K1VR's use of PVC on his gamma rod is a good example of a situation in
which you CAN'T use PVC. (No offence, Fred.) There can be several
thousand volts at the end of the gamma rod, all of it across a few
mm of PVC. Even if PVC were a good RF material, the voltage probably would
have been high enough to break it down. (I destroyed a ceramic feed-through
insulator that way.) On the other hand, if you use a horizontal piece of
PVC to hold the gamma rod away from the tower, that same voltage appears
across 1 foot or so of PVC, and there's no problem. As a very rough
rule, I'd recommend limiting the RF voltage across PVC to a couple volts
per mil (40-80 V/mm), below 30 MHz.
Scott K9MA
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