NOTES ON HANDBOOK VXO TRANSMITTER
                      BY DAVE NEWKIRK

Yes, the IARU transmitter design sometimes, maybe even often, 
exhibits a backwave.  Assuming a maximum output of 6 W, our 
spectrum analyzer shows it to be about 30 dB down from the key 
down output. (This is only about 6 mw of power, quite a bit 
stronger than the 100 mw that you are seeing.)

The backwave occurs for one or two of two possible reasons: (1) 
only one stage is keyed, and keying just one bipolar transistor 
doesn't give much isolation and (2) the design's buffer 
transistor (Q2) routinely oscillates (this may also involve 
feedback through the oscillator circuitry, even when the 
oscillator transistor is turned off).

It's a tricky phenomenon, because the oscillation usually doesn't
start up until the first time the key is pressed. Then it's in there
as long as the transmitter is connected to a power supply. I've
watched this scenario on a spectrum analyzer. A clue to it is that
Q2, a 2N2222 routinely runs hot.

Radical, but most effective: Throw out the original transmitter's Q1
and Q2 circuitry (its driver and final are FB). I did this with a
sheet-metal shear; it's very effective. Replace the original trans-
mitter's Q1 and Q2 circuitry with a *three*-transistor strip modeled
after the oscillator and buffer circuitry in ZL's QRP 3-Bander. ZL's
time-sequenced keying circuitry should also be included. A builder
will have to play with attenuation between this strip and the 6-W
transmitter's 2N3866 driver so as not to overdrive the 3866. Starting
with 18 dB of atten, decrease atten in 3-dB steps until power comes
up to max. Hardwire in that attenuation.

This arrangement should provide reasonable keying. Manual TR will
still be necessary.

I don't think the original circuitry can be easily fixed. It has
too few stages. You can't eliminate the backwave (and 
interelement Q2 oscillation) by keying the buffer and the driver, 
by the way, because doing so pulls the oscillator. (A buffer 
can't really buff if it's turned on and off without care.) An 
experienced experimenter, however, might be able to apply ZL's time-
sequence keying to the 6-Watter's osc and buffer/driver. 

*eof