I am in complete agreement. That was the point of my post. Unless the velocity factor is a lot more consistent than I would expect, that's all you'll be able to do with *any* TDR. And for practical RF tasks, all you need.
For measuring cable length to 100 m , 256 measurements from 1-256 MHz should suffice for 2 cm resolution if the phase angle is accurate to 10 degrees at 256 MHz.
As I have the great pleasure of a Tek 11801 and SD-24 along with a new coil of high quality coax, I'll do some tests once I get connectors. That setup has better than 5 ps TDR resolution. I'm going to make up a set of jumper cables. When I do, I'll be testing them and posting them to a thread on the testing RF connectors and cables I started on the EEVblog forum. I'll make a point of cutting each length under uniform tension so they are the same physical length. I've also got an HP 8753B. So I'll be solving the problem in both domains with 1st rate gear. Albeit rather old.
The nanoVNA has derailed a bunch of my projects because it's a $50 device and can do so much that was out of reach of post hams because of cost in the past.
Have Fun!
Reg
