[Lowfer] WC2XSR/13 rain problems today

[Ralph Hartwell]

I had an interesting afternoon, standing in the rain and watching sparks
fly!

At about 1500 CDT today, the transmitter PA current began taking erratic
dips for several seconds. Simultaneously, the RF forward power dropped
from 400 watts to 100 watts, and the reflected power went up from 10
watts to 100 watts, a sure sign of either a shorted or an open feedline.

Since it had just begun to rain, I thought the cause of these current
changes might be an arcing condition in the antenna system. An
inspection of the antenna showed nothing wrong, but the static gap at
the output of the antenna tuner was intermittently flashing over. There
was no lightning associated with this rain event, so I watched the gap
for a while to see what was happening.

After watching the static gap for a few minutes, I saw the cause of the
arcs. As rainwater ran down over the grounded leg of the static gap, the
water tended to collect and form hanging droplets on the underside of
the flap portion of the horizontal leg of the gap. After a few moments,
they would grow large enough and drop to the ground. If the droplet
reached critical size, and just before it fell away from the gap leg, if
the transmitter turned on at that moment, the high electric field would
attract the water droplet into the narrow portion of the gap itself.
This water intrusion into the gap would substantially reduce the
effective gap spacing, triggering an RF power arc, which would remain
there until the transmitter turned off again or the water drop
evaporated due to the heat of the arc. I will have to install a plastic
rain shield to prevent this in the future.

Another interesting thing was observed as well. After about 10 seconds
of the transmitter turning on with the QRSS30 modulation, I observed
what appeared to be smoke rising from the area of the static gap. Closer
inspection showed that it was not smoke, but steam!

Apparently what was happening is that during the off periods of the
QRSS30, the ceramic bowl insulator was becoming wet from the rain. When
the transmitter turned back on, the RF is heating the bowl insulator and
the water enough to create the steam cloud I saw. I will have to observe
this in the dark so that I can determine how much of the heating is
caused by dielectric loss in the ceramic and how much is caused by
surface corona discharge. This is just another interesting item that I
have noticed while working with high power LF energy.

73,

Ralph   W5JGV / WC2XSR / 13

http://home.att.net/~shmrg

http://home.att.net/~ralph.hartwell