Thanks all for the comments on tower resonance measurements.
The question was actually prompted by some recent measurements I made using
the ON4UN/W1FB method of a drop wire from the top of the tower.
Here the problem is the difference between theory and practice. I found
that as I moved the wire around, changed the spacing from the tower, changed
the attachment point height or rotated the beam on top, the apparent
resonant frequency moved around. I was using a impedance analyzer to
determine resonance from the zero reactance points.
If you are actually going to drive the shunt wire to excite the tower then
the measurements are realistic. If you interest is to see if the tower is
resonant in the neighborhood of a particular frequency where it might mess
up the pattern of an adjacent antenna, then great accuracy is not needed.
If the resonance is anywhere near the frequency of interest the you will
probably want to use the shunt wire to tune the tower to some other
frequency. In either case the measurement is accurate enough.
However, if you want to use the tower as a parasitic element in an array,
and not have the shunt wire a permanent part of the array, then an accurate
determination of the resonant frequency is needed. I was hoping that some
of the old BC hands on the reflector had dealt with that problem before and
could advise.
Thanks again,
73, Rudy N6LF
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