In article 490dad88.1002406@chupacabra,
Bob Dobbs wrote:

Brenda Ann wrote:

"Telamon" wrote in message
...
I'm not confused. Coax and waveguide are the same regarding conductor
spacing.

Waveguide has no 'conductor'. It's basically a pipe with the proper internal
diameter for a given wavelength. It does, therefor, indeed, get smaller for
higher frequencies.

The same is not true for coax. Skin effect is the most significant factor in
coax as frequency goes higher: there needs to be more surface area as the
frequency goes up.


Curious;
would coax behave better (velocity factor etc.) if it were sized closer
to the dimensions of the frequencies/wavelengths involved?
IOW: Coax the size of a culvert for VHF and so on.

Remember that for a specific frequency that wavelength varies depending
on the dielectric constant of the medium so the wavelength is longest in
vacuum (0), followed by air (1), followed by Teflon (2.2). The higher
the constant, the slower the propagation, the shorter the wavelength.
The dielectric constant of the inner insulator determines the velocity
factor in coax. Losses also go up with the dielectric constant. For
dielectric loss you also consider the frequency dependent loss tangent
of the material.

Waveguide has the lowest loss with air as the dielectric. It will beat
coax at any frequency but you would not want to use it for short wave as
it would be large, bulky, expensive, and hard to work with.

Coax is a real engineering marvel with a number of tradeoffs. You have
to determine what parameters are most important to your application. All
kind of things are done depending on what parameter was being optimized
for the application.

The two kinds of losses present in coax is conductor and dielectric.
Teflon is an old standby, which can be solid, foam, spiral, or periodic
inserts with a lower dielectric constant since air becomes a part of it.
The general rule is that as you create a design where the coax has less
loss and a flatter broadband response the less mechanically friendly it
becomes. You either can't bend it at all (hardline) or as sharply or
very often as lower electrically performing coax.

--
Telamon
Ventura, California