AG6K wrote:
> My first experience with the significance of dissipation-factor was when
> I used a General Radio Co. NYLON double-banana plug at the end of a home
> made open wire feedline. The nylon melted and began foaming. I changed
> the plug to one made from phenolic.
>
> My second experience was with PVC pipe near the end of a 40m dipole. As
> I was tuning up a home brew AB1/tetrode amplifier, the telephone rang.
> My neighbor said: "Rich: your antenna is on fire." My gut feeling was
> that the presence of flames and smoke indicated that it was quite
> possibly on fire.
> - [note: the amplifier was delivering about 1150v peak to the RG9
> feedline at the time]
> -Rich-
That's about 8 kW if the line is matched, but of course none of that
gives us any idea what voltage the delrin or nylon failed at.
Someone suggested a microwave, but again we would have no idea how
the material would behave at HF with an electric field (the microwave
is an electromagnetic wave, rather than a concentrated electric
field).
So here's a test I cooked up.
I used a three foot square PC board as a groundplane (I use it for
playing, it has a SO-239 at one end), and a large piece of B+W
miniductor (about 40 turns of number 10). I connected one end of the
miniductor to the SO-239 and the other end to a number 14 wire
parallel to the board.
The self resonant frequency was 13 MHz (about the middle of HF), and
the input impedance was ten ohms at resonance. Driving the miniductor
with 50 watts produced one inch streamers of corona off the wires
end into the air.
I put a two inch long half inch diameter Delrin spacer between the
PC board and the #14 stub, and turned up the power. Someplace at
about half the voltage where corona appeared, the Delrin got hot and
melted right at the wire (I cooked it for about half a minute in
steps of ten watts, it took 30 watts before the Delrin failed).
I used a signal generator and a Harris RF voltmeter (with a 500k ohm
input impedance probe) to measure the voltage. I estimate it took
about 30 kV peak to cook the Delrin, allowing for loading by the
probe. Nylon behaved almost the same way, but PTFE did not get hot.
A half inch spacer arced about the same voltage that caused
overheating.
This seems to be in line with what Rich found, since his "bad
experiences" were at the HV end of a dipole (fed with 8 kW or so).
Perhaps a good rule of thumb is if you are going to push the voltage
up anywhere near the voltage limit of corona, you'd better use a
good insulating material at that point. That could certainly apply to
some cases of open wire lines used with 8 kW PA's.
But in lower voltage applications the Delrin (or nylon) will work
fine IF it is just used for a support and the BULK of insulation is
air.
73, Tom W8JI
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