Hi Joe,
To measure tower resonances, all I'm doing is measuring the series impedance
of the sensing loop while it is attached to the tower.
Without the tower the loop is just an inductor up to the point where the
loop length approaches 1/4-wave resonance where it starts acting as a two
wire shorted transmission line. The coupling to the tower is very loose so
when you go past a tower resonance all you see is a small bump in the
impedance. In fact the deviation in |Z| (magnitude) is almost too small to
see, that's why I use the phase of Z. the deviation in phase is much easier
to see but it is still quite small. That's why I use a VNA for the
measurements.
There may be other instruments which will work but I haven't had time to
think about them. An AIM4170 impedance meter is good enough for the job,
but it's $400+.
This is why I've been very careful to tell everyone: my technique works but
is not, repeat not, a general answer to the problem. It's requires
knowledge and instrumentation that the average ham may not have. I'm hoping
that I or someone else will figure out a better and more general way to do
this. Actually I'm hard at work on that, lots of ideas but no
breakthroughs. For now the present less the optimum solution is it.
GL and 73, Rudy N6LF
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