Date: Mon, 29 Mar 2010 21:28:24 -0600
From: John Lyles <jtml@losalamos.com>
Subject: [Amps] coaxial stub on amplifier output
Has anyone seen anything in print about using a lambda/4 (Quarter
wavelength) stub on the output of an amplifier with a tube. Its a nice
way to bring water, tuning mechanisms into an output circuit, without
looking like a short circuit for the primary frequency. This same stub
is a half wavelength at second harmonic, so it reflects a short right
across the output at F2. This should cause some plate current waveform
modification, depending on where it is located physically with respect
to the plate, i.e., similar to Tyler circuits.
73
K5PRO
John
## I bought george's [ w2VJN] book from inrad a few yrs back
It's superb. There is also excellent material in the latest ON4UN
low band dx book. George's book show's other designs too.. all using
coax.
## apparently, whether you put the say shorted 1/4 wave stub right at the
output of the amp.. vs a 1/4 wave away from the amp depends on whether you
have a simple PI net... or a PI-L. They work great on the output of the 2 x
GS35B
6m amp. Attenutaion improves by a whole bunch, when bigger coax is used.
Huge
diff between 213 and heliax.... for the same 1/4 wave stub. Somebody
pointed out
to W8JI that they could hear his 2nd harmonic... when JI was TX on 160m...
and folks
could hear him easily.. when operating on 80m. JI installed the stub right
at the 160m ant
feed point... end of problem.
## I didn't think of the stub interacting with plate current waveform
modification. Will you
get the same waveform modification, when using a LP Filter... say with a cut
off of 33 mhz?
What if the LP filter has a cut off just a bit higher than the band in use...
like monoband LP
filters ??
Jim VE7RF
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