On Nov 10, 7:44*pm, "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

It may be that a 100 MHz scope has better than a 3.5 nsec risetime,
given that it is sped'ed for response flatness to that limit and its
response actually extends beyond !00 MHz. In retirement, I no longer
have access to test equipment that would support my point.