A few months back I bought a VNWA from SDR-Kits. Now this was a big purchase,equal to a new rig or well on the way to one. So what can it do? The answer to that is that I am still finding out and learning how to use it.
Today is a cold,wet and windy Thursday afternoon just after Christmas,exactly the time to make some BPF’s,band pass filters,for yet another home brew project. The new rig is using the R2Pro direct conversion RX modules with a Tayloe mixer. Originally it was designed to use a ring diode mixer with discrete LNAs,low noise amplifiers,for each band. I say each band but I think this was originally intended to be a single band RX.
The original LNA design was well filtered to stop local oscillator radiation as well as giving some selectivity. Using a Tayloe mixer means that the LO is running at four times the signal frequency i.e. at around 40-41 MHz for the 30m band. This means that a simple low pass filter should stop any of the LO sig from going up the antenna so the need for extra filtering in the RX front end is eliminated.
I want to make a three band transceiver covering 40m,30m and 20m. It will be mostly for /P use so needs to be small and light. There will be one RF amp with separate BPFs for each band. The last project used some very good toroid based. They were far from small and light so this time I wanted a different approach and decided to use canned coils from Spectrum Communications.
The photographs belows show the first tests of a simple BPF taken from a PDF information sheet from the GQRP club. The sheet shows the values of capacitor to use for TOKO coils which provided a close enough starting point to get instant success. In most cases all that was needed was to go the next value down for the resonating capacitors (C1 &C3.)
Getting back to the VNWA. It was really very easy to tune the coils. For the first time I could actually see what was happening and get figures for both forward and return loss and VSWR. This makes life much easier and produces a better filter response than is possible by tuning for maximum signal. The next step is to cascade 3 or 4 coils to try and improve the bandwidth although only operating at the CW end of the band means that I do not want full band coverage on 40 and 30. More fun on a wet Thursday afternoon!
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