Velocity Factors

[ptonsparky]

Anyone know where I can find a list of the velocity factors for urd, romex, etc? Another new to me tool.

 

[charlie b]

I have idea what a "velocity factor" could possibly mean. Can you explain the question?

 

[winnie]

Velocity factor is the speed that a signal propagates down a cable, expressed as a fraction of the speed of light. Normally we don't worry about this for electrical cables because we are not trying to send signals down them...

I am guessing from the 'another new to me tool' that Tom is playing with some sort of 'Time Domain Reflectometer' for detecting splices and breaks. In this case, you do send a signal down the cable, and then measure the time it takes the signal to bounce back from any discontinuity. If you know the velocity factor, you can get a distance to the discontinuity.

Unfortunately, knowing a handwaving bit of theory, I can't provide an answer for the OP

-Jon

 

[drbond24]

I've never seen a chart, but I have calculated the speed of propagation for several different types of cables, although never romex, and always get an answer around 65% the speed of light.

 

[ptonsparky]

New to me Time Domain Reflectometer. We can calculate the VF for each cable we work with but it will just take some time.

Tried it this morning with a known length of 12/2g UF cable. 11' 5". TDR calculated length of 11.4'. Close enough. We then wacked the cable with a hammer and suddenly we have a distance to the damaged insulation. Sweet. Good out to 3000 meters. Should show multiple damage spots as well.

Between this and the Fluke 43B, I am in hog heaven.:grin:

 

[rattus]

My memory tells me,

1 nanosec/foot in a vacuum

1.5 nanosec/foot in coax

Easy enough to measure though.

 

[cschmid]

wow another new toy..with ideal bench conditions it should be fine, on the surface it looks real cool, but in real life it needs some testing in the field..I will need a link so I can check this out..I believe I you should be able to get something like that from the manufacture of the wire..But I would start a journal now on it and keep accurate records so you can add in all the variables like temp, age, copper deterioration form age over heating ect..I am all ears on this and yeas it does look funny being all ears..

 

[drbond24]

My memory tells me,

1 nanosec/foot in a vacuum

1.5 nanosec/foot in coax

Easy enough to measure though.

Your memory tells you that electricity can move faster than light? :grin: :wink:

The speed of light in a vacuum is rougly 1 ft/ns. Actually, 0.983571087 ft/ns to be unnecessarily precise. Therefore my educated guess is about 0.64 ft/ns for speed of propagation in a cable. It isn't going to be faster than 1 ft/ns. Can't beat the speed of light.

 

[kc8dxx]

The velocity factor is the speed at which an RF signal travels through a material compared to the speed the same signal travels through a vacuum. The velocity of propagation is inversely proportional to the dielectric constant.

I have only seen velocity factors specified for coax cables and other wiring designed for high speed/high frequency signals. Like in the MHz and GHz. This information is available from the cable OEM. Typical values range from 0.6 (poor quality 50-ohm coax) to 0.8 (high quality coax) to 0.95 (twin lead or ladder line).

 

[rattus]

Pay attention to the units:

Pay attention to the units:

Your memory tells you that electricity can move faster than light? :grin: :wink:

The speed of light in a vacuum is rougly 1 ft/ns. Actually, 0.983571087 ft/ns to be unnecessarily precise. Therefore my educated guess is about 0.64 ft/ns for speed of propagation in a cable. It isn't going to be faster than 1 ft/ns. Can't beat the speed of light.

These units, commonly used in TDR, are time/distance rather than distance/time. In other words, we are expressing the delay per unit length rather than the velocity. So my memory is about right!

 

[drbond24]

These units, commonly used in TDR, are time/distance rather than distance/time. In other words, we are expressing the delay per unit length rather than the velocity. So my memory is about right!

I noticed you were using time/distance, but I thought you had just typed it wrong. :grin: You got me.

 

[drbond24]

I have only seen velocity factors specified for coax cables and other wiring designed for high speed/high frequency signals. Like in the MHz and GHz. This information is available from the cable OEM. Typical values range from 0.6 (poor quality 50-ohm coax) to 0.8 (high quality coax) to 0.95 (twin lead or ladder line).

http://en.wikipedia.org/wiki/Velocity_of_propagation

http://en.wikipedia.org/wiki/Velocity_of_propagation

Wiki references a table in the Radio Amateur's Handbook. You're right on the money.

As far as the OP, looks like if you wanted to track down this handbook you might get somewhere.

 

[dtiller]

In case you don't have the ARRL Handbook lying around, here's a link to a reasonably good description of the physics of velocity factor, along with a small table of typical insulation materials and their contribution to VF.

This site has a nice table of coax attenuation and power handling characteristics.

 

[rattus]

Units, units, units!

Units, units, units!

I noticed you were using time/distance, but I thought you had just typed it wrong. :grin: You got me.

Age wins out again!

I made a mistake once. Thought I was wrong!

 

[drbond24]

Age wins out again!

I made a mistake once. Thought I was wrong!

You can have your age. I just turned 28 and I like it that way. I'd rather be young than right, at least on this particular topic.