Communications: radio wave antennas – Antennas – Wave guide type
Reexamination Certificate
1998-09-30
2001-04-03
Wimer, Michael C. (Department: 2821)
Communications: radio wave antennas
Antennas
Wave guide type
C343S837000
Reexamination Certificate
active
06211834
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to communication systems, and is particularly directed to a new and improved multiband ring focus antenna architecture comprised of a common or shared pseudo parabolically shaped main reflector, and a plurality of diversely configured subreflector-feed pairs, that are interchangeable with each other to provide a reduced sidelobe envelope at a plurality of separate operational frequency bands.
BACKGROUND OF THE INVENTION
Satellite communication systems have customarily employed multi-reflector antenna architectures, often of center-fed Cassegrain configuration, in order to optimize the collection of electromagnetic energy within a prescribed frequency band transmitted over relatively long distances (e.g., earth station-satellite-earth station). Where the number and size of antenna components is not necessarily a major concern, such as a fixed, land-based facility that has ample room for the placement of one or more relatively large structures, it is common practice to employ a relatively large main reflector, and an associated subreflector that is on the order of several tens of wavelengths in diameter. Because of the substantial blockage associated with such a subreflector, the diameter of the main reflector may be in excess of five meters in diameter at C and/or X band. While such a large dimensioned subreflector—main reflector structure is capable of successfully performing its intended functionality for a given operational frequency band, if the earth station is to provide communication capability at separate bands, additional subreflector—main reflector pairs configured for operation at those bands must be installed.
In contrast, many communication systems, such as shipboard-mounted facilities, have only a limited amount of space for the installation of antenna components. In such spatially constrained environments, where antenna size is limited and its directivity pattern must typically comply with a very strict specification, it is not practical to install even one, much less multiple spatially large reflector structures. One proposal to deal with this space constraint problem is to employ a ring focus antenna, having a parabolic main reflector and a ‘shaped’ (i.e., ellipsoid) subreflector.
Advantageously, the conical properties of the ellipsoid-shaped subreflector provide a dual focus characteristic, with one of its foci displaced toward the vicinity of the aperture of the main reflector where a feed horn is installed. The other focus is symmetric about the antenna axis in the form of a ring, which enables the antenna to obtain a substantially uniform amplitude distribution in the aperture plane. As a consequence of this geometry characteristic, the antenna can is more compact than a conventional center-fed structure.
For non-limiting examples of documentation detailing the configuration and operation of a conventional ring focus antenna, attention may be directed to the following publications: “Amplitude Aperture-Distribution Control in Displaced-Axis Two-Reflector Antennas,” by A. Popov et al, Antenna Designer's Notebook, IEEE Antennas and Propagation Magazine, Vol. 39, No. 6, Dec. 1997, pp. 58-63; “The Theoretical Analysis of Shaped Dual-Reflector Antenna with Ring Focus,” by T. Wang et al, Conference Proceedings, 20th European Microwave Conference 90, pp 1553-1558; “Shaped Dual-Reflector Antenna with Ring Focus,” by R. Zhang et al, Science in China (Series A) Vol. 34, No. 10, Oct. 1991, pp 1243-1255; “Two-Reflector Antenna,” by Y. Erukhimovich et al, Radio Research Institute, Ministry of Posts and Telecommunications, USSR, pp. 205-207; and the Canadian Patent to Schwarz, No. 1,191,944, entitled “Improved Shifted Focus Cassegrain Antenna With Low Gain Feed,” and assigned to the assignee of the present application.
Now although a ring focus antenna, such as those described in the above literature, is intended to provide reduced subreflector blockage and thereby the overall size of the antenna structure to be smaller than a conventional Cassegrain architecture, its ellipsoid-shaped subreflector is still on the order of several tens of wavelengths in diameter, and is spaced apart from the antenna feed (horn) by similar electrical distance.
To minimize subreflector blockage, the size of the main reflector is still substantial; at C or X band, the main reflector may have a diameter on the order of three meters, depending upon gain and sidelobe requirements. This means that in order to provide communication capability at multiple spectrally separated bands, such as at each of C band and X band, the overall size of two ring focus antenna structures may extend to a diameter on the order of 16-20 feet. This not only places a strain on the space limitations of a facility such as a shipboard-mounted satellite communication system, but does not solve the hardware complexity and cost problems of having to install a separate ring focus pair for each operational band.
SUMMARY OF THE INVENTION
In accordance with the present invention, these problems are effectively obviated by a new and improved, reduced size, multiband, shaped ring focus antenna architecture that employs a single pseudo parabolically shaped main reflector, and a plurality of diversely configured subreflector-feed pairs, that are designed for operation at respectively different spectral bands. The geometric optical properties of the subreflector-feed pairs are such that they may be used with the same shaped main reflector. This allows the operational band of the antenna structure to be readily changed by simply swapping out the subreflector-feed pairs.
As will be described, the term ‘shaped’ as used to described the present invention is meant a subreflector and main reflector geometry that is defined in accordance with a prescribed set of (reduced sidelobe envelope) directivity pattern relationships and boundary conditions for a prescribed set of equations, rather than a shape that is definable by an equation for a regular conic, such as a parabola or an ellipse. As will be described, given prescribed feed inputs to and boundary conditions for the antenna, the shape of each of a subreflector and a main reflector are generated by executing a computer program that solves a prescribed set of equations for the predefined constraints. In a preferred embodiment, the equations are those which: 1—achieve conservation of energy across the antenna aperture, 2—provide equal phase across the antenna aperture, and 3—obey Snell's law.
While the boundary conditions may be selected to define a regular conical shape, such is not the intent of the shaping of the invention. The ultimate shape of each subreflector and the main reflector are whatever the parameters of the operational specification of the antenna dictate, when applied to the directivity pattern relationships and boundary conditions. As it turns out, because the main reflector produced by the shaping mechanism of the invention has a non-regular conical surface of revolution that is generally (but not necessarily precisely) parabolic, and its associated subreflector has a non-regular conical surface of revolution that is generally (but not necessarily precisely) elliptical, the shape of the main reflector may be termed ‘pseudo’ parabolic and the shape of the subreflector may be termed ‘pseudo’ elliptical.
Once the shapes of a subreflector and main reflector pair have been generated, the performance of the antenna is subjected to computer analysis, to determine whether the generated antenna shapes will produce a desired directivity characteristic. If the design performance criteria are not initially satisfied, one or more of the parameter constraints are adjusted, and performance of the antenna is analyzed for the new set of shapes. This process is iteratively repeated, until the shaped pair meets the antenna's intended operational performance specification.
This iterative shaping and performance analysis sequence is also conducted for another spectrally separate band, to obtain a set of subrefl
Durham Timothy E.
Gothard Griffin K.
Hibner Verlin A.
Lynch Michael J.
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Harris Corporation
Wimer Michael C.
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