[Lowfer] Dielectric loaded dipoles

[Ed Phillips]

Bill Ashlock wrote:
> 
> >From Lyle's post:
> >I would expect very little length reduction if you ran a 1750m dipole >down
> >the center of a 1 inch (or even 12 inch) PVC pipe full of water.
> And:
> >The surrounding dielectric has the same effect as connecting a >capacitor
> >between the ends of the dipole. It reduces the length >required for
> >resonance, but the "return" current in the capacitor is >in the opposite
> >direction from the current flowing in the dipole, and >it cancels most of
> >the radiation. So the net effect is a very >inefficient antenna.
> 
> Come-on guys...... there just ain't no magic at LF (and that's why most of
> us love this band). Nothing works better than just plain old paying
> attention to the theory that's been around for 100 years; identifying,
> testing, and resolving all the discreapancies, and, of course,.........
> waiting for those ego-building signal reports <G>.
> 
> Bill A

	This note should be repeated periodically!  The VLF antenna designs of
WW1 vintage have never been improved.  Super grounds with lots of
radials or ground plane meshes, and as tall as economics allow and top
loaded where possible.  In our case the height is fixed, so for an
antenna in the open the radials are about the only way to go.  There's a
picture on the cover of one 1921 issue of QST showing a multi-wire flat
top up at least a hundred feet above a house, with a massive
counterpoise at roof top level.  I suspect that would have made a
superlative 1750 meter installation!

	There have been a number of applications where antennas have been
operated in water to shorten the physical wavelength for specialized
purposes.  For example, George Southworth, who did pioneering work on
waveguide during the 30's, worked with waveguide in a swimming pool
because he didn't have usefully high-powered microwave sources.  Much
more recently a friend of mine built a theraputic heating setup for use
at 50 MHz, and apparently similar apparatus is used in a number of
hospitals.  The patient iss  surrounded with a tank of the purest water
that can be obtained. The wavelength at the SURFACE of the tank is just
over 2 feet, which apparently works out just fine for the intended
purpose.  Of course, the field falls off very rapidly with distance from
the surface.

Ed

Ed