Check the screws between the inside and outside trap tubes. I had the same
problem pop up after a few years on the OR coast, high SWR under higher
power on 15, and high SWR on all bands, particularly when it is raining. I
first suspected the issue was the balun (again, have lost 2) or a trap since
it almost immediately cleared up when the wx cleared and things had a chance
to dry for a day. If it had been coax and water had gotten into a bad
connector covering or the coax itself, it would have taken much much longer
to clear I thought. Nevertheless, the coax is very old and considering the
salt and UV here I am going to replace it. Using a tuner to get everything
to work required a very high inductance setting.
I'm preparing to lay over the HG-HD70 this week and begin repair and and
replacement of the bad parts and baluns (I also have a Cushcraft A3WS above
the TH7DX).
Now, let's discuss MFJ and Hy-Gain support. I raised the same question you
just posted to MFJ and I got a message from the ham tech who apparently mans
the Hy-Gain/Cushcraft desk.
This is what he said!!!!
------------------------------------------------------------------------------
"It may be the screw connection between the inner and outer trap tubes.
Remove the screw and inspect the connection point for corrosion or other
problems.
If the swr is over 10:1 it is in the driven element. IF it is around 3-5.1
at resonance, it is either the front (Note - I assume he meant director) or
rear trap (Note - Again I assume he meant reflector).
If you want to look inside the trap, find the dimples down the side of the
trap, take a drill bit the same size, and drill out the dimples. The outer
cover will slide off for inspection inside."
-------------------------------------------------------------------------------
Living on the salt water I have already lost two BN-86's to corrosion where
they swaged the ground side of the contact inside. Leaving the balun open
with the holes around the connector and the weep hole on the bottom is a
disaster waiting to happen.
I happen to have a spare TH7DX from my PDX location which has been setting
on sawhorses. I pulled a couple of traps off, removed the covers and low and
behold, a totally corroded screw and an open on one, and on the second also
a corroded screw. The beam has been down and setting for about 10 years. It
worked perfectly in PDX land.
So, when I get the beam off I am going to go through all of the traps and
fix the screw issue properly (I'm still contemplating the best solution for
here). I'm replacing the HY-Gain balun too. Don't know how the BN-4000 is
built but I suspect it also has an open case allowing for salt air intrusion
or bugs whichever is your environment(few bugs here). Maybe the airwound
balun or the use of the 2.4 in toroids would be better, have never used this
method before.
Both beams performed well inland but are obviously not designed for an
environment like mine (don't use stranded wire anymore either, have tried
all the 'cleaning' remedies and have yet to find one that really works
well).
Just as frustrated as you,
Don W7WLL
-----Original Message-----
From: Gary Marks
Sent: Monday, September 01, 2014 11:18 AM
To: towertalk@contesting.com
Subject: [TowerTalk] Traps and High SWR-troubleshooting problem ?
I have a Hy-Gain TH7-DX antenna that has a high SWR on 15M ONLY. With only
about 30 watts power, the SWR is 1.5 to 1 but as soon as about 300 to 400
watts are applied, the SWR jumps to over 4 to 1.
It works perfectly on 10M & 20M. I removed all the 15M traps looking for
carbon tracking, dis-assembled them and wire brushed the coils and cleaned
them with solvent.
After installing all the cleaned 15M traps I still have the same problem. At
that point I removed both the 10M and 15M traps and wire brushed all the
traps and re-installed them again with the same high SWR problem only on
15M. This time I kept my antenna tuner out of the system by installing a
barrel connector between the in and out coax cables.
While testing the SWR again on 15M, I just happened to touch the coax
connector's (yes they were tight) at the barrel connector and they were VERY
HOT.
At this point I replaced the BN-4000B balun with a new one and I still have
the exact same problem.....a 4 to 1 SWR jump from 1.5 to 1 when applying
over 300 watts or so on 15M ONLY.
Anyone have any idea what the problem might be ?
Gary
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