Today I did a rebuild of the TX only version of the WISPY 10m WSPR beacon to improve it in several ways. Switching to germanium diodes helps produce a much better looking DSB signal out of the mixer. With silicon diodes there was not quite enough drive. The second change was to remove the 2N3866 PA and replace it with a couple of paralleled up 2N3904s (each with a small emitter resistor so they share current properly). A 2N3866 is around £1.50 whereas a couple of 2N3904s are about 6 pence. Output is now a clean DSB signal, even without the low pass filter, with a conservative 200mW DSB out (100mW equivalent SSB signal in the WSPR band). This is now a very pleasing little design that can be built for even less. Within 5 minutes of turning it on it got a remarkable -3dB S/N report from EA8FF over 3000kms away.
3 comments:
Two ideas;
Would using Ge-diodes in the receivers Poljakov mixer have advantages as well. Would using more 2N3904 paralelled in the PA (e.g. qty 4) be possible.
Good luck
Ron PA2RF
Yes to both I think Ron. the change to Ge diodes made a world of difference on TX. On RX they would need less drive, so must be a "good idea".
Yes, I think one can add 2N3904s ad infinitum in the (low cost) PA. It certainly makes for a low cost design!
73s
Roger G3XBM
Mmm, very interesting. I recently bought qty 100 2N3904 on ebay for $2,50
:-)
Rgds
Ron
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