Measuring the phase length of a cable using a VNA

It's actually quite easy to use a VNA to measure the electrical length of a coax cable. You will use a  S11 phase measurement. The electrically longer cable will have a larger phase. The longer cable will have a phase that is CCW relative to a shorter cable when viewed on a polar plot.

Follow these steps:

1. Turn on the VNA and allow temperature to stabilise
2. Calibrate the VNA at the desired frequency you want to use the cables for i.e. 432.10Mhz
3. Chose the S11 measurement
4. Select S11 phase measurements
5. I use a marker at 432.10Mhz that displays the S11 phase (click on the picture on this page)
6. Measure the cable and record the result
7. Repeat for the other cables

Tips 

Use a scrap cable and measure the S11 phase and then cut the cable by say 3mm (1/8 inch) and then measure the change in phase to get an understanding of how the change in length will be displayed on the VNA.

Use S11 phase vs the TDR mode of a VNA for more stable results. The TDR maximum is wide and not as well defined compared to measuring the S11 phase directly.

A long cable i.e. greater than one wavelength (360 degrees) will have the same phase every 360 degrees. This is the modulo 360 phase wrap. You can see this by doing a very wide sweep of the cable i.e.1 to 500Mhz. Have a look at it. To avoid this I just have a span of 10Mhz at a centre frequency of 432.10Mhz