Toroidal Antennas 2

Greetings from my Keyboard,

I posted a provocative note on the reference topic on 9 November. I would
like to thank those that responded.

Jerry Burke wrote, concerning the CTHA antenna, that:
"basic sources, as with any antenna in the world, are electric currents,
and any argument about magnetic fields would be one way of looking at it.
If the antenna radiates like a vertical dipole it must have a net vertical
electric dipole moment. The only advantage that I can see is that if you
want to put it on the ground without a ground screen, it would be better to
have sources distributed over the toroid than concentrated on a dipole at
the same height. However, there are other ways to handle the problem".

Concerning reducing the size of the antenna, e.g. it was claimed that the
quarter-wavelength diameter of the prototype CTHA could be reduced by up
two orders of magnitude, Allen Davidson, who made a similar comment as
Jerry wrt the use of magnetic or electric sources, drew attention to the
fact that if efficiency is to be realized, as the antenna size (in
wavelengths) is reduced, the bandwidth must get vanishingly small.

A clear example of this, which is enjoying popularity amongst amateurs in
radio, an antenna I have written on [QST November 1993], is a compact loop;
since by using a "thick" conductor high Q can be realized.

L.B. Cebik, W4RNL commented on the G2AJV toroidal antenna, that instead of
plates one could run a wire, e.g. a half wave radiator (but then it would
not be a reduced size antenna) through the centre of the toroid, in which
case the displacement current will run along the wire. In fact this
comment had (apparently) been made in the late 70s, when Prof. Jennison
began his experiments, by Richard Feyman. Without this wire it appears
that the toroid with insulated plates is a very inefficient radiator.
According to experiments by P.D. Brooking, G4SHH the relative field
strength for the toroid was about 20 dB down on a small tuned loop of the
same approximate size [reference his letter in Radio Communication August
1994, p. 93].

Some of the physical structures described as "later developments" (but
really they are different antennas), in which two toroids, with a
connecting wire between them, and connected plates, arranged as a coaxial
end fed antenna or a dipole may work, but the described mode of radiation
is not, in G4SHH (and in my) view, responsible for them working.

I left out this follow on information deliberately, because I wanted a view
from someone on the circulation list concerning the fundamentals of the
curious original version of G2AJV's toroid with insulated plates, so
enthusiastically described as a Maxwell invented antenna.

Kok Chen, AA6TY asked whether the bandwidths of the of the G2AJV toroidal
antennas are infinitely narrow -- in fact they are not, being comparable to
a dipole for some versions, but rather narrow for other versions. AA6TY
also drew my attention to Prof. Jennison's article in the summer 1995 issue
of Communications Quarterly -- in fact I had copied that article for my
file but then forgot I did. On reading it yesterday the only comment I can
make is that I consider that the RadCom articles are (marginally) better
written, although I have problems with realizing his approachs to
understanding the fundamentals of radiation and particularly wrt his
toroidal antenna, and with following the logic of his various antenna
configurations.

Let me stop here, since discussion posted should in my view be brief and to
the point, and not extended by musings of the author. Again thanks to
those that replied to my note.

Jack, VE2CV

John S. (Jack) Belrose, VE2CV
Director, Radio Sciences
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2308
FAX 613-998-4077
Received on Wed Nov 15 1995 - 16:35:00 EST