[TowerTalk] Birds and PVC on Antennas

[Jim Lux]

At 03:52 AM 7/28/2006, Rolf Moberg wrote:
>K7LXC at aol.com 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
>better?


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.

Jim