Measuring Ladder Line With An Antenna Analyzer

Testing a ladder line for an electrical half wave length will yield meaningless results with a MFJ 259B Antenna Analyzer.  What works with 50 ohm coax will not work with any other impedance coax or ladder line (unless it’s 50 ohm ladder line).  The analyzer is a bridge circuit where the other three legs of the bridge are 50 ohms.  The fourth component must also be the electrical equivalent of a 50 ohm resistor for a meaningful readout. But not to fear.  It can still be accomplished with just a little work.  The steps to find an electrical half wave length for a ladder line are identical to that of a 50 ohm coax.

Using the same technique as would be used with a 50 ohm coax first find the electrical quarter wave frequency .  With the far end open and the near end attached to the analyzer tune for a dip at the lowest frequency. Going through several dips to get to the lowest one is normal.  Both resistance and impedance should dip as close to zero as possible. If impedance is zero but resistance is high keep tuning until both are zero. Note the frequency.

Now find the next frequency up where both resistance and impedance dip to zero.  This frequency should be triple the first frequency.  This will be another odd multiple of a quarter wave and it should be the three quarter wave frequency.  Make a mental note that the difference of these two frequencies is the electrical half wave frequency. For example the first frequency was one quarter wave length and the second frequency was three quarter wavelengths. The difference would be two quarter wave lengths or one half wavelength.  How do we get to meaningful dimensions?  We need the velocity factor.

The MFJ 259B has a built in advanced mode function called distance to fault.  The distance to fault will be the electrical distance to the end of the cable. It will be longer than the cable itself because the insulation slows down the electrons (by an amount known as the velocity factor!) To find the distance to fault hold down both the mode button and the gate button at the same time until advanced mode appears on the screen.  Press the mode button repeatedly until the distance to fault mode appears.

Repeat the steps above but this time enter the data into the analyzer.  Go back to the lower dip frequency and press gate.  This will become the 1st data.   Next rotate the frequency knob to the next highest dip and press the gate button again.  This is now the 2nd data.  Press the gate button a third time and the distance to fault will appears.

We need to know the physical length of the cable so measure the total length with a ruler.  Velocity factor is the quotient of the distances measured electrically by the analyzer and the actual measured length. Divide the physical length by the electrical length and the quotient is the velocity factor.  An example would be .88.


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