Q Ray, WA3CLD, asks, “I have an old Swan TB-3HA
tribander beam antenna. How can I check and recondition
the traps? It has 4 driven-element traps, 2 director traps
and no reflector traps. Can I adapt one of the driven element
pairs of traps for the reflector?”
A The biggest problem with old traps is corrosion, both ex ternal and internal. To “recondition” them, you’ll have to take them apart and thoroughly clean the inside. You can check a trap for internal corrosion without taking it apart by putting an ohmmeter across it—the resistance should be a relatively low value. If it is over 100 Ω, you probably have a corrosion problem.
Traps are designed to present a high impedance at the
“trapped” frequency and to act as a loading coil at lower frequencies, so you should indeed be able to use driven element traps for the reflector by adjusting the tubing lengths slightly.
If you have an antenna analyzer or a dip meter, you can check the resonance of just the reflector (it should be about 5% lower than the driven) by assembling it as a unit. You may need to get it at least 10 feet up in the air, however, because the effect of the nearby ground detunes a low antenna, changing the resonant point from where it will be when the antenna is installed later.
From QST February 2001
