disclosure of the oscillating circuit is to remove the condenser plates on the opposite ends of the straight coil. Such a system is called an open oscillatory circuit (Fig. 246, B). The image of the condenser plates at the ends of the open oscillatory circuit coil in Figure 246 is only conventional. In reality, the contour consists of a coil and a long wire - antenna. One end of the antenna is grounded, the second is raised above the surface of the earth.

The antenna coil has an inductive connection with the coil of the oscillatory circuit of the generator of the unlucky electromagnetic oscillations. The forced vibrations of high frequency in the antenna create an alternating electromagnetic field in the surrounding space. With the speed of electromagnetic waves propagate from the antenna.

The energy of the emitted electromagnetic waves with the same amplitude of the fluctuations in the current force in the antenna is proportional to the fourth degree of the oscillation frequency. At frequencies in tens, hundreds and even thousands of hertz, the intensity of radiation of electromagnetic waves is negligible. Therefore, electromagnetic waves are used to carry out radio and television communication with a frequency of several hundred thousand hertz to hundreds of thousands of megahertz.

Amplitude modulation.

When transferring on radio speech, music and other sound signals, various types of modulation of harmonic oscillations of high frequency are used.

To implement the amplitude modulation of electromagnetic oscillations of high frequency

(Fig. 247, a) In the electric circuit of the transistor generator, in series with a oscillating circuit, the transformer coil is included (Fig. 248). On the second coil of the transformer serves aC voltage Sound frequency, for example, from the microphone exit after the required gain. AC current in the second transformer coil causes an alternating voltage at the ends of the first transformer coil. A variable sound frequency voltage (Fig. 247, b) folds with a constant voltage of the current source; The voltage changes between the emitter and collector of the transistor lead to changes with the sound frequency of the amplitude of the high frequency current fluctuations in the generator circuit (Fig. 247, B). Such high frequency oscillations are called amplitude-modulated.

The radio transmitter antenna is inductively connected with the oscillatory circuit of the generator. Current current fluctuations

frequencies occurring in an antenna create electromagnetic waves.

Radio.

Electromagnetic waves emitted by an antenna radio transmitter cause forced oscillations of free electrons in any conductor. Voltage between the ends of the conductor, in which the electromagnetic wave excites the forced oscillations electric currentProportional to the length of the conductor. Therefore, for the reception of electromagnetic waves in the simplest detector radio receiver, a long wire is used - the receiving antenna 1 (Fig. 249). Forced oscillations in the antenna are excited by electromagnetic waves from all radio stations. In order to listen to only one broadcast, the voltage fluctuations are not directly directly on the amplifier input, and first fed to the oscillating circuit 2 with a changing natural frequency of oscillations. Changing your own oscillation frequency in the receiver circuit is usually made by changing the electrical capacity of the variable condenser. When the frequency of forced oscillations in the antenna with its own frequency of oscillations of the contour comes a resonance, while the amplitude of the forced voltage fluctuations on the circuit condenser plates reaches the maximum value. Thus, from a large number of electromagnetic oscillations excited in an antenna, fluctuations of the desired frequency are distinguished.

From the oscillatory outline of the receiver modulated oscillations

high frequencies come to the detector 3. You can use a semiconductor diode as a detector, transmitting a high frequency alternating current in only one direction. After passing the detector, the current strength in the chain varies in time by law shown in Figure 250, and. During each half-period of high frequency, current pulses charge the capacitor 4, at the same time the condenser is slowly discharged through the resistor 5. If the values \u200b\u200bof the electrical capacity of the capacitor and the electrical resistance of the resistor are chosen correctly, then the resistor will flow through the time via the sound frequency used by modulation oscillations in a radio transmitter (Fig. 250, b). To convert electrical oscillations to sound variable voltage, sound frequency is fed to the phone 6.

The detector radio receiver is very imperfect. It has a very low sensitivity and therefore can successfully take radio broadcasts only from powerful radio stations or from closely located radio transmitters.

To increase the sensitivity in modern radio receivers, the signal from the oscillating circuit enters the high frequency amplifier input (UHF), and from the output of the amplifier, high-frequency electrical oscillations arrive at the detector. To increase the power of the audio signal at the output of the radio receiver, the electrical oscillations of the sound frequency from the detector output arrive at the low frequency amplifier input (UNG).

A variable sound frequency voltage from the ONLC output is fed to the winding of the electrodynamic loudspeaker - the dynamics. The speaker converts energy alternating current Sound frequency in the energy of sound oscillations.

To enhance the electrical oscillations of high and low frequencies, circuits with electronic lamps or transistors can be used.

The diagram of the device of the simplest radio with high and low frequency amplifiers is presented in Figure 251.

To configure only one station in modern radio receptions, quite complicated electronic circuits, including electromagnetic oscillation generators. The addition of electrical oscillations from an internal receiver generator with oscillations excited in the receiver circuit with electromagnetic waves from transmitting radio stations allows you to configure the receiver to a very narrow range of received frequencies. The internal generator in the receiver is called a heterodyne, and the receiver with such a generator is called an eurgometroodyne radio.

A television.

Using radio filters, a transmission is transmitted to a distance of not only sound signals, but also images of the subject. Principle of transfer of moving black and white and color images with

the help of television transmitters and receivers is as follows.

To transmit one frame of a television image using a lens in a television chamber, an image of an object is obtained on the screen of a special electrovacuum instrument - the transmitting tube (Fig. 252). Under the action of light, the sections of the screen acquire positive charges. An electronic beam is sent to the screen inside the transmission tube, moving periodically from left to right to 625 horizontal lines - strings. During the running of the beam along the line, the neutralization of electrical charges occurs in separate parts of the screen and in the electrical circuit connecting the electronic gun and the screen; Current pulse proceeds. Changes in the strength of the current in the pulse correspond

changes in the illumination of the screen on the path of the electron beam.

High-frequency electromagnetic oscillations in the television transmitter are modulated by a pulse signal obtained at the output of the transmitting tube, and are fed to the transmitter antenna. Antenna emits electromagnetic waves.

In the television receiver - TV - there is an electrovacuum tube called a kinescope. In the kinescope, the electronic gun creates an electronic beam. Electrons under the action of the electric field are moving inside the tube to the screen coated with crystals capable of glowing under the blows of fast moving electrons. On the path to the screen, the electrons fly through the magnetic fields of two pairs of coils located outside the tube.

The magnetic field of one pair of coils causes the deviation of the electron beam horizontally, the second - vertically. Periodic changes in the strength of the current in the coils cause changes in magnetic fields, as a result of which the electronic beam per seconds runs 625 times on the screen from left to right and one time - from top to bottom (Fig. 253).

During the movement of the beam along the first line of the current force in the electronic beam, controls the signal received by the receiver from the transmitter while moving the beam in the transmitting tube on the first line; When the beam is moved along the second line of the current in the ray, the signal controls the signal from the second line, etc. As a result, the same image on the TV screen, which is built on the lens on the transmitting tube screen. Frames replace each other with a frequency of 25 frames per second, the sequence of replacing each other with a high frame rate is perceived by the human eye as a continuous movement.

Television transmissions are conducted in the range from 50 MHz to 230 MHz. In this range, electromagnetic waves apply almost only within the limits of direct visibility. Therefore, to ensure the transfer television signals High antennas are built on distant distances. The transmitting antennas of the Central Television Studios of the USSR are installed on the top of the Ostankino tower height such height ensures the reception of television gears at distances up to 120 km from Moscow.

The transfer of television signals to any point in our country is carried out using the relay artificial satellites of the Earth in the Orbit system.

Transmission and reception of color images require the use of more complex television systems. Instead of one transmitting tube, it is required to use three tubes that transmit three monochromatic images of red, blue and green colors.

Unlike a black and white TV, the color TV screen is covered with crystals of phosphors of three varieties. Some crystals are shoved in red light on them with red light, others are blue, the third is green. This crystals are located on the crane in a strict order. The signals come from the television transmitter to the three electron-beam guns.

On the color TV screen, three images of red, green and blue colors are created simultaneously. The imposition of these images consisting of small glowing points is perceived by the human eye as a multicolor image with all the shades of colors. The simultaneous glow of crystals in one place is blue, red and green is perceived by the eye as white color; Therefore, black and white images can be obtained on the color television screen.

Distribution of radio waves.

Radiocommunication is carried out on long medium short and ultrashort waves. Radio waves with different wavelengths are distributed differently at the surface of the Earth.

Long waves due to diffraction extend far beyond the visible horizon; Radio transmits on long waves can be taken at large distances outside the direct visibility of the antenna.

The average waves are less diffraction at the surface of the Earth and distribute due to the diffraction for smaller distances beyond the limits of direct visibility. Short waves are even less capable of diffraction at the surface of the earth, but they can be taken anywhere on the ground surface. The propagation of short radio waves over long distances from the transmitting radio station is explained by their ability to reflect the ionosphere.

The ionosphere is called the upper part of the atmosphere, which starts from the distance of about 50 km from the surface of the Earth and

running into an interplanetary plasma at distances 70-80 thousand km. A feature of the ionosphere is a high concentration in it free charged particles - ions and electrons. The ionization of the upper layers of the atmosphere is created by ultraviolet and X-ray radiation of the Sun. The maximum values \u200b\u200bof the amount of free electrons in the electrons ionosphere in a cubic centimeter are achieved at altitudes 250-400 km from the surface of the Earth.

The conductive layer of the earth's atmosphere - ionosphere is capable of absorbing and reflecting electromagnetic waves. From the ionosphere, long radio waves are well reflected. This phenomenon along with diffraction increases the range of distribution of long waves. Well reflected by ionosphere and short radio waves. Multiple reflections of short radio waves from the ionosphere and the earth's surface make possible radio communication on short waves between any points on the ground (Fig. 254).

Ultra-screw waves (VHF) are not reflected by the ionosphere and do not enhance the surface of the Earth as a result of diffraction (Fig. 255). Therefore, communication to VHF

it is carried out only within the direct visibility of the transmitter antenna.

Radar.

A large role in modern sea fleet, aviation and cosmonautics play radar communications. The radar base is the property of reflection of radio waves from conductive bodies.

If the radio transmitter is turned on at a very short time and turn off, then it can be possible to register the return of radio waves reflected from the conductive tel away from the radio station using the radio receiver.

Measuring using electronic equipment, the duration of the time interval between the points of the time of sending and returning the electromagnetic waves, one can determine the path passed by radio waves: where C is the speed of the electromagnetic wave. Since the waves passed the way to the body and back, the distance to the body, reflecting the radio wave, equal to half of this way:

To determine not only the distance to the body, in and its position in space, it is necessary to send a radio wave by a narrow-controlled beam. A narrow bundle of radio waves is created using an antenna having a shape close to spherical. In order for the antenna of the radar can create a narrow-controlled bundle of radio waves, ultra-screwed waves are used in radar

To determine, for example, the location of the aircraft the antenna of the radar is directed to the aircraft and in a very short time include an electromagnetic wave generator. Electromagnetic waves are reflected from the aircraft and returned to the radar. The reflected radio signal catches the same antenna disconnected from the transmitter and connected to the receiver (Fig. 256). At the angles of rotation of the antenna of the radar, the direction of the aircraft is determined. The radar installed on the plane allows the time of passage of radio waves to the surface of the Earth and to measure the height on which the aircraft is located.

Water and drying, dry and wet soil, urban structures and transport communications reflect radio waves in different ways. This allows with the help of radar devices by plane not only to measure the distance to

the surfaces of the Earth, but also to receive a kind of radar area of \u200b\u200bthe area, over which the plane flies. This map of the aircraft pilot receives day and night, in clear weather and with continuous cloudiness, as the clouds are not a barrier for electromagnetic waves.

Radar methods made the most accurate measurements of the distances from the ground to Louis and to the planets of Mercury, Venus, Mars and Jupiter.

Proved that electromagnetic energy can be sent to space in the form of radio waves, which pass through the atmosphere at about speed. This discovery helped to develop the principles of radio communications, which are used today. In addition, the scientist proved that radio waves have an electromagnetic nature, and their main characteristic is the frequency at which the energy varies between electrical and magnetic fields. The frequency in hertz (Hz) is associated with a wavelength of λ, which is a distance that radio wave passes within one oscillation. Thus, the following formula is obtained: λ \u003d C / F (where c is equal to the speed of light).

Radiocommunication principles are based on the transfer of radio wave carriers. They can transmit voice or digital data. For this, the radio station must have:

Device for collecting information into an electrical signal (for example, microphone). This signal is called the main frequency band in the usual sound range.

Modulator for making information in the signal bandwidth on the selected

Transmitter, signal that sends it to the antenna.

Antenna from conductive electricity rod of a certain length, which will emit electromagnetic radio wave.

Signal amplifier on receiver side.

A demodulator that will be able to restore initial information from the received radio signal.

Finally, the device for playing the transmitted information (for example, a loudspeaker).

The modern principle of radio communications was conceived at the beginning of the last century. At that time, the radio was mainly developed for voice and music. But very soon it was possible to use the principles of radio communications for transmitting more complex information. For example, such as text. This led to the invention of Morse telegraph.

Common for voice, music or telegraph is that the basic information is encrypted into which are characterized by amplitude and frequency (Hz). People can hear sounds ranging from 30 Hz and about 12,000 Hz. This range is called sound spectrum.

The radio frequency spectrum is divided into various each of which has specific characteristics regarding radiation and attenuation in the atmosphere. Allocate the communication applications described in the table below that operate in a different range.

It is difficult to imagine today, which would make humanity without a radio communications, which has found its use in many modern devices. For example, the principles of radio communications and television are used in mobile phones, keyboard, GPRS, Wi-Fi, wireless computer networks and so on.

The English scientist James Maxwell on the basis of the study of experimental work Faraday for electricity expressed a hypothesis about the existence of special waves in nature capable of spreading in vacuo. These waves Maxwell called electromagnetic waves. According to Maxwell's ideas: with any change in the electric field, a vortex magnetic field occurs and, on the contrary, with any change in the magnetic field there is a vortex electric field. Once, the process of mutual generation of magnetic and electric fields should continuously continue and capture all new and new areas in the surrounding space (Fig. 42). The process of the interaction of electrical and magnetic fields occurs in mutually perpendicular planes. The alternating electric field generates a vortex magnetic field, the alternating magnetic field generates a vortex electric field.

Electrical and magnetic fields may exist not only in substance, but also in vacuo. Therefore, the propagation of electromagnetic waves in vacuum should be possible.

The condition for the occurrence of electromagnetic waves is the accelerated movement of electrical charges. So, the change in the magnetic field occurs

When the current changes in the conductor, and the current change occurs when the charge rate changes, i.e., when moving with acceleration. The speed of propagation of electromagnetic waves in vacuo, according to Maxwell, should be approximately equal to 300,000 km / s.

For the first time, an electromagnetic wave of physicist Heinrich Hertz received electromagnetic waves, while using the high-frequency spark arrester (hertz vibrator). Hertz experimentally determined the speed of electromagnetic waves. It coincided with the theoretical determination of the speed of waves Maxwell. The simplest electromagnetic waves are waves in which electrical and magnetic fields make synchronous harmonic oscillations.

Of course, electromagnetic waves have all the main properties of the waves.

They are subject to the law of reflection of the waves: the angle of the fall is equal to the angle of reflection. When moving from one medium to another, the refractive effect of the waves is refracted: the ratio of the sinus of the angle of falling to the sinus of the refraction angle is the value constant for two data of the media and equal to the ratio of the velocity of electromagnetic waves in the first medium to the speed of electromagnetic waves in the second environment and is called the refractive index of the second media relative to the first.

The phenomenon of diffraction of electromagnetic waves, i.e. the deviation of the direction of their propagation from the straightforward, is observed at the region of the barrier or when passing through the hole. Electromagnetic waves are capable of interference. The interference is the ability of coherent waves to imposition, as a result of which the waves in some places are enhanced by each other, and in other places it is quenched. (Coherent waves are waves, the same frequency and phase of oscillation.) Electromagnetic waves have a dispersion, i.e. when the refractive index of the medium for electromagnetic waves depends on their frequency. Experiments with transmitting electromagnetic waves through a system of two lattices show that these waves are transverse.

When the electromagnetic wave is propagated, the vectors of the e and magnetic induction in perpendicular to the direction of propagation of the wave and mutually perpendicular to each other (Fig. 43).

The possibility of practical use of electromagnetic waves to establish communication without wires was demonstrated on May 7, 1895. Russian physicist A. Popov. This day is considered the day of the radio. To carry out radio communications, it is necessary to ensure the possibility of radiation of electromagnetic waves. If the electromagnetic waves occur in the contour from the coil and the condenser, the variable magnetic field turns to the coil, and the alternating electric field is concentrated between the condenser plates. This circuit is called closed (Fig. 44, a).

The closed oscillating circuit practically does not radiate electromagnetic waves into the surrounding space. If the contour consists of a coil and two plates of a flat capacitor, then these plates are deployed at the large angle, the more freely empty the electromagnetic field in the surrounding space (Fig. 44, b). The limit case of the disclosed oscillatory circuit is the removal of the plates on the opposite ends of the coil. Such a system is called an open oscillatory circuit (Fig. 44, B). In reality, the contour consists of a coil and a long wire - antenna.

The energy of the radiated (with the help of a generator of unsuccessful oscillations) of electromagnetic oscillations with the same amplitude of current fluctuations in the antenna is proportional to the fourth degree of the frequency of oscillations. At frequencies in tens, hundreds and even thousands of hertz, the intensity of electromagnetic oscillations is negligible. Therefore, electromagnetic waves are used to carry out radio and television communication with a frequency of several hundred thousand hertz to hundreds of megahertz.

When transmitting radio speech, music and other sound signals using various types of modulation of high-frequency (carrier) oscillations. The essence of modulation is that high-frequency oscillations produced by the generator are changed by the law of low frequency. This is one of the principles of radio transmission. Another principle is the reverse process - detection. When the radio received from the received antenna receiver, the modulated signal must be filtered by sound low-frequency oscillations.

Using radio waves, a transmission is transmitted to a distance of not only audio signals, but also images of objects. A large role in modern sea fleet, aviation and cosmonautics plays radar. The radar base is the property of reflection of waves from conductive bodies. (From the surface of the dielectric, electromagnetic waves are poorly reflected, and from the surface of the metals is almost completely.)