[TowerTalk] a HOT contest!
K6LL at adelphia.net
Wed Jan 17 16:35:36 EST 2007
This is a pretty wild story.
First, by way of background, my antennas are a KT-34XA at
50', a 40-2CD at 55', and an 80 meter half sloper fed at
So I'm operating in the RTTY Roundup on 80 meters using the
half sloper. The Russian tetrodes are putting out a solid
1500 watts, key down, on RTTY. I notice that the plate and
screen current on the tetrodes are drifting around after a
while on each transmission, but there's no time to
troubleshoot it now, because this is a CONTEST! At least the
wattmeter still says 1500 watts!
After a while, it's time to go back to 40 meters, using the
other radio, which has been tuned to 40 all the while. Oh
no, the radio is deaf! Signals are way down, and I figure
that I have somehow blown the front end. I finish the
contest with whatever equipment is left working.
Now it comes time to find out what went wrong. It turns out
that there is nothing wrong with the second radio, but it's
associated Dunestar filter box shows a dead short to ground!
I open up the Dunestar and find the little piece of RG-174
cable going to the SO-239 on the antenna side is totally
burned, melted, and shorted! After the RG-174 is replaced,
the filter is A-OK. Now there's a testament to Dunestar's
durability. So, how did so much 80 meter RF get onto the
Dunestar, which was connected to the 40 meter antenna?
The driven element of the 40 meter beam is insulated from
its mounting plate by a fiberglass tube. Looking at the tube
through binoculars, I can tell that the insulator is totally
black and burned. My guess is that the end of the boom is at
a high voltage point when I'm using the half-sloper on 80,
and the fiberglass just broke down, putting the 80 meter
energy onto the 40 meter feedline! This situation could have
ended very badly if the Dunestar 40 meter filter had broken
down. I hate to even think about it. I will add an open 1/2
wave stub across the 40 meter feedline as additional
insurance in the future to short out any 80 meter energy.
So, what do you think is the best way to fix the driven
element? I can get a replacement insulator from Cushcraft
(the old one is about 25 years old), but I would like to add
some additional high voltage insulation. Maybe I could wrap
the fiberglass with a flexible plastic sheet (how about a
flexible cutting board?), or maybe I could replace the
entire aluminum element mounting plate with a tough plastic
one? Any ideas would be appreciated.
Dave Hachadorian, K6LL
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