Boozo wrote:
Have a 100 foot roll(approx) of used coax cable which has no markings
printed on it.
Can anyone tell me how I can work out what it might be ? e.g. 75 ohm or
50 ohm or ?
Going by the size it looks similar to 50 ohm but I'm not sure.

Any suggestions would be appreciated.

You didn't mention what you have to make measurements with. The
simplest way to make a quick estimate is probably to guess at the
velocity factor first: solid polyethylene is about 0.66, and foam
polyethylene is roughly 0.78. The impedance in ohms is 60*velocity
factor*ln(D/d), where ln is the natural logarithm, and D/d is the ratio
of inner to outer conductor diameters. So for solid polyethylene
dielectric, D/d is about 3.5 for 50 ohm line, and about 6.2 for 72-75
ohm line. Even for fairly small line, it's not difficult to tell by
just looking at it. It's also possible to measure the capacitance of
the lenth of line and the electrical length in nanoseconds, and
calculate the impedance from those two. If you can measure RF
impedance, measure the line with the far end open, and with it shorted,
and take the square root of the product of those two.

Cheers,
Tom