[TowerTalk] Delrin End Insulators

[Michael Tope]

I just finished building and installing an 80/75 meter "fan" dipole
for the local club stations which uses pieces of black Delrin
for the center and end insulators. I had the black Delrin laying
around my shop and I figured it would be a good choice since
it is supposedly UV stable. Unfortunately, it was brought to my
attention that Delrin is a fairly lossy dielectric at HF frequencies
(DF = .027 at 1 MHz) and it also is quite prone to moisture
absorption. Should I be concerned at all about this. I am not
very concerned about the center insulator since the impedance
at that point is low. A quick calculation seems to confirm this.
If we assume about 5pF of stray C from the center insulator,
a 70 ohm feedpoint impedance, and 1.5 KW at 4 MHz, a DF
of .027 would correspond to about 350 milliwatts of dissipation
in the center insulator. Not enough to even cause a noticeable
rise in temperature. Perhaps water absorption might make
this worse, but I don't know if that will have much effect at 4MHz.

I am a little worried about the end insulators, however, as its
not clear to me how one goes about predicting the capacitance
and voltage at the ends of a dipole (e.g. what is the reactive
power flowing in the end insulator). The antenna will be installed
in an area that is full of very dry brush, so I don't want anything
going up in flames (max power will be 1.5 KW). In fact I will
probably switch over to ceramic just to be safe (starting a
wildfire is not the kind of empirical data I want to collect).
Still I am curious if anyone has any experience with this
material in that kind of environment?

Thanks,

Mike, W4EF..................................................