I'm further convinced of the need for shunt chokes on ungrounded driven
elements on shunt fed towers. See the recent QST article by N9NB.
Hopefully, this will be of use to someone else. It may even save someone
having to climb a tower in December.
My 70 foot tower has a D40 dipole at 75 feet, a JK Mid-Tri at 70 feet,
and an A3 at 50 feet fixed SE. Last night in the ARRL 160 contest, I
discovered that after a long run, the SWR of my 160 shunt feed shot up,
as if something were heating. Nothing was warm in the tuner box, so it
had to be something on the tower. (I did try it with a second
amplifier!) Both the upper tribander and the D40 have those shunt
chokes, but the lower A3 tribander, at 50 feet does not. That one is
presently waterlogged or ice-bound, so it's almost 5:1 on 20. However, I
discovered that right after the shunt feed failed, the A3 SWR went DOWN,
then quickly rose again as something cooled off. It must be the balun on
that antenna. I never had that problem before, but that was when all the
TH7 elements were connected to the boom, so the tower has less top
loading above now. In any case, disconnecting the A3 feedline at the
tower base helped: it took longer to overheat the balun, so I can
probably get away with that tonight. I wonder, though, how much of my RF
is going into heating that balun! I guess it's time to build another
shunt choke, and hope for a mild day to climb up there.
73,
Scott K9MA
--
Scott K9MA
k9ma@sdellington.us
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