Dipolantenner

Dipolantenner, or dipole antennas, are among the simplest and most widely used radio antennas. They consist of two straight conductors of equal length connected to a feedpoint at the center. The antenna is resonant when the total length is close to half the wavelength of the operating frequency, producing a standing wave and efficient radiation.

For a center-fed half-wave dipole in free space the input impedance is about 73 ohms, and the radiation pattern is strongest broadside to the element, with linear polarization along the conductor axis. The three-dimensional pattern resembles a doughnut around the wire, with reduced radiation along the axis. Bandwidth is relatively narrow around resonance, though it can be widened by using thicker elements, traps, or a folded configuration.

Common variants include the folded dipole, which has a higher input impedance (roughly 300 ohms) and is convenient for feeding with unbalanced lines, and multi-band designs that use traps or multiple wires. Dipoles can be mounted horizontally, vertically, or at other orientations and are typically fed with coax via a balun or with a balanced feedline to avoid currents on the outer conductor.

Length formulas: total length L ≈ 150 / f(MHz) meters, or 468 / f(MHz) feet, where f is the operating frequency. Applications span amateur radio, short-wave listening, and experimental communications; advantages are simplicity, low cost, and predictable performance; limitations include size at low frequencies and sensitivity to surroundings.