[Lowfer] Re: 600MRG> Link to some darn good antenna designs...

[Lyle Koehler]

Years and years ago, a friend and I were speculating about ways to
"artificially" increase the capacitance of a small antenna and thereby
increase the current in the radiating section. We knew that a parallel
capacitor in a ground return wire wouldn't work, for the reasons that
several people here have given (I particularly like Johan's concise
explanation). But what if you immerse the antenna in a high-dielectric
constant, insulating medium? That way there won't be any physical
"conductor" to carry the opposing current. Sounds too good to be true, but
we tried it anyway using a large-diameter PVC pipe with metal end plates and
a central radiating wire, filled with distilled water. Distilled water is
non-conducting but has a relative dielectric constant of about 80. Sure
enough, the capacitance between the end plates increased dramatically but it
didn't radiate worth a darn. Why? Because radiation occurs when you
accelerate electric charges. In a vacuum or in a material like air with
essentially the same dielectric constant as a vacuum, no charges are being
accelerated in what Johan calls the "space capacitance". In a medium like
water, it is in fact the movement of charges that give rise to the high
dielectric constant, and in our experimental antenna the radiation from
those charges was in opposition to the radiation from the central wire.

Bill Ashlock is correct in stating that if you make the capacitance in the
return wire very large, eventually you have created a loop antenna. However,
unless the enclosed area of the loop is very large (Bill's antenna must be
something like 225 square meters), it probably won't radiate as well as the
top-hatted vertical without the shunt capacitor. Since the author mentions
using 6-foot top radials, it doesn't sound like something that would make a
very efficient loop!

Lyle, K0LR