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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Radio waves. Radio wave bands. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur Suppose you pick up the handset of the telephone, dial or call the desired number. Soon you hear the voice of a comrade; and he is yours. What electrical phenomena occur during your telephone conversation? Sound, air vibrations created by you are converted by a microphone and electrical vibrations of sound frequency, which are transmitted via wires to your interlocutor's apparatus. There, at the other end of the line, they are converted with the help of a telephone into air vibrations perceived by your friend as sounds. In radio broadcasting, as in telephony, a microphone, a telephone or a loudspeaker head are the final links in the chain of radio transmission to radio reception. But the medium that connects them is not wires, but radio waves. The "heart" of the transmitter of any radio station is a high-frequency oscillator. It generates (generates) a current at a high, but strictly constant frequency for a given radio station. This current, amplified to the required power, enters the antenna and excites electromagnetic oscillations of the same frequency in the surrounding space - radio waves. The speed of removal of radio waves from the antenna of the radio station is equal to the speed of light: 300000 km / s, which is almost a million times faster than the propagation of sound in air. This means that if a transmitter was turned on at a certain moment in time at the Moscow Broadcasting Station, then its radio waves would reach Vladivostok in less than 1/30 s, and the sound during this time would only have time to propagate over 10m. Radio waves propagate not only in the air, but I am where it is not, for example, in outer space. In this they are fundamentally different from sound waves, for which air or some other dense medium, such as water, is absolutely necessary. When a broadcasting station begins its transmissions, the announcer sometimes announces that the radio station is operating on a wave of such and such a wavelength. We see a wave running on the surface of the water, and with a certain dexterity we can measure its length. The length of radio waves can only be measured with the help of special instruments or calculated mathematically if we know the frequency of the current that excites this wave. The length of a radio wave is the distance over which the energy of the electromagnetic field propagates during the period of current oscillation in the antenna of the radio station. This is how it should be understood. During one period of current in the transmitter antenna in the space around it, one radio wave arises. The higher the frequency of the current, the more consecutive radio waves are emitted by the antenna during each second. Let's say the frequency of the current in the antenna of the radio station is 1 MHz. This means that the period of this current and the electromagnetic field generated by the wells is equal to one millionth of a second. For 1 s, a radio wave travels a distance of 300000 km, or 300000000 m. In one millionth of a second, it travels a distance a million times less, i.e., 300000000:1000000. Therefore, the wavelength of this radio station is 300 m. The wavelength of a radio station depends on the frequency of the current in its antenna: the higher the frequency of the current, the shorter the wave and, conversely, the lower the frequency of the current, the longer the wave. current in its antenna. And in order, on the contrary, to find out the frequency of the current in the antenna of the radio station, it is necessary to divide the propagation speed of radio waves by the wavelength of the radio station. To convert the oscillation frequency in megahertz to wavelength in meters in reverse, it is convenient to use the following formulas:
where L is the wavelength; f-oscillation frequency; 300 is the speed of propagation of radio waves, expressed in thousands of kilometers per second. I want to warn you: do not confuse the concept of the diva of the wave on which the radio station operates with the range of its action, i.e. with the distance at which its transmissions can be received. The range of a radio station, however, depends on the wavelength, but is not identified with it. Thus, a transmission on a wave length of several tens of meters can be heard at a distance of several thousand kilometers, but is not always audible at closer distances. At the same time, the transmission of a radio station operating on a wave length of hundreds and thousands of meters is often not heard at such long distances that you can hear the transmission of shortwave stations. So, each broadcasting station operates on a specific frequency assigned to it, called the carrier. The wavelengths of different radio stations are not the same, but strictly constant for each of them. This makes it possible to receive transmissions of each radio station separately, and not all at the same time. Radio wave bands A very wide section of radio waves allocated for broadcasting stations is conventionally divided into several ranges: long-wave (abbreviated LW), medium-wave (MW), short-wave (HF), ultra-short-wave (VHF). In our country, the long-wave range covers waves from 735,3 to 2000 m, which corresponds to frequencies of 408-150 kHz; medium wave - radio waves with a length of 186,9 to 571,4 m, which corresponds to frequencies of 1605-525 kHz; shortwave - radio waves with a length of 24,8 to 75,5 m, which corresponds to frequencies of 12,1-3,95 MHz; ultrashortwave - radio waves with a length of 4,11 to 4,56 m, which corresponds to frequencies of 73-65,8 MHz, VHF radio waves are also called meter waves; in general, all waves shorter than 10 m are called ultrashort waves. Television broadcasts are conducted in this range, communication radio stations equipped on fire brigade vehicles, taxis, medical care for the population at home, and traffic safety operate. Shortwave broadcasting stations are unevenly distributed over the HF band: most of them operate on waves with a length of about 25, 31, 41 and 50 m. Accordingly, the shortwave broadcasting band is divided into 25, 31, 41 and 50-meter subbands. According to an international agreement, a 600 m (500 kHz) wave is reserved for the transmission of distress signals by ships at sea - 808. All marine emergency radio transmitters operate on this wave, the receivers of all rescue stations and lighthouses are tuned to this wave. Publication: N. Bolshakov, rf.atnn.ru
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