Question
How to measure impedance with non 50 Ohm cables?
Impedance measurement of non 50 Ohm cable
Answer
To measure the cable impedance, it is necessary to connect the cable to the port of a network analyzer. This is very easy with unsymmetrical 50 Ohm coax cables. It is not easy in the case of 18 Ohm cables, for example. In the case of a symmetric cable, an additional network is required to match the unsymmetrical analyzer to the symmetric cable.
A measurement can be performed as follows:
RESET
Set the analyzer to the desired start and stop frequency.
Calibrate the analyzer to 50 Ohm; full one-port is fine.
Terminate the cable with its impedance.
Try to connect the cable as well as possible to the analyzer.
Measure S11 in magnitude or in Smith chart. A direct marker readout is possible.
It is very important to have a clean cable termination. The termination must have exactly the cable's impedance. The deviation from the impedance can be measured by means of the marker function.
In the case of a symmetric cable, the network can be handled by embedding/deembedding. For this, you need to know the S-parameters of the network. A time-gated measurement can remove the reflection generated by the connectors. Embedding/deembedding and time-gated measurement are available as options.