Topband: Broadband vertical

[Earl W Cunningham]

Tom, W8JI, in response to my statement that the BW of a shunt-fed tower
was about the same as a series-fed tower, said:

"Actually not unless the shunt system is a VERY thick or wide conductor."
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As much as I respect Tom's opinions, I have to disagree here.

I know Tom doesn't necessarily believe in all modeling results, but my
shunt-fed tower modeling has been proven to be highly accurate in that it
agrees with real-world results as far as shunt wire parameters and BW is
concerned.  And most of the shunt feed modeling I have done uses #8 AWG
for the shunt wire.
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"To improve bandwidth, you can take steps that increase the amount of
capacitance used in the shunt feed capacitor. That would be a larger
diameter drop wire or a cage of drop wires. Rather than messing around
switching components, I'd use a multiple conductor drop system with wires
spaced as far apart as possible."
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Tom's first sentence here agrees with my statement that the broadest BW
occurs when the structure is electrically 1/4-wave.  In such a case, the
shunt-feed capacitor maxes out at approximately 750 pF.  On the other
hand, if the shunt-fed tower is electrically 1/2 wave, the capacitor is
less than 100 pF (and very narrow BW).

As for the rest of Tom's statement, a multiple-wire shunt system actually
increases the BW only slightly (about 5%, as stated in another of my
e-mails on this subject) and, IMO, not sufficiently enough to be worth
the effort.

73, de Earl, K6SE