May be I'm a bit o_ff t_opic, but rising to the top of a shunt fed element
with the fed wire and connecting there we obtain a folded unipole.
The ratio between the radiator diameter and the fed wire will act as an
impedance transformer, but at resonance the impedance measured at the bottom
(at feed point) will be in any case purely resistive or so.
Using such a system and an antenna analyzer, tuning from lower to higher
frequencies the first frequency where the imaginary part of the impedance
goes to zero is (close to) the structure lowest resonance (1/4 WL).
By direct experience this procedure works perfectly also with top loaded
towers when bringing the fed just below the capacitive hat.
Of course this is true if the reached height of the fed wire connection is
close the voltage node, whose thing generally happens at first "hat" if
"cuts off" electrically what's above.
Although there is no advantage in having the structure resonant, when it
happens it's also hard to match to 50 Ohm with a gamma using reasonable
ratios and spacings.
73,
Mauri I4JMY
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