Hi, I have 3 reference coax cables, and I'm trying to find an accurate way of measuring their electrical length. I understand a VNA is the best way to do this.

VP is 0.66 and the 3 lengths are 40m, 85m, 350m. I only have access to one end of the cable (other end is open)

The reason for wanting to know the exact electrical length of these cables is so that I can use them to verify the accuracy of a TDR under test.

I have an Agilent 8510C available to use for this test.

Would somebody be able to advise me on how to do this.

If you have time-domain, it is really easy to get the length. The 8510C, while a very nice analyzer, is quite old so not as convenient as any of the VNAs produced during this century. If your 8510C has option 10, you can just turn on the time domain response and look at the time delay, then convert to frequency. Dist=Rate*Time. Rate = 3e8 m/s * vel factor * time. But for 8510C the reflection is 2 way so you need to divide by 2. If you have the S21 delay, you can just use that. If you had a newer analyzer the marker can be configured to read out length directly.

If you have a reflection at the input, it will cause your response to be skewed, and you will want to do a compensation for that. For details, you can look at page 292 in second edition of my book that describes exactly how to do it.