Signal Lab

Why Your Tuner Cannot Fix a Jumping SWR

An operator wrote us this week with a symptom we hear every season: a multiband trapped vertical, a good automatic tuner, a clean match -- and then the SWR wandering all over the place from 17 meters on down. He assumed the tuner had failed. It had not. The tuner was doing its job perfectly on a target that would not hold still.

The Three Usual Suspects

Weathered traps and matching hardware. A trapped vertical stacks resonators and a matching box at the base. After years outdoors, the connections loosen and corrode; when SWR wanders in wind, that is often the elements moving and the joints making and breaking contact. Re-torque every joint, open the matching box, look for water.

Common-mode current on the feedline. The most common cause, and the least understood. A vertical with a small counterpoise loves to recruit the coax shield as the missing half of the antenna. When it succeeds, the feedline becomes part of the radiator, and SWR changes with how the coax lies, with moisture on it, with everything. Roy Lewallen, W7EL, wrote the definitive piece on why a current balun -- a common-mode choke -- is the cure; Jim Brown, K9YC, published the measured data on which ferrite chokes actually achieve it. Both are worth reading no matter whose antenna flies in your yard.

Water in a trap. If the jumping tracks rain or heavy dew, you have your answer, and a trap to open.

Why the Low Bands Suffer First

The symptom concentrates on the lower bands for a reason the masters wrote down long ago. John Kraus, W8JK, showed that radiation resistance falls with the square of an antenna's length in wavelengths -- so on 17 meters and below, a trapped vertical is electrically shortest exactly where it is being asked to work hardest, and every trap in the current path adds loss on top. The governing equation is unforgiving:

And Walter Maxwell, W2DU, made the point every tuner owner eventually learns the hard way: the tuner makes the transmitter happy at the shack end, but SWR is not efficiency. A 1:1 match tells you nothing about what fraction of your power is leaving the antenna. A tuner cannot stabilize an antenna system that is physically unstable, and it cannot recover power that a trap has already turned into heat.

Fix It Free First

We told the operator what we will tell you: try the choke and the re-torque before you spend a dime with anyone. A proper current choke at the feedpoint and tight, dry joints revive more trapped verticals than any new purchase. If that brings yours back, good -- a fellow operator helped, and that is the hobby working.

What We Build Instead, and Why

That physics is why our verticals carry no traps at all. An off-center-fed vertical dipole -- no traps, no radials, no matching box to corrode -- stands on the published work of Robert J. Zavrel Jr., W7SX (ARRL). Both halves of the dipole are up in the air where they belong, so there is no counterpoise for the feedline to substitute for, and a current choke at the feedpoint finishes the job. The field evidence is what convinces people: Bill K3WA ran the 160 meter portion of CW Sweepstakes from an HOA lot on our 24-footer -- 61 contacts, 23 sections -- on the band where trapped verticals give up entirely. One of our owners put Franz Josef Land in his log on 15 meters this month, and another in Naples, Florida took a direct hit from Hurricane Milton and reported, in his words, a mere baby wind.

Ham Radio is fun again! Pass it on...

73,
Jon KL2A
Greyline Performance -- Sun Valley, Idaho
435-200-4902