Built a lambda/4 stub out of 9 inch diameter EIA hard line, for
the big amps at work. It was build primarily to pass a tuning shaft
through the output line to the variable capacitor paddle for output
coupling to the plate/screen resonator cavity. Originally I thought it
might need water cooling, so that too was planned to pass through the
quarter wave stub. It took some modeling and measurement to get the stub
length to be high Z at the operating frequency as when the diameter of
the outer conductor approaches fractional wavelength then the geometric
length isn't exactly what works. This was all explained in Robert
Pound's chapter in the MIT Rad Lab textbook on coaxial line center
conductor supports. He figured out how to broadband the stubs so they
wouldn't have VSWR when the operating frequency shifted slightly.
http://www.ieeeghn.org/wiki/index.php/Oral-History:Robert_Pound
Anyway, my big stub worked very well on suppressing second harmonic.
Better than -50 dBc right out of the amplifier. Wish it still reflected
a short to the 4th harmonic, 805 Mhz, but due to being far overmoded (9
inch coax that is), it doesn't work.
Essentially it is forcing a short for the even harmonic at a particular
distance from the tee to the plate region of the tube. It modifies the
plate current or voltage waveform. This sort of modification is similar
to what is done with class F amplification, using the technique of Tyler
from Marconi, later used in RCA broadcast transmitters. In my amplifier,
I probed inside the main cavity in a safe location, and discovered that
there wasn't much second harmonic being generated, due to the quarter
wave stub on the output.
73
John
K5PRO
Date: Tue, 29 Apr 2014 02:17:38 -0700
From: "Jim Thomson" <jim.thom@telus.net>
To: <amps@contesting.com>
## Do you really require a LP filter ? You could also use a .25 wave long
coax stub..with far end shorted. You can also add additional stubs etc.
Paralleling caps is the real answer to your exploding cap problem. 2-4 caps in
parallel would be the ideal ticket. Your typ 2m ant will only be resonant on
its 3 and 5th harmonic. You can make stubs that will kill odd harmonics if I
remember correctly. W2VJN has a good book on stubs.
## Parallel some caps..and you wont have any more issues. I have used the
teflon sheeting sandwiched between 2 aluminum plates,,and that works superb...
provided it is constructed correctly.
## I like the idea of a LP filter......... then it kills everything beyond the
cutoff freq. A bandpass filter would even be better. You could also use a
LP filter
for TX....and a low power bandpass filter...for RX only.
Jim VE7RF
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