[TowerTalk] Too much antenna?
Sun, 17 Feb 2002 09:08:01 -0800
Congratulations on your antenna performance. Quite satisfying, isn't
I once had similar problems with RF in the shack. It is possible that
your feedline(s) and rotator cable and control cables are coupling common
mode currents from the antenna back to the shack.
1) Have you installed proper current mode choke baluns to EACH antenna
feedline at the antenna?
2) Have you installed a current mode choke on each feedline at the
3) Have you installed a current mode choke on the rotator cable and any
control cables from the tower at the shack?
Current mode baluns for the shack ends of the feedlines and cables can
be as simple as winding each feedline and each cable about 5 or 6 times
through seperate 2" or 2.5" ferrite RF toroids, #43 or #73/77 mix. I will
sometimes stack 2 or 3 toroids where I feel some "extra" choking impedance
is warrented. These ferrite components are available from Amidon and other
I have eliminated all traces of RFI in my shack and at a couple of
friends shacks using these techniques. It is amazing how those cables can
wreck havoc on our computers and other equipment. Let me know how it goes
John Petrich, W7FU
----- Original Message -----
Sent: Sunday, February 17, 2002 7:32 AM
Subject: [TowerTalk] Too much antenna?
> For 40 meters I now have a HF6V @ 10 feet with 4 elevated radials for 80
> 40, and my TH-7 boom @ 65 feet converted to 40 with an Omega match per
> Rig is an IC-765. Although both antennas are tuned to the CW segment of
> band there is enough difference between them that I have to retune the
> when changing antennas. I find an "clear" freq near the center of the
> segment, cut the power to 5w and go to RTTY mode. The 765s internal tuner
> quickly matches either antenna. I then crank it back to 100w and away I
> The problem I'm having is the TH-7 conversion makes my shack RF city at
> on 40m.
> During the ARRL DX CW contest I was S&Pn JAs on the vertical quite
> successfully until the sun was above the horizon when even the KW JAs
> hard to copy and they couldn't hear me at all. Still S&P I thought I heard
> VK5 with a big pile up going.
> I switched to the TH-7 boom, tuned it as above and got distracted by the
> for a bit. When I got back to the radio (about 5 minutes later) I could
> copy the VK and he was just working US west coast stations, although there
> were many east coast guys calling too. I decided to give it a shot and
> dropped my call. He came right back to me (I'm in TN) through the pile!
> During my exchange I noticed no RF at all on the monitor. I was still at
> After bit of playing I found that the monitor went nuts at 10w, but was FB
> 5w. I was still having trouble copying the rapidly fading JAs but they
> coming right back to me at 5 - 7 Watts! I had NO copy at all on the
> Anyone have any idea what's going on here?
> I've checked EVERTHING that I can think of and everything seems fine. I
> coax choke at the antenna, but it didn't make any difference at all. The
> tower's about 40 feet from the radio, the TH-7 proper and the conversion
> seperate RG-213 feeds with the coax shield grounded at the antenna and the
> bottom of the tower, they then run a couple of inches below the ground to
> This thing should ROCK at 100w, but the computers won't handle it --- They
> make a thumping sound on CW and just lock up on SSB. Running QRP there is
> indication of a problem.
> Maybe this thing is just toooooooo much antenna for LP? I'm suspecting the
> feet from the radio is the problem, but don't have a solution. Moving
> the tower or the shack is out of the question.
> Thanks in advance for any help,
> Don - K4BEV
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Where do you get ICE bandpass filters & beverage matching boxes? The
same place that pays for the hosting of this list: The eHam Store.
Order online at http://store.eham.net.
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Administrative requests: towertalk-REQUEST@contesting.com