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Re: [TowerTalk] Stubs

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Stubs
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Wed, 13 Nov 2013 10:39:46 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 11/13/2013 10:22 AM, Tom Osborne wrote:
Do I make the stub for the correct length, hooked to the tuner on the output
of the amp, or do I need to subtract the length from the output of the
exciter to the output of the tuner?

The length of the stub is to the point where it is Teed with the main coax. But that's not all -- it is VERY important to tune the stub at the frequency you want the stub to kill, using something like an antenna analyzer. The procedure is pretty simple -- if you want the stub to kill the 2nd harmonic, tweak the length of the stub until the analyzer sees a short at the second harmonic. If you want the stub to kill the fundamental of the band below you, tweak that stub at that lower frequency.

Another very important point -- if you are going to switch in stubs for multiple bands, you must make the measurement through the switch and the short length of coax between the switch and the Tee.

More detail about this in k9yc.com/Coax-Stubs.pdf

Before cutting your first stub, figure out what the Vf is for the coax you're using, and don't count on the published value being right. Most solid dielectric coax is close to 0.66, and most foam is in the range of 0.75 - 0.85.

73, Jim K9YC
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