SpamLover wrote:
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!

Not that I would discourge experimentation, but it would be worthwhile
to get a copy of something like the ARRL Antenna Book and read up on
top loading to learn what those capacitve hats actually do.

You will also find how to figure out the impedance of a top loaded system.

BTW, brazing rod is a bit more expensive but doesn't have the rust
problem. Use the plated MIG stuff for the prototypes, brazing rod for
the final build.

--
Jim Pennino

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