A shortwave broadband antenna is a radio antenna that can be used for transmission (and reception) of any shortwave radio band from among the greater part of the shortwave radio spectrum, without requiring any band-by-band adjustment of the antenna. Generally speaking, there is no difficulty in building an adequate receiving antenna; the challenge is designing an antenna which can be used for transmission without an adjustable impedance matching network.
An ideal “broadband” shortwave antenna would work continuously across much of, if not all of, the shortwave spectrum with good radiation efficiency and minimal compromise of the radiation pattern. Most practical broadband antennas compromise on one of the above: Either they only work on a few relatively narrow slices of the HF radio spectrum, or they work across the complete spectrum, without gaps, but are inefficient radiators on some or all of the frequencies. Other antennas provide adequate efficiency on some frequencies, but require a separate antenna tuner to function on others. A few designs remain omnidirectional on all frequencies but most “beam” antennas lose their directionality.
At the lower shortwave frequencies e.g. 1.8 MHz, the antennas need to be physically large to enable good coupling to "space" and hence efficient radiation. As an example, at 5 MHz a half wave dipole antenna is around 27 meters long (90 ft.), at 3.5 MHz nearly 41 meters (133 ft.), and 2 MHz it is 71 meters long (234 ft.). Half-wave horizontal dipoles are efficient radiators, if they are about half their length above ground or higher; if low to the ground relative to wavelength then horizontal dipoles suffer from large loss of signal in the earth and are inefficient radiators, despite their adequate length.[a] Half wave dipoles are narrow band (only work over a very small frequency range) before serious impedance mismatch occurs. This mismatch can be accommodated using an antenna tuner but these add to costs and modern shortwave communication often uses frequency hopping techniques. Even automatic antenna tuners will not work with frequency hopping signals.
A less ambitious idea of “broadband antenna” (often called “wideband”) is an antenna that continuously covers the proportionally widest amateur band, that spans 3.5–4.0 MHz (a 14% bandwidth),[b] without requiring an antenna tuner. There are many such designs, but those are not discussed here.[c]
Broadband shortwave base antennas traditionally fall into two main categories:
The challenge for many years has been to devise an antenna which is an efficient radiator, compact, and also inexpensive. Previous solutions include the Barker Williamson folded dipole, the Australian traveling-wave antenna and other designs by Guertler etc.
Some shortwave broadband antennas can even be used on the whole vaguely defined "shortwave" radio spectrum (usually 1.6–30 MHz) which consist of the upper part of the medium frequencies (MF band upper section = 1.6–3 MHz) and the whole of the high frequencies (HF band = 3–30 MHz).
There are two separate issues related to getting as many frequencies as possible out of an antenna:
Clearly, the optimum imaginable antenna would have both good traits, but achieving even one is a challenge, and achieving both is rarely possible with a single antenna, and essentially impossible with anything but an infeasibly large, expensive antenna. Further, different authors use the words "broadband",[2] "multiband",[3] and "wideband"[4] to mean one definition or the other, but the use of the words is inconsistent. Although "multiband" fairly unambiguously refers to meaning 2, both "broadband" and "wideband" are used erratically to refer to either of the two meanings. The reader needs to be aware of the two different issues, with two different words chosen by each author to designate them; either that or an author's description of an individual antenna design may use any of these three words, or others, but only address one of the issues.