Communications: directive radio wave systems and devices (e.g. – Directive – Including a steerable array
Reexamination Certificate
2000-12-12
2002-03-12
Phan, Dao (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a steerable array
C342S174000
Reexamination Certificate
active
06356233
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to array antennas, and more particularly to array antenna structures to aid in calibration of the active elements of the array.
BACKGROUND OF THE INVENTION
Our society has become dependent upon electromagnetic communications and sensing. The communications are exemplified by radio, television and personal communication devices such as cellphones, and the sensing by radar and lidar. When communications were in their infancy, it was sufficient to broadcast radio signals substantially omnidirectionally in the horizontal plane, and for that purpose a vertical radiator or tower was satisfactory. Early sensors attempted to produce directional results, as for example the directional null used for direction-finding in the Adcock type of antenna. When it became possible to produce short-wave signals such as microwave signals efficiently and relatively inexpensively, directional results became possible with shaped reflector antennas, which provided the relatively large radiating aperture required for high gain and directionality. Such antennas have been in use for over half a century, and they continue to find use because they are relatively simple to build and maintain. However, the shaped-reflector antenna has the salient disadvantage that it must be physically moved in order to move the antenna radiated beam or beams.
Those skilled in the art know that antennas are reciprocal elements, which transduce electrical or electromagnetic signals between unguided (radiating-mode) and guided modes. The “unguided” mode of propagation is that which occurs when the electromagnetic radiation propagates in “free space” without constraints, and the term “free space” also includes those conditions in which stray or unwanted environmental structures disturb or perturb the propagation. The “guided” mode includes those modes in which the propagation is constrained by transmission-line structures, or structures having an effect like those of a transmission line. The guided-wave mode of propagation occurs in rigid waveguides, and in coaxial cable and other transmission-line structures such as microstrip and stripline. The guided-wave mode also includes transmission guided by dielectric structures and single-wire transmission lines. Since the antenna is a transducer, there is no essential difference between transmission and receiving modes of operation. For historical reasons, certain words are used in the antenna fields in ways which do not reflect contemporaneous understanding of antennas. For example, the term used to describe the directional radiation pattern of an antenna is “beam,” which is somewhat meaningful in the context of a transmitting antenna, but which also applies to a receiving antenna, notwithstanding that conceptually there is no corresponding radiation associated with an antenna operated in its receiving mode. Those skilled in the art understand that an antenna “beam” shape is identical in both the transmission and reception modes of operation, with the meaning in the receiving mode being simply the transduction characteristic of the antenna as a function of solid angle. Other characteristics of antennas, such as impedance and mutual coupling, are similarly identical as between transmitting and receiving antennas. Another term associated with antennas which has a contemporaneous meaning different from the apparent meaning is the definition of the guided-wave port, which is often referred to as a “feed” port regardless of whether a transmitting or receiving antenna is referred to.
Array antennas are antennas in which a large radiating aperture is achieved by the use of a plurality of elemental antennas extending over the aperture, with each of the elemental antennas or antenna elements having its elemental port coupled through a “beamformer” to a common port, which can be considered to be the feed port of the array antenna. The beamformer may be as simple as a structure which, in the reception mode, sums together the signals received by each antenna element without introducing any relative phase shift of its own, or which in the transmission mode of operation receives at its common port the signal to be transmitted, and divides it equally among the antenna elements. Those skilled in the art know that the advantages of an array antenna are better realized when the signal transduced by each elemental antenna of an array antenna can be individually controlled in phase. When phase is controlled, it is possible to “steer” the beam of the array antenna over a limited range without physical slewing of the structure. Introduction of phase shifters into the feed path of the elemental antennas, and for that matter the beamformer itself, necessarily introduces unwanted resistive or heating losses or “attenuation” into the signal path. These losses effectively reduce the signal available at a receiver coupled to the array antenna feed port in the reception mode of operation, and also reduce the power reaching the antenna elements from the feed port when in a transmission mode of operation.
In order to maximize the utility of array antennas, it is common to introduce electronic amplifiers into the array antenna system, to aid in overcoming the losses attributable to the beamformer and to the phase shifters, if any, and any associated hardware such as filters and the like. In an array antenna, one such amplifier is used in conjunction with each antenna element. For reception of weak signals, it is common to use an amplifier which is optimized for “low-noise” operation, so as to amplify the signal received by each antenna element without contributing excessively to the noise inherent in the signal received by the antenna element itself. For transmission of signals, a “power” amplifier is ordinarily associated with each antenna element or group of antenna elements, to boost the power of the transmitted signal at a location near the antenna elements. In array antennas used for both transmission and reception, both receive and transmit amplifiers may be used.
Amplifiers tend to be nonlinear, in that the output signal amplitude of an amplifier is in a specific amplitude ratio to the input signal amplitude at input signal levels lying below a given level, but become nonlinear, in that the ratio becomes smaller (the gain decreases to a value below the small-signal level) with increasing signal level. Structures which are subject to such saturation or other nonlinear effects are termed “active.” It should be noted that an active element is often defined as one which requires or uses an electrical bias for operation; saturation tends to be inherent in such elements when the signal being handled approaches or equals the amplitude of the applied bias. Amplifiers are ordinarily not bidirectional, in that they amplify signals received at an input port, and the amplified signals are generated at an output port. Although bidirectional amplifiers are possible, the constraints required for bidirectional operation limit their utility, and unidirectional amplifiers are commonly used for array antennas. In the case of an array antenna used for both transmission and reception, each antenna element is associated with both a power amplifier and low-noise amplifier. Bidirectional, duplex or diplex operation, which is to say simultaneous operation in both transmission and reception, is accomplished by the use of circulators, which are three-port devices which allow connection of an antenna element to the output port of a power amplifier and to the input port of a low-noise amplifier. It should be noted that phase shifters which may be associated with each radiating element of an array in order to allow steering of the beam may be subject to saturation or nonlinear effects, and so may be considered to be “active” for this purpose, although these nonlinear effects may not be nearly so pronounced as in the case of amplifiers, and in some cases the saturation effects of phase shifters may be ignored. Some types of phase shifters rely on the interaction of discrete electronic element
Miller Richard Earl
Reese Robert Michael
Lockheed Martin Corporation
Meise W. H.
Phan Dao
Weinstein S. D.
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