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October 4th 03, 08:19 AM

(Richard) wrote in message . com...
Thanks for all the comments. I did it open and shorted and averaged
the results (very similar). The cable measured a VF of 0.63 and Z of
120 Ohms - was described as 75 Ohms. The difference caused a big
mismatch and an hour of head scratching and calculator button
pressing.

A comment on the Zo measurement: analyzers are commonly most accurate
for impedances near 50 ohms, and the errors for very low and very high
impedances may be fairly large. Better to do an iteration on the
measurement: put a 120 ohm resistor on the end of the line and see if
the impedance presented to the analyzer is independent of frequency,
or nearly so... or if you have a known 1/4 wavelength, see if at that
frequency you get the same result measuring the 120 ohm resistor and
the line when terminated in that resistor. If you get two different
results and the 120 measured as 120, then take the square root of the
product of the two and try again. When you find a load that gives the
same result directly and through 1/4 wave of line, you know that load
is the same as Zo (at that frequency).

Cheers,
Tom