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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Propagation of radio waves. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur In conclusion, it must be said that radio waves of different ranges have unequal properties that affect the range of their propagation. Waves of one length travel long distances, waves of another length are "lost" beyond the horizon. It happens that the radio signal is perfectly audible somewhere on the other side of the Earth or in Space, but it cannot be detected a few tens of kilometers from the radio station. If we tuned the receivers to nearby radio stations operating in the ranges of ultrashort, short, medium and long waves, then, moving away from the stations, we could observe the following phenomenon: already a few tens of kilometers away, the reception of ultrashortwave and shortwave stations would stop, after 800 - 1000 km would cease to hear the transmission of a medium-wave station, and after 1500-2000 km - and the transmission of a long-wave station. But at a greater distance, we could hear the transmission of a shortwave station.
How to explain this phenomenon? What affects the "range" of radio waves of different lengths? The earth and its surrounding atmosphere. Earth, as you already know, is a current conductor, although not as good as, say, copper wires. The earth's atmosphere consists of three layers. The first layer, the upper boundary of which ends 10-12 km from the Earth's surface, is called the troposphere. Above it, up to 50 kilometers from the Earth's surface, the second layer is the stratosphere. And above, up to about 400 km above the Earth, the third layer extends - the ionosphere (Fig. 1). The ionosphere plays a decisive role in the propagation of radio waves, especially short ones. The air in the ionosphere is very rarefied. Under the action of solar radiation there, many free electrons are released from the atoms of gases, as a result of which positive ions appear. There is, as they say, ionization of the upper layer of the atmosphere. The ionized layer is able to absorb radio waves and bend their path. During the day, depending on the intensity of solar radiation, the number of free electrons in the ionized layer, its thickness and height change, and this changes the electrical properties of this layer. Radio station antennas radiate radio waves both along the earth's surface and upwards at various angles to it. Waves that follow the first path are called terrestrial or surface waves, and the second path is called spatial. When receiving signals from long-wave stations, the energy of surface waves is mainly used, which well bend around the Earth's surface. But the Earth, being a conductor, absorbs the energy of radio waves. Therefore, as you move away from the long-wave station, the volume of reception of its transmissions gradually decreases and, finally, the reception completely stops. Medium waves bend around the Earth worse and, moreover, are absorbed by it more strongly than long ones. This explains the lower "range" of medium-wave broadcasting stations compared to long-wave ones. So, for example, the signals of a radio station operating on a wave length of 300-400 m can be received at a distance two to three times less than the signal of a station of the same power, but operating on a wave length of 1500-2000 m. To increase the range of these stations, it is necessary to increase their capacity. In the evening and at night, long-wave and medium-wave radio broadcasts can be heard at greater distances than during the day. The fact is that the part of the radio wave energy emitted upwards by these stations is lost without a trace in the atmosphere during the day. After sunset, the lower layer of the ionosphere bends their path so that they return to the Earth at such distances at which the reception of these stations by surface waves is no longer possible. Radio waves in the shortwave range are strongly absorbed by the Earth and poorly bend around its surface. Therefore, already a few tens of kilometers from such stations, their surface waves attenuate. But on the other hand, sky waves can be detected by receivers several thousand kilometers away and even at the opposite point of the Earth. The curvature of the path of spatial short waves occurs in the ionosphere. Having entered the ionosphere, they can go a very long way in it and return to Earth very far from the radio station. They can make a round-the-world "trip" - they can be received even at the place where the transmitting station is located. This explains the secret of good propagation of short waves over long distances even at low transmitter powers. But short waves also have disadvantages. Zones are formed where the transfers. shortwave station is not heard. They are called zones of silence (Fig. 1). The size of the silence zone depends on the wavelength and the state of the ionosphere, which in turn depends on the intensity of solar radiation. Ultrashort waves in their properties are closest to light rays. They mainly propagate in a straight line and are strongly absorbed by the earth, flora, various structures, and objects. Therefore, reliable reception of signals from ultrashort-wave stations by a surface wave is possible mainly when a straight line can be mentally drawn between the antennas of the transmitter and receiver, which does not encounter any obstacles in the form of mountains, hills, forests along the entire length. The ionosphere is "transparent" for ultrashort waves, like glass for light. Ultrashort waves pass through it almost unhindered. That is why this range of waves is used for communication with artificial satellites of the Earth, spacecraft and between them. But the ground range of even a powerful ultrashortwave station does not, as a rule, exceed 100-200 km. Only the path of the longest waves of this range (8-9 m) is slightly curved by the lower layer of the ionosphere, which, as it were, bends them to the ground. Due to this, the distance at which the VHF transmitter can be received can be large. Sometimes, however, transmissions from ultra-shortwave stations are heard at distances of hundreds and thousands of kilometers away. Publication: N. Bolshakov, rf.atnn.ru
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