Friday, 7 February 2014

Project TennaLady

So this is a new project, in effect a Wispy Mk II. The designated name comes from the fact that A) Its built for WSPR operation (and you'd only whisper about wearing TennaLady's wouldnt you?) on the 10m band (Tenna, geddit?), and is hopefully a more elegant solution (hence Lady),

that, and B) because it uses hermetically sealed Double Balanced Mixers, so I can avoid all the 'pissing about' with matching diodes and trifilar windings!

The Local Oscillator will use the 3rd overtone circuit built today, IF I can pull the crystal to 28.1246MHz. Unfortunately, overtone crystals dont like being pulled, so this might be a no go. If thats the case, I have a 28.060MHz fundamental crystal on order, and a 14.060MHz crystal. I KNOW the 14MHz will pull enough that when doubled it will work (thats what Wispy uses), but thats an extra transistor then! Maybe the 28MHz fundamental crystal will pull far enough, 65kHz to shift!

Its 18:40, its dark, and my 10m 500mW beacon is still being heard on the US east coast! And im still hearing five other US beacons!

One key aspect of Project TennaLady, somewhat in keeping with the nature of its namesake, will be shielding! I found today that the spectrum analyser was easily picking up the oscillator just by the proximity of the probe, so shielding the oscillator is essential to prevent LO leakage to beyond the mixer.

If I cant pull the crystal as far as the WSPR allocation, I will try for 28.120MHz, the PSK sub-band, thats just 60kHz to pull!

UPDATE - I tried taking a few turns off L1 on the oscillator, and it doesnt seem possible to pull the crystal more than a bit over 10kHz. The best I could get was 28.073MHz, before it jumped to 5th overtone. I'll just have to see what can be done with the 28MHz fundamental crystal.

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