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Re: [TowerTalk] Impedance at the Feedpoint

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Impedance at the Feedpoint
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Mon, 08 Nov 2010 10:15:02 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 11/6/2010 9:53 AM, Bill Gillenwater wrote:
> If you have a feedpoint impedance of 32 ohms on an inverted "L" for 1.8 mhz, 
> can anyone venture a guess as to what the impedance would be of the same 
> antenna at 3.5 mhz ??

VERY high -- if it's 32 ohms on 160M, it's close to a quarter wave, 
which would make it close to a half wave long wire on 80M.  WAG about 
10K ohms.

A good compromise is to make it a bit shorter (or longer) on 160, which 
makes the Z not nearly so high on 80 (because the Z rises VERY fast as 
the antenna hits a half wave).  How much is "a bit short or long?"  I'd 
say 10-20%. Several years ago, I had a Tee vertical that was close to 
resonant on 160, fed by about 30 ft of RG8.  I could load and run full 
power to it through my Ten Tec 229B on 80 CW, but not on 75M -- it was 
too close to a half wave there.

73, Jim K9YC
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