Abstract:
The present invention relates to active electronically scanned array antennas. A thin, low cost design is provided by coupling electromagnetic energy into periodically driven long slots ( 205 ) using circulators with integrated probes ( 107 ). The long slots ( 205 ) are formed as grooves ( 114 ) in a conductive base plate ( 103 ), each groove ( 114 ) bracketed on both sides by conductive strips ( 108 ). The circulators with integrated probes ( 107 ) are installed between the conductive strips ( 108 ) and the base plate ( 103 ), to reduce fabrication costs of the machined parts and to facilitate the making of connections between the circulators and the antenna electronics. The probes ( 128 ) protrude partway into the slots ( 205 ) and provide coupling to waves propagating in free space.

Description:
BACKGROUND 
     1. Field 
     Embodiments described herein relate to array antennas and in particular to active electronically scanned array antennas. 
     2. Description of Related Art 
     An active electronically scanned array (AESA) antenna is an antenna comprising multiple radiators, or elements, the relative amplitude and phase of which can be controlled, making it possible to steer the transmit or receive beam without moving the antenna. Such an antenna includes an aperture for transmitting or receiving waves traveling in free space, and it may include back-end circuitry, including electronics modules for generating signals to be transmitted and for processing received signals. Each element within the aperture may incorporate, or be connected to, a circulator, which separates the signals corresponding to transmit and receive channels, and which is connected to a transmit channel and a receive channel in the back-end electronics. The circulator may be fabricated as a microstrip circuit on a ferrite substrate, with a permanent magnet secured on or near the signal side of the substrate, and with a magnetic material, i.e., a material with a high magnetic permeability, on the ground plane side of the substrate to shape the magnetic field produced by the permanent magnet. 
     Prior art aperture structures include notch radiator arrays of the type described in U.S. Pat. No. 6,600,453, assembled from long, flat “sticks,” or “slats,” each including a series of notch radiators. In such an embodiment, a certain minimum notch depth may be required to achieve acceptable bandwidth, and the circulators may be installed in the plane of the sticks, resulting in a relatively deep aperture. 
     Another prior art aperture structure is disclosed in U.S. Pat. No. 7,315,288. This structure includes long slots spanning multiple array elements, periodically driven along their lengths. Probes in the form of current loops, located at intervals along each slot, excite the long slot. The probes, which are balanced transmission line or feed structures, are connected to single-ended transmit and receive electronics through baluns. In such a structure the baluns may be behind the radiators, and the circulators behind the baluns, and this combination may increase the depth of the antenna. Moreover the baluns may be a cause of electrical loss. 
     Especially in space-constrained applications such as in aircraft, it may be important to reduce the thickness and, thereby, the volume of an array antenna; moreover it is desirable to produce the antenna at a modest cost. Thus, there is a need for a low-cost, low-profile AESA antenna. 
     SUMMARY 
     Embodiments of the present invention provide a low-cost, low-profile array antenna. In an exemplary embodiment, the array antenna comprises an array of radiating elements, comprising a base plate having a surface comprising a plurality of grooves, a plurality of conductive strips on the base plate, and a plurality of circulators with integrated probes. Each circulator with integrated probe is coplanar with the base plate and secured between one of the conductive strips and the base plate. The conductive strips may be made of magnetic stainless steel, may have chamfers on their edges, and may be secured to the base plate using screws inserted through clearance holes in the conductive strips. The clearance holes in the conductive strips may be counterbored so that the screw heads do not protrude above the surface of the conductive strips, and oversized counterbores may be used to reduce the weight of the conductive strips. Additional lightening pockets may be formed in the conductive strips to further reduce weight. A wide-angle impedance matching (WAIM) sheet may be bonded to the front surface of the conductive strips. 
     In one embodiment of the invention, the circulators with integrated probes may be formed as microstrip circuits on ferrite substrates, with conductive pads at their transmit and receive ports. The array antenna may further include a multilayer printed wiring board (PWB) behind the array of radiating elements, and connections may be made between the multilayer PWB, and the conductive pads on the circulators with integrated probes, using straight coaxial conductor assemblies comprising floating spring pin center conductors. The antenna array may also include an eggcrate structure containing electronics modules, behind the multilayer PWB. The multilayer PWB may include a stripline translation layer to compensate for misalignments between connections in the electronics modules and the corresponding connections on the circulators with integrated probes. The multilayer PWB may also include a corporate feed network. The eggcrate structure may include a coolant manifold for cooling the electronics modules. The electronics modules may be held in place in the eggcrate structure by retainer springs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, aspects, and embodiments are described in conjunction with the attached drawings, in which: 
         FIG. 1  is a front exploded perspective view of a long slot aperture according to an embodiment of the present invention; 
         FIG. 2  is an enlarged fragmentary cross sectional view of a portion of a long slot aperture according to an embodiment of the present invention; 
         FIG. 3  is an enlarged rear perspective view of circulators on conductive strips, in a portion, situated within line  3  of  FIG. 1 , of the aperture; 
         FIG. 4  is an enlarged front view of a portion, situated within line  4  of  FIG. 1 , of the aperture; 
         FIG. 5  is an enlarged cross sectional view of a portion of a long slot aperture according to an embodiment of the present invention; 
         FIG. 6  is a rear exploded perspective view of a low profile long slot array antenna according to an embodiment of the present invention; and 
         FIG. 7  is an illustration of an electronics module and retainer spring according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of the presently preferred embodiments of a low profile array antenna provided in accordance with the present invention and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description sets forth the features of the present invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and structures may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention. As denoted elsewhere herein, like element numbers are intended to indicate like elements or features. 
     For the purpose of this description the surface of the antenna from which radiation may emanate will be referred to as the “front” of the antenna. Referring to  FIG. 1 , a long-slot aperture  100  may include a wide-angle impedance matching (WAIM) sheet  101 , conductive strips  108 , circulators with integrated probes  107 , and a base plate  103 . The assembly may be held together by screws  106  installed through counterbored holes  122  in the conductive strips  108  and clearance holes  118  in the base plate  103 , and threaded into threaded holes in a support plate such as the front wall  508  of a structure known as an “eggcrate” structure  503  ( FIG. 6 ). 
     The base plate  103 , which may be made of aluminum, contains several troughs or grooves  114  spanning its width, and several circulator cavities  116  immediately below, and spaced along, each groove  114 . The base plate  103  also has screw clearance holes  118 , and alignment pin holes (not shown). 
     Although the invention will function in any orientation, for the purpose of this description the orientation of the aperture, and of the antenna, will be that shown in  FIG. 1 . Each conductive strip  108 , except those in the bottom row, is installed against the base plate  103  so that its upper edge is flush with the lower edge of one groove  114 , and its lower edge is flush with the upper edge of the adjacent groove immediately below. The groove  114  and the edges of the adjoining conductive strips  108  form a slot  205  deeper than the groove  114  by itself ( FIG. 2 ). The edges of the conductive strips  108  may have chamfers  132  resulting in a slot  205  that flares at the front ( FIG. 2 ). The installation of the lowest row of conductive strips differs only in that there is no groove  114  below them in the base plate  103 . A conductive strip  108  is not needed above the uppermost groove if the base plate  103  has a thicker region or lip  120 , along its upper edge to provide the upper wall of the uppermost slot ( FIG. 2 ). 
       FIG. 2  shows an enlarged cross-sectional view through the slot  205 , showing circulator cavities  116  and lightening pockets  124  in cross section, as well as circulators with integrated probes  107 . 
     The aperture  100  also includes a plurality of circulators with integrated probes  107 . Referring to  FIG. 3 , each circulator with integrated probe  107  includes a circulator substrate  110 , a permanent magnet  126 , and a dielectric spacer  140  ( FIG. 2 ) installed between the permanent magnet  126  and the circulator substrate  110 . The substrate  110  may be made of ferrite. The magnet  126  may be bonded to the dielectric spacer  140 , and the spacer  140  to the substrate  110 , with a suitable adhesive, so that the dielectric spacer  140  will support the permanent magnet  126  at the desired separation from the ferrite substrate  110 . 
     The circulator with integrated probe  107  includes a circulator portion and a probe portion  128 . The circulator portion separates outbound waves from inbound waves at the antenna port, routing them from the transmit port of the circulator, or to the receive port of the circulator, respectively. The circulator portion may be constructed, for example, in the manner of the circulator disclosed in U.S. Pat. No. 3,935,548. The probe portion  128  of the circulator with integrated probe  107  couples waves traveling in a microstrip transmission line at the antenna port of the circulator to waves propagating in free space in front of the radiating aperture. The probe portion  128  may be formed as a conductive trace extending outwards from the circulator, on a tab formed for this purpose in the substrate  110 . In the assembled aperture  100 , the probe portion  128  may protrude into the slot  205  ( FIG. 2 ). 
     During assembly, the circulators with integrated probes  107  may be placed onto the conductive strips  108  and secured in place with an adhesive, such as conductive epoxy. This placement may be performed manually or robotically. The thickness of the adhesive layer may be approximately 0.002 inches (51 microns). 
     The conductive strips  108  are made of conductive material, which may also be magnetic, and which may have a coefficient of thermal expansion (CTE) similar to that of the substrate  110 . For proper function, in one embodiment each circulator with integrated probe  107  will be installed on the surface of a part made of a magnetic material. This part completes the magnetic circuit of the permanent magnet  126 , resulting in a suitable magnetic field in the circulator. The surface on which the circulator with integrated probe  107  is installed may also have a coefficient of thermal expansion similar to that of the substrate  110 . 
     In one embodiment, the conductive strips  108  are made of a magnetic stainless steel known as corrosion resistant steel (CRES). This material has all three desired properties: it is conductive, it is magnetic, and its CTE is similar to that of ferrite. In another embodiment the conductive strips  108  may be made of conducting material that is not magnetic, such as aluminum, and separate inserts made of a magnetic material with a suitable CTE may be installed between the circulators with integrated probes  107  and the conductive strips  108 . This embodiment may however result in increased fabrication cost. 
     The WAIM sheet  101  may be approximately 0.040 inches (1000 microns) thick, and it may be formed of a cyanate ester quartz laminate, fabricated from several sheets, each 0.005 inches (130 microns) thick, cured together. It provides an impedance match to free space, and it may also provide an environmental seal. 
     The clearance holes  118  in the conductive strips  108  may be counterbored so that in the assembly the heads of the screws  106  do not protrude above the front surfaces of the conductive strips  108 . This allows a flat WAIM sheet  101  to be bonded to the front surfaces of the conductive strips  108 . A polysulfide adhesive containing glass beads of uniform diameter may be used to bond the WAIM sheet  101  to the conductive strips  108 . For example, an adhesive containing 0.005 inch (127 micron) diameter beads will result in a 0.005 inch (127 micron) thick bond line between the WAIM sheet  101  and the conductive strips  108 . To reduce weight, the counterbored clearance holes  122  may have oversized counterbores, and lightening pockets  124  may be machined into the front face of each conductive strip  108 . Provided the lightening pockets  124  and counterbores are not too large, the contact area between the conductive strips  108  and WAIM sheet  101  may be adequate to form a strong bond between the conductive strips  108  and the WAIM sheet  101 , resulting in a mechanically robust assembly. The conductive strips  108  may have no machined features except for the counterbored holes  122 , chamfers  132 , lightening pockets  124 , and alignment pin holes, and, of these, only the alignment pin holes may require precision machining, which may result in low fabrication costs. The conductive strips  108  may be fabricated using computer numerical control (CNC) methods, such as fabrication on a CNC milling machine. 
     Referring to  FIG. 4 , the base plate  103  has two through holes in each circulator cavity  116  for two coaxial connectors, to form coaxial transmission line connections to the transmit and receive ports of the circulator with integrated probe  107 . Each coaxial connector  111  may consist of a dielectric cladding  134  holding a center conductor  305 , one end of which contacts a corresponding conductive pad  130  ( FIG. 3 ) on a circulator with integrated probe  107  when the antenna is assembled. The other end of the center conductor  305  may contact a conductive pad on a multilayer PWB  502 , which provides connections to antenna electronics modules  603  ( FIG. 6 ). In such an embodiment, the conductive wall of the through hole, the dielectric cladding  134 , and the center conductor  305  together form a coaxial transmission line. To provide contact pressure at both ends, the center conductor  305  in the coaxial connector  111  may be a floating spring pin, i.e., a compressible pin with an internal spring, fitting loosely within the dielectric cladding  134 . In another embodiment, the spring pin may comprise a non-floating central portion that is secured within the cladding  134 , and two spring-loaded contact pins, one at each end; however, this style of connector may be costlier to fabricate. 
     In an alternate embodiment, each circulator with integrated probe  107  may be installed with its permanent magnet  126  nearer the front of the antenna. In this case vias, or edge-wrap metallization, may be used to form connections between the front and back surfaces of the substrate  110 , and in particular to connect conductive traces on the front surface of the substrate  110  to the conductive pads  130  on the back surface of the substrate  110 . 
     The dielectric cladding  134  of the coaxial connector  111  may have a circumferential ridge  112  at each end ( FIG. 5 ). This circumferential ridge  112  may be nearly the same diameter as the hole, or slightly larger, so that it keeps the connector centered in the hole in the base plate. If, by design or as a result of manufacturing tolerances, the diameter of the circumferential ridge  112  exceeds that of the hole, then the circumferential ridge  112  will deform slightly during insertion of the coaxial connector  111 , resulting in a modest insertion force. If, instead of having circumferential ridges  112 , the dielectric cladding had a uniform outer diameter along its length, very tight fabrication tolerances would be required to simultaneously achieve accurate centering of the connector and an acceptable insertion force. 
     The base plate  103  may be made of aluminum and may be fabricated using a CNC machining process. In this application aluminum has several advantages over other materials: high electrical conductivity, low density, and being inexpensive to machine. In another embodiment the base plate may be made of a dielectric material with a conductive surface coating. 
       FIG. 5  shows a cross-sectional view of an exemplary embodiment of an aperture  100 , based on a cutting plane passing through two coaxial connectors  111 . Electromagnetic fields propagating along the coaxial connectors  111  and in the corresponding microstrip transmission lines on the substrate  110  form a transition between these transmission line structures. This transition represents an electrical discontinuity in the transmission line path, and precautions may be taken to minimize the reflections this discontinuity may otherwise cause. Such precautions may include adjusting the dimensions and shape of the center conductor  305  and ground conductors at and near the transition. They may also include adjusting the parameters of matching arms on the circulator with integrated probe  107 . These matching arms may include, for example, narrow or wide sections in the transmission lines connecting the circulator to the conductive pads  130 . 
     The details of the aperture design may be adjusted using software such as HFSS, sold by Ansys Incorporated, of Canonsburg, Pa. Using this software, a Floquet cell method, also known as a unit cell method or infinite array method, may be used to determine the electromagnetic fields in and in front of one antenna element within an infinite array. This solution then approximates the fields in and in front of an antenna element of a large finite array. Using this approach, detailed design parameters such as the dimensions of the slot  205 , the size and angle of the chamfers  132 , the dimensions of the microstrip sections in the matching arms, the shape of the conductive trace and the portion of the substrate in the probe  128 , the thickness of the WAIM sheet  101 , and the gap between the end of the probe  128  and the opposing wall of the slot  205  may be adjusted to obtain desired values, as functions of frequency and scan angle, for measures of performance such as the active reflection coefficient. 
     The aperture  100  may be integrated with an antenna back end, as shown in  FIG. 6 . Screws  106  extend through counterbored holes  122  in the conductive strips  108 , through clearance holes  118  in the base plate  103  ( FIG. 1 ) and in the multilayer PWB  502 , and into threaded holes in the front wall  508  of the eggcrate structure  503 , securing these parts together. Socket head cap screws, which unlike recessed cruciform screws may be installed with highly repeatable tightening torque, may be used. Alignment pins may be installed in corresponding holes in the conductive strips  108  and base plate  103  during assembly to ensure accurate registration of these parts. The circulators with integrated probes  107  may have been bonded to the conductive strips  108  in a prior assembly step ( FIG. 3 ). 
     Referring to  FIG. 7 , electronics modules  603  may be held in place in the eggcrate structure compartments by retainer springs  602 . Each retainer spring  602  has two wings  610 , each of which holds an electronics module  603  against the front wall  508  of the eggcrate structure  503 . Each retainer spring  602  also has two arms  608  that engage the undercut ends of a cutout  510  in the eggcrate structure wall separating the compartments containing the two electronics modules secured by the retainer spring  602 . 
     Referring to  FIG. 6 , a DC motherboard  505  may be secured to the rear surface of the eggcrate structure  503 , covering the compartments. The eggcrate structure  503  forms the structural backbone of the antenna, and its front wall  508  may contain a coolant manifold, which may be of the type disclosed in U.S. Pat. No. 7,032,651, comprising coolant cavities containing high density stamped or machined finstock. Coolant flowing through this manifold removes heat generated by the electronics modules  603 . Gaskets may be used between the electronics modules  603  and the front wall  508  of the eggcrate structure  503 , for the purpose of forming both a good electrical contact and a good thermal contact between the electronics module  603  and the front wall  508 . Such gaskets may contain a beryllium-copper foil with spring fingers for ensuring good electrical contact. They may also contain layers of thermally conductive material on both sides of the beryllium-copper foil for providing good thermal contact. The retainer springs  602  provide adequate pressure on the electronics modules  603  to compress the gasket. In one embodiment the retainer springs  602  may exert approximately 30 pounds (13.6 kg) of pressure on each electronics module  603  when installed. 
     Referring to  FIG. 6 , the multilayer PWB  502  may serve two purposes: it may provide a stripline corporate feed network and a stripline translation layer. This may be accomplished by using a PWB  502  consisting of four layers of dielectric and five conductive layers. Two layers of dielectric, e.g., the first two layers adjacent the front wall  508  of the eggcrate structure  503 , together with the three conductive layers in contact with them, may form a stripline corporate feed network. The remaining layers may form a stripline translation layer. The multilayer PWB  502  may be fabricated from copper conductive layers and dielectric layers made of a high molecular weight material such as CLTE, sold by Arlon-MED of Rancho Cucamonga, Calif. Other metals, or combinations of different metals, may be used to form the conductive layers. For example, it may be undesirable to have a copper layer in contact with an aluminum base plate  103  or an aluminum front wall  508 . In this case each outer conductive layer of the multilayer PWB  502  may instead be formed as a copper strike layer, plated with nickel and gold. Gold plating may improve corrosion resistance. 
     The translation layer compensates for offsets between conductive pads on the electronics modules  603  and corresponding pads  130  on the circulators with integrated probes  107 . For example, one electronics module  603  may have a pair of conductive pads which must be connected to a pair of conductive pads  130  on a circulator with integrated probe  107 , but the separation between the pads in each pair may be different, so that straight coaxial connectors  111 , perpendicular to the plane of the array, cannot be used. The translation layer resolves this difficulty by providing one pad, facing forward, aligned with a pad  130  on the circulator with integrated probe  107  and another pad, facing rearward, connected to the first with a stripline trace, aligned with the corresponding pad on the electronics module  603 . Straight coaxial connectors  111  can then be used to form connections between the translation layer and the circulators with integrated probes  107  and between the translation layer and the electronics modules  603 . In each case, coaxial connectors  111  with spring pin center conductors  305  may be used. 
     The corporate feed network distributes the outgoing signal to, and combines the received signal from, the electronics modules  603 . As with the translation layer, connections between the corporate feed layer and the electronics modules  603  may be made using straight coaxial connectors  111  with spring pin center conductors  305 . 
     The multilayer PWB  502  is sandwiched between two metal surfaces, viz., the surfaces of the base plate  103  and the front wall  508  of the eggcrate structure  503 . Thus, in another embodiment, one or both of the stripline layers in the multiplayer PWB  502  may be replaced with a channelized microstrip layer, by machining channels into the adjacent metal surface and modifying the PWB  502  accordingly. 
     Vias may be used in the multiplayer PWB  502  for several purposes. Signal vias may be used to bring a signal trace to the surface of the multilayer PWB  502 . A coaxial connector  111  may then form a connection with a surface pad surrounding such a signal via. The surface pad is preferably sufficiently large to ensure contact with the center conductor  305  of the coaxial connector  111  in the presence of manufacturing tolerances, but sufficiently small to avoid shorting against the wall of the hole holding the coaxial connector  111 . Ground vias, which connect ground layers together, may be used to provide electrical isolation between multiple signal paths in the multilayer PWB  502 , or to provide a uniform characteristic impedance for the transmission lines in the multilayer PWB  502 , especially at signal vias. Vias also may serve a mechanical purpose. Unlike dielectrics such as CLTE, vias have excellent dimensional stability in the presence of prolonged mechanical pressure. Absent the vias, the multilayer PWB  502  might become compressed after prolonged exposure to the clamping pressure of the screws  106 , allowing the entire assembly to loosen. Vias in the multilayer PWB  502  may prevent this from occurring. 
     Referring to  FIG. 6 , the DC motherboard  505  may provide low frequency functions such as supplying DC power to the electronics modules  603 , and it may be secured to the eggcrate structure  503  with threaded fasteners. Connections between the electronics modules  603  and the DC motherboard  505  may be formed using DC connectors  601  ( FIG. 7 ) which in one embodiment may comprise a dielectric body with multiple holes, and spring pin conductors installed in the holes to provide connections between corresponding contact pads on the electronics modules  603  and DC motherboard  505 . 
     Although limited embodiments of a low profile array antenna have been specifically described and illustrated herein, many modifications and variations will be apparent to those skilled in the art. Accordingly, it is to be understood that the low profile array antenna constructed according to principles of this invention may be embodied other than as specifically described herein. The invention is also defined in the following claims.