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
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