The NASA Glenn Research Center, in collaboration with the University of Colorado, Boulder, has developed a 952-element Space-Fed Lens Array (SFLA) for ground stations to use to communicate with low-Earth-orbiting satellites. The SFLA is an alternative to a phased-array antenna that replaces large numbers of expensive solid-state phase shifters with a single spatial feed network. The SFLA has lower cost in comparison to a phased array at the expense of total volume and complete beam continuity. These tradeoff parameters are not important for ground station applications and can be exploited to lower costs.
SFLA and feed-switching connections for a 16-beam feed network.
The basic SFLA scanning architecture is shown in the preceding diagram for the receiving mode only. The SFLA consists of three antenna arrays: the receiving-side array, the feed-side array, and the array of feed elements. The pairs of antenna elements on the two former arrays are connected through appropriate true-time delay lines. The front-end array (receiving-side) determines the beamwidth as in standard antenna array theory. The multiple beams are obtained with multiple spatial feeds. When a plane wave is incident from some direction (azimuth and elevation), the receive array antennas sample the wave front at approximately the Nyquist criterion, depending on the antenna element radiation pattern. Each sample is then appropriately time-delayed and reradiated by a second array of antennas, focusing the power onto the focal surface. The field is now sampled on the focal surface so that each feed antenna element preferentially receives incoming waves from a single direction. Thus, an SFLA can be viewed as a hardware discrete Fourier transform of an arbitrary linear combination of plane waves onto a focal surface. The SFLA can be implemented using standard printed-circuit-board technology and can incorporate power amplifiers for the transmit mode or low-noise amplifiers (LNAs) for the receive mode. The polarization can be chosen to be different on the two sides of the array for isolation purposes and for design simplicity. The LNAs also can be placed after the feed antennas, since the spatial feed does not contribute resistive loss.
The 952-element SFLA. Left: Receiving-side array. Right: Feed-side array, array feed elements, and switching network.
The SFLA is a low-cost approach using multiple relatively small SFLAs to replace a 10- to 11-m reflector and to achieve beam scanning nonmechanically. The photographs show the 952-element SFLA, which was designed to operate at 8.386 GHz with circular polarization on the receiving side and linear polarization on the feed side. The elements in the array are patch antennas, and there are two dielectric substrates with metallized vias1 interconnecting the corresponding antenna elements of the two arrays. The feed array, which is located along a focal arc, has 32 feed elements, with each feed receiving a beam from a different direction. In this architecture, scanning is accomplished by switching between independent beams, thus eliminating a need for microwave phase shifters. High-speed switches are not needed because the pass of a satellite allows many seconds per beam.
The SFLA was characterized using Glenn’s planar Near-Field Antenna Facility. The graph shows the radiation patterns along the scan plane for different beam positions. The scan loss was found to have a dependence with the scan angle θ of approximately cos θ. For the prototype demonstration, the beam was scanned only in one plane (elevation), and scanning in the other plane (azimuth) was accomplished mechanically. In general, scanning in the elevation plane is sufficient to track a satellite if it does not drift too far away from its orbital plane; however, a two-dimensional scan can be accomplished with a two-dimensional feed array over the focal plane. Our results demonstrate the feasibility of the SFLA for the aforementioned application.
Measured radiation patterns along the scan plane for various beam positions.
Glenn contacts:
Dr. Richard Q. Lee, 216-433-3489, Richard.Q.Lee@nasa.gov
Dr. Félix A. Miranda, 216-433-6589, Felix.A.Miranda@nasa.gov
University of Colorado contacts:
Zoya Popovic, 303-492-0374, zoya.popovic@colorado.edu
Sébastien Rondineau, 303-492-8719, sebastien.rondineau@colorado.edu
Authors:
Dr. Richard Q. Lee, Zoya Popovic, and Sébastien Rondineau
Headquarters program office:
Earth-Sun System Technology Office
Programs/projects:
Ground Network
Last updated: December 14, 2007
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