A popular way of multibanding antennas is to install parallel tuned circuits, or traps. There are two different techniques used. One technique is to resonate each trap in the middle of the ham bands, while the other is to resonate the traps between ham bands. The latter, while less intuitive, can offer more bands with fewer traps, if properly designed.
- Coaxial cable Antenna Traps
QST May 1981, pp. 15-17
Both the coil and capacitor of a parallel-resonant antenna trap can be made from the same length of coaxial cable. Sound intriguing? Here’s how.
- Build a Space-Efficient dipole Antenna for 40, 80, and 160 Meters
QST July 1992, pp. 35-36. Construction details in the September 1992 QST p. 88.
A new trap design, using only RG-58 and PVC pipe, yields better space efficiency than conventional coaxial traps.
- Two New Multiband Trap Dipoles
QST August 1994, pp. 26-29
W8NX details a new coax trap design used in two multiband antennas; one covering 80, 40, 20, 15, and 10 meters, and the other covering 80, 40, 17, and 12 meters.
- An Improved Multiband Trap Dipole Antenna
QST July 1996, pp. 32-34
You need this - traps with lower loss, higher Q, increased power-handling capability and four-band coverage. Also build a multiband dipole for 80-, 40-, 17-, and 10-meters only 84 feet long.
- Taming the Trap Dipole
QST March 2002, pp. 28-30
A trapped dipole for 10/15/17 meters