Patent Publication Number: US-6703980-B2

Title: Active dual-polarization microwave reflector, in particular for electronically scanning antenna

Description:
The present invention relates to a dual polarization active microwave reflector with electronic scanning, capable of being illuminated by a microwave source in order to form an antenna. 
     It is known to produce antennas comprising an active microwave reflector. The latter, also called a “reflect array”, is an array of phase shifters which can be controlled electronically. This array lies in a plane and comprises an array of elements with phase control, or a phased array, placed in front of the reflecting means, consisting, for example, of a metal ground plane forming a ground plane. The reflecting array especially comprises elementary cells each one producing reflection and phase shifting, variable by electronic control, of the microwave that it receives. An antenna of this sort provides considerable beam agility. A primary source, for example a horn, placed in front of the reflecting array emits microwaves toward the latter. 
     One aim of the invention is especially to make it possible to produce an electronic scanning antenna using an active reflecting array and operating with two independent polarizations. To this end, the subject of the invention is an active microwave reflector, capable of receiving an electromagnetic wave, comprising two imbricated waveguide arrays. The bottom of each guide is closed by a circuit carrying out the reflection and the phase shifting of the wave that it receives, one array being designed to receive one polarization and the other array being designed to receive a polarization perpendicular to the previous one. 
     One embodiment may be such that: 
     a first array comprises several sets of aligned guides, one row lying in a direction Ox and the set of rows lying in a perpendicular direction Oy, for the same row, the centers C of two consecutive guides being separated by a distance d, two consecutive rows being separated by a distance h, along Oy, and offset one with respect to the other by the distance d/2, along Ox; 
     the second array comprises several sets of guides aligned in the same way as in the first array, the rows being offset by an angle of 90° with respect to those of the first array; 
     a guide of one array is contiguous only with guides of the other array. 
     The subject of the invention is also an electronic scanning antenna comprising a reflector as defined above. This antenna may, for example, be of the “Reflect Array” type or of the Cassegrain type. 
     The particular advantages of the invention are that it makes it possible to obtain a compact, low-weight reflector, that it is simple to use and that it is economical. 
    
    
     Other characteristics and advantages of the invention will become apparent using the following description made with reference to the appended drawings which show: 
     FIG. 1, an exemplary embodiment of an electronic scanning antenna with an active microwave reflector; 
     FIG. 2, an illustration of the principle for producing a reflector according to the invention; 
     FIG. 3, an exemplary embodiment of a phase shift cell; 
     FIGS. 4 a ,  4   b  and  4   c , an illustration of a possible imbrication mode of the arrays of guides of a reflector according to the invention; 
     FIG. 5, by means of a sectional view, the possible layers constituting a reflector according to the invention; 
     FIG. 6, a possible embodiment of the arrays of guides of a reflector according to the invention; 
     FIG. 7, an additional embodiment especially making it possible to reduce the standing wave ratio. 
    
    
     FIG. 1 schematically illustrates an exemplary embodiment of an electronic scanning antenna with an active reflecting array with respect to an orthonormal coordinate system Oxyz. In this exemplary embodiment, the microwave distribution is, for example, of the so-called optical type, that is to say, for example, provided using a primary source illuminating the reflecting array. To this end, the antenna comprises a primary source  1 , for example a horn. The primary source  1  emits microwaves  3  toward the active reflecting array  4 , placed in the plane Oxy. This reflecting array  4  comprises a set of elementary cells producing the reflection and the phase shifting of the waves that they receive. Thus, by controlling the phase shifts impressed onto the wave received by each cell, it is possible, as is known, to form a microwave beam in the desired direction. With a reflector according to the invention, the primary source  1  may be with double polarization. 
     FIG. 2 illustrates the principle of producing a reflector according to the invention. The latter comprises two imbricated waveguide arrays  21 ,  22 . These guides are viewed along F, that is to say along an end-on view of the reflector  4 . The figure therefore especially shows the cross section of the guides in the plane Oxy, the walls of the guides lying in the direction Ox. Each guide belongs to an elementary cell, as mentioned above. A first guide array  21  is designed to receive the vertical polarization and a second guide array  22  is designed to receive the horizontal polarization. The incident microwaves  3  enter the guides. Each guide  21 ,  22  is short-circuited by a phase shifter, as described, for example, in French patent application No. 97 01326, which can be controlled with two to four bits or more. 
     FIG. 3 schematically illustrates a phase shift cell. This therefore comprises a guide  21 ,  22  and a phase shift circuit  31 , the latter being placed at the bottom of the guide in the plane Oxy. A phase shift circuit  31  comprises at least one conducting wire  32 ,  33 , itself carrying at least two semiconductors D 1 , D 2 , for example diodes, with two states. The conducting wires and the diodes are placed on a dielectric support  34 , the opposing face of which comprises a conducting plane reflecting the microwave. This conducting plane is, for example, in electrical contact with the walls of the guide  21 ,  22 . An elementary cell  31  then carries out the reflection and the phase shifting of the microwave  3  that it receives for the component of the wave whose polarization is substantially parallel to the conducting wires  32 ,  33 . By way of example, the cell of the sort illustrated in FIG. 3 acts on a wave polarized in the direction Oy parallel to the direction of the conducting wires  32 ,  33  of the cell. In horizontal polarization, only the guides designed to receive this polarization are active, the others being short-circuited. Similarly, in vertical polarization, only the guides designed to receive this polarization are active, the others being short-circuited. 
     FIGS. 4 a ,  4   b  and  4   c  illustrate a possible imbrication mode of the two guide arrays. FIG. 4 a  shows three guides  21  of the first array, representing a grid, designed for example to receive the vertical polarization. FIG. 4 b  shows three guides  22  of the second array, representing a grid, designed for example to receive the horizontal polarization. In any case, the two arrays are designed to receive waves with crossed polarizations, the second array of guides  22  being allocated to a polarization perpendicular to the polarization of the first array of guides  21 . The cross section of each guide comprises a midpoint C. Since this cross section is angular, the midpoint C is the intersection of its two mid lines. The cross sections of the guides are shown in the plane Oxy of the reflector. By way of example, the axis Ox is considered to correspond to the direction of a first polarization. Similarly, the axis Oy is considered to correspond to the direction of the second polarization, crossed with respect to the first. For the purpose of simplification and by way of example, hereinafter, the direction Oy may be considered equivalent to the vertical direction and the direction Ox to the horizontal direction. 
     FIG. 4 a  therefore shows a first array of guides  21  designed to receive the vertical polarizations. The array comprises several sets of aligned guides. One row of guides lies in the horizontal direction Ox and the set of rows lies in the vertical direction Oy. For the same row, the centers C of two consecutive guides  21  are separated by a distance d. Two consecutive rows are separated by a distance h, along Oy, and offset one with respect to the other by the distance d/2, along Ox. In other words, two consecutive mid lines  41 ,  42  are a distance h apart, the mid-lines being the mid-lines of the guides taken along Ox. Between two consecutive rows, there is an offset of d/2 of the midpoints of the guides. 
     FIG. 4 b  shows the second array of guides  22  designed to receive the horizontal polarization. The arrangement of the guides is similar to that of the array of FIG. 4 a , but with a rotation of the set by 90°. In this case, the rows lie along the axis Oy and the set of rows lies along the axis Ox. For the same row, the centers C of two consecutive guides  22  are separated by a distance d. Two consecutive rows are separated by a distance h, along Ox, and offset one with respect to the other by the distance d/2, along Oy. In other words, two consecutive mid lines  43 ,  44  are a distance h apart, the mid lines being the mid-lines of the guides taken along Oy. Between two consecutive rows, there is an offset of d/2 of the midpoints of the guides. 
     FIG. 4 c  defines the imbrication of the two arrays of guides by showing how a guide  22  of one array is positioned with respect to the guides  21  of the other array. This guide  22  is contiguous only with guides  21  of the other array. In the case of FIG. 4 c , the guide  22  is contiguous with four guides  21  of the other array. The midpoint C of this guide  22  is aligned with the midpoints of the two pairs of guides  21  surrounding the guide  22 . Thus a lattice, as illustrated in FIG. 2, is obtained. The internal dimensions of the waveguides  21 ,  22  are, for example, 0.6 λ and 0.3 λ (λ=length of the wave  3 ) in length and in width, respectively, the length of the guides lying along the rows of the arrays. The distance d between the midpoints C of two consecutive guides of the same row is then, for example, equal λ and the distance h between the mid lines  41 ,  42 ,  43 ,  44  of two consecutive rows is, for example, λ/2. By way of example, for a microwave 3 at 10 GHz, the internal dimensions of a waveguide are 1.8 cm and 0.9 cm, and the distances d and h are 3 cm and 1.5 cm, respectively. This lattice especially makes it possible for the beam reflected by the reflector  4  to be deflected over a cone of about 60°. 
     FIG. 5 shows, by means of a sectional view, the possible layers constituting a reflector according to the invention. It comprises at least three layers  51 ,  52 ,  53 . A first layer  51  comprises the microwave phase shift circuits, that is to say in particular the diodes D 1 , D 2 , the conducting wires which carry them and the associated connection circuits. The microwave circuits are for example supported by a substrate  54 . On the face opposite the microwave circuits, this substrate is covered with a metalized layer  56 , forming a conducting plane, which especially has the function of reflecting the microwaves  3 . In the X band, the thickness e h  of the substrate is, for example, about 3 mm, the relative dielectric constant ε r  being about 2.5. A second layer  52  comprises the circuits  55  for controlling the diodes D 1 , D 2  of the phase shifters. This layer moreover provides the connection between the control circuits and the diodes. To this end, it has, for example, the structure of a multilayer printed circuit comprising planes interconnecting the control circuits to the microwave circuits. Finally, a third layer  53 , placed facing the microwave circuits D 1 , D 2 , comprises the two waveguide arrays. 
     FIG. 6 shows a possible embodiment of the layer of waveguides  53 . This embodiment is especially easy to implement. The walls of the guides  21 ,  22  are made by plated-through holes  61 ,  62  oriented in the direction Oz. These plated-through holes could be replaced by conducting wires, that is to say rectilinear electrical conductors, oriented in the direction Oz. The guides thus produced have, for example, common wall parts, that is to say that plated-through holes  63 ,  64  are common to two guides. In this case, two neighboring guides have plated-through holes in common. The plated-through holes are made in a dielectric plate of thickness e g , this thickness constituting the length of the guides. The plated-through holes are sufficiently close to act as waveguide walls. These plated-through holes  61 ,  62  therefore pass through the entire third layer  53 . They extend into the microwave layer  51  in order to reach the conducting plane  56 . They thus make it possible moreover to electromagnetically decouple each phase shift circuit  32 ,  33 , D 1 , D 2  from its neighbors by forming an electromagnetic shield. There is then no wave propagation from one cell to the other. Advantageously, some plated-through holes  61 ,  64  may extend into the layer  52  comprising the control circuits. These extending holes especially make it possible to connect the control circuits electrically to the diodes of the phase shift circuits of the microwave layer  51 . These plated-through holes  61 ,  64  thus carry the control of the diodes and the electrical supply of the circuits. They are for example connected to the various interconnection planes of the control layer  52 . By way of example, the plated-through holes  61 ,  64  shown in black are also used for the supply and the control of the microwave circuits. These holes  61 ,  64  especially pass through the conducting plane  56  with no electrical contact therewith. The other holes  62 ,  63  stop, for example, at this conducting plane  56 , in electrical contact therewith. The thickness e g  of the waveguide layer is for example about one centimeter. It is necessary for example to provide hollows in this layer  53  of guides in order to house the diodes D 1 , D 2  of the microwave layer  51 . Advantageously, the weight of a reflector according to the invention is low because of the low weight of the various layers. Moreover, despite the waveguide layer, the reflector still remains compact. 
     FIG. 7 illustrates an additional embodiment making it possible especially to reduce the standing wave ratio (SWR) active in the guides. The input of the guides  21 ,  22  comprises an iris  71  with a rectangular opening, the assembly being closed by a dielectric plate  72 . In this embodiment, the waveguide layer  53  may be covered with a layer forming the irises, the assembly being closed by a dielectric layer. 
     A reflector according to the invention may be used for various types of antennas. It may be used as illustrated in FIG. 1 to form an antenna of the “reflect array” type. Similarly, it may be used in an antenna of the Cassegrain type. In the latter case, the primary source is placed at the center of the reflector and illuminates an auxiliary reflector. In its turn, the latter illuminates, by reflection, the reflector according to the invention. 
     A reflector or an antenna according to the invention are simple to use. They are also economical, since the components and the technologies used are cheap. Moreover, the invention provides all the advantages connected with dual polarization. An antenna according to the invention may thus, for example, be used for polarimetry measurements on targets, especially by emitting with one polarization and receiving with the other polarization. It may be used in telecommunications applications, for example dual-band applications.