Topband: Insulated elements atop shunt fed tower

[Pete Smith]

Thanks to Tom, Natan, Bill, Earl and Mauri for setting me straight on the
potential problem with insulated yagi elements atop a shunt-fed tower.
Somewhere in my amateur radio education I'd missed the idea that high
voltages always appear at the top of a vertical, regardless of its
electrical height.

That being said, I'm planning to run only 100 watts on 160 for the
foreseeable future.  The complexity of reworking both my 40-meter yagi and
my top C-3E to ground all the elements, coupled with uncertainty about the
effect on interaction between the two yagis, means that if I can't get away
with that power level then I'll have to drop the shunt feed idea altogether
and figure out something else.

W8JI:
>By changing height to 100 feet, no other changes, voltage 
>becomes 2576v RMS or 3642 volts peak between the element and 
>boom. Still high for the insulators I see on most antennas.

Thanks, Tom - I was wondering how you could quantify the voltage.  Sounds
fairly safe with a 100-foot tower and 100 watts -- maybe ~900 volts peak?
The Force 12 antennas use PVC pipe with ~1/8" wall as the insulation.

73, Pete N4ZR
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The World Contest Station Database 
is waiting for your input at
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