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 at www.nasatech.com/tsp  under the Electronics & Computers category.

NPO-21157

Topics:
Aircraft Antennas Electronic equipment Electronics & Computers Hazardous materials Hazards and emergency operations Radiation Unmanned aerial vehicles Vehicles
This Brief includes a Technical Support Package (TSP).
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Low-Sidelobe Microwave Reflector Antennas and Dichroic Plate

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

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NASA Tech Briefs Magazine

This article first appeared in the December, 2001 issue of NASA Tech Briefs Magazine (Vol. 25 No. 12).

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Overview

The document presents a technical overview of a low-sidelobe microwave reflector antenna system developed for the High Altitude MMIC Sounding Radiometer (HAMSR), designed to operate on remotely piloted aircraft. The primary objective of the HAMSR project is to create a miniaturized passive microwave radiometer that utilizes Monolithic Microwave Integrated Circuit (MMIC) receiver modules and solid-state components to perform atmospheric soundings.

The antenna system is engineered to receive signals at three specific frequency bands: 55 GHz, 118 GHz, and 183 GHz. The design incorporates a scanning flat reflector, a dichroic plate, and three offset-paraboloidal reflector antennas, each tailored for one of the designated frequencies. The dichroic plate, also referred to as a frequency-selective surface (FSS), is crucial for directing input radiation to the appropriate antennas. It is designed with a hexagonal array of subwavelength-diameter holes that selectively transmits 183-GHz radiation while reflecting 118-GHz radiation at an incidence angle of 22.5°.

The document details the analytical methods used in the design process, including full-wave mode-matching techniques for the feed horns and physical-optics theory for the paraboloidal reflectors. The directional radiation patterns of the reflectors were computed based on the near-field patterns of the feed horns. The analysis of the dichroic plate was conducted using the method of moments in conjunction with Floquet harmonics, with calculations performed using specialized computer programs developed at NASA’s Jet Propulsion Laboratory (JPL).

The system's design aims to achieve low sidelobes, which is essential for minimizing interference and enhancing the accuracy of atmospheric measurements. The antennas are oriented downward towards the Earth, allowing for effective temperature and humidity soundings at specific oxygen and water vapor emission lines.

The work was conducted by Vahraz Jamnejad and Abraham Riley from Caltech for NASA’s JPL, and the findings are documented in a report referenced as NPO-21157. This innovative antenna system represents a significant advancement in remote sensing technology, contributing to improved atmospheric data collection and analysis capabilities. The document emphasizes the collaborative efforts between NASA and Caltech in developing cutting-edge technologies for atmospheric research.