Communications: directive radio wave systems and devices (e.g. – Testing or calibrating of radar system – By monitoring
Reexamination Certificate
1998-03-16
2001-03-27
Issing, Gregory C. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Testing or calibrating of radar system
By monitoring
C342S372000
Reexamination Certificate
active
06208287
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to phased array antennas and more particularly to apparatus and methods used to calibrate such antennas.
As is known in the art, a phased array antenna includes an array of antenna elements adapted to produce a plurality of collimated and differently directed beams of radio frequency energy. These phased array elements may be corporate fed or space fed. In either case, the relative amplitude and phase shift across the array of antenna elements defines the antenna beam. This relative amplitude and phase state may be produced by controllable attenuators and phase shifters coupled to corresponding antenna elements or by beamforming networks disposed between a plurality of beam ports and the plurality of antenna elements, where each beam port corresponds to one of the beams.
In one such beamforming network phased array antenna system, the beamforming network has a plurality of array ports each one being coupled to a corresponding one of the antenna elements through a transmit/receive module. Each one of the transmit/receive modules includes an electronically controllable attenuator and phase shifter. During a receive calibration mode at the factory or test facility, a source of radio frequency (RF) energy is placed in the near field of the phased array antenna elements. The transmit/receive modules are sequentially activated. When each one of the transmit/receive module is placed in a receive mode and is activated, energy received by the antenna element coupled thereto is passed through the activated transmit/receive module and through the beamforming network. The energy at one of the beam ports is detected during the sequential activation. The detected energy is recorded for each of the elements of the array in sequence. The process is repeated for each of the beam ports. For each antenna element, a least mean square average is calculated for the detected energy associated with each of the beam ports. Thus, each antenna element is associated with an amplitude and phase vector. These measured/post-calculated vectors are compared with pre-calculated, designed vectors. If the antenna is operating properly (i.e., in accordance with its design), the measured/post-calculated vectors should match the pre-calculated vectors with minimal error. Any difference in such measured/post-calculated vector and the pre-calculated vector is used to provide a control signal to the controllable attenuator and/or phase shifter in the module to provide a suitably corrective adjustment. The calibration is performed in like, reciprocal manner, during a transmit calibration mode at the factory or test facility.
Thus, in either the transmit or receive calibration modes, errors in the relative phase or amplitude are detected and the controllable attenuator and/or phase shifter in the module is suitably adjusted. While such technique is suitable in a factory or test facility environment, the use of separate external transmit and receive antennas may be impractical and/or costly in operational environments. For example, when the antenna is deployed in the field it is sometimes necessary to re-calibrate the antenna after extensive use. Examples of such environments include, but are not limited to, outer space as where the antenna is used in a satellite, on aircraft including fixed wing, rotary wing, and tethered, and on the earth's surface.
A paper entitled “Phased Array Antenna Calibration and Pattern Predication Using Mutual Coupling Measurements” by Herbert M. Aumann, Alan J. Fenn, and Frank G. Willwerth published in IEEE Transactions on Antennas and Propagation, Vol. 37, July 1989, pages 844-850, develops mathematically and demonstrates a calibration and radiation pattern measurement technique which takes advantage of the inherent coupling in an array, by transmitting and receiving all adjacent pairs of radiating elements through two indent beamformers (corporate feeds). The technique utilizes an internal calibration source.
SUMMARY OF THE INVENTION
In Accordance with one feature of the invention, apparatus and method are provided for testing a phased array antenna. The antenna includes a plurality of antenna elements and a plurality of transmit/receive modules. Each one of the transmit/receive modules is coupled to a corresponding one of the antenna elements. The apparatus includes a calibration system having: an RF input port; an RF detector port; an RF detector coupled to the RF detector port; and an RF source connected to the RF input port. A switch section is included for sequentially coupling the antenna elements and the transmit/receive modules coupled thereto selectively to either: (a) the detector port during a receive calibration mode; or, (b) to the RF test input port during a transmit calibration mode. One, or more, (i.e., a predetermined set) of the plurality of antenna elements (i.e., calibration antenna elements) is also coupled to the switch section. The switch section couples each calibration antenna element selectively to either: (a) the RF test input during the receive calibration mode; or, (b) the RF detector port during the transmit calibration mode.
In accordance with another feature of the invention, apparatus and method are provided for testing a phased array antenna having a beamforming network. The beamforming network includes a plurality of array ports and a plurality of beam ports. A plurality of antenna elements and a plurality of transmit/receive modules are included. Each one of the modules is coupled between a corresponding one of the antenna elements and a corresponding one of the array ports. A calibration system is provided having: an RF input port; an RF detector port; an RF detector coupled to the RF detector port; and an RF source connected to the RF input port. A switch section is included for sequentially coupling each one of the antenna elements through the beam forming network and the one of the transmit/receive modules coupled thereto selectively to either: (a) the detector port during a receive calibration mode; or, (b) to the RF test input port during a transmit calibration mode. The switch section includes a switch for selectively coupling a predetermined one of the antenna elements (i.e., a calibration antenna element) selectively to either: (a) the RF test input of the calibration system during the receive calibration mode through a path isolated from the beamforming network; or, (b) to the detector port during the transmit calibration mode through a path isolated from the beamforming network. With such an arrangement, undesired coupling to the calibration antenna element through the beamforming network is eliminated.
In accordance with still another feature of the invention, the array of antenna elements is arranged in clusters, each one of the clusters having a predetermined antenna element (i.e, a calibration antenna element). With such an arrangement, each cluster is calibrated with the calibration antenna element in such cluster thereby enabling a relatively small dynamic range variation among the antenna elements in such cluster during the calibration of such cluster.
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Aumann et al., “Phased Array Antenna Calibration and Pattern Prediction Using Mutual Coupling Measurements”, IEEE Transactions On Antennas and Propagation, vol. 37, No. 7, Jul. 1989, pp. 844-850.
Aumann et al., “Phased Array Calibrations Using Measured Element Patterns”, Proc. 1995 IEEE AP-S International Symposium, pp. 918-921, Long Beach, CA Jun. 1995.
Fenn et al., “Mutual Coupling In Monopole Phased Array Antennas”, 1984 International Symposium Digest, Antennas and Propagation, vol. II, IEEE Catalog, No. 84C
Bedigian Oscar J.
Sikina Thomas V.
Daly, Crowley & Mofford LLP
Issing Gregory C.
RaytheonCompany
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