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Superb preselector for contesting |
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Contesting is very popular part of amater radio operation. But it’s not only the hardest test for operator but also for the equipment. In the same time are on the band hundreds of sigs. Our receiver must process a various signals from the weakest to the strongest from the stations in our area which are overloading the receiver. This is a hard job for every receiver.
If we got a huge QRM from the neighbour stations it’s a time to built the front-end filter called preselector. Preselector is a special, very selective RX filter circuit. Preselector is passing through only few kHz, all other signals are strongly rejected (at least –70 to –80 dB).
An excellent results we got with a low-cost crystal preselector that was designed by Peter G3IPV. On the 40m band is a tunning range 6 kHz, bandwith (-3 dB) around 500 Hz and stopband –80 kHz (4 kHz from the carrier). On the 20m band is tunnig range 12 kHz and bandwith 1 kHz.
Schematic diagram shows a six crystals in parallel with a five variable capacitors. Used crystals were in HC6U holder with a frequency 7020 kHz. Choose from a more six crystals with the same frequency (+/- 100 Hz). The C1, C2, C3 and C4 are miniature variable 20pF caps. C5 is a variable capacitor (around 75pF). Frequency-setting knob is from the insulating material. Otherwise the preselector should be affected with a capacity of our body. Preselector is fitted in the screened box and all wires make as short as possible!
Pre-tunning for this filter is necessary: turn the C1 – C4 for the minimal insertion loss. Be aware that preselector can be used only in the RX path. Transmitting into the preselector should damage it! Use a separate paths for RX and TX or relay. There could be also fitted preamplifier.
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