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Re: [TowerTalk] Stubs

Subject: Re: [TowerTalk] Stubs
From: Jim Brown <>
Date: Wed, 10 Oct 2012 12:13:24 -0700
List-post: <">>
On 10/10/2012 10:43 AM, K1TTT wrote:
For years I measured stubs with a tape measure.

Two potential problems with that approach. First, what kind of coax is being used? Foam or solid PE dielectric? BIG difference in Vf, and small variations from one coax type number to another. Second, Vf is NOT constant with frequency, but varies a few percent from 160M up to 10M (most of the change occurs between 160M and 40M). And there may be manufacturing tolerances. These differences, however small, can shift the null you're hoping will kill your transmitter harmonic, or the transmitter on the band below you, slightly higher or lower in the band, often enough that you don't have nearly as much attenuation where you wanted because you're up the slope of the attenuation curve, not at the null. .

Bottom line -- always cut stubs a bit long, then trim them by observing the frequency of the desired null, in the circuit where you plan to use them. I Tee them into a transmission line between a 50 ohm generator and 50 ohm voltmeter of some sort. One setup I use is an HP generator and HP spectrum analyzer. Another is a VNA.

There's a discussion of all of this in It includes a reprinting of the very useful stub info on the K1TTT website. :)

73, Jim K9YC.


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