coaxtest

Testing Coax Cable Using an SWR meter

Is my coax cable any good? If your like most hams, you've got lengths of coax that you've had for years. Now you have a new antenna but that long forgotten coax "looks" old. Can I still use it? Here is how to test it with an SWR meter. (bridge)

Hook up your HF rig and the SWR meter at the transmitter end. At the far end of the coax, short it out. This short must be "low inductance", so you need to use good sized wire. You can do this either using a mating coax connector with the center to shield shorted with say #14 or #16 wire, VERY short, or if you don't mind additional work, remove the existing coax connector, strip back the center and braid and expose say about an inch of the center. Wrap the shield over the center and either twist tightly and clamp, or solder. Either way, you need to short it out "good".

Use the lowest frequency available to your rig, 1.8 MHz if possible. The lowest frequency makes any stray "inductance" you might have in your "shorted end" less of a problem. If you don't have an HF rig, ask a ham who does have one to do this for you. Do not test the cable at VHF or UHF. Readings may be in error there. Apply just enough power to calibrate the "forward" direction. Switch to "Reflected" power and read the SWR. Compare to the chart below. The WORSE the SWR, the BETTER the cable is.

An SWR of 3:1 for example, with 3 db LOSS, means you will loose HALF your power just in the coax, and loss increases with frequency. As you increase frequency, the loss increases. I'd suggest if you see an SWR of GREATER than say 10:1, the coax is OK.

SWRLossSWRLossSWRLoss

40:10.225:1 1.761.8:15.44

30:10.294.5:1 1.961.6:16.37

20:10.434:1 2.221.5:16.99

15:10.583.5:1 2.551.4:17.78

12:10.733.2:1 2.811.3:18.85

10:10.873:1 3.011.2:110.41

9:10.972.8:1 3.251:1 13.22

8:11.092.6:1 3.52

7:11.252.4:1 3.85

6:11.462.2:1 4.26

5.5:11.602:1 4.77

Chart information courtesy of "Hints & Kinks", QST January 2003