A multiband dipole antenna, also called a doublet or half-wave dipole, is a radio antenna of electrically conducting rod or wire that is one-half a wavelength long. Split at the center with an insulator, the wire connects to a transmission line that has a radiation pattern with a maximum at right angles to the antenna. Dipole means “two poles," and is typically the simplest type of antenna, such as the familiar “rabbit ears” on a television. Multiband refers to radio communications across frequencies reserved for uses such as air band, public services, marine band, and citizens band, television, and commercial radio transmissions.
Multiband dipole antennas are frequently used in commercial and official radio technology. Shortwave radio enthusiasts often construct their own vertical or hanging antennas. These antennas allow users to listen to airwaves across the globe with accurate reception and stereo capability, and they cover a wide range of frequencies. Public agencies such as police or fire departments operate on their own dedicated frequencies, but in integrated operations where communication between agencies is necessary, multiband technology affords better coordination between departments.
Dipole antennas are typically movable, to adjust for changing local conditions. The familiar set-top television antenna can be adjusted in length and angle. Larger dipole antennas made from wire are hung in a V-shape, while shorter ones are hung vertically. A folded dipole antenna is folded back nearly to the feedpoint, and a shortwave dipole antenna can be slung over tree branches for portability. Scanners and radios that use multiband dipole antennas sometimes have external jacks to add booster antennas; they might use digital tuning and liquid crystal displays (LCDs) to indicate the frequency of the selected station or broadcast.
Besides accessing official transmissions, multiband technology also allows users to pick up entertainment broadcasts as well as weather scanning, emergency first-responder transmissions, and citizens band radio. A typical multiband radio might be capable of scanning dozens of channels per second, and 100 or more total channels. These antennas may appear on analog or digital equipment, for home, office, travel, hand-cranked emergency equipment, and units with global positioning technology. Military radio operators may carry several multiband dipole antennas for accessing different frequencies; these may be cut to the length required by the communications band, such as those frequencies used by medical evacuation, net control stations, or tactical stations. The effectiveness of the multiband receiver depends on the performance of the antenna.
