[TowerTalk] Feedpoint measurement question

jimlux jimlux at earthlink.net
Sun Jun 30 20:19:23 EDT 2019

On 6/30/19 4:55 PM, David Gilbert wrote:
> 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.

The tubing is *around* the driven element? or around the feedline?

So you're expecting it to provide a bit of dielectric loading and 
increase the C of the element?

PVC pipe doesn't have a particularly stable or consistent dielectric 
constant. If you're using a block of plastic, there are better plastics 
to choose that don't change epsilon with humidity, etc.  OTOH, a change 
in epsilon from 3.3 to 3.5 isn't going to make a huge difference. PVC is 
one of the lossier plastics as wel.

Delrin (acetal) is a popular choice, as is polypropylene, polyethylene 
or polystyrene - they're not hygroscopic, like nylon is, and cheap 
compared to, say, Teflon/PTFE.  Maybe you can find some Polypropylene 

It's worth checking on the dielectric properties, some of them are 
remarkably lossy at VHF/UHF frequencies (Rulon is a filled PTFE beloved 
of mechanical engineers for bearings and such because it's rugged 
mechanically, but the filler material is really lossy)

> 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.

I'd be more worried about the analyzer itself perturbing the 
measurement. But, since you're presumably going to do a "before and 
after" comparison, the effect will be similar.

But I'd do it with a have wavelength feedline and ferrite chokes on the 
feedline to turn it into a balun. Half a wavelength is only 7-8 ft, or 
so, depending on velocity factor.

> 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.
> 73,
> Dave  AB7E
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