Top input filter for contests – crystal preselector

Contests will definitely be the most difficult test for any receiver. In some areas, a large number of competitors are concentrated in a small area, sometimes also QRO. So that they can fully participate in contests, is important, so that they do not interfere with each other. Mutual interference leads to nervousness, when it is impossible to read a weak counter station in interference from a neighbor a few hundred meters away chirping fifteen kilohertz higher….

Big guns most of the contest “he presides” on one frequency. Their goal is to have the strongest possible signal in the target area and undisturbed reception, so they can read weak signals. When solving this question, it can be worked up to to a special crystal filter tunable in a narrow range with suppression of unwanted signals up to o -80 dB….

Peter Haylett G3IPV has been experimenting with selective filters at the receiver input for many years. The following circuit, which developed is characterized by top parameters and a low price. Retuning the filter is approx 6 kHz, width for drop o -3 dB ASI 500 Hz and the stopband reaches the value -80 dB (in the distance 4 kHz) on 7 MHz band. On 14 MHz, the detuning is doubled, on 3,5 MHz half. It's the same with bandwidth. For higher bands, there is a problem with finding suitable crystals and the results achieved tend to be weaker.

The wiring diagram is very simple. Crystals with series resonance at 7020 kHz v puzdre HC6U. Six crystals were selected from a larger number, so that their frequency differences are as small as possible (do 100 Hz).These crystals are commonly available in the UK at low cost. C1, C2, C3 and C4 are miniature capacitive trimmers 0 – 20 pF. Capacitor C5 has a capacitance 75 pF. So that the hands do not get out of tune, the tuning knob is connected to the axis of the capacitor by a non-conductive material.

In order to actually achieve suppression of unwanted signals o -80 dB is necessary, of course wall the entire filter. The components are not placed on the printed circuit board, but on the soldering iron. All connections must be as short as possible.

The filter setting consists in setting C1 – C4 to minimum attenuation. Of course, it is not possible to transmit to such a filter, only if in milliwatts. When using separate antennas for reception and transmission and the RX connector on the transceiver, it is done. Otherwise, it is necessary to arrange the relay as follows, so that even by mistake it is not possible to transmit to the filter…