If you need to find the velocity factor of a coax cable manually then the MFJ-259 will help you find that. You will need to have a piece of coax of a know length/measurement
Follow the steps below.
1. Disconnect both ends of the coax and measure the physical length of the line in feet.
2. Find the lowest frequency across all the bands at which the lowest SWR occurs.
3. Read the frequency from the frequency counter display. This is the 1/4 resonant wavelength frequency of your coax. Note that you will get low SWR reading at all odd multiples of 1/4 wavelength.
Example: On a 27 foot line the measured frequency was 7.3MHz.
4. Divide 246 by the measured frequency. This gives you the free space 1/4 wavelength in feet.
Example: 246 divided by a dip frequency of 7.3 MHz is 33.7 feet, the free space 1/4 wavelength
5. Divide the physical measured length of the coax in feet by the free space 1/4 wavelength calculated in number 4.
Example: 27 feet (physical length) divided by 33.7 feet (calculated length) equals .80 . The velocity factor is .80 or 80%.
Free space 1/4 wavelength = 246 / Low SWR frequency
Velocity Factor = Actual feedline length / Free space 1/4 wavelength