[Lowfer] Dielectric loaded dipoles

[Lyle Koehler]

To calculate the length of a half-wave dipole that is completely surrounded
by a dielectric material, I think you use the usual formula for a dipole in
free space and divide the length by the square root of the relative
permittivity of the dielectric. Some ceramics might have very high relative
permittivities, but are also expensive and difficult to work with if you're
talking about using them to surround a low-frequency antenna. Water has a
relative permittivity of 80, which is higher than most other materials, and
a fully submerged half-wave dipole for 1750m would only need to be about 98
meters long. But that's only if the surrounding water envelope is large
compared to the size of the antenna. 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.

There's one other catch: 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. Many
years ago an associate and I ran an experiment to see if a dielectric loaded
dipole would really work. We took a large plastic cylinder with two metal
end caps connected by a center-fed wire, and filled the cylinder with
distilled water. Sure enough, the water dielectric lowered the resonant
frequency, but the antenna was a very poor radiator.

Radiation occurs when you accelerate an electric charge. In a free-space
dipole, the "circuit" is completed by the so-called displacement current.
Because the displacement current exists in a vacuum, there are no
accelerating charges in the return path to cancel the radiation from the
current in the wire dipole. However, if the antenna is enclosed by a
dielectric material, there *are* accelerating charges in the surrounding
medium, even though they are bound so that they can only wiggle back and
forth, (that is, so that no DC current can flow).

This isn't to say that dielectric antennas can't work. Various forms of
dielectric antennas are used at microwave frequencies where the wavelength
is short enough so that the required dimensions are practical. One simple
form of dielectric antenna is a plastic or ceramic waveguide where radiation
can escape from the open end. I have a fiber-optic Christmas tree that has a
whole bunch of radiators like that!

Lyle, K0LR