If you need to find the lenght of a coax cable then the MFJ-259 will help you find that. You will need to know the velocity factor of the coax cable being measured. Often times this is a know measurement from the manufactor.
Follow the steps below.
1. Short one end of the coax.
2. Connect the other end of the coax to the MFJ-259 input.
3. Turn on the MFJ-259 and start at the lowest frequency range. Watch the RESISTANCE METER for a dip reading (zero ohms) and record the frequency.
4. Continue moving up in frequency to find a second dip, which should be twice the first (recorded) dip frequency.
5. Calculate the length: Divide 492 by the first dip frequency (in MHz) and multiply the result by the velocity factor of the coax cable being measured. The result should be in feet.