My secondary antenna for 80m/40m had been acting strange for the last
year or so, and I recently had some time to look into the issue.
The antenna is a untuned doublet of about 115 feet in length,
supported about 35 feet in the air. It is fed with 450 ohm window
line, which swings down to a remote balun under the porch, and then
coax into the shack.
I've been using this antenna in one form or another for 10 years.
Last year, it quit working on me -- I could hit the auto-tuner and it
would find a match, but then when I tried transmitting with more than
a few watts, it would show a bad match.
I disassembled the balun and found the answer. The balun is a
homebrew guanella type -- a T240-77 core wound with 9 turn bifilar
winding of insulated 16 gauge wire. Apparently the insulation between
the two wires cracked at a certain point, and the wires shorted -- it
only took a few watts to establish an arc between the two.
I fixed the problem by winding a W1JR-type balun. I used some RG-303
teflon coax and wound 10 turns on the T240-77 core. No more arcing.
I'm still not entirely happy with this doublet. I may opt for the 80m/
40m trap dipole after all.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr at arrl.net
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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