5.5.3 Suppressing the Signal of the Local Transmitter
From all the signals in the reception antenna, the one that is created by the local transmitter is by far the strongest one, due to the fact that it is hundreds, sometimes even thousands times closer than other radio transmitters. That signal can be so strong that it can jam normal reception of other stations. In case of simpler receivers its programme is heard,more or less, in all the positions of the variable capacitor. The solution for this problem is the so-called seal circuit, which serves to weaken the signal of the local transmitter, so that it doesn’t interfere (but is still strong enough for normal reception, when the receiver is tuned at it).
The seal circuit is a parallel oscillatory circuit which comprises the coil L1 and capacitor C1, as shown on pic.5.14-a. By means of C1 the resonance frequency of the circuit is set so that it corresponds to the carrier frequency of the local station. On that frequency, this circuit behaves as a huge resistor (see pic.3.2-b) and decreases the current that is created by the local transmitter signal. For other signals it has very small resistance and practically has no effect on them. The setup is done by tuning the receiver on the local station, and the reception is weakened enough by turning the C1. If the decay is too strong, a resistor should be added in parallel to C1.
Using a variable capacitor in the seal circuit (pic.5.14-a) isn’t an economical solution. It is much better, considering both economy and space, the solution given on pic.5.14-b. A block capacitor C1 and a variable inductance coil are used in the seal circuit. As shown on the framed part of picture, the coil is wound on the plastic body, with ferrite core. The number of quirks is found experimentally about couple of hundreds of quirks made with as thin copper wire as possible). The capacitance for C1 is also found experimentally (couple of hundreds of pF). The earlier mentioned IF transformer can also be used as a coil. With labelling acc. to pic.4.3-a, legs No.2 and 3 are used, the others are “hanging” (they are not soldered). C1 capacitance is also found experimentally. It is also possible to wind the coil on a piece of ferrite rod, as shown on pic.5.14-b, and setup to be done with trimmer Ct