What are radio waves? Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur
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And what do they represent radio waves? You can imagine it figuratively, but I wanted to know more about this phenomenon. I want to say right away that in everything I read there is nothing that will turn the world, or you. This is an article to remember, or to find out if you are a beginner.
Radio waves are divided into frequency ranges: long waves, medium waves, short wavesand ultrashort waves.
Long waves. The waves in this range are called long because their low frequency corresponds to a long wavelength. They can spread for thousands of kilometers, as they are able to bend around the earth's surface. Therefore, many international radio stations broadcast on long waves.
medium waves do not propagate over very long distances, since they can only be reflected from the ionosphere (one of the layers of the Earth's atmosphere). Medium wave transmissions are better received at night, when the reflectivity of the ionospheric layer increases.
Short waves are repeatedly reflected from the surface of the Earth and from the ionosphere, due to which they propagate over very long distances. Transmissions from a shortwave radio station can be received on the other side of the globe.
Ultrashort waves (VHF) can only be reflected from the surface of the Earth and therefore are suitable for broadcasting only over very short distances. On the waves of the VHF band, stereo sound is often transmitted, since interference is weaker on them.
In the pictures above, the wave is depicted as a strip, but here is how it actually looks.
Long waves - - - - - Short waves
150-300 kHz - - - - - 2300-26 100 kHz
(1000 - 2000 m) - - - - - (11 - 130 m)
Medium waves - - - - - Ultrashort waves
525 - 1700 kHz - - - - - 87 - 108 MHz
(180 - 570 m) - - - - - (2,5 - 3,5 m)
Well, let's say everyone understood this, let's talk about transmitters and antennas.
Transmitter emits radio waves modulated, i.e. changed so that they carry an audio signal.
Modulation. In order for radio waves to carry an audio frequency signal, they are modulated with this signal. Modulation is of two types: amplitude (AM) и frequency (FM). More on modulation below.
Amplitude Frequency
modulation modulation
Antenna. In the antenna, under the influence of radio waves, electrical oscillations occur at the same frequency as that of the radio wave. Let's say the antenna is located at the top of the tower of the transmitting radio center. Electric current passing up and down the antenna excites radio waves that diverge in all directions. Transmitting antennas are installed on high ground to increase the transmission range.
The word frequency was mentioned here, if someone forgot that:
Frequency is the number of repetitions of something per unit of time. The frequency of a wave is the number of its maxima passing through a fixed point in one second. The frequency is measured in hertz (Hz). One hertz is one repetition per second.
I'll tell you about the Amplitude, since you need to know this in order to understand AM and FM.
Amplitude - this is the maximum deviation from the equilibrium position during oscillations.
Thus, the amplitude of a wave traveling over the surface of water is equal to the height of its crest above the surface.
Amplitude modulation
With such modulation, the amplitude of the carrier wave is changed in accordance with the voltage of the audio frequency signal. The amplitude of the carrier wave increases as the audio signal voltage increases and decreases as the voltage decreases. Before modulation, the carrier wave has constant amplitude and frequency. Its frequency is much greater than the audio frequency.
Frequency modulation
With such modulation, the frequency of the carrier wave is changed in accordance with the voltage of the audio signal. The frequency of the carrier wave increases as the voltage of this signal increases and decreases as it decreases. With frequency modulation, there is less interference, but radio stations have to work in the VHF band. This is due to the fact that the frequency of the carrier wave must be many times greater than the sound frequencies.
Author: Valera, Kavalyu; Publication: cxem.net
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