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Re: [TowerTalk] Determining coax length?

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Determining coax length?
From: Jim Lux <jimlux@earthlink.net>
Date: Tue, 12 Nov 2013 08:29:11 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 11/12/13 8:06 AM, john@kk9a.com wrote:
I also cut the cables the same length before installing the antennas by
laying them flat on the ground. I build identical antennas and use the
same model balun. When connecting the baluns it is very important to
connect them identically (center of the coax to the same side of the
driven element for each beam).


There have been reports of the velocity factor varying substantially within a single spool of coax (other than a "once per turn on the spool"). It's not a parameter that is tightly controlled by the mfr.

If you have a lot of wavelengths, it can add up. Say you've got 200 feet of coax and you're phasing at 10 meters, so the coax is a bit over 6 wavelengths long. A 1 % change in velocity factor is 22 degrees.

Mind you, I think that other factors will probably have a bigger effect on the relative phasing (the self and mutual Z of the antennas are probably not that well controlled, as elements wave around in the wind, etc.)

I would put the thing up and try it. If the performance didn't seem right, I'd stick short lengths of coax in one side or the other to shift the relative phasing. What it's going to do is move the positions of the nulls in the various configurations, and I think that's generally quite "site dependent" in any case.



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