Barry wrote:
Jim,

I think you need to go back and read Spamhog's original question. He was
trying to determine whether the center conductor of a piece of coax had
migrated away from center. He knew where this might have happened - 10
feet from the end, and the migration would have occurred over less than
two inches. So the question of locating where the problem might be is
moot. What is needed is a measurement of the cable impedance in this
region.

First, let us get an estimate of what the impedance of the damaged
section might be. Spamhog was using RG-6 cable with a foamed
polyethylene dielectric. Its velocity factor is 0.85 making its relative
permittivity 1.384. The center conductor is 1 mm, and the normal
diameter of the center insulator is 4.7 mm. The thickness of the
insulator is 1.85 mm. We need to know the impedance if the center
conductor had migrated 0.925 mm toward the jacket.

For a quick estimate, use the formula for off-center coax
(http://www.microwaves101.com/encyclo...offcenter.cfm). This
gives an impedance of 69.8 ohms in this section compared to 78.9 ohms in
the non-distorted coax. A TDR displays the reflection coefficient
from -1 (short) to +1 (open). Here the reflection coefficient is -0.06.
So the TDR trace will drop from the center line by 6% for 200
picoseconds.

If your 100 MHz scope has a typical Gaussian response, its rise time is
at least 3.5 nanoseconds. Do you really think that your oscilloscope
trace will clearly show the 200 picosecond dip? Even with the
wide-screen magnifier that KB7QHC suggested, I think you will have great
difficulty seeing this.

73, Barry WA4VZQ

The bandwidth of the 'scope will make the trace have a ripple intead of
the nice, sharp bump you would get from a faster 'scope.

Spamhog's original statement was that the cable was crushed to half diameter
for about 2 inches as I recall.

I was able to see ripples in the display for cables with less crush then
that which were on the order of 1/4 wide.

The bottom line is the faster the 'scope you use and the faster the rise
time of the applied pulse, the better the meaurement.

And since this is a hobby and not building man rated space craft, I would
say try whatever you can get your hands on for free and see what happens.

Or spend eternity arguing whether or not it is possible to do.

--
Jim Pennino

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