The figure illustrates a lightweight composite-material dual-beam reflector antenna for the SeaWinds spaceborne scatterometer instrument, which operates in the Ku band and is designed to measure wind velocities at the surface of the ocean. The design of the antenna could also be of interest for terrestrial applications in that it addresses issues that are not unique to any particular application and that involve some overlap and competition between mechanical and electromagnetic-performance requirements.
The mechanical requirement is that the antenna be stiff enough to resist vibrations to an acceptable degree, as quantified by a vibrational-resonance frequency of at least 94 Hz. The basic electromagnetic-performance requirement is to generate two linearly polarized, independent beams at angles of 40° and 46° from the nadir when the reflector axis is aimed at 43° from the nadir. The inner beam (the one at 40°) must be horizontally polarized and have widths of 1.6° and 1.8° in the azimuth and elevation planes, respectively. The outer beam (the one at 46°) must be vertically polarized and have widths of 1.4° and 1.7° in the azimuth and elevation planes, respectively. The first sidelobe of each beam is required to be at least 15 dB below the peak of the beam.
The reflector surface is a paraboloid with an elliptical aperture. Two offset feeds (one for each beam) are mounted on a feed-support plate held by struts. To satisfy the stiffness requirement, the struts had to be made wider and the feed-support plate larger than in an ordinary design for an antenna of this type. It was found that, in the absence of corrective measures, (1) the increase in aperture blockage caused by enlargement of the feed-support plate and (2) interactions between the feed horns and the enlarged feed-support plate would result in sidelobe levels higher than allowed under the performance requirements, lower antenna gain, and altered beam widths.
By a combination of theory and experiment, it was found that suitable corrective measures would include coating the feed-support plate with an absorbing material, extending the inner-beam feed horn, and adding a choke around the outer-beam feed horn. Tests showed that these measures reduced sidelobe to the required levels and afforded some improvements in gain and beam widths.
This work was done by Ziad Hussein, Yahya Rahmat-Samii, and Kent Kellogg of Caltech for NASA's Jet Propulsion Laboratory. For further information,access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Electronic Components and Circuits category, or circle no. 113 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
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Lightweight dual-beam reflector antenna
(reference NPO20161) is currently available for download from the TSP library.
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