My specialty is UHF TV. We use bigger lines. 3 1/8 is plenty at 0.64 MHz.

www.eriinc.com/pubs/Catalog07_160-163.pdf

In article ,
Dave wrote:

My specialty is UHF TV. We use bigger lines. 3 1/8 is plenty at 0.64 MHz.

www.eriinc.com/pubs/Catalog07_160-163.pdf

Maybe I don't understand some of the ramifications of power but I do
know that cables and connectors are designed to keep the RF energy in
one TEM mode so conductor spacing is determined by frequency. The space
can't be so large at the frequency of operation that more one than
propagation mode is then possible. That means the line and connector
impedance will change dramatically with small changes in frequency and
power.

The rule here is that the higher the operating frequency the smaller the
conductor spacing has to be so then power will dictate the line diameter
by what would be required for a low loss conductor size.

--
Telamon
Ventura, California

Telamon wrote:
In article ,
Dave wrote:

My specialty is UHF TV. We use bigger lines. 3 1/8 is plenty at 0.64 MHz.

www.eriinc.com/pubs/Catalog07_160-163.pdf

Maybe I don't understand some of the ramifications of power but I do
know that cables and connectors are designed to keep the RF energy in
one TEM mode so conductor spacing is determined by frequency. The space
can't be so large at the frequency of operation that more one than
propagation mode is then possible. That means the line and connector
impedance will change dramatically with small changes in frequency and
power.

The rule here is that the higher the operating frequency the smaller the
conductor spacing has to be so then power will dictate the line diameter
by what would be required for a low loss conductor size.


You're confusing coaxial lines with waveguide. With coax, higher
frequencies require bigger cable; skin effect.

"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.

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.

--
Telamon
Ventura, California

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.

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

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