I have heard there is a process where coax cable is baked in an oven
to lessen the effects of temperature change of the cable on phase
shift. I havent been able to find any information on this process.
Direction to any information on the subject would be greatly
appreciated.

Jimmie

JIMMIE wrote:
I have heard there is a process where coax cable is baked in an oven
to lessen the effects of temperature change of the cable on phase
shift. I havent been able to find any information on this process.
Direction to any information on the subject would be greatly
appreciated.

Jimmie
mostly it's done by empirical methods..

The idea is to relieve internal stresses by several temperature cycles,
reducing at least one source of phase change.

In reality, what you want is "consistent and repeatable phase change vs
temperature without hysteresis"... hysteresis or randomness is usually
due to mechanical effects (stick/slip, etc.), so temperature cycling
fixes it.

google for phase-stable coaxial cable and you'll probably find
manufacturer sites that discuss it.

BTW, there's "low phase change cable" and "repeatable phase change
cable", and sometimes one is better than the other in a particular
application.

On Mar 6, 6:05*pm, Jim Lux wrote:
JIMMIE wrote:
I have heard there is a process where coax cable is baked in an oven
to lessen the effects of temperature change of the cable on phase
shift. I havent been able to find any information on this process.
Direction to any information on the subject would be greatly
appreciated.

Jimmie

mostly it's done by empirical methods..

The idea is to relieve internal stresses by several temperature cycles,
reducing at least one source of phase change.

In reality, what you want is "consistent and repeatable phase change vs
temperature without hysteresis"... hysteresis or randomness is usually
due to mechanical effects (stick/slip, etc.), so temperature cycling
fixes it.


Yes, I think this is the problem. There are multiple cables that must
remain phase matched fairly closely and they are exposed to the
weather. If there was a sudden change in phase of the signal on one
cable that would definatly be a problem. Any cable that does not have
hysterisis would probably work. I also realize now that the type of
cable being used is probably the worst possible choice. Just swiching
to a foam dielectic cable may solve the problem.


Jimmie

On Sat, 07 Mar 2009 07:34:32 -0800, JIMMIE wrote:

On Mar 6, 6:05Â*pm, Jim Lux wrote:
JIMMIE wrote:
I have heard there is a process where coax cable is baked in an oven
to lessen the effects of temperature change of the cable on phase
shift. I havent been able to find any information on this process.
Direction to any information on the subject would be greatly
appreciated.

Jimmie

mostly it's done by empirical methods..

The idea is to relieve internal stresses by several temperature cycles,
reducing at least one source of phase change.

In reality, what you want is "consistent and repeatable phase change vs
temperature without hysteresis"... hysteresis or randomness is usually
due to mechanical effects (stick/slip, etc.), so temperature cycling
fixes it.


Yes, I think this is the problem. There are multiple cables that must
remain phase matched fairly closely and they are exposed to the weather.
If there was a sudden change in phase of the signal on one cable that
would definatly be a problem. Any cable that does not have hysterisis
would probably work. I also realize now that the type of cable being
used is probably the worst possible choice. Just swiching to a foam
dielectic cable may solve the problem.


Jimmie

A little story of mine about phased coax....

No matter what the consultants tell you, make the two (or more) sample
lines the same length and exposed to the elements in the same manner.

I ran into a sample loop problem on an 5 tower AM directional array where
the coax from the sample loops were cut to the needed length and the
phase difference was compensated for in the antenna monitor. There was a
difference of several hundred feet between runs. Plus, one monitor loop
was a hundred feet higher up a tower.

Once I made the sample runs the same using phase matched coax, coiling
the same amount of coax on each tower then coiling the same access
underground, the readings changed drastically and the array became more
manageable. The insanity had ended!

Before installing the coax, measure each of them with connectors
installed using a signal gen and splitter. Align the antenna monitor for
0phase on all inputs with a signal gen and splitter, then add the coax to
the monitor with the signal gen and splitter on the other end before
installing onto the tower to confirm system integrity. Excessive bends
both in number and radius can change the phase of one line over the other
so be conscious of how you make the individual runs!

If your under the FCC, make sure you have a CP. :-)

I also realize now that the type of
cable being used is probably the worst possible choice. Just swiching
to a foam dielectic cable may solve the problem.

Jimmie

The physical dimensions of the coax changes with temperature. The
ratios of inner and outer diameter stays pretty much the same, so the
Z doesn't change, but the epsilon of the dielectric changes (it's less
dense at higher temperatures). since propagation speed goes as 1/sqrt
(epsilon), and the density goes as 1/t^3, the prop speed goes as 1/
(sqrt(1/t^3)) = t^(3/2)...

On top of that the coax gets longer as a function of T, so the time
delay/phase goes as T/(t^1.5) or 1/sqrt(T).

A CTE of 30ppm/deg C is about 0.05%/deg C for time delay. If your
coax is 1 wavelength long, a change of 5C is about a degree. (0.25% =
1 part in 400, 360 degrees/wavelength, etc.)

You can see that working at microwave frequencies, where coax might be
100 wavelengths long, trying to match to a degree is a challenge.