"Taking the worst case rain static situation – very heavy rain for say, about
30 minutes, the Top antenna is ~ @ S9, the 2nd @ S6-7, the 3rd @ S3 and the
bottom antenna quiet or almost quiet."
First we should note that it is not the rain that creates the noise... the rain
or snow is a symptom that may or may not accompany the noise. The noise is
caused by a high voltage gradient between the earth and cloud which causes
charges to accumulate on the higher parts of a tower/antenna and when enough
charge accumulates corona starts... it is the corona that you hear as the
static. It is very possible to have corona with no rain or snow at all, and it
is possible to have heavy rain or snow with no corona. It is possible to
predict with reasonable certainty if a particular storm will create corona,
after many years of watching radar and correlating with local conditions. I am
pretty good at predicting here, but wouldn't try to do it in other parts of the
country. Corona is most likely on ends of elements and sharp pointy things as
those are the places where the voltage gradient is the highest.
As far as preventing it, that can be difficult. Telrex tried adding rounded
balls to the ends of their elements for a while, but didn't round off corners
of t-match shorting straps or even the screws connecting the element sections
so did not solve the problem on their antennas. I think most of the quads that
claim to be quite probably had insulated wire, which combined with relatively
few pointy things like screws or hose clamps would reduce or prevent most
corona. Dipping a driven element in liquid rubber might be an interesting
experiment, but I would like to see a UV camera used on a Yagi during a storm
to see just where most of the corona is first, there may be simpler solutions,
or maybe not.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net:7373
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