Dipole Antennas

The dipole is one of the most fundamental antenna types and serves as a reference against which others are compared. In its common half-wave form it consists of two straight conductors in line, fed at the centre, with a total length close to half the wavelength of the operating frequency. At that length the antenna is resonant, presenting a convenient impedance to the feed line and radiating efficiently. Because wavelength depends on frequency, a dipole is cut to a specific band, and its physical size grows as the frequency falls.

A half-wave dipole has a well-defined radiation pattern. It radiates most strongly broadside, in the directions perpendicular to the wire, and very little off the ends, producing a doughnut-shaped pattern around the conductor. This modest directivity gives it a small gain over an isotropic radiator. The dipole is linearly polarised in the direction of the wire, so a horizontal dipole produces horizontally polarised waves and a vertical one produces vertical polarisation, a factor that affects how well it communicates with other antennas.

The folded dipole is a widely used variant in which a second parallel conductor connects the two ends, forming a narrow loop. This raises the antenna's feed impedance to about four times that of a simple dipole, which conveniently matches common balanced feed lines, and it also broadens the usable bandwidth. Folded dipoles are familiar as the driven element of Yagi antennas and in television receiving antennas. The dipole's simplicity and predictable behaviour make it the starting point for many more elaborate designs.

Frequently asked questions

How long is a half-wave dipole?

Its total length is close to half the wavelength at the operating frequency, slightly shorter in practice because of end effects. Lower frequencies require physically longer dipoles.

In which directions does a dipole radiate best?

A half-wave dipole radiates most strongly broadside, perpendicular to the wire, and least off its ends, giving a doughnut-shaped pattern around the conductor.

Why use a folded dipole?

The folded dipole raises the feed impedance to about four times that of a plain dipole, matching common feed lines, and it provides a wider bandwidth. It is often the driven element in Yagi and TV antennas.