Abstract:
An antenna array having a plurality of array elements is disclosed. The antenna array comprises a first array element ( 204 A) having a first suspended radiator ( 207 A) and a first ground conductor ( 206 A), the first suspended radiator being displaced from the first ground conductor. The antenna also comprises a second array element ( 204 B) being adjacent to the first array element, the second array element having a second suspended radiator ( 207 B) and a second ground conductor ( 206 B), wherein the second suspended radiator is displaced from the second ground conductor. In the antenna the first ground conductor is adjacent to and displaced from the second ground conductor and the first ground conductor is disposed on a first tier and the second ground conductor is disposed on a second tier to form an at least two-tiered ground conductor.

Description:
FIELD OF INVENTION  
       [0001]     The invention relates generally to antenna arrays. In particular, it relates to antenna arrays with array elements with a multi-tiered ground conductor.  
       BACKGROUND  
       [0002]     Mutual coupling between array elements of antenna arrays significantly affect the performances of these arrays in wireless communications applications. The affected performances include signal-to-interference-pulse-noise ratio (SINR) and direction-of-arrival (DOA) estimation in the case of an adaptive array.  
         [0003]     Therefore during the design of antenna arrays the problem of mutual coupling is an important consideration. Mutual coupling also adversely determines the dimensions of the arrays in addition to affecting the foregoing performances of the arrays.  
         [0004]     Typically, mutual coupling may degrade the radiation patterns for the arrays due to the increase in side lobe levels, the shift of nulls, and the appearance of grating lobes.  
         [0005]     Mutual coupling in plate antenna arrays is mainly attributed to space waves, higher-order waves, surface waves, and leaky waves. Generally for conventional plate antenna arrays with a common planar ground conductor, enlarging the spacing between plate array elements, or inter-element spacing, results in reducing or weakening mutual coupling. However, the larger inter-element spacing results in a larger lateral size of the arrays. The larger lateral size of the arrays leads to higher installation cost of wireless communications systems in which such arrays are applied.  
         [0006]     There is therefore a need for a laterally compact plate antenna array configured appropriately for reducing mutual coupling between plate array elements.  
       SUMMARY  
       [0007]     Embodiments of the invention are disclosed hereinafter for reducing the lateral size of an antenna array with reduced or weak mutual coupling by using a multi-tiered configuration. In particular, a common ground conductor, typically planar and single-tiered in a conventional antenna array, is multi-tiered by folding or corrugation to reduce the lateral spacing between plate array elements while maintaining the inter-element spacing.  
         [0008]     In accordance with one aspect of the invention, there is disclosed an antenna array having a plurality of array elements, the antenna array comprising a first array element having a first suspended radiator and a first ground conductor, the first suspended radiator being displaced from the first ground conductor. The antenna also comprises a second array element being adjacent to the first array element, the second array element having a second suspended radiator and a second ground conductor, wherein the second suspended radiator is displaced from the second ground conductor. In the antenna array the first ground conductor is adjacent to and displaced from the second ground conductor and the first ground conductor is disposed on a first tier and the second ground conductor is disposed on a second tier to form an at least two-tiered ground conductor.  
         [0009]     In accordance with another aspect of the invention, there is disclosed a method for configuring an antenna array having a plurality of array elements, the method comprising the steps of providing a first array element having a first suspended radiator and a first ground conductor, the first suspended radiator being displaced from the first ground conductor, and providing a second array element as adjacent to the first array element, the second array element having a second suspended radiator and a second ground conductor, wherein the second suspended radiator is displaced from the second ground conductor. The method also comprises the steps of disposing the first ground conductor adjacent to and displaced from the second ground conductor, and disposing the first ground conductor on a first tier and the second ground conductor on a second tier to form an at least two-tiered ground conductor.  
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0010]     Embodiments of the invention are described in detail hereinafter with reference to the drawings, in which:  
         [0011]      FIG. 1 ( a ) is an isometric view of a conventional plate antenna array with plate array elements and a planar ground conductor, and  
         [0012]     FIGS.  1 ( b ) and ( c ) are isometric views of two plate antenna arrays according to embodiments of the invention with plate array elements and corrugated ground conductors, whereby the lateral size of the plate antenna arrays is compared with the lateral size of the conventional plate antenna array of  FIG. 1 ( a );  
         [0013]     FIGS.  2 ( a ), ( b ) and ( c ) are respectively front elevation, side elevation and bottom views of adjacent plate array elements in a plate antenna array with a two-tiered ground conductor according to an embodiment of the invention;  
         [0014]      FIGS. 3 and 4  are plotted results of an investigation performed on the plate antenna array of  FIG. 2 ( a );  
         [0015]      FIG. 5 ( a ) is an isometric view of a rectangular plate antenna array according to an embodiment of the invention with rectangular plate array elements and a two-tiered, two-dimensionally corrugated ground conductor, and  
         [0016]      FIG. 5 ( b ) is an isometric view of a conventional plate antenna array with rectangular play array elements and a planar ground plate, in which the lateral size of the rectangular plate antenna array of  FIG. 5 ( a ) is compared with the lateral size of the conventional rectangular plate antenna array;  
         [0017]      FIG. 6  is an illustration of variations of the two-tiered ground conductor of  FIG. 2 ( c ); and  
         [0018]     FIGS.  7 ( a ) and  7 ( b ) are illustrations of plate antenna arrays with multi-tiered ground conductors according to embodiments of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0019]     Embodiments of the invention are described hereinafter with reference to the drawings for addressing the need for a laterally compact antenna array configured appropriately for reducing mutual coupling between array elements.  
         [0020]      FIG. 1 ( a ) shows the geometry of a conventional rectangular plate antenna array  102  with plate array elements  104  arranged in a single row along the length of the conventional rectangular plate antenna array  102 . The conventional rectangular plate antenna array  102  also includes a rectangular and single-tiered common ground conductor  106 .  
         [0021]     Each plate array element  104  comprises a suspended plate radiator and a corresponding ground patch, the ground patch being part of the common ground conductor  106 . The suspended plate radiator is fed with signals through conventional feeding means.  
         [0022]     Each plate array element  104  is also spaced apart from a nearest adjacent plate array element  104  by the distance D 1 , known hereinafter as inter-element spacing D 1 . In this case the inter-element spacing D 1  is equivalent to lateral spacing L 1 , which is spacing between nearest adjacent plate array elements  104  projected onto a plane parallel to the plane of the common ground conductor  106 .  
         [0023]     FIGS.  1 ( b ) and  1 ( c ) show two rectangular plate antenna arrays  112  and  122 , respectively, according to two different embodiments of the invention, which have smaller lateral sizes than the conventional rectangular plate antenna array  102  shown in  FIG. 1 ( a ). The plate antenna array  112  as shown in  FIG. 1 ( b ) includes plate array elements  114  arranged in a single row along the length of the rectangular plate antenna array  112 . The rectangular plate antenna array  112  also includes a rectangular and two-tiered common ground conductor  116  folded or corrugated longitudinally into alternating ridges  118  and grooves  119  of uniform widths. The ridges  118  are disposed on a same plane and form a higher tier or level with the corresponding plate array elements  114  while the grooves  119  are also disposed on a same plane and form a lower tier or level with the corresponding plate array elements  114 .  
         [0024]     Each plate array element  114  comprises a suspended plate radiator and a corresponding ground patch, the ground patch being plate-like and part of the common ground conductor  116 . The suspended plate radiator is fed with signals through conventional feeding means.  
         [0025]     Since the common ground conductor  116  is corrugated, inter-element spacing D 2  is greater than lateral spacing L 2  in relation to two nearest adjacent plate array elements  114 . By having the inter-element spacing D 2  being substantially equivalent to the inter-element spacing D 1  in the conventional rectangular plate antenna array  102 , mutual coupling between the plate array elements  114  in this case is not worsened or increased. This is true even though the lateral spacing L 2  is smaller than the lateral spacing L 1  in the conventional rectangular plate antenna array  102 .  
         [0026]     The plate antenna array  122  as shown in  FIG. 1 ( c ) includes plate array elements  124  arranged in a single row along the length of the rectangular plate antenna array  122  and has a symmetrical structure. The rectangular plate antenna array  122  also includes a rectangular and two-tiered common ground conductor  126  folded or corrugated longitudinally into alternating ridges  128  and grooves  129 A and  129 B, the grooves  129 A and  129 B not being of uniform widths. Specifically as shown in  FIG. 1 ( c ), in the middle of the rectangular plate antenna array  122  the central groove  129 A is wider than the side grooves  129 B as in the central groove  129 A two plate array elements  126  are disposed. The ridges  128  are disposed on a same plane and form a higher tier or level with the corresponding plate array elements  124  while the grooves  129 A and  129 B are also disposed on a same plane and form a lower tier or level with the corresponding plate array elements  124 .  
         [0027]     Each plate array element  124  comprises a suspended plate radiator and a corresponding ground patch, the ground patch being plate-like and forming part of the common ground conductor  126 . The suspended plate radiator is fed with signals through conventional feeding means.  
         [0028]     Since the common ground conductor  126  is corrugated, inter-element spacing D 3  between plate array elements  124 , other than those disposed in the central groove, is greater than lateral spacing L 3  in relation to two nearest adjacent plate array elements  124 . By having the inter-element spacing D 3  being substantially equivalent to the inter-element spacing D 1  in the conventional rectangular plate antenna array  102 , mutual coupling between the plate array elements  124  in this case is not worsened or increased. This is true even though the lateral spacing L 3  is smaller than the lateral spacing L 1  in the conventional rectangular plate antenna array  102 . In the case of the two plate array elements  124  in the central groove  129 A, inter-element spacing D 4  and lateral spacing L 4  are equivalent, and may also be equivalent to the inter-element spacing D 1  and lateral spacing L 1 , respectively.  
         [0029]     FIGS.  2 ( a ),  2 ( b ) and  2 ( c ) show geometrical and structural details of a rectangular plate antenna array  202  and two square plate array elements  204 A and  204 B therein according to an embodiment of the invention. Such an embodiment is constructed for investigation purposes, with reference to a coordinate system with X, Y and Z axes used for plotting results derived from the investigation, and forms a basic cell or unit from which larger plate antenna arrays according to the embodiments of the invention are formed. The investigation is therefore for providing results that are used hereinafter for substantiating design functionality and feasibility of the embodiments of the invention.  
         [0030]     The rectangular plate antenna array  202  includes plate array elements  204 A and  204 B that are arranged adjacently along the length of the rectangular plate antenna array  202 . The rectangular plate antenna array  202  also includes a rectangular and two-tiered common ground conductor  206  folded longitudinally into three planar and plate-like ground patches  206 A,  206 B and  206 C that are continuous and preferably unitary. The ground patches  206 A and  206 B form lower and higher tiers, respectively, and ground patch  206 C is a junction ground patch which connect the ground patches  206 A and  206 B located on different tiers.  
         [0031]     Each plate array element  204 A and  204 B comprises a suspended plate radiator  207 A and  207 B and the corresponding ground patches  206 A and  206 B, respectively. The suspended plate radiators  207 A and  207 B are fed with signals through feed points  208  via conventional feeding means. In this case the plate array elements  204 A and  204 B are fed via conventional means using coaxial probes  210  through surface mounted adapters (SMAs)  212 . The feed point  208  locations and heights of the suspended plate radiators  207 A and  207 B above the corresponding ground patches  206 A and  206 B, respectively, are determined for good impedance matching.  
         [0032]     The junction ground patch  206 C is inclined at an angle θ. The plate array element  204 B is located at a height H above the plate array element  204 A, and each of the suspended plate radiators  207 A and  207 B is located at a height h above the corresponding ground patches  206 A and  206 B, respectively.  
         [0033]      FIG. 3  shows the comparison between measured and simulated S parameters in relation to rectangular plate antenna array  202 , in which good correlation between measurement and simulation is obtained. The comparison of mutual coupling for the cases with a flat common ground conductor  206  (θ=0°) and a step-like common ground conductor  206  (θ=90°), where distance d=2 s is varied, is shown in  FIG. 4 . Mutual coupling in the case of the step-like common ground conductor  206  is weaker by greater than 10 dB than mutual coupling in the case of the flat common ground conductor  206  even for the smallest lateral distance d. For the step-like ground conductor  206 , the distance between such elements are much larger than the inter-element distance d due to the height H being preferably approximately 0.5λ τ , where λ τ  is the operating wavelength in free space.  
         [0034]     FIGS.  5 ( a ) and  5 ( b ) show a two-tiered, two-dimensionally corrugated plate antenna array  502  according to a further embodiment of the invention and a conventional planar plate antenna array  504 , respectively. Array elements of these arrays may be other types of radiators, such as microstrip patch antennas, tapered slot monopoles, or monopoles. The inclined angle of junction ground patches can vary from 0 to 90°.  
         [0035]     The anticipated reduction in the lateral size of the two-tiered, two-dimensionally corrugated plate antenna array in relation to conventional planar plate antenna arrays, both of which are square, while maintaining the same inter-element spacing, may be greater than 51% of the total lateral area or greater than 30% of each lateral dimension.  
         [0036]     Embodiments of the invention may be applied advantageously to antenna array applications, in particular, large-scale military phased arrays and commercial adaptive arrays and multiple-input-multiple-output subsystems. For example, the adaptive arrays presently and in the future may become very commonly used in wireless communications systems, such as 3G and beyond generations of cellular wireless communications systems. The reduced sizes and the suppressed mutual coupling benefits the antenna arrays and even systems with improvement in performances of the antenna arrays and the reduction in the installation space, resulting in low cost.  
         [0037]     In the foregoing manner, a laterally compact plate antenna array configured appropriately for reducing mutual coupling between plate array elements is disclosed. Although only a number of embodiments of the invention are disclosed, it becomes apparent to one skilled in the art in view of this disclosure that numerous changes and/or modification can be made without departing from the scope and spirit of the invention. For example, radiators in antenna arrays may be constructed from perfectly electrically conducting sheets of any shapes, such as rectangles, triangles, ellipses, polygons, annuli, or wires. Radiators may be installed at any angle with respect to corresponding ground patches. Radiators may be fed using a coaxial line, a microstrip line, aperture coupling, or waveguides. Junctions between two nearest adjacent ground patches at different tiers connecting the same may be of any shape, such as S, concave, convex, or multiple-step as shown in  FIG. 6 . Common ground conductors may also be folded or corrugated to form multi-tiers as shown in  FIG. 7 , therefore providing for multi-tiered antenna arrays. Common ground conductors may be constructed from perfectly electrically conducting and dielectric materials, or printed circuit boards (PCB). Antenna arrays may be planar or conformal with curviform surfaces, each tier being planar or conformal with curviform surfaces.