Time domain reflectometer and velocity factor

Thread Starter

asap3210

Joined Jan 14, 2021
2
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 Velocity Factor? (I have no way to find out the real VF, all I can read is "Noisless" and a logo)

Thank yhou
 

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dl324

Joined Mar 30, 2015
12,871
Welcome to AAC!
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 5.77 meters. Is this an error in the Velocity Factor? (I have no way to find out the real VF, all I can read is "Noisless" and a logo)
The velocity factor varies by cable type. If you don't have the number for the cable you're testing, then you can only estimate the distance.

A ham radio site tabulated velocity factors for some RG cables here.
 

Papabravo

Joined Feb 24, 2006
16,118
The velocity factor depends on the conductor separation and the dielectric properties of the insulating material. Without more information, I might suggest a bit of research on different lengths of the cable to see if the results are at least consistent. Age is also a factor.
 

MrChips

Joined Oct 2, 2009
23,519
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.
What is the number written on the coax cable?

5.55m = 11.1m round trip
assuming VF = 0.66
delay = 56ns

Repeat your experiment and double check the numbers.
 

Thread Starter

asap3210

Joined Jan 14, 2021
2
What is the number written on the coax cable?

5.55m = 11.1m round trip
assuming VF = 0.66
delay = 56ns

Repeat your experiment and double check the numbers.
Hi, thanks for your reply. There is no number in the coax. Yes, your numbers are what I expected.

I repeated the experiment with a different cable an this time worked fine. I guess the first cable had some issue.

Regards,
 

MrChips

Joined Oct 2, 2009
23,519
Just to verify, I ran a test with 16.15m of RG-62U.
Documented VF = 0.83

Based on this information
v = c x 0.83 = 0.248 m/ns
= 4.03ns/m

16.15 x 2 x 4.03 = 130.2ns

Measurement on the oscilloscope = 130ns which is within 1% accuracy
Even my measurement of the cable length could not have been within 1% accurate. Maybe I got lucky.
 
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