In article ,
Dave wrote:

Telamon wrote:
In article ,
"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.

You are confusing efficiency with mode. I for one have not been working
with over 6 inch diameter cable. The devices I work with operate up to
65GHz. The devices use an appropriate connector and coax or waveguide
depending on the band it operates in. that connector type is determined
by the max operating frequency.

If the spacing between the coax conductors is to large then the energy
with not stay TEM and can propagate in other modes. I've seen it happen.
All you have to do is make measurements on a cable past its rated band.
Once you pass the ratings of the connectors or cable you will see large
changes in the VSWR for instance and most of the energy is reflected or
absorbed.

That's all well and good, but we're talking about MW, HF, VHF and UHF
here. Of course, when you get to that magical sub-wavelength physical
size world, coaxial cable becomes a crappy waveguide. At one gigaHertz
the wavelength is 30 cm (a little over a foot). I'm not sure if a
half-wave (diametrically) would propagate in a coaxial cable, but I know
a 1/4 wave would.

I can tell you that 65GHz waveguide is a pain in the butt. A few tenths
of an inch dimensions (keyhole size) require location pins for flange
alignment.

--
Telamon
Ventura, California