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Re: [TowerTalk] Stubs and Reactances in parallel

To: "towertalk@contesting.com" <towertalk@contesting.com>
Subject: Re: [TowerTalk] Stubs and Reactances in parallel
From: Steve London <n2icarrl@gmail.com>
Reply-to: n2ic@arrl.net
Date: Wed, 24 Jun 2020 17:10:53 -0600
List-post: <mailto:towertalk@contesting.com>
The antennas are fed separately. The stub on the 80 meter dipole does kill the interaction to the 40 meter beam. However, the Z of the stub is around 1000 ohms 80 meters, so it affects the 80 meter dipole. Read my earlier writeup carefully for details.

73,
Steve, N2IC

On 6/24/2020 11:20 AM, Steve London wrote:

    Is there another solution I am overlooking ?


Hi Steve,

Yes. Feed the two antennas separately. Use a stub in the 80M feedline as detailed here to kill the harmonic of 80 on 40.

http://k9yc.com/LocatingStubs.pdf and
http://k9yc.com/StubPlacement.pdf

The same stub, properly placed, will apply a short at the feedpoint of the 80M antenna, which should kill the interaction. I learned this a few years ago doing an NEC model of our contesting trailer's antenna system k9yc.com/7QP.pdf I work with N0AX on the Handbook and Antenna Book, and when I told him what I'd learned he nodded his head yes, he'd known about that for years. The ancients keep stealing our inventions. :)


Also, remember that VF varies enough with frequency that stubs must be measured and tweaked to length at the frequency where you want them to be a short. The variation is enough to move their resonance to a different part of the band.


73, Jim K9YC
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