Well, impedance is primarily determined by the diameter of the center
conductor and the inside diameter of the outer braid or foil, with a
less sensitive dependence on the dielectric material. If you can
measure center conductor diameter, and the overall outer diameter, and
guess the kind of plastic used in the dielectric (teflon,
polyethylene, foam, air with spiral spacer, etc.) you can probably
identify it in a table of common types.
If you look at the characteristics of different coax
cables with the same impedance and dielectric type but with
various diameters, you will notice that the capacitance per
foot is nearly independent of the diameter. IIRC it is
about 22pF/ft for 50-ohm with a solid polythene inner and I
expect then that it will be just as constant across the
diameters for other types. This leads to the possibility of
identifying the impedance of a length of feeder by using a
simple DVM with a capacitance range. Of course the method
would be more accurate, the longer the test piece.
To measure a cable's impedance yourself would be fairly complex. Look for
an types of numbers on it to see if you can identify it from catalogue
The impedance is determined by the type of dielectric material that the
centre conductor is passing through and the shielding is around, and the
distance between the centre conductor and shield. The gauge of the centre
conductor, and outside diameter is also a determining factor.
By doing precise physical measurement, knowing the types of materials
involved, and the type of alloys used in the composition of the conductors,
a mathematical model of the characteristics of the cable can be determined.
This is very complex, and would not be accurate in all practicality for you
The best way would be to use a cable network analyser, or to use a sweep
signal generator and oscilloscope, with a RCL matching network to determine
the impedance curve over a range of frequencies. From this the actual
impedance can be determined.
Accurately measuring the characteristics of any type of cable or wire can
turn out to be a complex and expensive ordeal.
If your piece of cable is from a cable TV distributor, or from a video
distribution system, it is most likely a 75 ohm cable. If it is from an
older Ethernet system, it is probably a 150 ohm cable. If it is from the
antenna system used on communications radio, it is most likely a 50 ohm