I used to operate/maintain some very large HF RLPs that use delrin
insulators. Freq range 3-30 MHz and power is over 5KW. Many of these
antennas have been in use for over 20 years. I know cause I installed
several of em. Little different than a fan dipole but we never had any
problems with moisture absorption. We had a problem with UV resistance
and they'd crack after a few years. But the manufacturer corrected that
and we haven't had a problem since. If you have insulators that absorb
moisture, you probably don't have delrin or they've changed from those
I'm familiar with.
Regards
Noel
Mike wrote:
--SNIP: 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.
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