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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING All-wave small-sized TV antenna. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Television antennas The problem of building a compact television antenna that operates on all MB and UHF channels is still very relevant. To this end, viewers are constantly experimenting with both indoor and outdoor antennas. The author offers another solution to the problem. Very often, a small and easy-to-make all-channel television antenna is needed, which could be placed on the window or hung on the wall of the house outside, next to the window. The basis of the proposed antenna was a broadband wave vibrator DM V, equipped with a split reflector. The latter is also used as a broadband vibrator MB. As is known from the theory of antennas, a symmetrical vibrator has a main resonance (half-wave) at a length equal to 0,45 ... 0,47X, where X is the length of the received waves. The power points (drop cable connections) are at the maximum (antinode) of the current, the input resistance r is active and equal to 73 ohms for a thin vibrator, dropping to about 50 ohms for a thick vibrator. As the frequency increases, a second resonance (wave) can also be detected, when half of the wave fits on each half of the vibrator, while the input resistance becomes active again, but very high. It is approximately equal to W2/r, where W is the wave impedance of the vibrator conductors, defined as the square root of the ratio of linear inductance and capacitance: W = (L/C)1/2 A thin wave vibrator can have a resistance of up to several kiloohms. When detuned away from the resonant frequencies, a reactive component also appears in the input resistance, the larger, the larger W. For this reason, thin vibrators, for example, like those of a common two-pin room antenna ("whiskers"), do not work well in a wide frequency band, requiring changes in their length when moving from channel to channel. For broadband antennas, it is advantageous to use thick vibrators, the wave impedance of which is small (150 ... 300 Ohms). At the same time, the reactance that appears during detunings is also small. The input impedance of the wave vibrator also decreases. Moreover, it is not necessary to make it voluminous; flat structures made in the form of a lattice of conductors also work well. Such considerations formed the basis of the proposed design. A sketch of the antenna in two views (front and top) is shown in fig. one.
The antenna receives horizontally polarized radio waves coming from the front. Each half of the vibrator is made up of three straight pieces of wire with a diameter of 3...5 mm (the thicker, the better) and a length of 18...20 cm, diverging in a fan of power points X - X. The middles and ends of the conductors are connected by jumpers from the same wire . In the UHF range (at a wavelength of 40 ... 60 cm), the vibrator works like a wave vibrator. In the middle of the halves of the vibrator, metal racks are installed, with which the vibrator is attached to the reflector. Electrical contact at the points of attachment of the racks to both the vibrator and the reflector is mandatory. A conventional non-resonant reflector would be a continuous reflective surface. In the described antenna, it is done differently: the reflector consists of two parts, each of which has a size slightly larger than the UHF half-wave. Such a split reflector on the UHF is no less effective. In the UHF range, the considered antenna has a good directivity and provides a gain of 7 ... 8 dB compared to an antenna without a reflector. In the MB range, halves of the reflector complement the active vibrator and, together with it, form a single broadband dipole. Its directional properties are approximately the same as those of a conventional half-wave vibrator (figure eight pattern). The zeros of the radiation pattern lie in the plane of the vibrator, i.e., it does not receive from the lateral directions. With a symmetrical vibrator, it is best to use a symmetrical two-wire feeder line. Ribbon cable with a wave impedance of 300 ohms works well, but ordinary plastic-insulated telephone wire ("noodles"), whose wave impedance is close to 240 ohms, is also quite suitable. To match the unbalanced 75-ohm TV input, an industrial 300/75 ohm transformer adapter on a ferrite ring was used. When tested on the window of the ninth floor, overlooking the television center (the distance to it is about 17 km), the antenna showed excellent results on almost all channels used in Moscow, except for the sixth and first. It was possible to improve the performance of the antenna on the sixth channel by including between the halves of the reflector at points Y - Y, the position of which in the gap is not critical, a series oscillatory circuit L1C1, shown in Fig. 2a. It consists of a trimmer capacitor KPK-1 and a coil containing three turns of wire 15 mm in diameter with a diameter of 1 ... 1,5 mm. By squeezing or pushing the turns and adjusting the capacitor, the best image is achieved on the sixth channel. With the circuit, the vibrator turns into a shunt-fed half-wave continuous dipole. At the frequencies of other channels, the resistance of the circuit is high and it does not participate in the work. For the frequencies of the first channel, the dimensions of the vibrator are too small, but there is a way out here too. Since the halves of the reflector have a rather large capacitance, the vibrator can be tuned to the frequency of the first channel by connecting the L2 coil (Fig. 2,b), and even together with the already tuned L1C1 circuit. Coil L2 contains 6 ... 8 turns with a diameter of 20 ... 25 mm of the same wire as coil L1. The circuit formed by the capacitance of the vibrator and the inductance of the coil is adjusted by compressing and expanding the coils.
Since the antenna is very broadband, its dimensions are completely uncritical. For its manufacture, you can use a variety of improvised materials. As a reflector, for example, two shelves fit perfectly - lattices from an old refrigerator. To increase the mechanical strength at points X - X and in the gap between the halves of the reflector, strips of insulating non-hygroscopic material are installed. If the antenna is placed on the wall of the house facing the television center, the distance from the reflector to the wall is chosen (selected) within 0,8 ... 1,5 m so that the wall serves as an additional reflector at MB frequencies. Another way to improve the performance of the antenna on long-wave channels (first or third) is to install another large split reflector, fastened in the same way as in it with the existing one. Author: V.Polyakov, Moscow
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