On Sun, 7 Mar 2004 21:18:46 -0600 (CST),
(Richard Harrison) wrote:

Jimmy wrote:
"The last description I saw of a quarter wave antenna was that of a
parallel circuit. Isn`t that basically how a capacity hat shortens an
antenna, by increasing the parallel capacitance?"

Parallel or series hardly makes any difference.

Part of this debate has ignored that all resonant circuits can be
analyzed as both parallel and series. That is, barring your and my
observations.

To force the parallel resonant observation upon the quarterwave
vertical, all that need be done is to move the drive from the base to
the tip. The same current distribution will be observed, the same
radiation characteristic will persist, and as such nothing has really
changed.

This may raise hosannas from Art in that he has been redeemed by this
move - but at a cost. Moving that drive to that point necessarily
brings a lead that is a quarterwave long to accomplish this mission.
We then find ourselves in a situation where the solution has become
part of the problem. Do we really have a drive at the top, or another
radiator? Art, I am sure, would dismiss this necessary lead as "not
part of the antenna" and would close the books before the audit is
complete. It would be in fact the classic folded dipole. To escape
that and maintain the mystery of driving from the top, we would have
to accept unipolar RF sources (soon to be patented).

However, if we were to return to Jimmy's question/observation of the
top hat; that structure resides at the point we speculatively drove
(the distal tip), and with respect to its own contribution looking
back towards ground, it sees an entirely different circuit topology
than does the drive at the base. This is not exactly the same
situation as moving the drive. The top hat does not grace a full
quarterwave vertical as it would be redundant to that mission. Such
an addition would end up instead throwing the design into a quasi
3/8ths tuning, or such, to dubious purpose. Thus the analysis becomes
murky (for further debate suitable to efficiency per unit length). We
can shorten the quarterwave by small intervals and find the top hat
appears to replace that missing length; but as we shorten, the system
becomes capacitively reactive and we hardly need more.

One of the language problems with the name Capacity Top Hat, and the
expectation of adding more capacitance is that the short antenna is
already excessively capacitive. Logically, the addition of more
capacity does not lead to resonance. The purpose of a top hat
transcends notions of resonance to answer problems of radiation
characteristics. Resonance, as always, is answered through other
devices (inductors) that reside there to serve that same problem of
radiation.

After 10 years of reading a spectrum of discussion, I have never,
ever, read any post that purported a mission to build a 100pf top hat.
I have never read anyone ask how big a hat was needed to resonate
such-and-such inductance. The structure is often too big to qualify
as a lumped capacitor and calling it by this name is a convention, not
a reality.

73's
Richard Clark, KB7QHC