Q Gene, WA2FLN, asks, “For the last 20 years I’ve been feeding my antenna with a length of Belden RG-8U coax. If I place a dummy load at the antenna I still measure a 1:1 SWR in the shack, but I wonder whether I should replace the old feed line anyway. What do you think?”
A The conductors inside the cable have a tendency to oxidize over time and the dielectric may begin to absorb some moisture. All of this leads to increased loss, which will make your SWR look better than it actually is. I would consider 15-20 years to be the useful limit for coax, but it will vary quite a bit depending upon the environment and the materials used to make the coax.
If you really want to evaluate the condition of your coax, the best thing to do is to measure the RF power at the input of the cable, and at the dummy load on the other end. Subtract the input power from the power measured at the dummy load and you’ll know how much RF you’re losing in the cable. Compare the total loss to the specifications for your cable. This will tell you how much your 20- year-old coax has departed from its original pristine condition. For RG8U, the power loss in a 50-foot section at 14 MHz should be roughly 0.4 dB, which is about 9 W out of 100 W.
From QST June 2000
