Communications: radio wave antennas – Antennas – Wave guide type
Reexamination Certificate
2001-07-31
2003-08-05
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
Wave guide type
C343S7810CA
Reexamination Certificate
active
06603437
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not Applicable.
FIELD OF THE INVENTION
The present invention relates generally to antennas and, more particularly, to reflector antennas.
BACKGROUND OF THE INVENTION
Conventional reflector antenna designs require a tradeoff between high efficiency and low sidelobes. In general, the aperture illumination is tapered to minimize near in sidelobes. A −15 dB edge taper from the peak is a typical aperture distribution to minimize sidelobes adjacent to the main beam. However, tapering the aperture distribution reduces the illumination efficiency of the antenna aperture. For example, a −15 dB taper can reduce the aperture efficiency by about 25 percent and result in a 1.2 dB loss in antenna gain.
FIG. 1
shows a prior art reflector antenna
10
having a main reflector
12
that reflects energy from the feed
14
. Far-out sidelobes due to so-called spillover energy
16
, which exits the feed
14
but does not reach the reflector
12
, and so-called edge diffraction
18
, must also be taken into account.
One prior art attempt shapes a subreflector to redistribute a high taper feed pattern to almost uniform distribution on the main reflector aperture. However, with such a main reflector distribution, the near-in sidelobes are typically too high to meet standard commercial sidelobe requirements, e.g., 29-25 log
10
(&thgr;) dBi, where &thgr; is the angle from antenna boresight.
Another attempt to provide low sidelobes and high efficiency includes synthesizing dual-shaped reflectors to produce aperture power distribution defined by 1-(1-taper)(r/a)**2, where taper is the amplitude taper, r is the radial variable, and a is the main reflector radius. This arrangement does not provide low near-in sidelobes without relatively low illumination efficiency.
It would, therefore, be desirable to provide a reflector antenna system that provides relatively low near-in and far-out sidelobes and high aperture efficiency.
SUMMARY OF THE INVENTION
The present invention provides an antenna system having a main reflector and a subreflector having geometries that optimize antenna efficiency. While the invention is primarily shown and described in conjunction with a truncated Gaussian distribution over a circular aperture, it is understood that the invention is applicable to other antenna shapes and configurations.
In one aspect of the invention, a method for synthesizing a dual reflector antenna includes selecting certain parameters for the antenna such as reflector size, feed location, sub reflector midpoint location, main reflector midpoint location, and synthesis interval. The method further includes mapping energy from a known feed pattern to a selected analytical aperture distribution. From the initial locations of the feed and reflector midpoints, the shapes of the main and sub reflectors are synthesized using wavefront parameters to determine surface normals for each surface point. The resultant reflector shapes are adjusted as necessary to correct an computational errors.
The actual aperture field distribution is modified from the initial truncated Gaussian field distribution, for example, for the final synthesis of the shaped reflectors to allow the near in sidelobes to bump against a predetermined sidelobe requirement for optimizing overall antenna efficiency.
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The Microwave Journal; Jul. 1965; pp. 182-184.
Per-Simon Kildal; “Synthesis of Multireflector Antennas by Kinematic and Dynamic Ray Tracing”; IEEE Transactions on Antennas and Propagation; vol. 38, No. 10; Oct. 1990; pp. 1587-1599.
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Kildal, Per-Simon, Synthesis of Offset Dual-Reflector Antennas by Solving Linear Equations, Antennas and Propagation Society International Symposium 1988, XP 10073278, Syracuse, NY, USA, Jun. 06-10, 1988, vol. 1, No., pp. 136-139.
Daly, Crowley & Mofford, LLP.
Dinh Trinh Vo
Raytheon Company
Wong Don
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