| This is a method to find out the transmission line Velocity Factor
of an unknown coaxial cable, twin lead etc. You need a VNA or other impedance
analyzer. 1. Connect the cable of interest in the DUT port. The cable length should be more than full wave lengths compared to the highest frequency of your VNA. 2. Leave the cable end open (do not terminate the end). 3: Run reflection measurement and select impedance plot (|Z| Ohms). Adjust the scan frequency upper limit so that you can clearly see the second impedance peak. 
Here is an example velocity factor measurement of an unknown coaxial
cable with OD of 5 mm.
a) Check the second impedance peak frequency = full wave frequency. In this example the full wave frequency is 30.73 MHz. b) Calculate the full wave length 300 / f (MHz) (In this example 300 / 30.73 = 9.76 m) c) Measure the physical cable length (In this example 7.48 m) d) Calculate the velocity factor = physical cable length / full wave length (In this example velocity factor is 7.48 / 9.76 = 0.77) You can use this method to cut a transmission line cable to a desired electrical length at a specific frequency eg 1/4 wave (first dip), 1/2 wave (first peak) etc. Update 2022-03-03 OH7SV Back to OH7SV projects |