Measuring ¼ Wave Lines – by Dan, N5AR


Hams use quarter wave lines for many purposes, impedance matching, stub filtering, etc. There are several methods to cut these lines to the proper length. A method was described to me by K5GW which is simple and seems to be more accurate than others I have tried. It uses equipment which is present in nearly all ham shacks. If you have a dummy load, a T connector, an SWR meter, and your radio you can do it.

1. Connect the T connector to your dummy load.
2. Connect your SWR meter to the T connector and to your transceiver through short lengths of coax.
3. Key the rig up and note the SWR, it should be 1 or very close to 1.
4. Connect the ¼ wave line to the other side of the T connector.
5. Short the far end of the coax. I have been able to short it with a needle pushed thru the coax line.
6. Sweep the frequency until you see a SWR of 1.
7. Change the shorting point until the SWR=1 point is at the desired frequency.
8. Cut and short the coax at that point.

Of course this method will work for any odd multiple of quarter wave line. It should also work to measure half wave lines with open rather than shorted ends but I have not tried that mode.

73, Dan, N5AR