Bill wrote:
"What is the general effect of moisture on antennas and feedlines as
power is increased?"

I worked for years in a shortwave broadcasting plant where transmitters
ranged in power between 3KW and 100 KW. Moisture had little apparent
effect on the 600-ohm open-wire feedlines or curtain and rhombic
antennas at any power level.

We can speculate that wet insulators were slightly conductive when wet
due to disolved impurities. Ohm`s law says current and heat are higher
at high power.

The climate was dry for most of the year but the rainy season was
intense resulting in occasional floods. As I recall, transmission line
flashovers rarely happened in wet weather but dry was the norm and arcs
were infrequent in any case even at 100 KW with 100% high-level
modulation.

From my mediumwave experience I would say that most antenna system
arcing resulted from approaching moisture, not to its arrival.

Approaching thunderstorms are often preceded by charged air sweeping
across the antenna system. Tower guy segments, separated by insulators,
charge until they arc across the insulators with a loud report (Bang!).
This triggers ionized paths which short antenna system parts. This
overloads the transmitter which removes itself from the air to clear the
overload and protect itself. This often repeats rapidly until the storm
actually arrives and the rain starts.

Best regards, Richard Harrison, KB5WZI