Henry Koleanik, WD5JFR wrote:
"Anyone really know theory behind these things?"

The Isotron is a small tuned loop. Tuning gets rid of reactance which
would cripple performance were it not removed from the loop.

A small loop has a null perpendicular to the plane of the loop. Energy
suppressed in directions of the loop`s axis is radiated in the plane of
the loop giving a directivity power gain of about 1.5 (not dB, where
it`s just under 2 dB gain) over an isotropic radiation in the plane of
the loop as in other directions as well.

The bad news is that even made with a large surface area, the small
loop`s loss resistance looms large as compared its radiation resistance.

The 3rd edition of Kraus` "Antennas" is a clear source of single-turn
circular loop information. Fig.7-10 on page 209 gives radiation
resistance versus loop circumference. Fig. 7-11 gives directivity.

If radiation resistance were 0.5 ohm and r-f resistance were 0.5 ohm,
efficiency would be:
0.5/1=50%

Kraus has Fig. 7-17 on page 217 which gives radiation efficiency as a
function of frequency for a 1-m-diameter single-turn 10mm copper tubing
loop in air. At 1 MHz, the loss is about 40 dB. At 10 MHz, the loss is
about 6 dB. The radiation resistance of the loop is rising more rapidly
than is the loss resistance as frequency goes up. The loss is enormous.
This is ok for reception in most cases, but it`s very dear for
transmitting.

Best regards, Richard Harrison, KB5WZI