[TowerTalk] Feedpoint measurement question
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.
> Dave AB7E
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