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
tubing?
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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