How to measure the impedance of a transmission line?
Updated: Feb 26, 2020
If you have a transmission line (coax or open line) with an unknown impedance, I will explain here how to measure this.
Above: far end of the transmission line.
Left: beginning of the transmission line.
Photo of the total installation.
Method: put an adjustable resistor on the far end of the transmission line.
Calibrate the measuring cable (open, short and loaded) and connect the transmission line to be tested.
Use a low frequency range (stop frequency = 30 MHz) to prevent non-resistive behavior of the adjustable resistor.
Adjust the resistance until you have a constant reflection level. In this case, you have terminated the transmission line with the same value as the impedance. It looks like the transmission line never stops because there ar no impedance jumps (resistor value = impedance value).
When you have a constant return loss (SWR) the value of the resistor is the same as the impedance value of the transmission line under test. To be sure, just measure the resistor value with an ordinary multi meter.