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[TowerTalk] Tuning coax stubs w/ shack test equipment

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Subject: [TowerTalk] Tuning coax stubs w/ shack test equipment
From: (George Cutsogeorge)
Date: Tue, 30 Mar 1999 15:26:26 -0800
I have made well over 100 stubs of various kinds and with several
types of coax and would like to offer some real numbers measured
with a network analyzer.  First of all, a stub cut for the CW end
of a band will lose several dB of attenuation on the SSB end of
the band.  Here are a couple of examples with RG-213:

20 meter open 1/2 wave gives 28 attenuation at it's 7020 null.
At 7200 it gives 22 dB.
20 meter shorted 1/4 wave gives 29 dB attenuation at 28020.  At
28500 it gives 17 dB.

The null is so sharp at 28 MHz that 1/4 inch cut off will move it
substantially.  The null can be positioned to 5 or 10 kHz with a
network analyzer.  I don't have an Autek analyzer, but it is
impossible to get this kind of reading on an MFJ249.

The null obtained with RG-213 is roughly equivalent to a 1 ohm
impedance.  Seven ohms would give less than 20 dB of attenuation.

If optimum attenuation is needed on both CW and SSB, a pair of
stubs can be stagger tuned and coupled with another cable.  This
can provide 40 dB or more attenuation on both parts of the band.

It is possible to get good resolution with the inexpensive
analyzers but you need to use an external detector.  This could
be a wide band scope or a DVM with a high Z probe.


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