I'm thinking of taking the insulator and center conductor from coax and
sliding it in and out of a copper tube, maybe with threaded rod to hold
position and facilitate fine adjustment. I'm figuring on copper tape on a
hula-hoop as the loop, with the cap mounted directly to the hoop, and
copper foil tying the sides of the cap to the loop so I don't have to
halve the capacitance with a dual-plunger reimplementation of the
butterfly. What kind of capacitance per inch can I expect? Would a 36
inch diameter loop at 5 watts make enough voltage to puncture rg-58?
rg-8?
If it'll take a lot of sliding, I'll have to switch to dual-plunger
to avoid excessive conductor length and flexing.

I plan to mount it on a backpack, using dowels, or maybe PVC, to elevate
it for a foot or so seperation from the metal pack frame - cap at
top(another reason to keep the cap light). Will tuning change a lot
depending on whether the pack is worn? I really don't want to have to add
motorized tuning so I can tune it while worn... weight, don't you know?

One other possible deviation - I'm considering carrying it flat. My width
will be unchanged, but it'll lower height by 18" or allow another 18" of
elevation, and give me an omnidirectional horizontally-polarized pattern,
right?

At 5W, I'm not concerned about heating, and even any RF burns should be
manageable, but I want to be as efficient as I can be within the
constraints of /PM operation.

If I can successfully build this, I'll try to document it well, and make
the design available to others. At the very least, maybe we'll get some
cool ideas.

73,

tim