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Re: [TowerTalk] FW: Antenna resonance question

To: towertalk@contesting.com
Subject: Re: [TowerTalk] FW: Antenna resonance question
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Tue, 13 Dec 2016 11:23:56 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On Tue,12/13/2016 11:06 AM, Matt wrote:
I just don't know if the tuning stubs will change the resonance
of the antenna itself?   Does this sound like a valid methodology?

You're asking a good question, but finding the answer is not easy. :) The transmission line does, indeed, affect the resonance of the antenna, including both the stub and whatever is connected to it in the shack. If you've designed the stub matching, you probably have done so by measuring the complex Z somewhere, plugged that data into Smith Chart software, and done the design. To answer your question, I would disconnect the coax from the antenna, measure the coax at the antenna with the shack end connected to your receiver, then plug that value of impedance into your NEC model.

Then add the RX antenna to the model and look at its pattern with and without that measured impedance in the model.

BTW -- there's an excellent paper by N6LF showing that an antenna doesn't have to be very close to resonance to disturb the pattern of a nearby antenna. I have a 45 ft tower with a 15 ft mast holding up 3-el 20M and 4-el 15M monobanders about 60-80 ft from a Tee vertical for 160M. A quick NEC model shows it resonant around 2.4 MHz (those Yagis with elements connected to their booms do a lot of top-loading), and the model shows a bump of several dB in the pattern for the Tee vertical. Google on N6LF to find Rudy's website.

73, Jim K9YC


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