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 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, a reflective array antenna is an antenna in which multiple driven elements are mounted in front of a surface designed to reflect the radio waves in a desired direction.

Reflective array antennas usually have many driven elements working in conjunction with a flat, electrically large reflecting surface to produce a unidirectional beam, increasing antenna gain and reducing radiation in unwanted directions. These are also called "billboard antennas". The reflector may be a metal sheet or more commonly a wire screen. They may contain parasitic elements as well as driven elements.

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.

Actually Reflect array antennas came into full existence from Year 1980 as a cross breed from the parabolic reflectors and array antenna theory. Utilizing Micro strips ,dipoles as preliminary radiating elements without power division transmission lines fed through Horn antenna reflect array antennas gained lot of popularity in terms of the Low weight and surface tolerances in compare to parabolic reflectors.

The feed element radiates the plane which is normal and the plane re radiates back the incident power onto the reflect array elements with electrical phases required to form the planar wave front. The re-radiating plane can be any polarizer with dielectric substrate filled or slotted wave guide.

The Polarizer helps to achieve either of one polarization in the sense if the feed is illuminated with the Horizontal polarization you can see Vertical polarization in the free space and vice versa , In order to achieve circular polarization the reflect array elements needed to have special properties such as two transmission line stubs with proper phase delay [2].

References

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