Abstract:
An antenna formed of multiple sub-arrays, each having rows of interconnected radiating elements. One row of radiating elements is shared between two sub-arrays by a coupler which isolatingly couples one row of radiating elements to each of two sub-arrays allowing the feed to the two sub-arrays to be isolatingly applied to the shared row of radiating elements while suppressing grating lobe generation and providing high sub-array isolation.

Description:
BACKGROUND 
     The present invention relates, in general, to phased array antennas and, in particular, to phased array antennas that require grating lobe suppression. 
     A phased array antenna is a plurality of sub-array antennas coupled to a common source or load in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. 
     A limited scan antenna system scans a narrow beam only a few beam widths. Grating lobe suppression is a difficult design task for limited scan antennas where sub-arrays are employed. Few techniques have been developed to reduce the level of spurious grating lobes. One approach is to use non-constant sub-array separations which disrupt the coherent summation of radiation in the grating lobe directions. However, the resulting side lobes are higher. 
     Another approach is overlapped sub-arrays that interleave the radiation elements. For a fixed sub-array separation, overlapping sub-arrays allow a larger sub-array aperture, resulting in a narrower beam width of the sub-array pattern. The grating lobes of the array can be placed completely within the side lobe region of the sub-array pattern, giving grating lobe suppression. This method works well when the radiation elements are relatively short in the vertical direction according to the orientation shown in  FIG. 1   
     However, for long element arrays, the coupling between elements and, hence, the sub-arrays, due to interleaving, become stronger. The consequence is that the sub-array patterns are degraded resulting in lower gain and higher side lobes, and sub-array port-to-port isolation deteriorates. 
     It is desirable to provide a novel solution for a partially overlapped sub-array antenna approach, for both short and long element arrays, which provides high isolation between the sub-array ports and desired sub-array patterns can be achieved in a simple and low cost structure. 
     SUMMARY 
     An antenna includes a plurality of radiating elements, a first sub-array defined by a plurality of rows of serially interconnected radiating elements, all connected by a first signal feed port, a second sub-array defined by a plurality of rows of serially interconnected radiating elements, all connected by a second signal feed port, a first coupler isolatingly coupling the radiating elements of one row of the first sub-array and the radiating elements of one row of the second sub-array as a shared row of radiating elements, wherein a signal feed through the first and second feed ports is respectively, applied to the shared row of radiating elements of the first and second sub-arrays. 
     The coupler can include one feed port connectable to the radiating elements in the antenna and first and second isolated ports. 
     This phased array antenna provides improved sub-array patterns with higher gain and lower side lobes, and increased sub-array port-to-port isolation. This is achieved in a simple, low cost structure and finds particular advantageous use in antennas with long radiating element arrays. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The various features, advantages and other uses of the disclosed partially overlapped phased antenna can be had by referring to the following detailed description and drawing in which: 
         FIG. 1  is a pictorial representation of the prior art partially overlapped phased array antenna using interleaving elements; 
         FIG. 2  is a graph depicting the sub-array pattern of the prior art antenna shown in  FIG. 1 ; 
         FIG. 3  is a pictorial representation of a partially overlapped phased array antenna using couplers in the feed network; 
         FIG. 4  is an enlarged pictorial representation of the coupler, feed network and radiation elements of the antenna shown in  FIG. 3 ; and 
         FIG. 5  is a graph depicting the sub-array pattern of the antenna shown in  FIGS. 3 and 4 . 
     
    
    
     DETAILED DESCRIPTION 
     In order to clarify the understanding the features of the partially overlapped sub-array phased array antenna described hereafter, a brief reference will be had to  FIGS. 1 and 2  which depict a prior art partially overlapped sub-array antenna  10  using interleaving elements. For clarity, the antenna  10  is pictorially shown without the substrate, which can be a printed circuit board, or intervening dielectric insulating layers between a radiation layer, a coupling aperture in a middle layer, and a feed network in a bottom layer. The bottom layer is shown overlaying the radiation layer. 
     The antenna  10  is formed of a plurality of phased sub-arrays A, B and C. Each sub-array A, B and C, is formed of a plurality of rows of serially connected radiation elements  12 . The number of radiation elements in each vertical row as well as the number of rows in each sub-array A, B and C can vary according to the particular antenna application. Thus, it will be understood that three sub-arrays A, B and C are shown by example only as the antenna  10  will typically include greater or lesser numbers of sub-arrays. 
       FIG. 1  depicts a prior art approach to grating lobe suppression in which overlapped sub-arrays interleave the radiation elements. Sub-array A is formed of rows R 1 , R 2 , R 3  and R 5  of serially connected radiation elements  12 . Sub-array B is formed of rows R 4 , R 6 , R 7  and R 9  of radiation elements  12 . Sub-array C is formed of rows R 8 , R 10 , R 11  and R 12  of radiation elements  12 . 
     Row R 4  of sub-array B is interleaved between rows R 3  and R 5  of sub-array A. Rows R 6  and R 7  of sub-array B are interleaved between rows R 5  and R 8  of sub-arrays A and C, respectively. Rows R 9  of sub-array B is interleaved between rows R 8  and R 10  of sub-array C. The radiating elements  12  may be linearly offset as shown in  FIG. 1  in separate sub-arrays. 
     Signal feed ports  20 ,  22  and  24 , each having parallel port connections  26 ,  28 ,  30  and  32 , respectively, are connected through the coupling apertures to the radiating elements  12  in each sub-array A, B and C to supply feed signals through feed ports I, II and III. 
     For a fixed sub-array separation, the sub-array overlapping for the antenna  10  shown in  FIG. 1  allows a larger sub-array aperture resulting in a narrower beam width of the sub-array pattern. The grating lobes of each array A, B and C can be placed completely within the side lobe region of the sub-array pattern for grating lobe suppression. 
     This method works adequately when the rows of radiating elements are relatively short in the vertical direction. However, for long element arrays, the coupling between radiating elements  12  and, hence, the sub-arrays A, B and C, due to interleaving become stronger. The consequences are that the sub-array patterns are degraded with lower gain and, higher side lobes, and sub array port-to-port isolation deteriorates. This is evidenced by the graph of the sub-array pattern of the antenna shown in  FIG. 2  which shows an undesired pattern shape. 
     Referring now to  FIGS. 3 and 4 , there is depicted a phased array antenna  40  formed of a plurality of sub-arrays A, B and C. Each sub-array A, B and C is formed of a plurality of rows R 1 -R 10 , each row being formed of a plurality of serially interconnected radiating elements  42 . 
     It will be understood that the number of sub-arrays forming the antenna  40  as well as the number of rows in each sub-array and the number of radiating elements in each row can be varied to suit the application requirements of the antenna. 
     By example only, the sub-arrays A, B and C in the antenna  40  are each formed of four rows of serially interconnected radiating elements  42 . The sub-arrays are partially overlapped with one row, such as row R 4 , being shared by sub-arrays A and B through the use of a unique coupler means  44 . The sub-array overlapping is achieved through sharing of the radiating elements  42  in row R 4 . Since there is no radiating element  42  interleaving, the sub-array to sub-array coupling is very small even for long radiating elements. In addition, since the left and right arms of the coupler  44  are well isolated due to the nature of the coupler  44 , the port-to-port isolation between two sub-arrays A, B or B, C is further enhanced. 
     A similar coupler means  45  may be employed to couple a shared row of radiating elements  42 , such as row R 7  in sub-arrays B and C, and so on for any additional sub-arrays in the antenna  40 . 
     A signal input through the first sub-array feed port I is fed by the channel  46  of port I to the radiating elements  42  in rows R 1 , R 2 , R 3  and R 4  through two channels  52  and  54  of a power splitter through coupling apertures in the middle layer of the antenna  40  to the radiating elements  42  in the top layer of the antenna  40  stack. 
     Channels  51  and  53  are connected between the channels  52  and  54 , respectively, to a channel  50  connecting the radiating elements  42  in row R 1  and to the coupler  44  which provides a connection to the radiating elements  42  in row R 4  when an input signal is received through port I of the sub-array A. 
     Input port II for sub-array B has a similar configuration with a channel  48  split into channels  52  and  54 , which are coupled to the radiating elements  42  in rows R 5  and R 6 . Side channels  51  and  55  extend from the port II power splitter  48  to two couplers  44  and  45 . Thus, port I feeds the radiating elements  42  in rows R 1 , R 2 , R 3  and R 4 . Port II feeds the radiating elements  42  in rows R 4 , R 5 , R 6  and R 7 . The first coupler  44  provides feed isolation and sharing between the two sub-arrays A and B in row R 4 . The second coupler  45  provides feed isolation and sharing between sub-arrays B and C in row R 7   
     The coupler means  44  can be any suitable microwave or radio frequency power splitter-divider or coupler that has two isolated ports and a common feed port. For example only, the coupler means  44  is illustrated in  FIGS. 3 and 4  as being a rat-race type coupler. The coupler means  44  can also be any other type of coupler, power divider, combiner or power splitter, such as hybrid branch coupler, a parallel-line coupler, a Wilkinson power divider etc. 
     The couplers  44  and  45  have a port with an impedance matching tail  56  that has RF absorbing material to be applied thereto. 
     It will be understood that additional sub-arrays can be added to the antenna  40  with the same radiating element row sharing by the use of additional couplers  44 . 
     The four rows of radiating elements in each sub-array A, B and C can be fed with a desired amplitude taper for low side lobes. The shared rows R 4 , R 7 , etc. of radiating elements  42  always have low power amplitude due to the requirement of low side lobes, limiting the power lost to the matched load of the couplers  44 . 
     As depicted in  FIG. 5 , twenty-five dB sub-array side lobes with the desired pattern shape have been achieved. Such side lobe patterns will effectively suppress grating lobes beyond the sub-array main beam. Measurements indicate that the antenna  40  has more than thirty dB sub-array isolation. 
     It will be understood that the radiating elements  42  can be any type of radiator, not limited to the illustrated rectangular patch elements. 
     Further, while the antenna  40  has been described as a phased array antenna, it will be understood that this antenna type is by way of example only as the use of a coupler and a shared row of radiating elements can be used in other types of antennas, such as printed board antennas, etc.