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Re: [TowerTalk] Change in Frequency with Height above Ground

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Change in Frequency with Height above Ground
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Thu, 26 May 2016 20:37:09 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On Thu,5/26/2016 8:24 PM, Don W7WLL wrote:
Lowering the height of an antenna above ground, such as a beam, will cause the 
resonant freq to decrease, increase?

Decrease. But not always -- I have a 10M beam at about 15 ft that goes up! The variable here is mutual coupling between elements and between elements and ground.

Another error in using SWR as the indicator of resonance is that feedpoint Z changes with height, and in a very complicated way (and because of mutual coupling to ground). In general, low dipoles (as a fraction of a wavelength) are lower Z, high dipoles are higher. AND -- feedpoint Z is also affected by the quality of the ground.

SO -- we may THINK the resonance is affected by the height as indicated by the low value for SWR, but it's actually the feedpoint Z changing!

73, Jim K9YC
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