Features of the microwave range and its use

The microwave range (SHF) occupies the short-wave section of radio waves in frequencies from 300 MHz to 3000 GHz and in wavelengths, respectively, from 1 m to 0.1 mm (see Fig. 1). At the low-frequency edge, it borders on the VHF radio range, and on the high-frequency edge, it borders on the infrared range of optical radiation. Symbols of radio wave ranges are shown in fig. 2.

The propagation paths of radio waves are illustrated in a simplified way in Fig. 1.3. Traditional radio waves (RW) with frequencies below 300 MHz bend around the earth (surface wave 1 ) or are reflected from the ionosphere (sky wave 2 ). Such reflections can occur repeatedly. The absorption of radiation by the atmosphere is small and depends on its state, time of day, etc. In this case, the waves do not go beyond the ionosphere and are able to completely go around the globe.

Microwave radiation propagates within the line of sight (like optical radiation), can penetrate the ionosphere and go beyond it, providing communication with space objects (waves 3 , 4 and 5 ). Waves with a wavelength of more than 1 cm are absorbed by the atmosphere relatively weakly; in the range of centimeter waves, the lowest level of noise of various origins is observed. In the EHF range, atmospheric absorption increases significantly, but there are “transparency windows” where the losses are relatively small (areas near 35 GHz, 94 GHz, etc.). Optical waves also propagate within the line-of-sight and are capable of going into space, but they are characterized by very high losses in the atmosphere under conditions of cloudiness, fog, precipitation, etc., which makes such communication through the atmosphere unreliable.

Rice. 1.3. Propagation of radio waves

The main areas of application of radio waves are listed in fig. 1.4.

100 – 10 km office communication; communication with submarines
10 – 1 km broadcasting; navigation
1 – 0.1 km broadcasting; navigation; office communication
100 – 10 m broadcasting; long-distance communication with moving objects; amateur radio
10 – 1 m TV; RL; close communication with moving objects; broadcasting; navigation
100 – 10 cm TV; RL; radio relay communication; landing systems; cellular telephone connection; microwave heating
10 – 1 cm RL; radio astronomy; communication with space objects; satellite TV
10 – 1 mm close radar; radio astronomy; Medical equipment
1 – 0.1 mm space communications, physical research

Rice. 1.4. Main applications of radio waves

The widest and most important field of application of microwave waves, as well as longer radio waves, is communication technology. Let us consider the features of the microwave range in comparison with the rest of the radio ranges, bearing in mind the named area of their use.

It is known that the transmission of information over a radio channel requires a certain frequency band near the central “carrier” frequency allocated for this channel. Thus, the minimum bandwidth required for the transmission of audio signals with moderate quality reproduction is about 20 kHz. When transmitting a television signal, about 8 MHz is already required. The transition to digital communication systems improves the quality and reliability of transmission, but requires a significant expansion of the occupied frequency band. It follows from this that the capacity of the “traditional” radio bands (including the VHF meter wave band) is insufficient for the use of modern systems of high-quality television signal transmission, multichannel telephone cellular communication systems, etc. The possibilities for their implementation opened up only after mastering the microwave band.

The use of microwave communication lines made it possible to increase the speed of digital data transmission many times over and to transmit a large amount of various information (including many video signals in real time) simultaneously over one channel from the transmitter to the receiver. In this case, the width of the operating frequency band can be only about 1% of the carrier frequency. From what has been said, it is clear why the development of radio engineering is accompanied by a constant struggle for the development of ever shorter wavelength bands.

When using radar systems and in most cases of special communications, highly directional antennas with a beam width of a few degrees or even fractions of a degree are used. This makes it possible to dramatically increase the communication range for a given transmitter power. A narrow beam is necessary to achieve high accuracy and resolution of radars. In addition, in order to detect objects using radar systems, it is necessary that the radiation wavelength does not exceed the size of the object, otherwise the beam will go around it without giving an effective reflected signal. Therefore, to solve the most important problems in this field of technology, it is required to work at a wavelength of no more than 1 m.

The directivity of an antenna is proportional to the area of its mirror and inversely proportional to the square of the radiation wavelength. Therefore, with an increase in the operating frequency, the gain due to the use of highly directional antennas increases significantly, which in the microwave range can reach about 10 5 , such directivity values characterize both transmitting and receiving antennas. This makes it possible to judge the advantages of microwave waves for communication systems, radar, radio astronomy, etc., in comparison with longer radio waves.

As shown in fig. 1.3, only microwave radio waves are capable of penetrating the earth’s atmosphere and providing communication with space objects. Therefore, all activities related to the exploration and use of space are based on the use of communication systems of this range.

The most important advantages of the microwave range compared to longer wavelength radio bands (except item 5) are listed in fig. 1.5.

Rice. 1.5. Advantages of the microwave range

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