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).
NPO-20161
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Lightweight dual-beam reflector antenna
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Overview
The document presents a technical support package detailing the design and performance evaluation of a lightweight dual-beam reflector antenna developed for the SeaWinds spaceborne scatterometer instrument. This antenna operates in the Ku band and is specifically designed to measure wind speed and direction over the Earth's ocean surface.
The design features a composite material construction, which contributes to its lightweight nature, making it suitable for space applications. The antenna is characterized by an elliptical aperture and is calibrated for high performance in measuring wind velocities. The document outlines the mechanical and electromagnetic performance requirements that the antenna must meet. Notably, the antenna must maintain a vibrational-resonance frequency of at least 94 Hz to resist vibrations effectively.
The antenna generates two linearly polarized beams at angles of 40° and 46° from the nadir, with specific requirements for beam widths and sidelobe levels. The inner beam, which is horizontally polarized, has widths of 1.6° and 1.8° in the azimuth and elevation planes, respectively. The outer beam, vertically polarized, has widths of 1.4° and 1.7° in the same planes. The first sidelobe of each beam is required to be at least 15 dB below the peak of the beam, ensuring high-quality signal reception.
To achieve the necessary structural integrity and performance, the design incorporates larger-than-nominal dimensions for the struts and feed support plate. This approach, while enhancing the fundamental resonant frequency, can degrade the antenna's RF performance. To mitigate this, the design includes absorbing material coated on the feed support plate and the addition of chokes around the feed horn. These modifications aim to reduce RF interaction with the support plate and improve the antenna's dual-beam RF performance.
The document emphasizes the potential for the antenna's design to be applicable in terrestrial environments, addressing common challenges in mechanical and electromagnetic performance. Overall, the lightweight dual-beam reflector antenna represents a significant advancement in technology for remote sensing applications, particularly in meteorology and oceanography. The research and development efforts detailed in this document reflect a collaborative effort between the Jet Propulsion Laboratory and the University of California, Los Angeles, showcasing innovative engineering solutions for spaceborne instruments.
