Q Jay Pryor, K4OGG, asks, “I just put up an inverted-L antenna for 160 meters. It is fed with 150 feet of RG-213 coaxial cable. When I measure the SWR at the antenna feed point, I get an SWR of 1.7:1. When I measure the SWR at the rig end of the feed line, the SWR is about 3:1. I’ve used two different pieces of RG-213 with the same results. I have also replaced the PL-259 connectors at both ends. I can use an antenna tuner at the rig, which makes the antenna usable, but is there something else I could do to get the same SWR at the rig that I have at the feed point?”
A What you are running into is covered in some detail starting on page 26-14 of the 18th Edition of The ARRL Antenna Book. The part under the heading “SWR Change with Common-Mode Current” is particularly relevant to your own case. What is happening is that your inverted-L is radiating common-mode current onto the shield of the coax going to your shack. Such a current can only flow on the shield (since the coax’s center conductor is shielded), and hence the fields inside the coax are not equal and opposite in phase as they should be in a transmission line. As a result, the fields inside the coax are distorted by the presence of the common-mode current. The amount of this stealthy, undesirable common-mode current varies depending on the placement of the coax under the antenna and the length of the coax. The impedances seen along the coax will vary. When the impedance varies, your SWR meter will show different readings at different places along the coax. In extreme cases, common-mode currents can cause the SWR protection circuitry in a transmitter to shut down everything, protecting against what looks like excessive SWR.
The way to cure this problem is to employ common-mode chokes, often called choke baluns to “choke off” the current. See The ARRL Antenna Book or The ARRL Handbook for details on these devices. Your first step would be to put one right at the feed point of the inverted-L. That will probably knock the problem down somewhat, but you may have to place other chokes at 1/4-wave increments down the coax to really tame the beast. Another good trick is to bury the coax, especially if it runs right under the top L portion of the antenna. In fact, coax runs that are elevated off ground (for convenience or to keep them away from mowers, small children and animals) is particularly prone to this “varying SWR” problem.
From QST September 1999
