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Re: [TowerTalk] Measuring Coax Length

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Measuring Coax Length
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Tue, 28 Oct 2014 17:05:47 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 10/28/2014 2:39 PM, Jim Lux wrote:
For TDR (or looking for integer half wavelengths as you sweep the frequency on an analyzer) you have to assume a particular propagation factor,

Mike,

Measuring the length of a long piece of transmission line is tricky for several reasons, most important because Vf (the velocity factor) varies a bit with frequency. Study k9yc.com/Coax-Stubs.pdf

An easy way to get close to the actual length is to put a connector on one end, leave the other end open, and measure the impedance with an antenna analyzer. At every frequency where it's some odd multiple of quarter waves long, it will look like a short. So sweep it with whatever analyzer you own, write down those frequencies, and do the arithmetic.

73, Jim K9YC


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