Topband: Shunt feed - how high is high enough?

[Maurizio Panicara]

Hi Pete,

>is there any reason
> from a loss or field strength standpoint to prefer the higher tap?

I'd rather say how low is low enough....
When a tower is self resonant the current distribution is linear and a pure
voltage shunt feed will drop into a folded unipole.
When the radiator is not resonant, but short and/or top loaded, the current
distribution along it is not linear.
In this case, it is raccomandable to keep the shunt tap as much as it is
possible down in the antenna portion, in an area where the current
distribution would approach that of a resonant antenna.
Consequently, and generally speaking, it's not the best idea to tap the
gamma close to the capacitive hat.
Unless the antenna is very small, consider that the effective hat is mostly
represented by the lower antenna of the tower.
In very short radiators where the current distribution is almost the same
along the whole antenna it's not raccomandable to shunt feed, but rather to
series feed. Shunt feeding in this cases lead to realize an antenna that
approaches a tuned rectangular loop, mostly similar to a so called
"magnetical loop" rather than a shunt feed vertical.
On the other hand it has to be identified what's the limit beyond which the
"lowering tap" process is producing no enhancements and possibly delivering
problems or losses.
The solution with gamma ratio and spacing and gamma capacitor for any tap
position is single but useful tap positions are many.

73,
Mauri I4JMY




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