Broadband TV antenna for UHF. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Television antennas
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For high-quality reception of broadcasts from multi-program television centers operating on various UHF channels, it is necessary to have a broadband directional antenna. Log-periodic antennas meet these requirements to the greatest extent, due to which they are widely used. Unfortunately, the manufacture of some varieties of log-periodic antennas by radio amateurs at home is difficult.
The variant of the log-periodic antenna described in [1, 2] is distinguished by its constructive simplicity. According to the description, the antenna has a gain of at least 4 dB in the frequency band 470...638 MHz, i.e. allows you to receive TV channels from 21 to 41 and can be used in the near field as an indoor antenna.
Ris.1
When manufacturing several copies of such antennas from duralumin sheets with a thickness of 1,5 to 2 mm in accordance with Fig. 1 and 2, and testing them for a long time as outdoor ones, it turned out that the design has significant resistance in the most adverse climatic conditions, for example, when significant mass of frost and strong wind.
Ris.2
The positive qualities of the antenna listed above, as well as simple matching with the power cable, determined the author's interest in this design, and he tried to improve it by adding a reflector to the structure, as shown in Fig. 3. At the same time, the performance of the antenna has improved, and moreover, it has become suitable for quite satisfactory reception of high-frequency channels in the meter range.
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Ris.3
Some additional explanations for the construction thus obtained. The vibrator consists of two toothed blades 1 and 2 (Fig. 1) made of duralumin sheets 2 mm thick. Figure 1 shows their unfolded configuration and shows the dimensions in millimeters. The canvases are fixed at an angle of 45° with respect to each other on an organic glass insulator 3 (Fig. 2) with a thickness of at least 10 mm. For fastening, screws 4 with M3 thread are used, which must also be cut into four holes in the insulator. The drop cable is attached to the lower web using a metal bracket 5. To connect the cable to the webs, it is better to use special petals 6 up to 1 mm thick with a width and length corresponding to the dimensions of the insulator. They can be made, for example, from tin plate or other metals that allow soldering and, at the same time, are compatible with duralumin. Otherwise, additional noise and corrosion problems may arise.
The vibrator is mounted on a U-shaped metal profile or corner 7 (Fig. 3), welded to the tube with brackets 8. The carrier mast 9 is also the basis for attaching the elements of the reflector 10. It can be made of wood, while the cross section of the mast should not less than 30x40 mm. As reflector elements, duralumin rods or tubes with a diameter of at least 4 mm can be used. With reflector dimensions A=350 mm and B=400 mm, it is desirable that it consists of at least 8 elements [3]. If necessary, the elements 10 are fixed with small screws.
The distance B between the reflector and the emitter is best determined experimentally, because it depends on the specific combination of received channels on which it is desirable to achieve the highest quality reception. Approximately this distance corresponds to 0,15L. With a smaller value, the antenna gain may increase, but its TWV will deteriorate.
Literature
- Kapchinsky L.M. TV antennas. 2nd ed. - M.: Energy, 1979.
- Kapchinsky L.M. Design and manufacture of TV antennas. - M.: Radio and communication, 1995, S. 73-81.
- Kismereshkin V.P. TV antennas for individual reception. - M.: Radio and communication, 1982, S.42-44.
Author: V. Efremov; Publication: N. Bolshakov, rf.atnn.ru
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