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
http://audiosystemsgroup.com/Coax-Stubs.pdf It includes a reprinting
of the very useful stub info on the K1TTT website. :)
73, Jim K9YC.
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