time domain reflectometry

Hello, first post here. I've built the TDR with the 74ac14 as shown at wae2w yt video () It seems to work fine, but when it comes to test it with a coax 5.55 meters long, I read 72 nS, which according to the average velocity factor of 0.659 it results in 6.24 meters. Is this an error in the...

Hai Friends, I would like to design a soil moisture sensor using TDR technology since TDR based moisture sensors are more accurate than capacitive or resistance-based moisture sensors. But I don't have any idea about it. Can anyone help me in finding a solution for it?