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
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Stew Perry Topband Distance Challenge coming on December 29th.
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