TV filter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Filters and matching devices

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The number of amateur radio stations is steadily growing every year and the problem of combating interference with television during the operation of an amateur radio station is becoming one of the most pressing. It is especially relevant for beginner radio amateurs and for those who live in settlements located at a considerable distance from transmitting television centers and repeaters, where the television signal is rather weak and therefore more susceptible to interference. Interference in the reception of television programs mainly occurs due to spurious radiation of the transmitter during the nonlinear operation of its cascades, poor shielding or a poorly tuned antenna (SWR> 1), as well as when harmonic components of the main signal enter the television channel band. Sometimes there are nutritional problems.

To effectively suppress harmonic components, low-pass filters (LPF) are often used at the transmitter output. Such a filter should pass frequencies up to 30 MHz without significant attenuation and significantly attenuate signals of higher frequencies. At the same time, its input and output resistances must be matched with the wave impedance of the feeder line. The number of filter sections is chosen depending on the required amount of spurious radiation suppression.

According to the recommendation of the CCIR and the requirements of GOST, the average power of any spurious radiation entering the feeder should be 40 dB lower than the average power of the main signal, but not more than 50 mW [1]. Therefore, a sufficient degree of suppression of spurious radiation in radio stations with an input power of up to 200 W will be completely provided by a five-section filter, the degree of suppression of which is determined by the formula: b=10lg(P1/P2):n where Pi is the output power; P2 - input power; n is the number of filter links. A five-link filter, the scheme of which is shown in Fig. 1, was proposed by S. G. Bunin and L. P. Yailenko [2].

Ris.1

It did not receive proper distribution due to the fact that the authors did not provide the brands of radio components used and data on the dimensions and manufacturing technology of the filter. The proposed technology for manufacturing such a filter is quite simple and does not require much experience and scarce parts, as well as special devices for manufacturing and tuning. It is quite within the power of beginner radio amateurs. Structurally, the filter is made in a box of double-sided foil fiberglass with a thickness of 1,5 mm (the use of fiberglass of a different thickness is undesirable due to changes in the filter's constant capacitances). The height of the box is not critical and in this case is 40 mm. On the main board (Fig. 2), which is the bottom of the box, foil plates are formed by cutting or etching according to the indicated dimensions for a 75-ohm cable (dimensions for a 50-ohm cable are indicated in brackets).

Fig.2 (click to enlarge)

These plates form finely tuned filter capacitors.

Loop coils are made of ordinary copper wire PEV-2 with a diameter of 2,0 mm. The winding is frameless. The inner diameter of the coil is 12,5 mm. Winding pitch - 3 turns / cm (i.e. L-2 cm, etc.). The filter parameters are given in Table. one.

The coils are soldered to the connectors and to the capacitor plates. The place of soldering is not critical. Connectors G1 and G2 - type СР-50, СР-75 and the like. You can use the filter without connectors by including it in the cable break. In this case, instead of a connector, the coil is attached to an insulator and the central core of the cable is soldered to it. The braid, threaded inside the box, is soldered around the perimeter. If the dimensions are correctly maintained, the filter does not require adjustment. It should be noted that the filter does not eliminate interference caused by poor contact between the antenna and the power line. Experimentally, the filter was tested on random antennas fed by a coaxial cable with a significant SWR and a power of more than 200 watts. There was no TV interference. This filter was repeated by more than a hundred radio amateurs. Without tuning, he showed good results.

Literature

1. "Radio", No. 10, 1983, pp. 17-20.
2. S.G. Bunin, L.P. Yaylenko. Handbook of a radio amateur-shortwave. Kyiv: Technique, 1978, p.190.
3. "Amateur Radio", Nos. 5-6, 1976

Author: A. Ronzhin, Vladimir region, Petushki; Publication: N. Bolshakov, rf.atnn.ru

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