The figure depicts the basic geometric features of an antenna system designed to be part of a miniature atmosphere-sounding passive microwave radiometer to be flown at high altitude aboard a remotely piloted aircraft. The system includes (1) a scanning flat reflector; (2) a dichroic plate; and (3) three low-sidelobe, offset-paraboloidal reflector antennas for reception in three frequency bands that include, and are denoted by, nominal frequencies of 55, 118, and 183 GHz, respectively.

The Basic Geometry of the Antenna System is shown here in simplified form for the sake of clarity.
In operation, the antennas are aimed downward toward the Earth. The 55-GHz reflector receives input radiation directly, while input radiation is directed to the 118- and 183-GHz antennas via the scanning flat reflector and the dichroic plate. The dichroic plate [also known as a frequency-selective surface (FSS)] is a plate that contains a hexagonal array of subwavelength-diameter holes with dimensions chosen precisely so that at an angle of incidence of 22.5° the plate transmits most of 183-GHz radiation while reflecting most of the 118-GHz radiation. The paraboloidal main reflector in each antenna concentrates the incident radiation into a corrugated feed horn, followed by a circular-to-rectangular transition, followed by a rectangular waveguide that couples the radiation into a receiver operating in the designated frequency band.

The feed horns were analyzed and designed by use of a full-wave mode-matching technique. The paraboloidal reflectors were analyzed and designed by use of physical-optics theory. The directional radiation patterns of the reflectors were computed by use of the near-field patterns of the feed horns. The analysis and design of the dichroic plate for the instant case of oblique incidence was based on the method of moments in conjunction with Floquet harmonics. The calculations for analysis and design were performed by use of computer programs developed at NASA's Jet Propulsion Laboratory and elsewhere for the analysis of reflector systems, corrugated horns, waveguide junctions, and frequency-selective surfaces.

This work was done by Vahraz Jamnejad and Abraham Riley of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line atwww.nasatech.com/tsp  under the Electronics & Computers category.

NPO-21157


This Brief includes a Technical Support Package (TSP).
Low-Sidelobe Microwave Reflector Antennas and Dichroic Plate

(reference NPO-21157) is currently available for download from the TSP library.

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