On Tue, 20 Apr 2004 07:35:50 GMT, zeno wrote:

When transmitting what effect does moisture and/or
precipitation have on various kinds of antennas, also what is
the effect of rain on say uninsulated ladder line.

I've heard rain affects balanced line, but I use standard insulated
450 ohm ladderline to feed a dipole, and I can't detect any SWR
changes when the line is wet or dry.

Bob
k5qwg


What is the general effect of moisture on antennas and feed
lines as power is increased?

Bill

Bob Miller wrote:
I've heard rain affects balanced line, but I use standard insulated
450 ohm ladderline to feed a dipole, and I can't detect any SWR
changes when the line is wet or dry.

Some (biased) experiments have been performed with twinlead laying
on a wooden deck wetted by a soapy solution. Moral: Avoid soapy
horizontal ladder-line. :-)

I use the length of the ladder-line to tune my antenna system to
resonance. I have to change the length by up to 2% to compensate
for the rain, i.e. rain has a negligible effect on the tuning of
my antenna system.
--
73, Cecil http://www.qsl.net/w5dxp



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Bill wrote:
"When transmitting what effect does moisture and / or precipitation
have on various kinds of antennas---?"

I worked off and on for 10 years in a medium wave broadcast station with
a 4-tower directional array (separate day and night patterns) fed by
open-wire 6-wire skeletal coax.
Every half hour we recorded the sampling loop currents associated with
each tower and the phase relationships between the towers using an RCA
WM-30A phase monitor. Precipitation and fog didn`t amount to a hill of
beans in nearly all cases. Currents and phases were nailed in place.

Best regards, Richard Harrison, KB5WZI

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