Yes, but it normally will provide a reasonable result unless you are doing
something very particular like the 160m matching transformer and stub I just
built... See the homework problem and one answer at:
http://wiki.k1ttt.net/2012%20Maintenance%20and%20Upgrade%20Blog.ashx#160m
Part of that shows what jim points out, my original 1/2 wave line that was
probably just measured by length was a bit long... though it had been
performing just fine for years before I decided to upgrade the array.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net
-----Original Message-----
From: Jim Brown [mailto:jim@audiosystemsgroup.com]
Sent: Wednesday, October 10, 2012 19:13
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Stubs
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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