A The traps in a trap dipole are parallel-resonant tuned circuits that “trap” an RF signal to prevent it from passing beyond a specific point on a conductor (a wire, a metal tube, etc). At some other frequencies, however, the traps no longer act like “traps” and instead allow the RF to pass.
An antenna trap is designed for a particular operating frequency, and there may be several traps in the overall system, each designed for a specific frequency. Therefore, a 40- through 15-meter trap antenna, like the trap dipole shown in Figure 3, might contain traps for 20 and 15 meters. When you’re operating on 15 meters, the 15-meter traps effectively “shorten” the antenna by blocking the RF from traveling beyond them. If you switch to 20 meters, the 15-meter traps suddenly become transparent to the 20-meter RF, effectively “lengthening” the antenna. The 20-meter traps, however, are a kind of impedance roadblock to RF, keeping the signal from traveling farther. On 40 meters, all of the traps are “absorbed” into the system to become part of the overall 40-meter dipole antenna.
Because of the loading effect of the traps, the 40-meter portion of the antenna will be somewhat shorter than a full-sized 40-meter dipole without traps. The effective bandwidth on each band will be narrower than that of a standard dipole, too.
A trap-style antenna is not quite as efficient as a full-size dipole, but if the traps are well designed, the losses are not significant. Most hams consider the losses a fair trade-off for the convenience of having an antenna that presents a 50-Ω match to coax on several bands.
Figure 3—A typical 3-band trap dipole antenna. The traps block RF at a specific frequency, or allow it to pass. From an electrical standpoint, this effectively lengthens or shortens the antenna.
From QST June 1999
