Trap Dipoles and Parallel Dipoles
For multiband applications, you’ll often find the trap dipole and the parallel dipole. Traps are tuned circuits that act somewhat like automatically switched inductors or capacitors, adding or subtracting from the length of the antenna according to the frequency of your signal. The parallel dipole uses a different approach. In the parallel design, several dipoles are joined together in the center and fed with the same cable. The dipole that radiates the RF is the one that presents an impedance that most closely matches the cable (50 ohms). That matching impedance will change according to the frequency of the signal. One dipole will offer a 50-ohm match on, say, 40 meters, while another provides the best match on 20 meters.
Obviously, these designs are somewhat more complicated than single-band dipoles, although many hams do choose to build their own. If you don’t have time or desire to tackle a trap or parallel dipole, you’ll discover that many QST advertisers sell prebuilt models.
Click on the gallery images below for diagrams.