An effort to develop large-aperture, wide-angle-scanning reflectarray antennas for microwave radar and communication systems is underway. In an antenna of this type as envisioned, scanning of the radiated or incident microwave beam would be effected through mechanical rotation of the passive (reflective) patch antenna elements, using microelectro-mechanical systems (MEMS) stepping rotary actuators typified by piezoelectric micromotors. It is anticipated that the cost, mass, and complexity of such an antenna would be less than, and the reliability greater than, those of an electronically scanned phased-array antenna of comparable beam-scanning capability and angular resolution.

Passive Patch Antenna Elements in an array would be mounted on shafts of MEMS stepping rotary actuators that, in turn, would be mounted on a common substrate. The patch elements would be circularly polarized, and would be phase-sensitive in the sense that each would alter the phase difference between incident and reflected radiation by an amount that would depend on the actuator shaft angle.
In the design and operation of a reflectarray, one seeks to position and orient an array of passive patch elements in a geometric pattern such that, through constructive interference of the reflections from them, they collectively act as an efficient single reflector of radio waves within a desired frequency band. Typically, the patches lie in a common plane and radiation is incident upon them from a feed horn. Certain phase-sensitive types of such elements can be clocked to predetermined angles, relative to those of their neighbors, to modify the phase of the radiation incident from the feed horn and reflected from the elements so as to, for example, make the a flat array of patches act as though it were a parabolic reflector.

Another reflectarray characteristic, essential to the present development, is that if the patch elements are rotated in unison, then the beam radiated by the antenna can be steered in elevation and azimuth through angular displacements of as much as ±50°. In an antenna of the type under development, the patch elements would be phase-sensitive in the sense mentioned above, would be circularly polarized, and would be mounted on the shafts of MEMS stepping rotary actuators (see figure). The maximum range of element rotation needed for wide-angle beam scanning would be only about ±180°, and scanning could be effected by use of relatively coarse rotational steps.

This work was done by Houfei Fang, John Huang, and Mark W. Thomson of Caltech for NASA’s Jet Propulsion Laboratory.


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Wide-Angle-Scanning Reflectarray Antennas Actuated by MEMS

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This article first appeared in the March, 2009 issue of NASA Tech Briefs Magazine.

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