>
> This makes the physical lenght of the line shorter than freespace size.
>
> Take a Yagi, and fill the space between elements with a material which its
> dielectric constant is less than 1.0 (air's).
>
The amount of size reduction you get out of dielectric loading depends on
the effective dielectric constant of a large volume surrounding the antenna.
If you built a yagi in an infinite space of solid teflon, it would be
smaller than its full size counterpart by the difference in velocity factor
between teflon and air.
If you simply coat the elements with a thin layer of teflon, you only get a
couple percent shortening.
In order to approach the "velocity factor shortened" amount, you'd have to
build a big enough dielectric chunk to make sure the majority of the near
field electric field passed through the dielectric. In coaxial cable, this
is a snap. The electric field of the guided TEM mode that's responsible for
transmitting signals through coax is entirely inside the coax, and most coax
is **totally full** of whatever the dielectric is.
The equivalent scenario for an antenna really is "all of space is full of
dielectric," and you could approximate this with a gigantic block, but
that's not very practical. Furthermore, you'd have to take into account the
effect of reflection and transmission at the dielectric/air interface.
Since good, low loss dielectrics with different enough dielectric constant
to do anything are generally much denser than air and much weaker in
mechanical strength than aluminum, it's not likely that there's a good way
to build dielectrically loaded HF antennas that have any advantage over any
other kind of loading.
At microwave frequencies, pure dielectric antennas and cavities start to be
straightforward, and those are sometimes used. A "polyrod" antenna is just
a stick of solid polystyrene; you excite the back end with a waveguide and
it fires a beam of RF with a nice pattern off the tip of the stick:
http://personal.ee.surrey.ac.uk/Personal/D.Jefferies/miscellaneous/antennasmeasurements/med/polyrod-antenna-040304.jpg
The equivalent HF antenna would weigh many, many tons and might not even be
possible at all.
There's a reason that at HF we use aluminum tubing with capacitance hats and
loading coils (and all the stuff that antenna marketers think up to replace
capacitance hats and loading coils that probably don't work quite as well).
Antenna resonance is all about storing just the right amount of energy in
the right form in the fields around the antenna so that the sloshing of that
stored energy "pushes" on the electrons in the antenna in phase with your
driving power from the transmitter. As you make an antenna smaller and
smaller, you need to start doing things to help store a lot of energy in a
small volume. Dielectrics are good at temporarily storing more electric
field than air/vacuum but are certainly not the most weight efficient way to
achieve extra electric field energy storage! Once you get to microwave
frequencies when strength and weight are not primary concerns, you can use
dielectric loading pretty freely.
73
Dan
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