IMPROVED THINNED ARRAY ANTENNA SYSTEM

BACKGROUND OF THE INVENTION

Field of the invention

[0001] The invention in general relates to a phased array antenna, and more particularly to thinned array comprised of subarrays for tracking targets.

Description of related art

[0002] Extremely large antenna arrays exist for tracking targets such as missiles. These arrays must provide high transmit/ receive sensitivity as well as high angular accuracy. Further, in order to prevent jamming and to reduce clutter, the antenna beams must have very low sidelobes and no grating lobes. The antenna system of the present invention meets these requirements.

SUMMARY OF THE INVENTION

[0003] The present invention is a thinned antenna array having identical subarrays in a core area and in a peripheral area surrounding the core. Each subarray includes antenna elements with all of the antenna elements of the core subarrays being active and participating in signal processing operations, while less than all of the antenna elements in the peripheral area participate in signal processing operations. The active antenna elements of a subarray have their antenna signals combined in a manifold that provides, for peripheral subarrays, at least a first output signal, and for core subarrays, three output signals. The manifold output signals are provided to respective first, second and third digital receivers,

each for converting the RF antenna output to digital form and to apply a predetermined weighting function to the result. AU of the first digital receiver outputs are utilized in a digital beamformer to generate a sum beam. The outputs of the first and second digital receiver are used by the beamformer to generate an azimuth difference beam and the outputs of the first and third digital receiver are used by the beamformer to generate an elevation difference beam. These beams may then be used by a radar processor to track targets.

[0004] Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description and specific example, while disclosing the preferred embodiment of the invention, is provided by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art, from the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The present invention will become more fully understood from the detailed description provided hereinafter and the accompanying drawings, which are not necessarily to scale, and are given by way of illustration only, and wherein:

[0006] Fig. 1 is a simplified presentation of a one dimensional antenna system to explain certain principles.

[0007] Fig. 2 is a more detailed view of a T/R (transmit/ receive) component of Fig. 1.

[0008] Fig. 3 illustrates a manifold component of Fig. 1.

[0009] Figs. 3A and 3B are waveforms associated with Fig. 3.

[0010] Fig. 4 illustrates sum and difference antenna beams.

[0011] Fig. 5 is a presentation of a two dimensional antenna array.

[0012] Fig. 6 is a block diagram of one embodiment of the present invention.

[0013] Fig. 7 illustrates a manifold component of Fig. 6.

[0014] Figs. 7A, 7B and 7C are waveforms associated with Fig. 7.

[0015] Figs. 8A to 8E illustrate certain digital weighting functions.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0016] The antenna system 10 of Fig. 1 includes a plurality of subarrays 12 each having a one dimensional line array of antenna elements 14. All of the subarrays are identical and a typical subarray 12 includes antenna circuitry 16 connected to each antenna element, with each antenna circuitry 16 being connected to a manifold 18. Manifold 18 provides first and second output signals at respective U and L output ports 20 and 21. These first and second output signals are operated on by respective digital receivers 22 and 24, the collective outputs of which are provided to a digital beamformer 26. A radar processor 28 utilizes the beam signals provided by the digital beamformer 26 to, for example, track a particular target or targets.

[0017] Each digital receiver 22 or 24 basically converts the RF manifold signal, which is produced in response to combining the antenna output signals as a result of a radar return, from an analog to a digital signal. In addition, the digital receivers are also operative to modify the amplitude of the manifold signal applied to them, by application of a weighting function. The weighting function information, which may be different for the various

digital receivers, is provided to each digital receiver by the digital receiver control circuit 30 via lines 31.

[0018] The antenna system of the present invention may be a passive receive only system, or it may be a part of a radar system which also transmits a radar pulse. If the latter, then a transmit pulse is provided to the antenna elements 14 of each subarray 12 via transmit lines 32 and via antenna circuitry 16, which would be constituted by a T/ R module.

[0019] As will be seen, the antenna circuitry, or T/ R module 16 includes components which receive certain control signals via control lines 34 from attenuator and phase shifter control 36. A typical T/ R module 16 is illustrated in Fig. 2. The T/ R module 16 includes a transmit channel 40 and a receive channel 41. A transmit pulse at input 44 is provided to the antenna element 14 via adjustable attenuator 46, adjustable phase shifter 47, power amplifier 48 and circulator 50.

[0020] The antenna element 14 provides a receive signal as a result of a radar return and this antenna signal is provided to output 52 via circulator 50, low noise amplifier 54, adjustable phase shifter 55, and adjustable attenuator 56.

[0021] A typical end fed manifold 18 is illustrated in more detail in Fig.

3. Manifold 18 includes a plurality of input ports 58 for receiving the respective antenna element signals from T/ R modules 16. Transmission lines 60 are included, with signals being coupled between transmission lines by means of a plurality of four-port couplers 61 arranged in a cascaded series geometry. The arrangement additionally includes termination resistors 62 and at least one delay line section 63 which provides 180° phase shift. Horizontal transmission lines 60 terminate in respective output ports 20 and 21. This type of manifold was developed by Alfred E. Blass and is sometimes known as a Blass RF circuit, various designs of which are well- known to those skilled in the art. Other manifold topologies, for example,

center fed and equal path corporate fed manifolds can also be considered for the typical manifold 18.

[0022] Operation of the manifold 18 may best be understood by application of the reciprocity theory. That is, if a voltage is applied to output port 20, the design of the manifold 18 is such that a voltage distribution appears across the array of antenna elements 14 as illustrated in Fig. 3A. For this case, in response to the input voltage at output port 20, the left most antenna element 14 produces a voltage of +V and each subsequent antenna element 14 provides the same voltage +V so that the curve of Fig. 3 A is uniform, hence the U designation for this port 20.

[0023] If a voltage is applied to the other output port 21, the design of the manifold 18 is such that a voltage distribution appears across the array of antenna elements 14 as illustrated in Fig. 3B. For this case, in response to the input voltage at output port 21, the left most antenna element 14 produces a voltage of +V and each subsequent antenna element 14 provides a progressively smaller voltage with the right most antenna element 14 producing a voltage of -V. The curve of Fig. 3B is linear (with a slope), hence the L designation for this port 21.

[0024] Referring once again to Fig. 1, all of output signals at the U output ports 20 of all of the manifolds 18 are operated upon by respective digital receivers 22, and all of output signals at the L output ports 21 of all of the manifolds 18 are operated upon by respective digital receivers 24. The digital receiver outputs are provided to the digital beamformer 26 which produces a σ, or sum beam and a δ, or difference beam.

[0025] A sum beam 68 is shown in solid line in Fig. 4, and a difference beam 70 is shown dotted. It is to be noted that the width of the beams are greatly exaggerated so that the two beams may be readily seen. A typical sum beamwidth may actually be on the order of ±1 ° at the 3dB points.

[0026] A one-dimensional antenna element line array has been described for purposes of illustration. In actuality, a practical antenna system will have two antenna elements 14 in two dimensions for tracking targets in space. A two dimensional antenna 80 for such system is illustrated in Fig. 5.

[0027] The antenna 80, shown circular by way of example, is comprised of a plurality of identical subarrays 82, each having a two- dimensional array of N antenna elements 14. In accordance with the concept of a thinned array, subarrays within a core 84 are fully populated with antenna elements 14 providing output signals which are subject to further signal processing.

[0028] However, predetermined antenna elements 14 in subarrays surrounding the core 84, in a peripheral area 86, are disabled so as not to contribute to any further signal processing operations. That is, all N antenna elements 14 of core 84 subarrays are utilized for beamforming while less than N antenna elements 14 of predetermined peripheral subarrays 86 are utilized. The active antenna elements 14 participating in the beamforming operation are indicated by the small black dots in Fig. 5. Thinned arrays are well-known and in addition to significantly lowering the cost of the antenna system, they also reduce beam sidelobe levels.

[0029] Fig. 6 is a block diagram of an antenna system utilizing the antenna 80 of Fig. 5, in accordance with one embodiment of the present invention. The illustrated antenna system includes a plurality of subarrays 82 within core 84, as well as subarrays 82 in the peripheral area 86, some of which are shown. Each subarray 82 includes antenna elements 14 connected to antenna circuitry 90, the outputs of which are provided to a respective manifold 92. If the antenna is part of a radar system which also transmits, then, with N antenna elements 14 per subarray, the antenna circuitry 90 would be constituted by N T/ R modules, similar to that shown in Fig. 2, and a transmit pulse would be supplied via lines 94. An

attenuator and phase shifter control circuit 96 supplies control signals to the antenna circuitry 90 via lines 98.

[0030] Manifolds 92 of the core 84 subarrays respectively provide three output signals at three output ports, a U output port 100, an LA output port 101 and an LE output port 102. Manifolds 92 within the peripheral area 86 provide only outputs at the U output ports 100, since adequate beam formation may be achieved without the additional LA and LE outputs from these latter subarrays. AU of the U outputs are utilized to form a σ _? or sum beam, all of the U outputs along with all of the LA outputs are utilized to form a δAZ, or azimuth difference beam and all of the U outputs along with all of the LE outputs are utilized to form a δEL, or elevation difference beam. The σ and δAZ beams would be similar to beams 68 and 70 of Fig. 4, while the δEL would be as curve 70, except rotated by 90 °, these beams being in three dimensions.

[0031] The U, LA and LE outputs from the manifolds 92 of the core subarrays are provided to respective digital receivers 106, 107 and 108 while the peripheral subarrays utilize only a single digital receiver 106 for the U output. Appropriate weighting values are supplied to the digital receivers by means of digital receiver control 110 and lines 111.

[0032] The output signals from all of the digital receivers 106, 107 and

108 are provided to digital beamformer 114 for formation of the σ, δAZ and δEL beams which are used by the radar processor 116 to track a particular target or targets. The signal processing performed by the digital receivers further reduces unwanted beam side lobes more than that accomplished with thinning alone. The σ and δ beams as described herein are an example of the types of radar beams possible with the present invention. It should be noted that other beams are possible given the degrees of freedom provided by the digital receivers 106, 107 and 108.

[0033] A typical manifold 92 is illustrated in more detail in Fig. 7.

Manifold 92 is an end fed, series arranged Blass RF circuit which includes a plurality of transmission lines 120 with signals being coupled between transmission lines by means of a plurality of four-port couplers 121. The arrangement additionally includes termination resistors 122 and a plurality of 180° delay line sections 123. Antenna signals (via the T/R modules) are applied to the manifold 92 at respective inputs 124. The antenna signals are combined in the manifold 92 such that the top most transmission line provides the LB output, the middle line, the U output and the bottom line, the LA output. Manifolds for peripheral subarrays could be identical just using the U output of the three outputs, or they may be designed to just provide a U output alone.

[0034] The design of manifold 92 may be explained by again applying the reciprocity theory. That is, if a voltage is applied to the U output port 100, the manifold design is such that a uniform voltage distribution appears across all N antenna elements, as illustrated in Fig. 7A. If that same voltage is applied to LE output port 102, the voltage distribution across the antenna elements would be linear (and sloping) as in Fig. 7B, and Fig. 7C illustrates the voltage distribution for the voltage applied to the LA output port 101.

[0035] The weighting function distribution for performing the beamforming operation is illustrated in Figs. 8A to 8E. As shown in Fig. 8A the weighting function is that as applied to all of the digital receivers 106 of the entire array for calculating the σ beam. The weighting function has a minimum value near zero and a maximum value of WMAX.

[0035] Fig. 8B shows the weighting function distribution, which in conjunction with the weighting function distribution of Fig. 8C, is utilized for calculating the δAZ beam. The weighting function of Fig. 8B, having a maximum value of WMAX and a minimum value of WMIN, is applied to all of the U digital receivers 106, while the weighting function of Fig. 8C is that applied to all LA digital receivers 107 within core 84.

[0036] Fig. 8D shows the weighting function distribution, which in conjunction with the weighting function distribution of Fig. 8E, is utilized for calculating the δEL beam. The weighting function of Fig. 8D, having a maximum value of WMAX and a minimum value of WMIN, is applied to all of the U digital receivers 106, while the weighting function of Fig. 8E is that applied to all LE digital receivers 108 within core 84.

[0037] The foregoing detailed description merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope.