Richard, Cecil and Tom

I should explain the "problem" somewhat.

The transmitter designs I believe are not very good. Perhaps they were
designed to work on a different or narrower frequency and were hastily
adapted by beancounters instead of designers. grin What is happening
is that at a certain critical band of freqs (around 11.2GHz) the output
from the detect diode is very close to zero and hence the power control
(via ALC) and calibration table dont work too well. Thinking it was a
design issue I actually moved the ref diode to the limit of its pads and
the problem resolved itself. The effect was of course to make the sensor
length electrically shorter by doing this. I was trying to establish
whether I had a 1/2 or 3/4 wavelength sensor section to maybe take it up
with engineering. (No doubt for another frequency I would move it in the
opposite direction)

I suspect there is also another issue whereby the resonant sensor
section is actually absorbing more power than it should and dissipating
it in the resistor. The effect of this being a loss of available output
power at the SMA connector.

Keep in mind that I am a production line tech rather than a designer. I
am not suppose to modify things, only test and align! (Am actually an IT
professional but had trouble finding that line of work!)

Many thanks for your input. As usual with you gents I am going to have
to work through it all slowly!

To answer you specifically Richard, the PCB is a multilayer fibreglass
thing with a largish ground being under both the output and sensor
striplines. I was surprised that the sensor line was so long and so
close when I first saw it thinking some major overcoupling might be
occurring. The tracks are maybe 3mm wide and about 1.5mm (edges) apart.
I am thinking also that the 50 ohm output is not being preserved as it
goes past the sensor stripline. I think the freq coverage for this model
is about 10.5 to 11.5GHz. Keep in mind that this really is the frist
time I have seen microwave TX's up close so my gut feelings about track
sizes/spacing may be way off.

Cheers Bob VK2YQA