During the night I made a few more tests...
Iso xfmr at PA output. 14T:14T on FT-114-JC core. Made the problem
slightly WORSE.
Iso xfmr at amp input. 4T:4T on 3 x BN73-202. Made the problem
slightly WORSE.
Removed iso xfmr from amp output, left iso on PA input. Slightly
better than without iso anywhere, but still bad.
Tried again all configurations of iso xfmrs (at antenna, at PA
output, at PA input) using HP 3325B to drive the PA. No difference
from using the Ultimate 3S.
When I use an iso xfmr at the antenna the problem is WORSE than when
I have the coax connected to the antenna ground. It's not a big
change, but measurable and repeatable.
Considered this might be an instrumentation problem with RF
affecting the HP 1980B oscilloscope, but I can see the signal
quality degrade in a receiver when the problem occurs so I don't
think so.
I can try common mode chokes instead of isolation transformers, but
will this really be better? Seems if the problem is common mode
current the iso xfmrs should kill it...??
I still have a couple of ideas to try: Replace 50uH choke in PA with
different type core. Use completely separate bias supply. However
since the PA works perfect into a dummy load I am not sure the PA is
the problem.
I will try to make a shielded box for the PA. That will take some time.
The Ultimate 3S is in the metal case sold for it. This is not great
because of the paint (or powder coat) at mating surfaces. I will try
to work on that but since using the HP 3325B as a PA driver does not
make any change I have doubts this will help.
Paul N1BUG
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