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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Radio communication on VHF. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Civil radio communications VHF is widely used for service and amateur radio communications. Radio communication on VHF has the following advantages: independence from weather, time of day, year, low level of interference, small dimensions of equipment and antennas. Service radio communications on VHF are carried out using mobile, portable and fixed radiotelephone stations. They are used in construction, agriculture, oil fields, forestry enterprises, and transport. In cities, ambulances, fire and emergency vehicles, and taxis are equipped with radiotelephone stations. As antennas, vertically arranged quarter-wave asymmetric or half-wave symmetrical vibrators are usually used. The directivity pattern of the vertical vibrator in the horizontal plane has the shape of a circle, which makes it possible to successfully use it to establish communication in any direction along the earth's surface. In the vertical plane, the antenna has a main radiation maximum located along the earth's surface, which is especially beneficial in the case of low-lying antennas, which are so characteristic of moving objects. The most convenient antenna of a moving object in terms of design is a quarter-wave pin. Increasing the field strength at the receiving point is possible by using transmitting antennas that create even more directional radiation in the vertical plane. One of the well-known ways to create such antennas for stationary radio stations is the design of multi-tiered coaxial antennas. Their amplification is 2,5-3,5 times greater than that of a vertical symmetrical vibrator. For comparison, the radiation patterns in the vertical plane of these antennas are shown in Fig. one.
On fig. 2 shows a coaxial antenna (a) and current distribution along it (b), which consists of three symmetrical vertical half-wave split vibrators I, II and III, located on a common vertical mast.
Each of the arms of the vibrators has a length of about l/4, where l is the average wavelength of the operating range. Vibrators are made of a metal tube with an outer diameter of 12-30 mm. The coaxial cable runs inside the shoulders b - e. The central core and the internal insulation of the cable are continuous along its entire length. The central core is soldered to shoulder a at point 2. The cable sheath ends at point 3 and has gaps between points 6-7 and 10-11. At points 3, 6, 7, 10,11 it is soldered to the tubes. Vibrators can also be made of a cable braid of the same or another brand, for which the braid removed from the cable should be put on in the form of a stocking on the outer insulation of the feeder. The use of vibrators from the cable sheath makes the antenna lighter, and in the absence of tubes and an excess of cable, it speeds up its manufacture. The central core of the cable and arm a can be considered as a single wire of finite length. A standing current wave with a knot at the end is installed in such a wire. Wave spreading over the shoulder а, does not experience shortening. A wave propagating in a coaxial cable is shortened (for the calculation of the shortening coefficients, see the journal "Radio", 1964, N "7, pp. 31-32). Due to the absence of a cable braid between points 6-7 and 10-11, the wave is not here undergoes shortening, however, due to the small length of the sections (50-70 mm), this can be neglected with sufficient accuracy for practice. The arms b, c, d, e are fed as follows. Under the influence of the current of the central core, a current is induced on the inner surface of the braid, which The current induced on the inner surface of the braid is out of phase with the current of the central core When it reaches the surface of the tube, the current changes its direction by 180 ° and becomes in-phase with the current of the central core at the feed points of the antenna. а is a continuation of the current of the central core, the current of the arm e is a continuation of the current of the inner surface of the feeder braid. It is known that vibrators fed in phase and located on the same line create radiation concentrated in a narrow beam lying in a plane passing through this line. For in-phase power supply of half-wave vibrators I, II and III, the distance between the vibrators is chosen equal to the wavelength in the cable lk. The task of balancing the antenna is solved by the shoulders of the vibrators b - e, each of which, together with the outer surface of the braid, forms a balancing "glass" known in radio engineering. The input impedance of one half-wave vibrator is 75 ohms. With an increase in the number of series-fed vibrators, the input impedance of the antenna drops somewhat and, with three vibrators, reaches 50 ohms. Therefore, standard coaxial cables with impedances of 75 or 50 ohms can be used to power the antenna. The total length of the antenna L (distance between points 1-12) is equal to L =l/2+(N-1)lk, where N is the number of symmetrical half-wave antenna vibrators. In the long-wave section of the KB range, the total length of the antenna can reach large sizes. If this creates a design inconvenience, then you can limit yourself to two vibrators I and II with a slight decrease in gain. The antenna mast is made of wood or other non-conductive material. The tubes are attached to insulators. The cable between points 4-5, 8-9 is attached to the mast using brackets. At points 3, 6, 7, 10, 11, the cable braid is laid out evenly over the entire surface of the tube cut and soldered to it. The gaps 2-3, 6-7, 10-11, as well as the gaps between the tube and the cable at points 4, 5, 8, 9, 12, are sealed with plasticine. Hole 1 is closed with a stopper and also sealed. Antenna Calculation Example The task. Calculate a three-tier antenna for operation in the range of 144-146 MHz. fav=145 MHz. l=300/145=2,07 m. Let's use a tube with outer diameter D=28 mm and inner diameter d=25 mm. We determine the length of the arm of the vibrator, taking into account the shortening l=k(l/4), where k is the shortening factor, depending on the ratio of the wavelength to the diameter of the tube l/D. The values of this coefficient are given in the table. Table 1
l/D=2070/28=74, Interpolating, we find k=0,95. Consequently, l=0,95(2070/4)=490 mm. We choose a cable PK-50-7-12 with a wave impedance of 50 ohms and an outer diameter of 11,2 mm. The internal insulation of the cable is made of polyethylene having e=2,5. We determine the distance between the feed points of the vibrators:
Antenna length: L=l/2+(N-4)lк=1,04+(3-1)*1,38=3,8 м. The described antenna was designed and tested on the long-wave section of the VHF band. The results of operation over a number of years have confirmed its effective properties. Literature:
Publication: N. Bolshakov, rf.atnn.ru
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