Reflective array antenna

Reflective array 'billboard' antenna of the AN-270 radar, an early radar system used in 1941 at Pearl Harbor. It consists of 32 horizontal half wave dipoles mounted in front of a 55 ft. high screen reflector. With an operating frequency of 106 MHz and a wavelength of 3 meters this large antenna was required to generate a sufficiently narrow beamwidth to locate enemy aircraft.

In telecommunication and radar, a reflective array antenna is a class of directive antennas in which multiple driven elements are mounted in front of a flat surface designed to reflect the radio waves in a desired direction. They are often used in the VHF frequency band, and these versions often resemble a highway billboard, so they are sometimes called "billboard antennas". The curtain array is a larger version used by shortwave radio stations.

Reflective array antennas usually have a number of identical driven elements, fed in phase, in front of a flat, electrically large reflecting surface to produce a unidirectional beam, increasing antenna gain and reducing radiation in unwanted directions. The reflector may be a metal sheet or more commonly a wire screen. A metal screen reflects radio waves as well as a solid metal sheet as long as the holes in the screen are smaller than about one-tenth of a wavelength, so screens are often used to reduce weight and wind loads on the antenna. The individual elements are most commonly half wave dipoles, although they sometimes contain parasitic elements as well as driven elements.

Since the 1980s, versions for use at microwave frequencies have been made with patch antenna elements mounted in front of a metal surface.

References

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).

A modern form of reflective array is the "bow tie" UHF television antenna. This example has two crossed-dipole driven elements in front of a grill reflector oriented for vertical polarization.

Enormous reflective array antenna of the Duga-3 or "Steel Yard" over-the-horizon (OTH) radar system, Chernobyl, Ukraine, part of the Soviet early-warning network. It transmits at frequencies between 7 and 19 MHz. The pairs of cylindrical cages at right are the half wave dipole driven elements, fed at their center by vertical feed lines. Behind them is a reflector screen of horizontal wires, just visible in center.