John,

The highest operating frequency of a magloop occurs when the tuning
capacitor is set to minimum or zero and the circumference of the loop
is approximately 1/2-wavelength. That's rather obvious.

100 feet circumference = 30 metres. So greatest working frequency is
150 / 30 = 5 MHz.

A small coupling loop is far and away the best method of feeding a
magloop. Nothing could be mechanically more simple. There are no
additional components and the loop is isolated from the main loop.
Furthermore its diameter for a good impedance match to 50 ohms tends
to be independent of operating frequency. The wire diameter of the
coupling loop need be no larger than that of the inner conductor of
the coaxial feedline to which it is connected.

I've never heard of Ted Hart. But, of course, a gamma match will
work. However the direct connection to the main loop unbalances it and
encourages radiation from the feedline. In comparison with a small
coupling loop it is also mechanically complicated.
----
Reg, G4FGQ