Marc,

That's a fine picture of a variable capacitor, but how are we supposed
to know how the connections inside are made, or what the capacitance
values are? You may be able to tell if you inspect it carefully from
the outside. Even if I have one that _looks_ similar (which I may well
have) I would not rely on its having the same connections or
capacitance values as yours.

Have you done any analysis of the loop to see what capacitance you need
to tune it to the frequencies you care about? Reg Edwards has on his
website a program that's useful for analyzing loop antennas. I believe
it's only set up for square loops, but if you enter side lengths to
give the same loop area as you have, you should get quite close to the
same performance. You may find it's helpful to use a larger
cross-section wire in the hula-hoop, for lower resistance.

How will you tune the antenna to different frequencies if the capacitor
is sealed in the box? Will you provide a hole for adjustment? How
will you feed the antenna output to the receiver? What frequency range
do you hope to cover?

I fear there are more questions than answers...but questions are good
if they get you thinking about the whole situation before you put a lot
of work into something that may not do what you expect or what you
want.

Cheers,
Tom