Abstract:
TEM horn array apertures are disclosed, comprises a plurality of stamped sheet segments interconnected to form an “egg crate” like array. Each stamped sheet comprises a plurality of TEM horn antennas with a series of cooperating slots, enabling the sheets to be assembled into an “egg crate” design that allows quick and inexpensive building of a TEM horn antenna array. Moreover, the stamped sheet segments are structurally and dimensionally stable, and thus the assembled array may be connected to a guide plane without the need for additional aligning or structural elements. A method of assembling the array is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation-in-part of U.S. nonprovisional patent application Ser. No. 10/290,733, filed Nov. 7, 2002, now U.S. Pat. No. 6,891,511 By Marc T. Angelucci, titled “Method of Fabricating a Radar Array,” the entire contents of which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to the field of microwave antenna arrays. 
   BACKGROUND 
   As the frequency of operation of radar antennas increases, the spacing between the radiating elements that make up the aperture becomes smaller. For example, the spacing may be less than 1.0 cm (0.400″) center-to-center at 16 GHz (Ku band). In addition, effective phased array radars can have 10,000 or more radiating elements. The radiating elements in these assemblies have critical alignment requirements. They also require isolation between adjacent radiating elements and excellent grounding. 
   Previous designs require manufacturing individual horn elements, each of which must be produced by a complicated machining process. The individual horn elements must then be mechanically interconnected to adjacent elements to assemble a horn array of a desired size. Such a process is complex both in the machining of the individual horn elements, as well as in the alignment and interconnection of adjacent elements. Thus, there is a need for an improved arrangement and assembly process for fabricating a horn array that reduces or eliminates complex machining steps, and which also simplifies the array assembly process. An improved array structure and method of making the array is thus disclosed. 
   SUMMARY OF THE INVENTION 
   An antenna array is disclosed, comprising a first strip of first horn segments, the first horn segments each including a radiating portion and a ground plane connection portion. A second strip is also provided having a plurality of second horn segments, the second horn segments each including a radiating portion and a ground plane connection portion. The first and second strips further can have cooperating slots disposed between the radiating and ground plane connection portions of each horn segment which are configured to interlock to allow the first and second strips to be assembled into a dimensionally stable lattice having an upper radiating end and a lower ground plane connecting end. A connector is associated with each horn segment of the first and second strips for connecting the horns to a transmit/receive distribution network. The lower ground plane connecting end of the lattice is configured to engage a ground plane for direct fixation thereto. 
   In one embodiment, when the first and second strips are interlocked, the plurality of first horn segments are disposed substantially perpendicular to the plurality of second horn segments. The first and second strips can be secured together by epoxy. 
   The connectors can be integrally formed with an associated fin line feed element. The plurality of first and second sheets are stamped or machined to form the associated plurality of horn segments. The plurality of first and second sheets can be made of aluminum or a metallized plastic. The metallized plastic sheets can be formed by a suitable molding or injection molding process. Alternatively, the plurality of first and second sheets can be made of copper or a chromium-copper alloy. 
   An antenna array is disclosed, comprising a plurality of first and second horn strips, where each horn strips can have a plurality of horn segments, and each of the horn segments can comprise a radiating portion and a ground plane connection portion. The first and second horn strips each can comprise cooperating vertically-disposed slots configured to allow pairs of first and second strips to lock together into a dimensionally stable lattice. A plurality of connectors can be associated with the horn segments of the plurality of first and second strips for connecting each horn segment to a transmit/receive distribution network. The lattice can have a ground plane connecting end can be configured to engage a ground plane for direct fixation thereto. 
   The plurality of first and second horn strips can be stamped or machined to form the associated plurality of horn segments. Also, the plurality of horn segments of each first strip in the lattice can be disposed substantially perpendicular to the plurality of second horn segments of each second strip in the lattice. The plurality of first and second strips can be secured together by epoxy. The connectors can be coaxial connectors. The plurality of first and second horn strips can comprise aluminum or a metallized plastic, or alternatively they can comprise copper or chromium copper alloy. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the present invention will be more fully disclosed in, or rendered obvious by, the following detailed description of the preferred embodiment of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts, and further wherein: 
       FIG. 1  is an isometric view of an exemplary microwave antenna array  100 ; 
       FIG. 2  is a partial assembly view showing a multi-element strip (of the type shown in  FIG. 1 ); 
       FIG. 2   a  is a perspective view of a fin line and connector assembly of the multi-element strip of  FIG. 1 ; 
       FIG. 3  is an isometric view showing the step of mounting an upper multi-element strip (of the type shown in  FIG. 1 ) on the strip of  FIG. 2 ; 
       FIG. 4  is an isometric view of the partial assembly of  FIG. 3  after both lower and upper multi-element strips are in position; 
       FIG. 5  shows the insertion of a plurality of upper multi-element strips into a plurality of lower multi-element strips; 
       FIG. 6  is a top plan view of a fractional array assembly; 
       FIG. 7  is a cross sectional view of the intersection of two multi-element strips, taken along section line  11 — 11  of  FIG. 4 ; 
       FIG. 8  shows an array comprising a plurality of array assemblies of the type shown in  FIG. 6  assembled to form a larger array; 
       FIG. 9  is an isometric view of a variation of the fractional array shown in  FIG. 1 . 
   

   DETAILED DESCRIPTION 
   In the accompanying drawings, like items are indicated by like reference numerals. 
   This description of the preferred embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description of this invention. In the description, relative terms such as “lower,” “upper,” “horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivative thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the apparatus be constructed or operated in a particular orientation. Terms concerning attachments, coupling and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. 
     FIG. 1  is an isometric view of an exemplary microwave antenna array assembly  100 . Array  100  may be a complete aperture or a fractional portion of a larger array suitable for use in a wide-band phased array radar system. In the exemplary array  100 , radiating elements are assembled vertically above a metal ground plane in a lattice of orthogonal pairs. The structure visually resembles an “egg crate” as shown in  FIG. 1 . This representative section of a large phased array aperture has 128 horns  134 ,  154  on a metal ground plane  110 . 
   The array assembly  100  comprises a ground plane  110  and a plurality of multi-element strips  130 ,  150 . The ground plane  110  may be a plate of suitable metal, such as aluminum or copper. The multi-element strips include at least one first (lower) strip  130  having a plurality of horns  134  ( FIG. 2 ) and a first plurality of slots  138 . The multi-element strips include at least one second strip  150  ( FIG. 3 ) mounted on the at least one first strip  130 . The second strip  150  has a plurality of horns  154  and a second plurality of slots  157  that mate with the slots  138  of the at least one first strip  130 . In the example of  FIG. 1 , there are a plurality of lower multi-element strips  130  and a plurality of upper multi-element strips  150  arranged in a lattice array. 
   Preferably, the strips  130 ,  150  are made of metal, such as aluminum, or metallized plastic to minimize the overall weight of the assembled array. Metallized plastic strips can be formed using a suitable molding or injection molding process. In such cases, conductive adhesive (e.g., conductive epoxy) may be used to form secure physical and electrical connections among the strips. Importantly, however, it should be possible to manufacture the strips using stringent tolerances so that they can simply be fit together without the need for adhesives. 
   Alternatively, the strips  130 ,  150  can be made of a solderable material (e.g., copper or a chromium-copper alloy), or have a solder or indium plating thereon. The strips in this example are made of chromium-copper (C18200 alloy). Solder plating on all surfaces provides solder volume and also protects the strips from the environment. When the multi-element strips  130 ,  150  are assembled, the solder can be reflowed to form secure physical and electrical connections among the strips. The solder may be tin-lead solder, for example. Other solder compositions or indium may be used. In other embodiments, solder can be applied in situ after assembly of the strips  130 ,  150  onto the ground plane  110 . 
   The strips  130 ,  150  are preferably individually stamped or machined from unitary pieces of material (again, aluminum, metallized plastic, etc.). The advantage of providing stamped or machined metal strips is that they can be fabricated to precise dimensions with tight tolerances that will remain stable when subject to the long term operational environment. Such dimensional stability ensures that when the strips  130 ,  150  are assembled together, the horns  134 ,  154  will be precisely positioned with respect to each other, and that this precise positioning will be maintained throughout operation. Thus, no adhesives should be required to maintain this positioning, nor should additional alignment hardware (e.g. positioning clips) be required. 
   The strips  130 ,  150  are self-aligning to ensure that, when assembled (as described below) assembly, the adjacent horns  134 ,  154  are accurately positioned vertically, laterally, and rotationally with respect to each other. The rotational alignment of the strips  130 ,  150  is provided by the multiplicity of interconnections between the slots  138 ,  157  that must be aligned in order for the various strips  130 ,  150  to lie straight. 
     FIGS. 3 and 4  show a method for assembling a sub-assembly having one lower multi-element strip  130  and one upper multi-element strip  150 . 
   As shown in  FIG. 3 , the strip  130  has a plurality of identical horns  134 . The horns  134  can be formed by stamping or machining as previously described. Each horn  134  has a connector  136  for connecting the element to the transmit/receive distribution network (not shown). An exemplary connector is a Gilbert SK-1896-2 edge-launch male connector manufactured by the Corning Gilbert Corporation of Glendale, Ariz., which can be used to connect a fin line feed  1136  ( FIG. 2   a ) directly to a coaxial cable. As shown in  FIG. 2   a , The fin line feed  1136  can be a microstrip circuit  1139  bonded to a carrier  1138 . The microstrip circuit  1139  can comprise a ceramic (e.g. alumina) or duroid substrate  1139   a  having a thin layer of etched copper  1139   b  disposed thereon. The fin line feed  1136  with connector  136  can be attached to the associated horn  134  using any appropriate technique known in the art, such as brazing, epoxy, etc. The carrier  1138  may have a vertical slot  1137  configured to correspond to a slotted portion  135  of the associated horn  134 . The microstrip circuit  1139  itself will overlie the vertical slot  1137  and part of the slotted portion  135  of the associated multi-element strip  130 . 
   The lower multi-element strip  130  has a plurality of slots  138  with a respective slot between each pair of adjacent horns  134 . Slots  138  are located at an edge  132  distal from the ground plane  110 , to receive bridge portions  152   b  of the upper strips  150 . In alternative embodiments (not shown) having more than two circuit boards intersecting at a single point, the slot  138  may receive connecting portions of two or more strips (which may be accomplished by providing a longer slot  138 , or shorter connecting portions on the upper strip). 
   The bottom edge  132   a  of strip  130  abuts the ground plate  110  to locate the strip  130  for properly seating the connector  136  to mate with the distribution network (not shown). 
     FIG. 4  shows an assembly step in which the upper multi-element strip  150  is being moved into position with the lower multi-element strip  130 . The upper multi-element strip  150  is similar to the lower multi-element strip, with significant differences explained below. Otherwise, like features of strip  150  have the reference numerals of corresponding features of strip  130 , increased by 20. These like features include horns  154 , connector  156 , and fin line feeds. 
   In strips  150 , the slot  157 , extending from the bottom edge  152   a  proximate to the ground plane  110  is longer than the slot  138  of the lower strip  130  by an amount that is approximately the height of the connecting section  132   c  of the lower strip. The bridge (attachment portion)  152   b  is at or adjacent to the distal (top) edge  152  of the strip  150  from the ground plane  110 . There is no need for a second slot above the bridge  152   b , and in preferred embodiments, there is none (although alternative embodiments—not shown—may optionally include a second slot, for example, to accommodate one or more additional strips). The bridge  152   b  is received by the slot  138  of the strip  130 . 
   Slots  138  and  157  intersect like a pair of intersecting combs. When the connecting strip  150  is in its final position, the bridge section  152   b  may optionally abut the top of connecting section  132   c , or a small space may be allowed between them. When assembled in this manner, the multi-element strips  130 ,  150  including the horns  134 ,  154  are self-aligned in all axes to pre-established reference locations on the ground plane  110 . These pre-established reference locations can, for example, consist of slots or holes in the ground plane  110  into which corresponding tabs or dowels on the strips  130 ,  150  can fit. In one embodiment, the tabs or dowels can be configured to snap into the corresponding reference locations, further facilitating assembly of the array, by eliminating the need for additional hardware or soldering steps to fix the strips to the ground plane. Appropriate ratchet surfaces or other snap-in features can be used for this purpose. Alternatively, the corresponding mechanical engagement features of the strips and ground plane can be sized and toleranced to be press-fit together. As will be appreciated, other similar connection schemes can be used to engage the strips with the ground plane. 
     FIG. 4  shows the lower strip  130  and upper strip  150  in the assembled configuration. In the example, the horns  134  and  154  are positioned at the same height, with the connectors  136  and  156  also at the same height. The metal-to-metal contact between strips  130 ,  150  provides isolation and prevents leakage between adjacent horns  134 ,  156 . A cross-sectional plan view of a connection “node”  160  between strips  130 ,  150  is shown in  FIG. 7 . 
   Although  FIGS. 2–4  show a structure and method for assembling two perpendicular multi-element strips into an array of square cells, other embodiments may have strips that are not orthogonally, e.g., to form a lattice of polygonal cells (e.g., diamond or hexagonal shaped cells). 
   Furthermore, although the arrangement of  FIGS. 2–4  have only two strips intersecting at each junction or “node”  160  ( FIG. 7 ), in other embodiments, three or more multi-element strips can intersect at a single junction. 
     FIG. 4  shows the assembly of an individual joint, however, the design of the strips  130 ,  150  permits an entire horn array to be preassembled in one process. This may be accomplished by assembling multiple multi-element strips  130 ,  150  containing a plurality of horns, to create a lattice as shown in  FIG. 5 , and then placing the lattice on the ground plane  110  as shown in  FIGS. 5–6 . As previously noted, the ground plane  110  can have a plurality of pre-established reference locations (e.g., slots, recesses or other features) that correspond to specific portions of strips  130 ,  150  and which guarantee accurate positioning of the strips (and thus the array) on the ground plane  110 . Alternatively, when using strips configured to be soldered together, or to be soldered to the ground plane  110 , the array of horns  134 ,  154  can be joined together and the solder in the assembly  100  can then be reflowed en masse, thus connecting the strips  130 ,  150  to each other and to the ground plane in one reflow step. 
     FIGS. 5 and 6  show an exemplary method of assembling the array shown in  FIG. 1 . Successive rows of strips  130 ,  150  are interlaced so that their slots  138 ,  157  are oriented at ±45 degrees from the vertical edge of the ground plane  110 , and ±45 degrees from the horizontal edge of the ground plane. As a result, as best seen in  FIG. 6 , the horns are oriented at ±45 degree angles from horizontal. In  FIG. 6 , four different strip lengths (corresponding to two, four, six and eight horns, respectively) are sufficient to form a 64 element array  100  having a rectangular form factor. The number of different strip lengths used for any given array  100  depends on the lesser of the number of rows and the number of columns of horns. An array  200  having a desired number of horns can be formed using a plurality of such arrays  100 . 
   Multiple individual arrays can also be assembled together to form a larger array. For example, as shown in  FIG. 8 , an  1152  horn array  200  can be formed of 18 individual array sections  100 . Due to the tight fit between the individual strips  130 ,  150 , and the fact that the strips are made of metal, the arrays  100  are expected to be sufficiently rigid that they will not need to be physically connected to one another. The junctions between the fractional arrays can, however, be joined using epoxy, or solder. 
   Referring again to  FIG. 5 , the first strips  130  may have respectively different lengths, so as to provide the array  100  with any desired shape (e.g., rectangular, square or approximately octagonal or circular), with a respective one of the plurality of clips at each corner of each polygonal cell. 
     FIG. 5  shows a plurality of second (upper) strips  150  being inserted on the first strips  130 , with the strips  150  parallel to each other, and normal to the first strips  130 . The intersecting lower and upper strips  130 ,  150  form an array of square cells, with respective horns  134 ,  154  within each of the cells. The second (upper) strips  150  have a plurality of horns  154  and a first plurality of slots  157 . As shown in  FIG. 5 , the second strips  150  may also have respectively different lengths, so as to provide the array  100  with any desired shape (e.g., rectangular, square or approximately octagonal or circular). 
   The advantage of the present design is that a large array can be manufactured from sheets of metal (or metallized plastic) without the need for constructing fractional array elements which themselves must be fit together to make the large array. In one example, using aluminum, it is expected that individual strips could be manufactured, each having 100 horn elements  134 ,  154 , and that a large array could be constructed from these individual strips. 
   Although the invention is expected to be advantageous for forming large arrays without the need for smaller fractional arrays, such smaller arrays can still be made. Thus,  FIG. 6  shows and exemplary assembled array  100 . 
     FIG. 7  is an enlarged detail of  FIG. 1 , showing a pair of adjacent cells in detail. A conductive joint or “node”  160  is formed along each edge of the slots  138 ,  157  of the first and second multi-element strips  130 ,  150 . These “nodes”  160  consist of the metal-to-metal interface between the slots  138 ,  157  in the upper and lower strips  130 ,  150 . The tight tolerances used to form the strips  130 ,  150 , and the fact that the metal or metalized plastic construction of the strips is expected to hold these tolerances for an extended term, even under shock conditions, result in structural “nodes” in the array  100  which make the “egg crate” structure of the array very rigid. 
     FIG. 9  shows another arrangement of the lower strips  130  and upper strips  150 . Although the embodiment shown in  FIGS. 1–7  has a triangular lattice pattern, other configurations are contemplated. For example, in alternative embodiments such as that of  FIG. 9 , the rows and columns of strips  130 ,  150  are all aligned in a plain rectangular grid  300 , which may be square. In this configuration, each lower multi-element strip  130  has the same first number of horns, and each upper multi-element strip  150  has the same second number of horns, where the first and second numbers may be the same as, or different from, each other. 
   In the examples of  FIGS. 1–7  and  9 , each collinear arrangement of horns is provided using a single multi-element strip  130  or  150 . In other embodiments (not shown), a relatively long collinear arrangement of horns may be provided using two or more aligned multi-element strips, where the abutting ends of each strip comprise a respective half of the slots  138  (for lower strip  130 ) or  157  (for upper strip  150 ). This technique may be used to reduce the number of different multi-element strip sizes in an array having a large number of rows and a large number of columns of horns. 
   A method for fabricating an array  100  will now be described, with reference to the foregoing figures. 
   First, a plurality of multi-element strips  130 ,  150  are stamped or machined from individual pieces of selected material (e.g. aluminum, metallized plastic). The fin line feeds  1136  are then constructed by bonding a microstrip  1139  to a carrier element  1138  using, for example, a thermosetting adhesive, and then attaching a coaxial connector  136 ,  156  to the microstrip  1139 . The fin line feeds  1136  can then be bonded, for example, using a thermosetting adhesive, to each radiating element  134 ,  154  of the multi-element strips  130 ,  150 . 
   For embodiments in which a solder material is used to connect the strips together, or to the ground plane  110 , solder can be applied to the bottom edges of the multi-element strips  130 ,  150  and/or to the inside edges of the slots  138 ,  157 . If solder is not used, then this step is skipped. 
   Gaskets (not shown) may then be inserted into each connector-receiving hole in the plate  110 . The gaskets can be positioned so that, in the finished assembly, the gasket lies beneath the associated connector  136 ,  156 . The gasket can provide EMI shielding, a weather seal for the marine environment, and a light pressure seal. The gasket may be, for example, a Cho-Seal 1298 corrosion resistant EMI gasket manufactured by Parker Chomerics of Woburn, Mass. 
   The upper strips  150  are then pressed into position on the corresponding lower strips  130  ( FIGS. 3–5 ). For embodiments in which the strips are epoxied together, a suitable epoxy can be applied to the slot surfaces prior to pressing the strips together. 
   Once the strips  150 ,  130  have been assembled to create an array of the desired size, the connectors  136 ,  156  can be fit into the associated openings the ground plane  110 . For the preferred embodiment in which pre-established reference locations (e.g. slots or holes) are provided on the ground plane  110  to engage corresponding features in the strips  130 ,  150 , the assembled strips can be fit (e.g. snap-fit, press-fit, or the like) into the reference locations, thus aligning the array on the ground plane  110 . 
   In one alternative embodiment, the connectors can be used to fix the strips to the ground plane. Standard hardware can be used for this purpose (e.g., screws and washers located on the bottom surface of the ground plane  110 ) and can be torqued to fix the connectors  136 ,  156  (and thus the strips  130 ,  150 ) to the plate  110 . 
   In alternative embodiments employing solder connections between strips and/or between the strips and the ground plane  110 , the solder can be reflowed to form the desired electrical and mechanical connection. In some embodiments, the entire array  100  can be placed in a reflow oven (not shown) for this purpose. In other embodiments, a local heating tool can be used to reflow the solder locally only at the boundaries. In some embodiments, a reflow tool (not shown) can be used to apply radiant heat at the desired location. An example of a reflow tool includes a plurality of heating elements, each including a cartridge heater at the center of a ceramic insulator. The insulators may have cutouts to direct the radiated heat. These heating elements may be configured in a one or two dimensional array. 
   Where soldered arrays are used for developing a larger array, the individual arrays  100  can be formed (i.e. soldered) independently. Alternatively, the strips  130 ,  150  for each individual array  100  can be connected together, and the individual arrays  100  positioned adjacent one another, followed by a mass reflow of the entire array. The interaction between the slotted multi-element plates  130 ,  150  facilitates the use of a reflow process that can assembly very large arrays of horns. A reflow process of this type is advantageous for high frequency-wide band radiating elements where a small lattice—for example &lt;1.0 cm (&lt;0.400″) center to center at 16 GHz—limits the working space when using local soldering or epoxy attachments that require alignment tooling. 
   Some advantages of the attachment method described above are that the solid metal strips with slots eliminate the need for alignment tooling or alignment clips to position the horns vertically, laterally, or rotationally. 
   Furthermore, although the radiating elements  134 ,  154  have been illustrated as being elliptical in shape, other configurations are also possible. For example the radiating elements  134 ,  154  could have a stepped or corrugated configuration. 
   Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the invention, which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.