[TowerTalk] Measuring electrical length of coax...

[Tom Rauch]

> Where it really IS important to get the electrical length
correct is when one
> is making coaxial stub filters.  The nulls are pretty
sharp, much less than
> the width of the HF bands, so you really have to decide
whether you want
> a particular stub to be a phone or a CW stub.  Or use
multiple stubs.

I had a few complaints about a very weak 40M harmonic when
working 80. It was well within FCC level limits, but of
course even though 65 dB down was technically still illegal
if it caused a problem.

In the process of measuring 80 meter 1/4 wl shorted stubs, I
was surprised at the poor Q using good RG213 coax. As I
recall Rp was about two thousand ohms at resonance on 80,
and just under two ohms on 40M. Reactance climbed from zero
at resonance to 3 ohms + - 150kHz. Real part remained just
under two ohms.

Stubs work a good bit better when placed at a point where
the impedance at the harmonic is very high. They are
certainly a lot less critical when placed at those points.

73 Tom