At 03:52 AM 7/28/2006, Rolf Moberg wrote:
> > How about putting some slightly oversize PVC tubing over the elements?
> > They'd NEVER be able to perch on that antenna - they'll just roll off!
>You should never install PVC to your antenna systems. PVC and RF field
>"are not friends", even some US manufacturers (SteppIR) uses PVC in
>amateur antennas. I cannot explain is it extra loss due to permeability
>of PVC or which magnitude it is. Maybe somebody understands this topic
Not all PVC pipe is the same, and especially so for PVC in general. PVC is
fairly low loss at RF (e.g. a heck of a lot better than, say, glass) but
has two notable "gotcha's":
1) It can be hygroscopic, and when it absorbs a lot of water, then the
water might increase the conductivity of the plastic making it more lossy
and reducing the dielectric strength
2) PVC, as used in pipe, typically has fillers added to it to change the
mechanical properties. Those fillers may or may not be good for RF (e.g.
carbon black isn't wonderful). ANd just because the pipe is white on the
outside does not mean that it's white inside (as I found when turning a
piece of 3" PVC down on a lathe to "clean it up"). As has been pointed out
in many other contexts on this list, pipe is made to be pipe, not a
structural member or a RF insulator. It only has to hold the pressure, etc.
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