Turnstile antenna

A turnstile antenna, or crossed-dipole antenna, is a radio antenna consisting of a set of two identical dipole antennas mounted at right angles to each other and fed in phase quadrature; the two currents applied to the dipoles are 90° out of phase. The name reflects the notion the antenna looks like a turnstile when mounted horizontally. The antenna can be used in two possible modes. In normal mode the antenna radiates horizontally polarized radio waves perpendicular to its axis. In axial mode the antenna radiates circularly polarized radiation along its axis.

Specialized normal mode turnstile antennas called superturnstile or batwing antennas are used as television broadcasting antennas. Axial mode turnstiles are widely used for satellite ground station antennas in the VHF and UHF bands, as circular polarization is often used for satellite communication since it is not sensitive to the orientation of the satellite antenna in space.

The turnstile antenna was invented by George Brown in 1935 and described in scholarship in 1936. The patent history reveals the popularity of the turnstile antenna over the years.

The antenna can be used in two different modes: normal mode and axial mode.

In directions perpendicular to its axis the antenna radiates linearly polarized radio waves (horizontally polarized when the antenna's axis is vertical). This is called normal mode. The radiation pattern, a superposition of the two dipole patterns, is close to omnidirectional but actually "cloverleaf shaped", with four small maxima off the ends of the elements. The pattern departs from omnidirectional by only ±5 percent. The radiation in these horizontal directions is often increased by vertically stacking multiple turnstile antennas (called "bays") fed in phase. This increases the gain by strengthening the radiation in the desired horizontal directions but causes partial cancellation of the radiation in vertical directions, reducing power wasted radiated into the sky or down toward the earth. These stacked normal mode turnstile antennas are used at VHF and UHF frequencies for FM and television broadcasting.

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