Thank you everyone!

65 mph
We have enough problms with all those flying saucers annoying people
from time to time. I promise I won't create more.


So, it seems that high capacitance is not a no-no for electrical
reasons. If you look at naval radio masts, sometime you'll see
manufacturers boasting of their "high capacitance" - like that
double-drainpipe-thick-mast stainless steel monster that I guess won't
cost less than $20k - but takes 40 kW, and won't stretch an automated
ATU to seek an impossible match.
(http://www.valcom-ottawa.com/Guelph/...415_photo.html
et al.)

From a mechanical standpoint I'd have no problem putting up a 1ft.
diameter wire "disk" or a similar structure. I have been
experimenting with a bizarre material: MIG copperflashed steelwire.
Dirt cheap, springy, solders extremely well, can be hand-shaped. It
can be used to put together small, super-light, bouncy, 3-dimensional,
lattice structures. Rusts like hell, but nothing that can't be dealt
with using an ordinary nitric acid paint and laquer.

I had already planned to use that stuff in my next VHF discone, now I
am tempted to use a high capacitance disk as pickup in a mobile
broadband HF active antenna.

Is there some good soul here who can help me figure out the impedance
I need to match?

I would like to interpose either a T + Pi network or a Chebyscheff
between the antenna and a classic FET gate, to kill off RF below 3 MHz
and above 30 MHz.

My first try would be either a 1 ft disk with 16 braced 1mm radials
about 8" above a car roof, or a cone of the same dimensions pointing
down into the amplifier case. Hmmm... that would be a structure
similar to those monster HF broadband designs - just 50 times smaller!