At 08:42 AM 12/4/2006, Dan Zimmerman N3OX wrote:
>In the spirit of "some antenna is better than no antenna at all," I'm
>going to add a base loading coil to for 160m to my 40 foot vertical
>(http://www.n3ox.net/projects/lowbandvert).
>
>I know it's a substantial compromise over other forms of loading, but
>I'm willing to accept the efficiency penalty for convenience in this
>particular case. What I want to know about is whether or not grey PVC
>electrical conduit is suitable for coil forms at 1.8MHz. I searched
>the archives and found a discussion on PVC formers for HF and that
>they're not unacceptable unless you're running many close-spaced
>turns, but I wonder if people have tried grey vs. white and what
>they've found. I know the microwave trick is routinely used but I
>don't think absorption at 2.4GHz says much about the MF properties of
>the material.
I've used a fair amount of PVC pipe over the years for insulators and
here are my observations:
The exterior color says nothing about what's actually inside the pipe
wall. I turned some White PVC down on a lathe and found a remarkable
array of grey, white, and black colors inside.
The grey stuff is intended as electrical conduit, so has some
manufacturing standards as far as dielectric strength (i.e. it's got
to be an insulator at DC) and mechanical properties, and, maybe UV resistance.
PVC isn't particularly lossy at RF, BUT, there can be significant
amounts of crud (including metal shavings) inside the pipe wall that
might cause a problem. There is no way to tell this from the outside.
PVC is somewhat hygroscopic, so I wouldn't use it for a form for a
precision capacitance or inductance standard, because the dielectric
constant will probably change with the weather. The surface
breakdown voltage will also change with humidity.
One could conceivably test your pipe by winding a small coil on a
slightly larger form, hooking it up to something like an antenna
analyzer, and then passing your proposed form through the middle and
seeing what effect it has, if any. (this has been used to do things
like measure moisture content of lumber) The problem I would see is
that I doubt you could see 1% loss, and if you planned to run a
kilowatt, that 1% would be 10 watts, which might melt the PVC.
I'd actually worry more about the coil wires getting warm (from IR
losses) and melting/softening the PVC. Your application will
presumably have the windings exposed to the air, so it's probably not an issue.
And, of course, you could agonize about it longer than it would take
to just try it and see how well it works. Groove the pipe, wind the
coil, measure it and see if it looks like it will work.
Jim, W6RMK
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