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5.2.6. Measuring the characteristic impedance
The
characteristic impedance
is one of the main parameters of any coaxial cable.
Usually, its value is printed on the cable by the manufacturer. However, in certain
cases the exact value of the characteristic impedance is unknown or is in question.
To measure the characteristic impedance of a cable,
1.
Connect a non-inductive resistor to the end of the cable. The exact value of this
resistor is not important. However, it is recommended to use 50 to 100 Ohm
resistors.
Example 1: RG-58 cable with 51 Ohm resistor at the far end.
Example 2: Unknown cable with 51 Ohm resistor at the far end.
2.
Enter the
R,X graph
mode and make measurement in the full frequency range.
Example 1: RG-58 cable
Example 2: Unknown cable
3.
Changing the display range and performing additional scans, find a frequency
where R (solid line) reaches its maximum, and another frequency with
minimum. At these points, X (dotted line) will cross the zero line.
Example 1: 975 kHz – max., 1.838 MHz – min.
Example 2: 1.988 MHz – max., 4.425 MHz – min.
