I think the formula you've used for impedance is for a circular
coaxial line. In the amp, the metalwork forming the 'outer' is a
rectangle, so much of it is spaced further away from the pipe
meaning the the Zo is higher.
I can't see why the excess pipe can't be trimmed back if you use
the original size clamp at the ground end. That blocks off most of
the area so there won't be much field getting past it into the
bottom section - but be prepared for the tuning to be a bit different.
I think I remember a 220MHz version with shorter pipes.
If you're basing your design on the Antennaworks version, don't
forget to add glitch resistor(s), meter protection and all that
good stuff.
Steve
Putting some parts together for a 2M amp. Like the W6PO design. Antennaworks
has a nice write up of the design choices in their manual online:
http://antennaworks.com/
I have 2 questions.
1) On page 21 of the manual, it states that the Zo of the resonator is
"approximately 130 ohms". I don't have the referenced ARRL Handbook, but my
calculations come up with about 70 ohms. I have used 2 formulas, Zo=138
log(R2/R1), and Zo=60 ln(R2/R1). What am I missing?
2) The bottom of all these W6PO designs have excess lengths of pipe
extending beyond the short. Notwithstanding the need for some excess to
allow tuning, is there any reason to not shorten the pipes and the excess
cavity? Maybe shorten the height to about 8 inches?
Thanks for the input,
Charlie N6CFH
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