John,

In principle, the coupling loop can be located in the same plane and
anywhere within the main loop. So it is most convenient to locate it
at the bottom of the main loop within an inch or two of the main loop.

But make no direct connection between the two loops or between the
coax and the main loop. There has to be some sort of support for the
coax and coupling loop. So you can make use of the insulating jacket
of the coax feedline and sticky insulating tape. If any other
supporting insulating materials are needed any plastics will do.

You can, of course, make a beautiful engineering job out of it which
you can be proud of. But, radio-wise, the antenna will not work the
slightest bit better. ;o)

A comment about a motor-driven variable capacitor at the top of the
main loop. To allow the main loop to maintain self-balance about
ground, insert a multi-wire choke in the control wires and any other
wires feeding the motor. Or whatever form the motor may take.

Remember, the main loop should be allowed to take up its own natural
balance about ground to reduce "radiation from the feedline."

In the case of magloops, radiation from the feedline is a matter of
loss of radiation from the antenna. Waste not, want not!
----
Reg, G4FGQ