Communications: radio wave antennas – Antennas – Spiral or helical type
Reexamination Certificate
1999-11-16
2001-06-05
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
Spiral or helical type
C343S766000, C343S853000
Reexamination Certificate
active
06243052
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to communication systems, and is particularly directed to a new and improved, low profile, panel-configured helical phased array antenna architecture, that is configured for use with a mobile (e.g, land vehicle) platform, and which contains an integrated pseudo-monopulse based, beam-aiming (tilting) subsystem, that is coupled to a platform positioning system so as to facilitate pointing of the antenna along the path of, a (low earth orbit) satellite.
BACKGROUND OF THE INVENTION
In order to be certified for acceptance with a given (satellite) communication system, the directivity pattern of an antenna relative to a target (e.g., satellite) must conform with prescribed main lobe and sidelobe characteristics. Where the antenna is to be installed at a fixed, land-based location, and there are no restrictions on the physical parameters and cost of the antenna, satisfying a given performance specification may be readily accomplished by suitable design of a conventional (parabolic) dish antenna and associated monopulse hardware configuration. However, where the environment in which the antenna is to deployed is mobile and potentially hostile, a variety of physical parameters come into play, which effectively negate the use of a large dish and its associated beam steering components.
For example, in a tactical (mobile) environment, where detection and therefore survivability of a communication system may depend upon the effective profile or observable footprint of the antenna, it is highly desirable to make the antenna as small as possible. However, as the size of the antenna is reduced, so is its available energy collecting aperture. A further complication is the fact that it may be necessary to dynamically position or orient the antenna, in order to follow or track a (low earth orbit) satellite. Even if a reduced diameter dish architecture is employed, its moment of inertia and observable profile is further enlarged by the auxiliary (azimuth and elevation sum and difference horns) and waveguide and stripline ‘plumbing’ of the associated (monopulse) tracking control subsystem. Moreover, should it be necessary to change the operational parameters of such a dish-based architecture, major disassembly and retrofitting of its associated waveguide hardware is required.
SUMMARY OF THE INVENTION
In accordance with the present invention, such shortcomings of conventional, relatively massive parabolic (e.g., Cassegrain) antenna architectures are effectively obviated by a new and improved ‘low profile’, panel-configured helical phased array antenna and integrated beam-aiming (tilting) subsystem architecture. As will be described this architecture not only readily lends itself to being implemented with commercial off the shelf (COTS) components, to reduce its cost, but it may be operated in a ‘pseudo’-monopulse mode, to facilitate operation of a mobile platform-mounted positioner, and exhibits a performance that conforms with industry standards, such as the DSCS (defense satellite communication system) specification.
For low observability, the helical antenna arrays and RF circuit components of the antenna are mounted to a generally flat plate or panel. Transmit and receive arrays of tapered pitch helical antenna elements are mounted side-by-side upon a front side of the panel, while RF circuit components associated with the transmit and receive arrays are mounted to a rear side of the plate, which avoids aperture blockage. The parameters of the tapered pitch helices and their respective locations are preferably defined to constrain the sidelobes of the antenna's directivity pattern within with the DISA envelope of DSCS certification requirements.
Each of the respective transmit and receive arrays is configured as a compact, spatially periodic distribution of tapered pitch helical antenna elements to minimize the height of the antenna. Element-to-element spacing is minimized for maximum aperture efficiency. In a preferred embodiment, each array geometry is that of a circular truncation of an equiangular (60°) triangle-based lattice into sixty-four locations, subdivided into four quadrants of sixteen elements/quadrant. To achieve a substantial reduction in the sidelobe envelope for complying with the DISA specification, the lattice geometries of the arrays have a ‘rotated’ orientation on the support plate.
By ‘rotated’ orientation is meant that each of the three sets of parallel rows of the 60° lattice geometry of a respective array is rotationally offset relative to both the target travel path and the normal to that path projected in the plane of the array. The projection of the antenna's scan plane upon the array is defined by the orientation of the plate in azimuth (AZ) and elevation (EL), under the control of the associated positioning subsystem upon which the plate is mounted, and corresponds to the projection upon the array of the travel path of the satellite being tracked.
To adjust the antenna boresight, the support plate is mounted to an associated positioning subsystem, such as but not limited to an associated azimuth AZ and elevation EL (&thgr;/&PHgr;) positioning subsystem. Such a subsystem may effect a change in elevation by rotating the plate some angle &PHgr; about an axis that is parallel to upper and lower parallel edges of the plate. To effect a change in azimuth, the positioning subsystem rotates the plate some angle &thgr; about an axis, the normal projection of which upon the plate is parallel to its two parallel side edges. Positioning control commands for driving the positioning subsystem are supplied by an associated system supervisory host computer.
The RF components for the transmit array on the rear side of the support plate are comprised of COTS components, and include a four-way power divider coupled to four, sixteen-way power dividers, whose outputs are coupled to feed ports of an associated set of sixteen antenna elements within the four spatial quadrants of the transmit array. For the adjacent receive array, the output ports of each of the sixteen antenna elements of its four quadrants are coupled to respective ones of a set of four sixteen-way microstrip power combiners, whose outputs are directly coupled to associated combine filters to suppress the RF band of the signals emitted by the adjacent transmit array. These combine filters are coupled through low noise amplifiers to a four-way phase shifter and combiner.
In accordance with the invention, the phase shifter and combiner is operative, under control of the host processor, to impart a controlled amount of phase shift to each receive array quadrant signal path. It then sums the resulting (phase-shifted) inputs from the four quadrants of the receive array. The output of the four-way combiner is coupled through a further combine filter-LNA stage and routed therefrom to downstream transceiver circuitry.
To selectively impart a controlled amount of phase shift to each input path, the four-way combiner includes four digitally controlled, single-bit, quadrant phase shifters. The phase shift imparted by each phase shift element is programmable; whether that control voltage is applied to the phase shift element is determined by the value of the single bit. The use of digitally controlled phase shifters facilitates adjustments to the associated pseudo-monopulse tracking subsystem, and allows the main beam to be electrically selectively scanned, or sequentially stepped up to a prescribed offset angle (e.g., 1°) from boresight, in order to extract azimuth and elevation error signals used by the positioning subsystem to correct, as necessary, the pointing of the antenna. At times other than this tracking mode, the digital programmability of each of the phase shifters allows the beam pattern of the array to be controllably electrically tilted at a selected inclination angle off boresight, under user control. This feature allows a trade-off between and simplifies optimization of tracking performance and gain/thermal noise ratio (G/T).
REFERENCES:
Daffron William C.
Fejzuli Alen
Goldstein M. Larry
Offner James B.
Svatik, Jr. Emil G.
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Harris Corporation
Nguyen Hoang
Wong Don
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