A parameter of coaxial cable is velocity factor (v.f.). This value relates the physical length to the electrical length and is a scalar quantity. By sheer physics the number should be less than 1.0. Why would you want to calculate velocity factor you ask? When you need to install antenna’s in a pre-determined phase relationship, you need to space them a known electrical amount apart. The physical distance is one thing, but they must be the proper electrical distance apart too. This is one reason you might want to calculate velocity factor.
If you have an antenna analyzer that can measure SWR like the MFJ-259 or, in my case, the Autek RF-1, this procedure and included calculator (below) will make short work of it.
- Measure the length of the cable you want to calculate velocity factor for in feet.
- Connect the open-circuit (not shorted, unterminated) cable to your analyzer.
- Starting at the LOWest frequency possible, raise the frequency while watching SWR. When the SWR reaches a minimum this represents 1/4 wavelength of cable. Note the frequency (MHz)
- Plug the numbers into the calculator below and press the calculate button.
The formula to calculate velocity factor is:
Velocity Factor = Physical Length / Electrical (free space) Length
The electrical length is 246 / f (SWR minimum, open cable; MHz)
Putting it all together we get:
Velocity Factor = Physical length * f / 246
One more thing to think about, what are the limits?
Knowing the formula we can figure out what we can reasonably expect the device can be expected to do. In other words, how short or how long of a piece of coax can it accommodate when used to calculate velocity factor? All we need to do is a little algebraic gymnastics and re-arrange the equation thus:
Physical length = VF * 246 / f
Instead of calculating velocity factor using my Autek Research RF1 analyst, let’s determine the maximum and minimum lengths we should be able to determine with it. We find the following from the data sheet supplied with the Autek Research RF1:
- Minimum frequency is ~1.2 MHz
- Maximum frequency is ~34 Mhz
If we assume a velocity factor of 0.6 in the long, or low-frequency, case we get about 120 ft if the velocity factor is a “perfect” 1.0 then we could theoretically measure just 200 ft. Doing the same analysis at the high frequency, the shortest length we could measure is somewhere between 4.5 and 7 ft.
What are typical values of velocity factor you can expect to find? I’ve never seen less than 0.60 or more than 0.90, but that doesn’t mean there isn’t one like that. Common values are typically between 0.66 and 0.85.
VELOCITY FACTOR (VF) OF COMMON COAXIAL CABLES
Please let me know what you think by commenting below.