[Amps] 2M W6PO Amp Cavity Zo calculations

[Steve Thompson]

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