Broadband antenna for TV reception. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Television antennas
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Shortwave radio amateurs have long used the T2FD broadband antenna (Top terminated folder dipole - a loop vibrator loaded at the top) in their practice [1]. It is very similar to the classic loop vibrator, except for the load resistor Rn introduced into the middle of the upper part, as shown in Fig. 1.
The figure shows that the antenna is smaller than a half-wave dipole. It is calculated on the lowest frequency television broadcast channel received in a particular area. With a load resistor of 700 ... 750 ohms, the input impedance of the antenna is 675 ohms, therefore, to match with a 75-ohm feeder, a Tcc transformer balancing-matching system with a transformation ratio of 9 is used.
The transformer balancing-matching system is made according to the scheme shown in fig. 2, and consists of three RF transformers T1-T3. Their windings are wound simultaneously with two PEV-2 wires with a diameter of 0,5-0,7 mm on ferrite rings with a magnetic permeability of 20 VCh and an outer diameter of 10-12 mm. They contain ten turns. You can also use the so-called "binoculars" - matching devices from television antennas "Taiga" and the like. Those who want to independently calculate the balancing-matching system can use the relations indicated in [2, 3].
For example, the antenna for receiving TV programs from the 4th channel of meter to the 30th channel of decimeter waves is made of copper wire with a diameter of 2,5 mm, as shown in Fig. 3. On a plate of one-sided foil fiberglass with a thickness of 2 mm and dimensions of 130x70 mm, areas are cut out, to which halves of the vibrator, a load resistor, HF transformers of the balancing-matching system and a feeder are soldered.
The load resistor (its dissipation power does not matter) with a resistance of 750 Ohm and the balancing-matching system must be closed with a dielectric cover to protect it from atmospheric precipitation. In order to increase rigidity, on each half of the vibrator, plates with dimensions of 48x20 mm are installed from one-sided foiled fiberglass, on which the foil is removed in the middle, and the vibrator halves are soldered to the edges. The cable is secured with two clamps. The antenna is screwed to the wooden mast with four M3 screws. As a rule, the antenna does not need to be tuned.
Five such antennas were made. They are placed on the roofs of houses. All meter channels and part of decimeter channels are received with good quality at a distance of 10 km from the TV center.
Literature
- Rothammel K. Antennas. - M.: Energy, 1969, p. 75, 76.
- Zakharov V. Matching devices on ferrite magnetic circuits. - Radio, 1987, No. 6, p. 26-29.
- London S. E., Tomashevich S. V. Handbook of high-frequency transformer devices. - M.: Radio and communication, 1984, p. 100-102.
Author: A. Ivanov, Tomsk
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