Saturday, 16 August 2014

What a Muppet

Yesterday evening, I tried to test my 10m Wispy on my Marconi 2955 test set, with shocking results. This morning, whilst I had a few moments spare, I fed the test signal direct into the RF amp, bypassing the filters, but had no better luck. I then fed it direct to the mixer, with still no luck.

It wasnt until a good while later that the niggle at the back of my mind (which sadly has had to contend with quite a few niggles these past weeks) resolved itself, whilst I was reviewing the details of the operation of the Polyakov mixer - This mixer has good AM rejection!

Bugger! Ive been testing with an on-frequency AM signal!

Part of the problem for me, I think, is that I tried an off-air receive test first, and didnt get a single received signal. But, propagation is poor on 10m at the moment, and although I looked at WSPRnet and noted not many signals, I didnt really correlate the lack of spotting by stations in central and northern UK with my lack of success, leading me subconsciously to think I had a fault.

A look back through Roger G3XBMs design notes, showed he achieved a -124dBm sensitivity without an RF amp, but with over 70dB of AM rejection, the AM only becoming detectable at -50dBm or so. Compare then with my, unaligned, -22dBm detection point of a 60% AM signal!

So, when I next get in the workshop, I will retest - this time using an actual carrier! Rather than set the test set to the actual tuned frequency of 28.1246MHz with a 1.5kHz modulated AM signal, I will instead just set the test set, with modulation switched off, to 28.1261MHz.

I do hope this is the only real issue. The RF amp is a grounded gate FET, and Roger used an MPF102. I didnt have any at the time of building, so substituted a J310 FET. This shouldnt be a problem. The only other change is to one audio coupling capacitor, and a bit of additional screening.

Last nights 472kHz reception was just as good as the previous session, best DX again 1141km into Germany, and again spotting Rogers tiny 5mW signal from over 190km from the small hours and into daylight, finally losing him around 06:00.

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