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Re: [TowerTalk] real world formula for the length of a quarter wave tran

To: towertalk@contesting.com
Subject: Re: [TowerTalk] real world formula for the length of a quarter wave transmission line
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Thu, 14 Feb 2013 06:39:05 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 2/14/2013 5:33 AM, Jim Lux wrote:
I think the OP was looking for a impedance transformer, and for that application, I think small errors in length won't have a big effect. Say you were using 1/4 wavelength of 75 ohm line to transform 50 ohms to 112.5 ohms.. If the line were 7/32 wavelength, would it be that much different?

You're right, it would not, but the effect is not zero. AND -- the resonance that a length of line like this provides can do some VERY interesting and useful things in broadbanding resonant antennas. That piece of line is a quarter wave at the design frequency, but on one side of the design frequency it's inductive, and on the other side it's capacitive. Depending on the antenna and the rest of the system, the error could help or hurt.

One can learn a lot by building a simple NEC model of a dipole, exporting it to SimSmith, and seeing what happens with various lengths of line of various impedances.

73, Jim K9YC
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