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Multiband HF verticals
Trap, tuned and autotuned vertical designs.
Trap, tuned and autotuned vertical designs.
One vertical that works on several HF bands. A full-size quarter-wave is different on every band (32.7 ft on 40 m, 16.5 ft on 20 m), so a multiband vertical uses a trick to look the right length on each: traps, switched or tuned sections, a motorized coil, or an automatic tuner at the base. Each trick has a different price.
A trap is a parallel coil and capacitor tuned to a band. At its resonant frequency it blocks RF, so everything above it is cut off and the lower section is a quarter-wave on that band. Below that frequency the trap acts like a coil: the RF passes and the trap loads the antenna, so the whole vertical is electrically a quarter-wave on the lower band while being physically shorter. Add more traps for more bands.
It is a simple fixed antenna with no tuning. The trade-offs are a little loss in each trap, and a narrower bandwidth on the lowest bands, where the antenna is shortened the most.
| Approach | How it works | Trade-off |
|---|---|---|
| Traps | Parallel-resonant traps isolate sections | Fixed bands, trap loss, narrow low-band bandwidth |
| Parallel or switched elements | A separate resonant wire or relay-selected section for each band | More wire or complexity, but each band can be full-size |
| Motorized coil | A motor moves a tap on a coil to retune on command | One band at a time; the short loaded radiator is narrow and lossier |
| Base tuner on a long radiator | A remote tuner matches whatever frequency you choose | Works on any band, but the radiator's efficiency is still set by its length and ground |
A tuner at the base, or a motorized coil, makes the antenna match the radio; it does not make a short radiator more efficient. See Antenna tuners.
Whatever the trick, a vertical works against its ground: radials or a counterpoise. A different set of radials for each band, or a large uniform radial field, is the usual answer; see Radials and ground planes. A shortened vertical on 80 m or 160 m with a poor ground can waste most of the power, because its radiation resistance is only a few ohms; see Loaded and short antennas.