Sunday, April 19, 2020

Using nanoVNA to determine true Velocity Factor at operating frequency

A recent clean-up of the shack revealed several lengths of RG-58U coaxial cable, each with a connector on only one end. I realized that they would make ideal candidates for using my nVNA's ability to determine their velocity factor.

No other info is printed on the cable jacket except "RG-58/U" and a 12 digit number. Looking up "RG-58/U" reveals several possible velocity factors and I could probably Google the long number and get more info, but where's the fun in that? After all, I have a $65 VNA.

NanoVNASaver has a TDR function but using it to determine the length of a cable requires the velocity factor to be known. Rearranging this function, I can use the measured length of the cable to determine its VF. In other words, how much does the physical length of the cable differ from the electrical length? This difference (expressed in percent) is the velocity factor.

Tape measure in hand, I found the physical length of the cable to be 19 feet, 9-3/4 inches - or 6.039
Actual length: 19' 9.75" or 6.039 meters

The next job was to attach the cable to the nVNA and run a sweep of roughly 10 to 25 MHz.

Why that range? 6.039 meters represents a 1/4 wavelength (in free space) for a frequency of 12.525 MHz; I chose a range encompassing that frequency since the VNA will show pure resistance at that point, ie no reactance, or a phase of 0 degrees.

Once I saw the zero-crossing, I ran a narrower sweep of 15-18 MHz for greater resolution of where the zero R actually occurred (screenshots below).

The sweep revealed a crossing of the zero reactance point occurs at 16.8475 MHz. This will repeat every 1/4 wavelength. Dividing 12.525 MHz by 16.8475 MHz reveals the VF - 74%. VF varies slightly with frequency; this cable is probably its rated VF of .73 at a much higher frequency.

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