Topband: single wire Dielectric covering.

[Tom W8JI]

> Dielectrics affects/increases capacitance.
>
> Question:  Has anyone made a study as to how it lowers the frequency of 
> the wire that it is covering?.
> My thought.... It is probably is a linear capacitance along the wire 
> itself.

While the dielectric and conductors inside the shield are not involved in 
affecting electrical length or signal reception in the antenna mode, 
multiple factors are at work outside the shield. The same is true for a 
single wire.

How things affect the system depends on the electric field distribution in 
the "stuff" surrounding the conductor.

It is impossible to insulate or isolate the electric field because it passes 
through dielectrics. This is why buried open wire line, even when suspended 
in a plastic pipe, has much higher loss and slow Vp than the same line does 
with air surrounding the line for several wire spacings away.

An antenna is typically a single wire transmission line, so it is even worse 
than two-wire open line in a balanced mode. It has nothing to cancel the 
external field except distance. Keeping things spaced away at great 
distances relative to wire length, electric field intensity, and other 
things that reduce or allow the field to spread can reduce effects.

If the outer jacket were solid infinitely thick polyethelyne, the velocity 
factor would be about the same as solid dielectric coax with the same 
dielectric. In both cases all of the field would be in the dielectric. If a 
large portion of the electric field is in air, the velocity factor moves 
towards that of air. This is why velocity factor outside a cable (antenna 
mode) is normally faster than velocity factor inside a cable (which can only 
be transmission line mode, because common mode cannot exist with any 
significance inside a cable with a shield many skin depths thick).

An example of where this trips people up is the coaxial collinear antenna 
published in QST and the ARRL books. That antenna, which copies the concept 
used by Phelps-Dodge, does not work. The reason is Phelps-Dodge encased the 
coax for a few inches in thick bee's wax when they used solid dielectric 
cables. This made the Vf identical inside and outside the cables, so a 1/2 
wave stub section was 1/2 wave electrical inside and out. When P-D switched 
to air coax, they removed the bee's wax and used nearly all air outside the 
cable, so the outside dielectric for a few inches (which is a large fraction 
of a wavelength on VHF) matched the inside. The author of the QST and ARRL 
handbook coaxial collinear antennas never realized that, and built an 
antenna that did not work properly.

All of this applies to 160 meters, or any other band. It is the distance in 
wavelengths and other factors that determine how far out the electric field 
goes, and what really happens when we play with antenna insulation. On two 
meters, the thickness of a given dielectric is 80 times more than 160 
meters. Also the electric field intensity, based on voltage distribution, 
also affects the results. Obviously any rule of thumb that applies to one 
antenna on one band in one environment cannot be applied to all antennas in 
all environments.

Why would we expect insulation to have the same effect on a 10 meter dipole 
one wavelength from anything else as a 160 meter longwire laying right on 
lossy dirt? With a 160 antenna laying on the dirt, most of the field is in 
the dirt. On a ten meter dipole way up in the air, most of the field is in 
air. It isn't likely even the same band-by-band in the same environment.

The only thing we do know is in antenna or common mode, it is what is 
outside that matters. In differential mode or transmission line mode, it is 
what is on the inside of a two-conductor line that matters.

73 Tom