10 Jan 2010

Gnat-1: 1 transistor transceiver from SPRAT 138

Am I alone in struggling with this design from SPRAT 138?

Today I did a breadboard version for 80m on the bench. On TX I had no problems (as long as I grounded the transistor's emitter directly on TX), but on RX the performance was diabolical and no better than a crystal set with injected carrier. I cannot quite fathom out the function of the transformer T2 or why the key is grounding the secondary of T2. As described in the article I would have thought T2 could be removed and simply key the emitter to ground on TX and decouple a larger (variable) emitter resistor on RX.

Has anyone had success with this design on RX? It may well be me!

2 comments:

SM7FBJ said...

I made mine work, but the receiver had poor signal.....when I had a QSO with a friend 40km away, I used my ICOM receiver

SM7FBJ/Barney

Roger G3XBM said...

Chris Trask, who designed the Gnat-1 says he's never measured the RX sensitivity! He says T2 is critical to avoiding the need to switch oscillator bias thereby avoiding frequency shift betweent TX and RX. Keying to short out the secondary winding T2 apparently does not change the current, just the closed-loop gain. I'd like to find SOMEONE who has successfully made this as a full transceiver.