Toroidal Antennas and other Remarks

An antenna is sometimes described as a coupling device, coupling
electromagnetic energy to space, and following on with this line of
thinking, antenna amateurs/specialists have wondered whether there were
ways of achieving this process in more efficient ways than provided by use
of monopoles, dipoles and loops.

Most ideas that have so far emerged are however hoaxes. In one, where one
considers an antenna as an opened out transmission line, it is claimed that
the characteristic impedance of the antenna should be made equal to the
intrinsic impedance of free space (377 ohms). In another, one equates the
radiation resistance to the free space impedance. This latter statement
has even less meaning, since the antenna's radiation resistance depends on
where you decide to reference it on the antenna structure.

In 1989 (see Electronics & Wireless World, March, July and November, 1989)
and in following years, Maurice C. Hatley, GM3HAT described a cross-field
antenna, in which quadrature E and H fields are separately generated.
Reversing the form of Maxwell's equations led to the "realization and
development of this revolutionary new antenna system". Quite small
versions of it have been devised and tested. And, while tests have been
made, to demonstrate its ability to efficiently couple to space, in my view
this antenna fits the category of a hoax.

Recently I have learned about two new revolutionary (??) antennas, both
toroidal types.

In one, Roger Jennison, G2AJV (see Radio Communications (RSGB magazine),
April, May and August, 1994) described his toroidal antenna as follows. He
had also looked at Maxwell's equations, and following a particular line of
reasoning, he concluded if an RF current were applied to a toroidal coil,
it would curl around the space inside the torus. And, since the pertinent
Maxwell equation states that if a magnetic field curls it will produce a
displacement current at right angles to the magnetic field. This
displacement current can collapse back whence it came, or it can radiate
into space. He further concluded that the more efficient a toroidal coil
would be in trapping the magnetic field, the greater would be the electric
field produced. So he wound a small air-cored toroid of about 20-turns,
which he resonated at 14 MHz with a small capacity, and fed via a 2-turn
primary. He placed two small circular plates above and below the toroid,
but insulated from the toroid, so that stray fields would be a minimum. He
achieved a good match, and when power was fed to it, the toroid did not get
hot -- so where was the power going? It must have been radiated.

The displacement current, dD/dt, is strongest where H is most tightly
curled (Maxwell's equation), near the inner radius and, if the coil is
resonant, it will be greatest where the field is strongest. This gives
rise to extremely high potentials on the insulated free plates. Subsequent
experiments showed that indeed a small G2AJV toroidal antenna for the 20M
band was as efficient as a center-loaded mobile whip; and versions of it
have been fabricated by amateurs in radio for bands 80M to 2M.

 Another toroidal antenna has more recently been described by James E.
Smith, Center for Industrial Research, West Virginia University,
Morgantown, WV. It is Contrawound Toroidal Helical Antenna (CTHA). This
toroidal helical antenna is described as a low profile, resonant antenna.
It has a torroidal geometry, and a pair of contra-wound helical windings.
Its small size (1/60th the height of a dipole) results from both the
circular geometry, and the effect of its helical windings, which retard the
propagation of EM waves around the structure. Signals are fed to the
antenna through up to four networks which attach to the structure at
evenly-spaced locations. It is claimed that the resulting magnetic fields
act as if they are solely produced by a ring of pure magnetic currents; in
other words, the contributions due to electric currents are canceled. The
planar ring of magnetic current is electromagnetically equivalent to a
linear electric current. It is further claimed that a VHF marine band
model provided a gain of 4.12 dBi, compared to the 2.15 dBi for a half-wave
dipole. The size of the quad-contra was 1.6 cm high; 49.4 cm diameter.
However it is stated that the antenna's diameter can be reduced by
two-orders of magnitude by increasing the number of turns or by using
ferrite cores -- but no gain claim is made for these very small CTHA
antennas.

I would be pleased to receive views on these new "revolutionary" toroidal
antennas. Correspondence concerning performance of toroidal antennas has
ranged from skeptical to beyond belief!!

73, 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 Thu Nov 09 1995 - 21:12:00 EST