[TowerTalk] Feedpoint measurement question

David Gilbert xdavid at cis-broadband.com
Sun Jun 30 19:55:22 EDT 2019

I plan to run a quick experiment to see how much effect a few inches of 
surrounding PVC at the feedpoint (homebrew boom to element connector) 
has on the SWR curve of a 6m resonant dipole built with 1/2 inch 
tubing.  EZNEC+ says five inches of PVC roughly 0.2 inches think will 
shift the SWR curve down a hundred KHz or two on the yagi design I plan 
to use.

With a bit of care I can actually get my little FA-VA5 analyzer as close 
(or closer) to the feedpoint as I can the end of a piece of coax that 
has some sort of common choke on it.  Since in the first case there 
would be virtually no feedline of any appreciable length, does balanced 
versus unbalanced really have any meaning here?  Yes, the FA-VA5 
nominally has an unbalanced BNC input, but I don't think that has much 
relevance inside the analyzer or that close to it.

Am I right or am I wrong?  We're talking 50 MHz here and a fraction of 
an inch between the analyzer and the actual feedpoint of the dipole.

I'd appreciate any knowledgeable comments.

Dave  AB7E

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