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Doublets and the G5RV
Open-wire-fed multiband antennas.
Open-wire-fed multiband antennas.
A doublet is a center-fed wire dipole, fed with balanced open-wire or ladder line instead of coax, and matched with a tuner. The G5RV is one specific doublet design, named after its designer's call sign: a 102 ft flat top, about 34 ft of open-wire line, then a balun and coax. Both are multiband wires built on the same idea.
When an antenna is not matched to the line, the line carries standing waves, and its own loss is applied to power that bounces back and forth. Open-wire line has very low loss at HF, so it can carry a high SWR with little penalty. The tuner then matches that mismatch to the radio. That is what lets one wire cover many bands.
Open-wire line is also balanced, like a dipole, so it keeps RF off the line, provided it is fed from a balanced tuner or through a balun. See Baluns and ununs. At higher bands the flat top is many half-waves long, so the pattern divides into several lobes; the antenna works on those bands, but not as a clean broadside figure-8.
The G5RV is built around 20 m. There the flat top is about 1.5 wavelengths long and the line section is roughly a half wavelength. A half-wave section repeats the impedance at its far end, so on 20 m the coax sees roughly what the wire presents. On other bands the section transforms the wire's impedance to something else, so most operators use a tuner.
The honest trade-offs:
A doublet has no fixed rules: choose a flat top at least a half wavelength on the lowest band (longer gives more freedom), run ladder line to the shack, and tune. A center-fed 1.25 wavelength dipole is the extended double Zepp, with a little more gain than a half-wave dipole. A half-wave fed at its end is a Zepp; see End-fed half-wave antennas.