[CQ-Contest] [TowerTalk] Coax Stubs for SO2R

[Jim Brown]

On Wed,9/9/2015 4:07 PM, David Robbins wrote:
> Yes, 1/4 wave (measured on 40m) shorted stub goes on the 40m tx.

An important quibble, David. Because Vf varies slightly with frequency, 
the MEASUREMENT of a harmonic suppression stub at the frequency of the 
harmonic. For a typical RG213 coax, the difference is enough to shift 
the resonance of the stub 60 kHz higher than you intended.

Let's say we wanted max suppression at 14,060 kHz and we are using 
Commscope 3227, a coax whose Vf I've carefully measured. Vf is .8385 at 
7 MHz and .8415 at 14 MHz. Measuring at 7,030 rather than at 14,060 
would move the stub up to 14,120 kHz.

Likewise, if we have a half wave open stub on 20 to kill the 40M 
fundamental, we should measure on 40M.

AC6LA's excellent ZPlots spreadsheet allows computation of Vf, Zo, and 
attenuation vs frequency from complex Z measurements of a suitable 
length of any transmission line (with a Vector Analyzer). You measure 
the line open and shorted over a fairly wide frequency range, then plug 
the two sweeps into ZPlots.

73, Jim K9YC