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Fan and trap dipoles
Covering several bands with one antenna.
Covering several bands with one antenna.
One wire antenna, one feed line, more than one band. There are three ways to get there: lean on harmonics, put a separate pair of wires on the antenna for each band (a fan), or add traps. Each trades something away.
A half-wave dipole is also resonant near 3, 5, 7 times its frequency, because the wire is then 3, 5, 7 half-waves long and the center is still a current peak. The feed-point impedance stays low, so coax can feed it. At 2 or 4 times the frequency the center lands on a current null: impedance is very high and coax feeding is poor.
A wire cut for 7.1 MHz is therefore three half-waves near 21.3 MHz, inside the 15 m band, but it does not work well in the 20 m band at twice the frequency. Real wires resonate a little off the exact multiple, and on the higher band the pattern breaks into several lobes instead of one broadside figure-8.
A fan is just several dipoles sharing a feed point. On each band the matching pair is resonant and the others carry little current. There are no lossy parts, but the wires interact slightly, so trimming is repeated and slow, and the spread ends need supports.
A trap is a parallel inductor and capacitor tuned to the higher band. At that frequency it presents a very high impedance and electrically cuts off the outer wire. On the lower band the trap acts like a coil, which loads the wire so the whole antenna can be shorter than full size. The costs are some loss in the traps, usually less bandwidth on the lower band, and weight and weatherproofing.
| Approach | Strength | Cost |
|---|---|---|
| Odd harmonics | no extra parts | skips even-multiple bands; multi-lobed pattern |
| Fan | simple, no trap loss | several supports, interacting wires |
| Traps | compact, one wire | loss, narrower bandwidth, weather and weight |
Any antenna that works well on several bands can also radiate the harmonics of your transmitter on them, so good filtering at the radio still matters (see Harmonics and spurious emissions). Where the match is poor on a band, a tuner can fix what the radio sees, but not the antenna's losses. Other multiband wires: the Off-center-fed dipoles, End-fed half-wave antennas and doublets.