Boozo ha escrito:

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.

About coax impedance,

When you just have a (VHF) transmitter with 50 Ohm VSWR meter and
external dummy load, you can do the following:

Check whether the insulation (between inner and outer conductor) is
solid or not (may be foam).
Check whether the insulation (between inner and outer conductor) melts
(changes color to transparent) at relative low temperature.

If solid, the velocity factor is about 0.66, when foam, it will be
about 0.79..0.84
If it melts at relative low temperature it is very likely Polyethyleen
(PE) insulation, if not, it is PTFE (Teflon).

Based on your transmission frequency and the velocity factor, cut an
electrically quarter wave. Terminate the cable with the 50 Ohms dummy
load and connect the cable to the VSWR meter. Measure the VSWR of the
cable/dummy load combination.

When VSWR is almost one, you can be sure it is 50 Ohms cable. When it
is about 2.2, it will be 75 Ohms cable. When it has VSWR close to 3.5,
it is probably 93 Ohms cable. This is based on: Zload*Zinput = Zcalbe^2
for a quarter wave line.

When these measurements matches the mechanical identification of Tom's
posting, you can be very sure about the impedance of the cable.

Best Regards,


Wim
PA3DJS