I needed some 75ohm cable to make impedance transformer harness for my VHF/UHF cross yagi setup. Found a 20m roll of RG179 which was procured from China long ago. I thought it was wise to test the cable (many times similar looking product came from Aliexpress which often doesn’t match given specification.).
Impedance:
To find the impedance of a coaxial cable, put a 50ohm non-inductive resistor (dummy load) at the end of the cable, an ordinary 0.25 watt carbon resistor will do. Connect the other end of the cable to the first port (S1) of the nanoVNA. Make a sweep and check the smith chart. note the resistive part (R) of the point where it first time crosses the center line. Lets say it is Z_crossing.
(Smith Chart Screenshot)
Z_load = 50 ohm.
Z_cable = squrt (Z_load * Z_crossing)
As an example lets say we get the value 120ohm at the crossing point, then
Z_cable = Squrt (50*120)
Z_cable = 77.4 Ohm
So we can see that its a 75 ohm coaxial cable with some tolerance.
Velocity Factor:
To find the VF we need to know the exact length of the cable. Then we can do a TDR measurement. NanoVNA Saver application has TDR option.
Once TDR is determined, select common VF to match the length shown with the exact length of the cable.
I found the RG179 I have has VF or 0.69
