Tunable trap filters. 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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To reduce the effect of interference to television reception from the harmonics of radio stations, idle trap filters are used in the form of open quarter-wave ("Radio", 1961, No. 7, p. 20) or short-circuited half-wave ("Radio", 1961, No. 4, p. 59) segments of coaxial cables. A significant disadvantage of such trap filters is that tuning them to the noise frequency is difficult, since it is done by changing the geometric length of the segment. In addition, the possibility of tuning filters to a different frequency is practically excluded.

These shortcomings can be eliminated by including a variable capacitance or variable inductance at the end of the cable segment.

Almost all types of coaxial cables can be used to suppress harmonic interference in the frequency ranges of 1-12 television channels. When using segments of coaxial cables with polyethylene filler (e = 2,2-2,8) and wave impedance r = 75 Ohm for tunable filters (RK-1, RK-3, RK-4, RK-8, RK-20, RK-49) the lengths of these segments, as well as the values ​​​​of the minimum and maximum capacitances of the trimmer capacitor (Cmin and Cmax) and the inductance of the trimmer coil (Lmin and Lmax) are given in Table. 1.

In the case of the use of coaxial cables with polyethylene filler (e=2,2-2,3), but the characteristic impedance r' is not equal to 75 Ohm (RK-2, RK-6, RK-19, RK-48, RKV), the required cable lengths for 1-12 television channels are equal to those given in the same table, and the minimum and maximum capacitance or inductance values ​​must be calculated using the following formulas:

Tunable notch filters must be connected in parallel with the television receiver antenna. Structurally, the filter is made in the form of a cable segment, to one end of which a television antenna plug is attached, and a trimmer capacitor or coil with a variable inductance is connected to the other (Fig. 1).

Ris.1

The design of the variable inductance coil is shown in fig. 2. The frame of the coil is made of organic glass and consists of two separate parts - fixed 1 and movable 2. On the fixed half there is a thread for screwing the nut 3. The change in the coil inductance occurs due to a change in the winding pitch when unscrewing or screwing the nut.

Ris.2

To prevent the rotation of the movable half of the frame when the nut is rotated, a groove 4 is made on the fixed half, in which the pin 5 goes, inserted into the movable half. Since the change in the length of the coil when unscrewing the nut occurs only due to the elastic state of the turns of the coil, it is advisable to use a bronze wire with a diameter of 1 mm to wind the coil.

The winding data of the coils for various television channels are given in Table. 2.

To turn on a piece of cable parallel to the antenna on TVs that have only one antenna socket, you need to install the second one and turn it on in parallel with the main one. On TVs that have two antenna sockets, one of them can be used to turn on a piece of cable. To do this, from the socket that is not used to turn on the antenna, it is necessary to unsolder the parts and connect this socket in parallel with the one used. In the absence of interference, the filter must be disconnected from the TV, as it attenuates the useful signal.

When it is necessary to attenuate the harmonics emitted by the HF transmitter, a tunable notch filter is soldered directly to the antenna feeder (see "Radio", 1961, No. 7). Since the fundamental frequency of the transmitter is usually much lower than the resonant frequency of the filter, it practically does not consume the useful power of the transmitter and there is no need to turn it off. The cable segments of the filters connected to the transmitters must have a high operating voltage, since due to the voltage resonance that occurs when the filter is tuned, breakdown of the cable insulation is possible.

Author: N. Semenyuta; Publication: N. Bolshakov, rf.atnn.ru

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