Abstract:
Interconnections are made through a planar circuit by a monolithic short-circuited transmission path which extends from a circuit portion of the planar circuit to the opposite side. The opposite side is ground sufficiently to remove the short-circuiting plate, thereby separating the previously monolithic conductors, and exposing ends of the separated conductors of the transmission path. Connection is made between the exposed conductors of the transmission path and the registered contacts of a second planar circuit by means of electrically conductive, compliant fuzz buttons. The transmission path may be a coaxial path useful for RF.

Description:
This application is a division of application Ser. No. 09/070,033, filed Apr. 30, 1998. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to RF (including microwave) interconnections among layers of assemblies of multiple integrated circuits, and more particularly to interconnection arrangements which may be sandwiched between adjacent circuits. 
     BACKGROUND OF THE INVENTION 
     Active antenna arrays are expected to provide performance improvements and reduce operating costs of communications systems. An active antenna array includes an array of antenna elements. In this context, the antenna element may be viewed as being a transducer which converts between free-space electromagnetic radiation and guided waves. In an active antenna array, each antenna element, or a subgroup of antenna elements, is associated with an active module. The active module may be a low-noise receiver for low-noise amplification of the signal received by its associated antenna element(s), or it may be a power amplifier for amplifying the signal to be transmitted by the associated antenna element(s). Many active antenna arrays use transmit-receive (T/R) modules which perform both functions in relation to their associated antenna elements. The active modules, in addition to providing amplification, ordinarily also provide amplitude and phase control of the signals traversing the module, in order to point the beam(s) of the antenna in the desired direction. In some arrangements, the active module also includes filters, circulators, andor other functions. 
     A major cost driver in active antenna arrays is the active transmit or receive, or T/R module. It is desirable to use monolithic microwave integrated circuits (MMIC) to reduce cost and to enhance repeatability from element to element of the array. Some prior-art arrangements use ceramic-substrate high-density-interconnect (HDI) substrate for the MMICs, with the substrate mounted to a ceramic, metal, or metal-matrix composite base for carrying away heat. These technologies are effective, but the substrates may be too expensive for some applications. 
     FIG. 1 illustrates a cross-section of an epoxy-encapsulated HDI module  10  in which a monolithic microwave integrated circuit (MMIC)  14  is mounted by way of a eutectic solder junction  16  onto the top of a heat-transferring metal deep-reach shim  18 . The illustrated MMIC  14 , solder  16 , and shim  18  are encapsulated, together with other like MMIC, solder and shim assemblies (not illustrated) within a plastic encapsulant or body  12 , the material of which may be, for example, epoxy resin. The resulting encapsulated part, which may be termed “HDI-connected chips” inherently has, or the lower surfaces are ground and polished to generate, a flat lower surface  12   ls . The flat lower surface  12   ls , and the exposed lower surface  18   ls  of the shim, are coated with a layer  20  of electrically and thermally conductive material, such as copper or gold. As so far described, the module  10  of FIG. 1 has a plurality of individual MMIC mounted or encapsulated within the plastic body  12 , but no connections are provided between the individual MMICs or between any one MMIC and the outside world. Heat which might be generated by the MMIC, were it operational, would flow preferentially through the solder junction  16  and the shim  18  to the conductive layer  20 . 
     In FIG. 1, the upper surface of MMIC  14  has two representative electrically conductive connections or electrodes  14   1  and  14   2 . Connections are made between electrodes  14   1  and  14   2  and the corresponding electrodes (not illustrated) of others of the MMICs (not illustrated) encapsulated within body  12  by means of HDI technology, including flexible layers of KAPTON on which traces or patterns of conductive paths, some of which are illustrated as  32   1  and  32   2 , have been placed, and in which the various layers are interconnected by means of conductive vias. In FIG. 1, KAPTON layers  24 ,  26 , and  30  are provided with paths defined by traces or patterns of conductors. The layers illustrated as  24  and  26  are bonded together to form a multilayer, double-sided structure, with conductive paths on its upper and lower surfaces, and additional conductive paths lying between layers  24  and  26 . Double-sided layer  24 / 26  is mounted on upper surface  12   us  of body  12  by a layer  22  of adhesive. A further layer  30  of KAPTON, with its own pattern of electrically conductive traces  32   2 , is held to the upper surface of double-sided layer  24 / 26  by means of an adhesive layer  28 . The uppermost layer of electrically conductive traces may include printed antenna elements in one embodiment of the invention. As mentioned above, electrical connections are made between the conductive traces of the various layers, and between the traces and appropriate ones of the MMIC contacts  14   1  and  14   2 , by through vias, some of which are illustrated as  36 . The items designated MT 0 , MT 1 , MT 2 , and MT 3  at the left of FIG. 1 are designations of various ones of the flexible sheets carrying the various conductive traces. Those skilled in the art will recognize this structure as being an HDI interconnection arrangement, which is described in U.S. Pat. No. 5,552,633, issued Sep. 3, 1996 in the name of Sharma. 
     As illustrated in FIG. 1, at least one radio-frequency (RF) ground conductor layer or “plane”  34  is associated with lower layer  24  of the double-sided layer  24 / 26 . Those skilled in the art will realize that the presence of ground plane  34  allows ordinary “microstrip” transmission-line techniques to carry RF signals in lateral directions, parallel with upper surface  12   us  of plastic body  12 , so that RF signals can also be transmitted from one MMIC to another in the assembly  10  of FIG.  1 . 
     Allowed U.S. patent application Ser. No. 08/815,349, in the name of McNulty et al., describes an arrangement by which signals can be coupled to and from an HDI circuit such as that of FIG.  1 . As described in the McNulty et al. application, the HDI KAPTON layers with their patterns of conductive traces are lapped over an internal terminal portion of a hermetically sealed housing. Connections are made within the body of the housing between the internal terminal portion and an externally accessible terminal portion. 
     One of the advantages of an antenna array is that it is a relatively flat structure, by comparison with the three-dimensional curvature of reflector-type antennas. When assemblies such as that of FIG. 1 are to be used for the transmit-receive modules of an active array antenna, it is often desirable to keep the structure as flat as possible, so as, for example, to make it relatively easy to conform the antenna array to the outer surface of a vehicle. FIG. 2 a  illustrates an HDI module such as that described in the abovementioned McNulty patent application. In FIG. 2 a , representative module  210  includes a mounting base  210 , to which heat is transferred from internal chips. A plurality of mounting holes are provided, some of which are designated  298 . A contoured lid  213  is hermetically sealed to a peripheral portion of base  212 , to protect the chips within. A first set of electrical connection terminals, some of which are designated as  222   a ,  224   a , and  226   a  are illustrated as being located on the near side of the base, and a similar set of connection terminals, including terminals designated as  222   b ,  224   b , and  226   b  are located on the remote side of the base. FIG. 2 b  is a perspective or isometric view, partially exploded, of an active array antenna  200 . In FIG. 2 b , the rear or reverse side (the non-radiating or connection side) of a flat antenna element structure  202  is shown, divided into rows designated a, b, c, and d and columns 1, 2, 3, 4, and 5. Each location of array structure  202  is identified by its row and column number, and each such location is associated with a set of terminals, three in number for each location. Each array location of antenna element array  202  is associated with an antenna element, which is on the obverse or front side of structure  202 . Each antenna element on the obverse side of the antenna element structure  202  is connected to the associated set of three terminals on the corresponding row and column of the reverse side of the antenna element array  202 . Each antenna element of active antenna array  200  of FIG. 2 b  is associated with a corresponding active antenna module  210 , only one of which is illustrated. In FIG. 2 b , active antenna module  210   b   3  is associated with antenna element or array element  202   b   3 . Active module  210   b   3  is identical to module  210  of FIG. 2 a  and to all of the other modules (not illustrated) of FIG. 2 b . Representative module  210   b   3  has its terminals  222   a ,  224   a , and  226   a  connected by means of electrical conductors to the set of three terminals associated with array element  202   b   3  of antenna structure  202 . The other set of terminals of module  210   b   3 , namely the set including terminals  222   b ,  224   b , and  226   b , is available to connect to a source or sink of signals which are to be transmitted or received, respectively. It will be clear that the orientation of module  210   b   3 , and of the other modules which it represents, will, when all present, will extend for a significant distance behind or to the rear of the antenna element support structure  202 , thereby tending to make the active antenna array  200  fairly thick. Also, the presence of the many modules will make it difficult to support the individual modules in a manner such that heat can readily be extracted from the mounting plates ( 212  of FIG. 2 a ). Also, the presence of many such active modules  210  will make it difficult to make the connections between the terminal sets of the active modules and the terminal sets of the antenna elements. The problem of thickness of the structure of FIG. 2 b  is exacerbated by the need for a signal distribution arrangement, partially illustrated as  290 . Distribution arrangement  290  receives signal from a source  292 , and distributes some of the signal to the near connections of each of the modules, such as connections  222   b .  224   b , and  226   b  of module  210   b   3 . 
     A further problem with the structure of FIG. 2 b  is that the connections between the active module  210   b   3  and the set of terminals for array element  202   b   3  is by way of an open transmission-line. Those skilled in the art of RF and microwave communications know that such open transmission-lines tend to be lossy, and in a structure such as that illustrated in FIG. 2 b , the losses will tend to result in cross-coupling of signal between the terminals of the various array elements. 
     A further problem with interconnecting the structure of FIG. 2 b  is that of tolerance build-up between the antenna terminal sets on the reverse side of the antenna element structure  202 , the terminals of the modules  210 , and the terminals of beamformer  290 . 
     Improved arrangements are desired for producing flat HDI-connected structures which can be arrayed with other flat structures. 
     SUMMARY OF THE INVENTION 
     A short-circuited transmission line according to an aspect of the invention includes a monolithic, electrically conductive structure including (a) a solid center conductor having a circular cross-section about a central axis. The center conductor terminates at a first plane and has a first diameter at the first plane in a direction transverse to the central axis, and a second diameter, greater than the first diameter, at a second plane parallel to the first plane. The diameter of the center conductor tapers monotonically between the first and second diameters. The length of the center conductor is defined by the separation of the first and second planes. The monolithic structure further includes (b) a plurality of mutually identical solid outer conductors. Each one of the outer conductors has a circular cross-section about a longitudinal axis. The longitudinal axes of the outer conductors are parallel with the central axis of the center conductor. Each of the outer conductors terminates at the first plane, and has a third diameter at the first plane, and a fourth diameter, greater than the third diameter, at the second plane. The diameter of the outer conductors tapers monotonically between the first and second diameters. The outer conductors have their longitudinal axes equally spaced from each other at radii which make equal angles with adjacent radii. The monolithic structure also includes (c) a solid short-circuiting plate interconnecting the center conductor and the outer conductors at the second plane. 
     In a particular embodiment of the invention, the third diameter equals the first diameter, and the fourth diameter equals the second diameter, and the taper of the diameters of the center and outer conductors is linear. In another embodiment of the invention, the short-circuiting plate has a thickness no greater than the length of the center conductor. The periphery of the short-circuiting plate may be defined by a radius measured from the central axis of the center conductor, which radius is equal to the sum of (a) one of the radii plus (b) half of the greater of (i) the second diameter and (ii) the fourth diameter. In yet another embodiment, the length of the center conductor is no greater than the diameter of the dielectric insulator. 
     In one embodiment, a disk-like dielectric insulator encapsulates the monolithic structure. The insulator defines a central axis coincident with the central axis of the center conductor, a thickness sufficient to enclose that portion of the center and outer conductors lying between the first and second planes, and a periphery defined, at least in part, by a radius from the central axis sufficient to encapsulate the sides at the greatest taper, which is a radius which is greater than the sum of (a) one of the radii plus (b) half of the greater of (i) the second diameter and (ii) the fourth diameter. In one embodiment of the invention, the second and fourth diameters are equal, so the radius of the encapsulating insulator is equal to the radius of the circles on which the outer conductor axes lie, plus half the diameter of a conductor at the second plane. The insulator surrounds at least portions of the center and outer conductors, for insulating the center conductor from the outer conductors and the outer conductors from each other, except at the short-circuiting plate. The dielectric insulator may be either rigid or deformable, as an elastomer. 
     A method for producing a flat antenna array according to another aspect of the invention includes the step of affixing a plurality of microwave integrated-circuit chips to a planar support, with connections of the chips adjacent to the support. A short electrical transmission-line is procured. The electrical transmission-line includes 
     (i) a monolithic, electrically conductive structure which includes 
     (a) a solid center conductor having a circular cross-section about a central axis, and terminating in a first end at a first plane. The center conductor has a first diameter at a first plane transverse to the central axis, and a second diameter, greater than the first diameter, at a second plane parallel to the first plane. The diameter of the center conductor tapers monotonically between the first and second diameters. The length of the center conductor is defined by the separation of the first and second planes, 
     (b) a plurality of mutually identical solid outer conductors. Each one of the outer conductors has a circular cross-section about a longitudinal axis, and the longitudinal axes of the outer conductors lie parallel with the central axis of the center conductor. Each of the outer conductors terminates at a first end at the first plane, and the first ends of the outer conductors have a third diameter at the first plane. The outer conductors have a fourth diameter, greater than the third diameter, at the second plane. The diameter of the outer conductors tapers monotonically between the third and fourth diameters. The outer conductors have their longitudinal axes equally spaced from each other at radii which make equal angles with adjacent radii. 
     (c) a solid short-circuiting plate interconnecting the center conductor and the outer conductors at the second plane. 
     In a particular embodiment of the invention, the electrical transmission line also includes 
     (ii) a disk-like dielectric insulator defining a central axis coincident with the central axis of the center conductor, and a thickness sufficient to enclose that portion of the center and outer conductors lying between the first and second planes. The periphery of the disk-like dielectric insulator is defined, at least in part, by a radius from the central axis which is sufficient to enclose the all the outer conductors at their greatest diameter. This radius is no less than the sum of (a) one of the radii plus (b) half of the greater of (i) the second diameter and (ii) the fourth diameter. The insulator, when used, surrounds at least portions of the center and outer conductors in the axial direction, for insulating the center conductor from the outer conductors and the outer conductors from each other, but no electrical insulation between the conductors exists at the short-circuiting plate. 
     The method includes the step of applying the short transmission-line to the support with the first ends adjacent the support, and encapsulating the chips and the short transmission-line in rigid dielectric material, to thereby produce a structure including an encapsulated chip and transmission-line. At least portions of the support are removed from the encapsulated chip and transmission-line, to thereby expose at least portions of a first side of the encapsulated chip and transmission-line, including at least the connections of the chips and the first ends of the center and outer conductors of the short transmission-line. If the support lacks conductive traces, a layer of flexible dielectric sheet carrying a plurality of electrically conductive traces is applied to the first side of the encapsulated chip and transmission-line. At least one of the connections of at least one of the chips is interconnected with the first end of the center conductor of the transmission-line, and at least one other of the connections of the one of the chips is interconnected to the first ends of all of the outer conductors of the transmission-line, by way of some of the traces and through vias, to thereby produce a first-side-connected encapsulated arrangement. At least so much material is removed from that side of the first-side-connected encapsulated arrangement which is remote from the first side as will expose separated second ends, remote from the first ends, of the center and outer conductors of the transmission-line, to thereby produce a first planar arrangement having exposed second ends of the center and outer conductors of the transmission-line. A planar conductor arrangement including a plurality of individual electrical connections is applied over the first planar arrangement, adjacent the side of the first planar arrangement with exposed second ends of the center and outer conductors. The electrical connections of the planar conductor are selected so that, when the planar conductor arrangement is registered with the first planar arrangement, the electrical connections are registered with the center and outer conductors of the transmission-line. The planar conductor arrangement is registered with the first planar arrangement, and electrical connections are made between the second ends of the center and outer conductors of the transmission line of the first planar arrangement and the connections of the planar conductor arrangement. 
     In a particular method according to an aspect of the invention, the step of making electrical connections includes the steps of placing a compressible floccule of electrically conductive material between the second ends of each of the center and outer conductors of the transmission line of the first planar arrangement and the registered ones of the electrical connections of the planar conductor arrangement, and compressing the compressible floccule of electrically conductive material between the second ends of the center and outer conductors of the transmission lines of the first planar arrangement and the registered ones of the electrical connections of the planar conductor arrangement, to thereby establish the electrical connections and to aid in holding the compressible floccules in place. In a preferred embodiment of the invention, the method encapsulates the chips and the short transmission-line in the same dielectric material used in the dielectric disk. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a simplified cross-sectional view of a portion of a prior-art high-density interconnect arrangement by which connections are made between multiple integrated-circuit chips mounted on a single supporting substrate; 
     FIG. 2 a  is a simplified perspective or isometric view of a prior-art module which contains HDI-connected integrated-circuit chips, and FIG. 2 b  illustrates how a flat or planar antenna array might use a plurality of the modules of FIG. 2 a  to form an active antenna array; 
     FIGS. 3 a  and  3   b  are simplified plan and elevation views, respectively, of a short transmission-line, and FIG. 3 c  is a cross-section of the structure of FIG. 3 a  taken along section lines  3   c-   3   c;    
     FIGS. 4 a ,  4   b ,  4   c ,  4   d ,  4   e ,  4   f ,  4   g , and  4   h  illustrate steps, in simplified form, in the fabrication of an RF HDI structures using a short transmission-line as in FIGS. 3 a ,  3   b , and  3   c  to interface to another planar circuit, illustrated as a beamformer or manifold; 
     FIG. 5 illustrates an arrangement similar to that of FIG. 4 h  with a cold plate interposed between the HDI-connected chips and the beamformer, and using a rigid fuzz button holder; 
     FIG. 6 a  is a simplified plan view of a compressible or conformable short transmission line, FIG. 6 b  is a simplified cross-section of the arrangement of FIG. 6 a  taken along section lines  6   a-   6   a , FIG. 6 c  is a simplified perspective or isometric view of the short transmission line of FIGS. 6 a  and  6   b , with the fuzz button conductors illustrated in phantom, and FIG. 6 d  is a simplified perspective or isometric view of a representative fuzz button; 
     FIG. 7 is a simplified cross-sectional representation of an assemblage including a cold plate, in which a compressible fuzz button holder is used; 
     FIG. 8 is a simplified perspective or isometric view, exploded to reveal certain details, of the assemblage of FIG. 7; 
     FIG. 9 a  is a simplified perspective or isometric view of a short-circuited transmission line according to an aspect of the invention, FIG. 9 b  is a side or elevation view of the transmission line of FIG. 9 a , FIG. 9 c  illustrates the arrangement of FIG. 9 a  in encapsulated form, and FIG. 9 d  is a side elevation of the encapsulated structure of FIG. 9 c;    
     FIG. 10 a  illustrates the result of certain fabrication steps corresponding to the steps of FIGS. 4 a ,  4   b ,  4   c , and  4   d  applied to the short-circuited transmission line of FIGS. 9 c  and  9   d , and FIG. 10 b  illustrates the result of further fabrication steps applied to the structure of FIG. 10 a;    
     FIG. 11 illustrates a short-circuited multiple transmission line which may be encapsulated as described in conjunction with FIGS. 9 c  or  9   d , and used for interconnecting planar circuit arrangements at frequencies somewhat lower than the higher RF frequencies, such as the clock frequencies of logic circuits; 
     FIG. 12 is a perspective or isometric view of a structure according to an aspect of the invention, including a planar plastic HDI circuit, a bipartite separator plate, and a second planar circuit, some of which are cut away to reveal interior details; 
     FIG. 13 is an exploded view of the structure of FIG. 12, showing the planar plastic HDI circuit associated with one portion of the separator plate as one part, the second portion of the separator plate, and the second planar circuit as other parts of the exploded structure; 
     FIG. 14 is an exploded view of a portion of the second part of the separator plate, showing rigid and compliant transmission lines, and other structure; and 
     FIG. 15 is a more detailed cross-sectional view of the structure of FIG.  12 . 
    
    
     DESCRIPTION OF THE INVENTION 
     In FIGS. 3 a    3   b , and  3   c , a short transmission line or “molded coaxial interconnect”  310  is in the form of a flat disk or right circular cylinder  311  having a thickness  312  and an outer diameter  314  centered about an axis  308 . Thickness  312  should not exceed diameter  314 . An electrically conductive center conductor  316  is in the form of a right circular cylinder defining a central axis which is concentric with axis  308 . A set  318  of a plurality, in this case eight, of further electrical conductors  318   a ,  318   b ,  318   c ,  318   d ,  318   e ,  318   f ,  318   g , and  318   h , are also in the form of right circular cylinders, with axes which lie parallel with the axis  308  of the flat disk. The further electrical conductors have their axes equally spaced by an incremental angle of 45° on a circle of diameter  320 , also centered on axis  308 . The main body of short transmission line  310  is made from a dielectric material, which encapsulates the sides, but not the ends, of center conductor  316  and outer conductors  318   a ,  318   b ,  318   c ,  318   d ,  318   e ,  318   f ,  318   g , and  318   h . The diameter of circle  320  on which the axes of the outer conductors lie is selected so that the outer conductors lie completely within the outer periphery of the dielectric disk. A first end of the center conductor and the outer conductors lies adjacent a plane  301 , and a second end of each lies adjacent to a second plane  302 . In a particular embodiment of the short transmission line, the thickness  312  is 0.055 in., and the diameter is 0.304 in. In another embodiment, the diameter is the same, but the thickness is 0.115 in. In both embodiments, the axes of the outer conductors of set  308  are centered on a circle of diameter 0.192 in., and the conductors have diameters of 0.032 in. The material of the dielectric disk is Plaskon SMT-B-1 molding compound, and the conductors are copper. As described below, these short transmission lines are used for interconnecting RF circuits. The characteristic impedance of the short transmission line of FIGS. 3 a ,  3   b , and  3   c  is selected to substantially match the impedances of the signal source and sink, or to substantially match the impedances of the stripline or microstrip transmission lines to which the short transmission line is connected in an HDI circuit. The impedance Z 0  of the short transmission line is determined by                Z   0     =       (     138     ɛ       )            log   10          (       D   0       D   i       )               1                              
     where 
     ∈ is the dielectric constant of the dielectric disk; 
     D o  is the diameter of the inside surface of the outer conductor; and 
     D i  is the outer diameter of the center conductor. To produce a 50-ohm characteristic impedance, with center conductor wire diameter of 0.032″ and epoxy encapsulation material having a dielectric constant of 3.7, the axes of the outer conductors should be on a circle having a diameter of 0.192 inches. 
     FIGS. 4 a ,  4   b ,  4   c ,  4   d ,  4   e ,  4   f , and  4   h  illustrate steps in the fabrication of an RF HDI structure. In a step preceding that illustrated in FIG. 4 a , one or more short transmission lines  310  are fabricated, and monolithic RF circuits  14  are assembled with heat-transferring metal deep-reach shims  18 . In FIG. 4 a , the chip/shim assemblages  14 / 18  and the short transmission lines  310  are mounted face-down onto an adhesive backed KAPTON substrate  410 . FIG. 4 b  illustrates the encapsulation of the assemblages  14 / 18  and the short transmission line  310  within an epoxy or other encapsulation to form a structure with encapsulated chips and transmission-lines. The structure of FIG. 4 b  with encapsulated chips and transmission lines then continues through conventional HDI processing. As illustrated in FIG. 4 c , vias are laser-drilled to die bond pads  14   1  and  14   2  and to the conductors of the short transmission line  310  which are against the substrate  410 . Conductive traces are then patterned on the exposed substrate  410 , making the necessary electrical connections. FIG. 4 d  illustrates the result of applying a plurality (illustrated as three) of layers of conductive-trace bearing flexible HDI connection material designated together as  424 , with the traces appropriately registered with the connections  14   1  and  14   2  of the chips  14 , and with the center conductor  316  and the set  318  of outer conductors of the short transmission line  310 . 
     Following the step illustrated in FIG. 4 c , plated through-vias  36  are formed in the conductive-trace bearing flexible HDI connection material  424 , with the result that the chip connections are made, and the connections to the short transmission line  18  are made as illustrated in FIG. 4 e . The metallization layers  32  connect the short transmission line to at least one of the chips  14 , so that one connection of a chip connects to center conductor  316  of short transmission line  310  of FIG. 4 e , and so that a ground conductor associated with the chip connects to the set  318  of outer conductors of the short transmission line. FIG. 4 f  represents the cutting off of that portion of the encapsulated structure (the structure of FIG. 4 e ) which lies, in FIG. 4 f , above a dash line  426 . This produces a planar structure  401 , illustrated in FIG. 4 g , in which the connections among the chips  14 , and between the chips and one end of the short transmission lines, lie within the conductive-trace layers  424  on the “bottom” of the encapsulated structure, and in which a heat interface end  18   hi  of each of the heat-conducting shims  18 , and the ends of the center conductor  316  and of the set  318  of outer conductors of a coaxial connection structure  490  at the end of the short transmission line, are exposed on the “upper” side of the structure as contacts. The center conductor contact is illustrated as  316   c , and some of the outer conductor contacts are designated as  318   a   c  and  318   f   c . 
     FIG. 4 h  illustrates a cross-section of a structure resulting from a further step following the step illustrated in conjunction with FIGS. 4 f  and  4   g . More particularly, the structure of FIG. 4 g  is attached to an RF manifold or beamformer  430 , which distributes the signals which are to be radiated by the active array antenna. The surface  430   s  of manifold  430  which is adjacent to the encapsulated structure bears conductive traces, some of which are designated  432 . In order to make contact between the conductive traces  432  on the RF distribution manifold and the exposed ends of the center conductor  316  and the set  318  of outer conductors of the short transmission line, compressible electrical conductors  450 , termed “fuzz buttons,” are placed between the conductive traces  432  on the distribution manifold  430  and the exposed ends of the center conductor  316  and set  318  of outer conductors of each of the short transmission lines  310 . The manifold  430  is then pressed against the remainder of the structure, with the fuzz buttons between, which compresses the fuzz buttons to make good electrical connection to the adjacent surfaces, and which also tends to hold the fuzz buttons in place due to compression. Appropriate thermal connection must also be made between the manifold and the shims  18  to aid in carrying away heat. Thus, in the arrangement of FIGS. 4 a-   4   h , electrical RF signals are distributed to the ports (only one illustrated) of the distribution manifold  430  to a plurality of the ports (only one of which is illustrated) represented by short transmission lines  310  of planar circuit  401  of FIG. 4 g , and the signals are coupled through the short transmission lines to appropriate ones of the metallization layers  32   0 ,  32   1 , and  32   2 , as may be required to carry the signals to the MMIC for amplification or other processing, and the signals processed by the MMIC are then passed through the signal paths defined by the paths defined by conductive traces  32   0 ,  32   1 , and  32   2  to that layer of conductive traces which is most remote from the distribution manifold  430 . More particularly, when the distribution manifold  430  is in the illustrated position relative to the encapsulated pieces, the uppermost layer  32   2  of conductive traces may itself define the antenna elements. Thus, the structure  400  defined in FIG. 4 h , together with other portions which appear in other ones of FIGS. 4 a-   4   g , comprises the distribution, signal processing, and radiating portions of a planar or flat active array antenna. 
     The fuzz buttons  450  of FIG. 4 h  may be part no. 3300050, manufactured by TECKNIT, whose address is 129 Dermodry Street, Cranford, N.J. 07016, phone (908) 272-5500. 
     If the conductors  32   2  of metallization layer MT 2  of FIG. 4 h  are elemental antenna elements, the RF manifold  430  can be a feed distribution arrangement which establishes some measure of control over the distribution of signals to the active MMICs of the various antenna elements. On the other hand, the structure of FIG. 4 h  denominated as RF manifold  430  could instead be an antenna array, with the elemental antennas on side  430   p , while the metallization layers  32   1  and  32   2  would in that case distribute the signals to be radiated, or collect the received signals. Thus, the described structure is simply a connection arrangement between two separated planar distribution sets. 
     It will be noted that in FIG. 4 h , the region  460  about the fuzz buttons  450  is surrounded by air dielectric, which has a dielectric constant of approximately 1. Since the fuzz buttons  450  have roughly the same diameter as the center conductor  316  and the outer conductors  318 , the characteristic impedance of the section  460  of transmission line extending from exposed traces  432  to short transmission line  310  is larger than that of the short transmission line. If the short transmission line has a characteristic impedance of about 50 ohms, the characteristic impedance of the region  460  will be greater than 50 ohms. Those skilled in the art know that such a change of impedance has the effect of interposing an effective inductance into the transmission path, and may be undesirable. 
     FIG. 5 represents a structure such as that of FIG. 4 h  with a cold plate  510  interposed between the HDI-connected chips  10  on structure  12  and the beamformer  430 . The cold plate  510  has an interface surface  510  is which makes contact with the adjacent surface of the plastic body  12  of the HDI circuit  10 . The cold plate may be, as known in the art, a metal plate with fluid coolant channels or tubes located within, for carrying heat from heat interface surfaces  18   hi  to a heat rejection location (not illustrated). Those skilled in the art know that a heat conductive grease or other material may be required at the interface. As illustrated in FIG. 5, a fuzz button housing  512  has a thickness about equal to that of the cold plate, for holding fuzz buttons  450  in a coaxial pattern similar to that of center conductor  316  and outer conductors  318 , for making connections between the center conductor  316 /outer conductors  318  and the corresponding metallizations  432  of the beamformer  430 . More particularly, the outer conductors  318  and the outer conductor fuzz buttons  450  lie on a circle with diameter d 192 . The dielectric constant of the material of fuzz button housing  512  is selected to provide the selected characteristic impedance. As also illustrated in FIG. 5, fuzz button housing  512  is not quite as large in diameter as the cut-out or aperture in cold plate  510 , in order to take tolerance build-up. Consequently, an air-dielectric gap  512   g1  exists around fuzz button housing  512 . The axial length of fuzz button housing  512  is similarly not quite as great as the thickness of the cold plate  510 , resulting in a gap  512   g2 . Gaps  512   g1  and  512   g2  have an effect on the characteristic impedance of the transmission path provided by the fuzz buttons  450  which is similar to the effect of the air gap  460  of FIG. 4 h . In an analysis of an arrangement similar to that of FIG. 5, the calculated through loss was 0.8 dB, and the return loss was only 10.5 dB. 
     The fuzz button housing or holder  512  is made from an elastomeric material, which compresses when compressed between the HDI-connected chips  10  and the underlying beamformer  430 , so as to eliminate air gaps which might adversely affect the transmission path. FIGS. 6 a ,  6   b , and  6   c  are views of a compressible or compliant RF interconnect with fuzz button conductors. In FIGS. 6 a ,  6   b , and  6   c , elements corresponding to those of FIGS. 3 a ,  3   b , and  3   c  are designated by like reference numerals, but in the  600  series rather than in the  300  series. As illustrated in FIGS. 6 a ,  6   b , and  6   c , compliant RF interconnect  610  includes a fuzz button center conductor  616  defining an axis  608 , and a set  618  including a plurality, illustrated as eight, of fuzz button outer conductors  618   a ,  618   b ,  618   c ,  618   d ,  618   e ,  618   f ,  618   g , and  618   h , spaced at equal angular increments, which in the case of eight outer conductor elements corresponds to 45°, about center axis  608 , on a radius  620  having a diameter of 0.200″. Dielectric body  611  has an outer periphery  611   p , and is made from a silicone elastomer having a dielectric constant within the range of 2.7 to 2.9, and has an overall diameter  614  of about 0.36″, and a thickness  612  of 0.10″. As can be best seen in FIGS. 6 a  and  6   c , the dielectric body  611  has two keying notches  650   a  and  650   b . Dielectric body  611  also has a flanged inner portion  648  with a diameter of 0.30″, and the maximum-diameter portion  652  has a thickness  654  of about 0.44″. The fuzz buttons  616 ,  618  have a length  613  in the axial direction which is slightly greater (0.115″ in the embodiment) than the axial dimension  612  of body  611  (0.10″). FIG. 6 d  illustrates a representative one of the outer conductor fuzz buttons, which is selected to be fuzz button  618   f  for definiteness. In FIG. 6 d , outer conductor fuzz button  618   f  is in the form of a right circular cylinder centered on an axis  617 , and defines first and second ends  618   f   1  and  618   f   2  which are coincident with planes  601  and  602 , respectively, of FIG. 6 b . The cylindrical form of fuzz button  618   f  of FIG. 6 d  defines an outer surface  618   fs  lying between the first and second ends  618   f   1  and  618   f   2 . 
     FIG. 7 is similar to FIG. 5, and corresponding elements are designated by the same reference numerals. In FIG. 7, the compliant RF interconnect  610  is compressed between the broad surface  430   fs , of beamformer manifold  430  and the broad surface  712   ls , of HDI-connected chip arrangement  10 , and is somewhat compressed axially, to thereby eliminate the gap  512   g2  which appears in FIG.  5 . This, in turn, eliminates the principal portion of the impedance discontinuity at the interface which is filled by the compliant RF interconnect  610 . The axial compression of the dielectric body  611  of the compliant RF interconnect  610 , in turn, tends to cause the compliant body  611  to expand radially, to thereby somewhat fill the circumferential or annular gap  512   g1 , which further tends to reduce impedance discontinuities at the interface. A further advantage of the axial compression of body  611  is that the compression tends to compress the body  611  around the fuzz button conductors  616 ,  618 , to help in holding them in place. Analysis of the arrangement of FIG. 7 indicated that the through loss would be 0.3 dB and the return loss 28 dB, which is much better than the values of 0.8 dB and 10.5 dB calculated for the arrangement of FIG.  5 . 
     As illustrated in FIG. 7, a heat-transfer interface surface  18   hi  on the broad surface  712   ls , of HDI-connected chip structure  10  is pressed against cold plate  510 . 
     In the view of FIG. 7, the fuzz button conductors  616  and  618  of the compliant coaxial interconnect  610  are illustrated as being of a different diameter than the conductors  316 ,  318  of the molded coaxial interconnect  310 , and the outer conductors  618  are centered on a circle of somewhat different diameter than the outer conductors  318 . The difference in diameter of the wires and the spacing of the outer conductor from the axis of the center conductor is attributable to differences in the dielectric constant of the epoxy which is used as the dielectric material in the molded coaxial interconnect  310  and the silicone material which is the dielectric material of compliant interconnect  610 . In order to minimize reflection losses, both interconnects are maintained near 50 ohms, which requires slightly different dimensioning. This should not be a problem, so long as the diameters of the circles on which the outer conductors of the molded and compliant interconnects are centered allow an overlap of the conductive material, so that contact is made at the interface. 
     A method for making electrical connections as described in conjunction with FIGS. 6 a ,  6   b ,  6   c ,  7 , and  8  includes the step of providing or procuring a first planar circuit  10  including at least a first broad surface  712   ls . The first broad surface  712   ls  of the first planar circuit  10  includes at least one region  490  defining a first coaxial connection. It may also include at least a first thermally conductive region  18   hi  to which heat flows from an active device within the first planar circuit. The first coaxial connection  490  of the first planar circuit  10  defines a center conductor contact  616   c  centered on a first axis  608  orthogonal to the first broad surface of the first planar circuit  10 , and also defines a first plurality of outer conductor contacts, such as  618   a   c  and  618   f   c . Each of the outer conductor contacts such as  618   a   c ,  618   f   c  of the first coaxial connection  490  of the first planar circuit  10  is centered and equally spaced on a circle spaced by a first particular radius, equal to half of diameter d 192 , from the first axis  608  of the center conductor contact  616  of the first coaxial connection  490 . The first broad surface  712   ls  of the first planar circuit  10  further includes dielectric material electrically isolating the center conductor contact  616   c  of the first planar circuit  10  from the outer conductor contacts, such as  618   a   c ,  618   f   c , and the outer conductor contacts, such as  618   a   c ,  618   f   c , from each other. The method also includes the step of providing a second planar circuit  430 , which includes at least a first broad surface  430   fs . The first broad surface  430   fs  of the second planar circuit  430  includes at least one region  431  defining a coaxial connection. The coaxial connection  431  of the second planar circuit  430  includes a center conductor contact  432   c  centered on a second axis  808  orthogonal to the first broad surface  430   fs  of the second planar circuit  430 , and also includes the first plurality (eight) of outer conductor contacts  432   o . Each of the outer conductor contacts, such as  432   co ,  432   o , of the coaxial connection  431  of the second planar circuit  430  is centered and equally spaced on a circle spaced by a second particular radius, close in value to the first particular radius, from second axis  808  of the center conductor contact  432   c  of the coaxial connector  431  of the second planar circuit  430 . The first broad surface  430   fs  of the second planar circuit  430  further includes dielectric material electrically isolating the center conductor contact  432   c  of the second planar circuit  430  from the outer conductor contacts, such as  432   co ,  432   o  of the second planar circuit  430 , and the outer conductor contacts, such as  432   co ,  432   o  of the second planar circuit  430 , from each other. A compliant coaxial connector  610  is provided, which includes (a) a center conductor  616  which is electrically conductive and physically compliant, at least in the axial direction. The compliant center conductor  616  has the form of a circular cylinder centered about a third axis  608 , and defines an axial length  613  between first  617   f1 , and second  617   f2  ends. The compliant coaxial connector  610  also includes (b) an outer electrical conductor arrangement  618  including a set  618  including the first plurality (eight) of mutually identical, electrically conductive, physically compliant outer conductors  618   a ,  618   b ,  618   c ,  618   d ,  618   e ,  618   f ,  618   g , and  618   h . Each of the compliant outer conductors  618   a ,  618   b ,  618   c ,  618   d ,  618   e ,  618   f ,  618   g , and  618   h  is in the form of a circular cylinder centered about an axis  617 , and each has an axial length  613  between first  617   f1  and second  617   f2  ends which is equal to the axial length  613  of the compliant center conductor  616 . The axes  617  of the compliant outer conductors  618   a ,  618   b ,  618   c ,  618   d ,  618   e ,  618   f ,  618   g , and  618   h  are oriented parallel with each other, and with the third axis  608  of the compliant center conductor  616 . The first ends  617   f1  of the compliant center conductor  616  and the compliant outer conductors  618   a ,  618   b ,  618   c ,  618   d ,  618   e ,  618   f ,  618   g , and  618   h  coincide with a first plane  601  which is orthogonal to the axes  608 ,  617  of the compliant center conductor  616  and the compliant outer conductors  618   a ,  618   b ,  618   c ,  618   d ,  618   e ,  618   f ,  618   g , and  618   h , and the second ends  617   f2  of the compliant center conductor  616  and the compliant outer conductors  618   a ,  618   b ,  618   c ,  618   d ,  618   e ,  618   f ,  618   g , and  618   h  coincide with a second plane  602  parallel with the first plane  601 . The compliant outer conductors  618   a ,  618   b ,  618   c ,  618   d ,  618   e ,  618   f ,  618   g , and  618   h  have their axes  617  equally spaced from each other at the particular radius from the axis  608  of the compliant center conductor  616 . The compliant coaxial connector  610  further includes (c) a compliant dielectric disk-like structure  611  defining a fourth center axis  608  coincident with the third axis  608  of the compliant center conductor  616  and also defining an uncompressed axial length no more than about 10% greater than the uncompressed axial length of the compliant center conductor  616 . The compliant disk-like structure  611  also defines a periphery  611   p  spaced from the center axis  608  by a second radius which is greater than both (a) the first radius (half of diameter  620 ) and (b) the axial length  613  of the compliant center conductor  616 . The compliant dielectric disk  611  surrounds and supports the compliant center conductor  616  and the compliant outer conductors  618   a ,  618   b ,  618   c ,  618   d ,  618   e ,  618   f ,  618   g , and  618   h  at least on side regions  618   fs  thereof lying between the first  618   f1  and second  618   f2  ends of the compliant center conductor  616  and the compliant outer conductors  618   a ,  618   b ,  618   c ,  618   d ,  618   e ,  618   f ,  618   g , and  618   h . The compliant dielectric disk-like structure  611  does not overlie the first  618   f2  ends of the compliant center conductor  616  and the compliant outer conductors  618   a ,  618   b ,  618   c ,  618   d ,  618   e ,  618   f ,  618   g , and  618   h , so that electrical connection thereto can be easily established. 
     The method described in conjunction with FIGS. 6 a ,  6   b ,  6   c ,  7 , and  8  also includes the further step of placing the first broad surfaces  712   ls ,  430   fs  of the first and second planar circuits  10 ,  430  mutually parallel, with the first axis  8  passing through the center of the center conductor contact  316   c  of the first planar circuit  10  and orthogonal to the first broad surface  712   ls  of the first planar circuit  10 , and coaxial with the second axis  808  passing through the center of the center conductor contact  432   c  of the second planar circuit  430  orthogonal to the first broad surface  430   ls  of the second planar circuit  430 , with the first and second planar circuits  10 ,  430  rotationally oriented around the coaxial first and second axes  8 ,  808  so that a fourth axis  880  orthogonal to the first broad side  712   ls  of the first planar circuit  10  and passing through the center of one of the outer conductor contacts  318   cc  of the first coaxial connector  431  of the first planar circuit  10  is coaxial with a fifth axis  882  orthogonal to the first broad side  430   fs  of the second planar circuit  430  and passing through the center of one of the outer conductor contacts  432   cc  of the first coaxial connector  431  of the second planar circuit  430 . The compliant coaxial connector  310  is placed between the first and second planar circuits  10 ,  430 , with the third axis  608  of the compliant center conductor  616  substantially coaxial with the mutually coaxial first and second axes  8 ,  808 . The compliant coaxial connector  610  is oriented so that a sixth axis  884  of one of the compliant outer conductors  618   a ,  618   b ,  618   c ,  618   d ,  618   e ,  618   f ,  618   g , and  618   h  is coaxial with the mutually coaxial fourth and fifth axes  880 ,  882 . Force is applied to translate the first and second planar circuits  10 ,  430  toward each other until the compliant coaxial connector  610  is compressed between the first broad surface  712   ls  of the first planar circuit  10  and the first broad surface  430   fs  of the second planar circuit  430  sufficiently to make contact between the center conductor contacts  316   c ,  432   c  of the first and second planar circuits  10 ,  430  through the compliant center conductor  616 , and to make contact between outer conductor contacts  318   a   c ,  318   f   c  of the first planar circuit and corresponding outer conductor contacts  432   a   c ,  432   f   c  of the second planar circuit  430  through some of the compliant outer conductors  618 . 
     In a particular version of the method described in conjunction with FIGS. 6 a ,  6   b ,  6   c ,  7 , and  8  also includes the further step of procuring a first planar circuit  10  in which the first broad surface  712   ls  includes a first thermally conductive region  18   hi  to which heat flows from an active device within the first planar circuit. In this version of the method, before the step of applying force to translate the first and second planar circuits  10 ,  430  toward each other, a planar spacer or cold plate  510  is interposed between the first broad surface  712   ls  of the first planar circuit  10  and the first broad surface  430   fs  of the second planar circuit  430 . In this method, the step of interposing a planar cold plate  510  between the first broad surfaces  712   ls ,  430   fs  comprises the step of interposing a planar cold plate  510  having an aperture  810  with internal dimensions no smaller than twice the second radius of the compliant dielectric disk-like structure  610 , with the outer periphery of the aperture  810  surrounding the compliant coaxial connector  610 . 
     FIG. 9 a  is a simplified perspective or isometric view of a short monolithic (one-piece without joints) conductive short-circuited transmission line or RF interconnect  900  according to an aspect of the invention, FIG. 9 b  is a side or elevation view of the transmission line of FIG. 9 a , and FIGS. 9 c  and  9   d  illustrate the arrangement of FIG. 9 a  in encapsulated form. In FIGS. 9 a  and  9   b , the short-circuited transmission line or RF interconnect  900  has an air dielectric, and is made by machining from a block, or preferably by casting. Transmission line  900  includes a center conductor  916  centered on an axis  908 , and having a circular cross-section. Center conductor  916  ends at a plane  903  in a flat circular end  916   e , and each of the outer conductors  918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f , and  918   h  also has a corresponding flat circular end  918   ae ,  918   be ,  918   ce ,  918   de ,  918   ee ,  918   fe , and  918   he . The cross-sectional diameters of the center conductor  916  and the outer conductors  918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f , and  918   h  taper from a relatively small diameter d i  of the circular ends at plane  903  to a larger diameter d 2  at a second plane  902 . At (or immediately adjacent to) plane  902 , a short-circuiting plate  907  interconnects the ends of the center conductor  916  and the outer conductors  918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f , and  918   h  which are remote from plane  903 . In FIGS. 9 a  and  9   b , the axes of outer conductors  918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f , and  918   h , only one of which is illustrated and designated  918   aa , lie on a circle illustrated as a dash line  921 , which lies at a radius  920  from axis  908  of center conductor  916 . The periphery lip of short-circuiting plate  907  is illustrated as being circular, with a diameter or radius measured from axis  908  which is just large enough so that the outer edges of the various outer conductors of set  918  are coincident or tangent with periphery lip at plane  902 . 
     While not the best mode of using the short-circuited transmission line of FIGS. 9 a  and  9   b , FIGS. 9 c  and  9   d  illustrate the short-circuited transmission line  900  of FIGS. 9 a  and  9   b  encapsulated in a cylindrical body  911  of dielectric material corresponding to the dielectric body  311  of FIG. 3, to form an encapsulated short-circuited transmission line  901 . As illustrated in FIG. 9 c , the encapsulating body  911  does not cover the ends  916   e  and  918   ae ,  918   be ,  918   ce ,  918   de ,  918   ee ,  918   fe , and  918   he  of the center and outer conductors, thereby making them available for connections. As also illustrated in FIG. 9 c , the diameter of dielectric body  911  of encapsulated short-circuited transmission line  901  is the same as the diameter  914  of the short-circuiting plate  907 , so the side of the short-circuiting plate  907  is exposed. The diameter of the dielectric encapsulating body could be greater than diameter  914  of the short-circuiting plate  907 , in which case the plate  907  would not be visible in FIG. 9 c.    
     With the unencapsulated short-circuited transmission-line  900  made as described in conjunction with FIGS. 9 a ,  9   b , or with the encapsulated short-circuited transmission line  901  made as described in conjunction with FIGS. 9 a ,  9   b ,  9   c , and  9   d , the unencapsulated ( 900 ) or encapsulated transmission line ( 901 ) can then be made a part of a planar circuit. The unencapsulated short-circuited transmission line  900  of FIGS. 9 a  and  9   b , or the encapsulated transmission line  901 , is placed on a substrate  410  as illustrated for circuit  310  in FIG. 4 a , with its exposed conductor ends  916   e ,  918   ae ,  918   be ,  918   ce ,  918   de ,  918   ee ,  918   fe , and  918   he  adjacent substrate  410 . The steps of FIGS. 4 b ,  4   c , and  4   d  are followed. 
     FIG. 10 a  is a simplified representation of the result of applying the steps of FIGS. 4 a ,  4   b ,  4   c , and  4   d  to the encapsulated transmission line  901  of FIGS. 9 a ,  9   b , and  9   c . In FIG. 10 a , elements corresponding to those of FIG. 4 e  are designated by like reference numerals, and elements corresponding to those of FIGS. 9 a ,  9   b ,  9   c , and  9   d  are designated by like reference numerals. As illustrated in FIG. 10 a , the planar circuit structure  1000 , which may be an antenna array, has the location of the short-circuiting plate  907  below the parting plane  426  at which a cut is made to expose a newly formed end  1016   e  of the tapered center conductor and to also expose newly formed ends of the set of outer conductors  918 , respectively. As illustrated in FIG. 10 a , the parting plane lies between planes  903  and  902  associated with the RF interconnect  900 . FIG. 10 b  is a simplified cross-section of a structure generally similar to that of FIG. 4 h , in which the structure of FIG. 10 a  is the starting point; elements of FIG. 10 b  corresponding to those of FIG. 10 a  are designated by like reference numerals, and elements corresponding to those of FIG. 4 h  are designated by like reference numerals. It will be apparent to those skilled in the art that the structure of FIG. 10B is equivalent to that of FIG. 4 h , with the sole difference lying in the tapered diameter of the center conductor  916  and of the outer conductors represented by  918   b  and  918   f  between the small ends  916   e  and newly formed large ends  1018   be  and  1018   fe , respectively. This taper may change the characteristic impedance somewhat between the ends of the RF interconnect, but this effect is mitigated by the relatively small taper, and because the axial length of the RF interconnect is selected to be relatively short in terms of wavelength at the highest frequency of operation. Naturally, if one or more unencapsulated short-circuited transmission lines  900  are used to make the planar circuit according to the method described in conjunction with FIGS. 4 a ,  4   b ,  4   c ,  4   d ,  10   a , and  10   b , the dielectric constant of the encapsulant material of the transmission line is the same as that of the planar circuit itself. If an encapsulated transmission line such as  901  is used to make the planar circuit of FIG. 10 b , it is desirable that the encapsulating materials be identical. 
     FIG. 11 illustrates a monolithic electrically conductive structure which forms multiple short-circuited transmission paths, each consisting of at least one conductor paired with another; as known to those skilled in the art, one of the pair may be common with other circuit paths, and may be used at somewhat lower frequencies than the coaxial structures, down to zero frequency. In FIG. 11, the multiple short-circuited transmission paths take the form of a monolithic electrically conductive structure  1110 , including a baseplate  1112  and a plurality, eleven in number, of tapered pins or posts  1114   a ,  1114   b ,  1114   c ,  1114   d ,  1114   e ,  1114   f ,  1114   g ,  1114   h ,  1114   i ,  1114   j , and  1114   k . The short-circuited multiple transmission-line structure is used instead of the coaxial arrangement  900  in the method described in conjunction with FIGS. 4 a ,  4   b ,  4   c ,  4   d ,  10   a , and  10   b , to make a planar structure. Those skilled in the art know that antenna array/beamformer combinations require not only connection of RF signals, but also require transmission between elements of power and control signals, which can be handled by the structure made with the multiple transmission paths of FIG.  11 . 
     FIGS. 12,  13 ,  14 , and  15  illustrate a planar plastic HDI circuit  10  similar to those described in conjunction with FIGS. 3 a ,  3   b ,  3   c ,  4   a ,  4   b ,  4   c ,  4   d ,  4   e ,  4   f , and  4   g . More particularly, planar plastic HDI circuit  10  includes a molded interconnect  310  such as that described in conjunction with FIGS. 3 a ,  3   b , and  3   c , assembled to the substrate  12  as described in conjunction with FIGS. 4 a ,  4   b ,  4   c ,  4   d ,  4   e ,  4   f , and  4   g . The planar plastic HDI circuit  10  is mounted on a stiffening plate  510   a , which is part of a bipartite separation plate  510 . First portion  510   a  of the bipartite separation plate  510  has an aperture  810  formed therein to accommodate the flanged disk-like body of compliant interconnect  610 , with the fuzz-button conductors  616 ,  618  of the compliant interconnect registered with the conductors of molded interconnect  310  so as to be in contact therewith. 
     Second portion  510   b  of separation plate  510  of FIGS. 12,  13 ,  14 , and  15  has a through aperture  1312  including a cylindrical portion, and also including a recess  1214   2  adjacent side  1310   b  of second portion  510   b  of separation plate  510 , which recess accommodates a hold-down flange  1214 . Through aperture  1312  also includes a lip or flange  1314  adjacent side  1310   c , which aids in holding the body of a rigid coaxial transmission line  1210  in place. Rigid coaxial transmission line  1210  is similar to molded interconnect  310 , but may be longer, so as to be able to carry signals through the first and second portions of the separation plate  510 . Aperture  1312  also defines a key receptacle  1316  which accepts a key  1212  protruding from the body of rigid transmission line  1210 . The number of conductors of rigid transmission line  1210  is selected, and the conductors are oriented about the longitudinal axis of the rigid transmission line, in such a manner as, when keyed into the aperture  1312  in separation plate  510 , the conductors each match and make contact with corresponding conductors of compliant interconnects  610   a  and  610   b . Compliant interconnect  610   a  is compressed between molded interconnect  310  and rigid coaxial transmission line  1210 , and is oriented to make the appropriate connections between the center fuzz button  616  of molded interconnect  610   a  and the center conductor  1210   c , and between the outer fuzz buttons  618  of molded interconnect  610   a  and the outer conductors, one of which is designated  1210   o , of the rigid transmission line  1210 . 
     Molded interconnect  610   b  of FIGS. 12,  13 ,  14 , and  15  is compressed between a surface  1210   s  of rigid transmission line  1210  and face  430   s  of second circuit  430 , and, when the second circuit  430  is registered with separation plate  510 , the center and outer metallizations  1332  and  1334 , respectively, of its coaxial port  1331  are registered with the corresponding center fuzz button  616  and outer fuzz buttons  618  of compliant interconnect  610   b . The second compliant interconnect  610   b  is held in place by flange  1214 , which in turn is held down by screws  1216   a  and  1216   b  in threaded apertures  1218   a  and  1218   b , respectively. 
     It will be clear from FIGS. 12,  13 ,  14 , and  15  that when the center axis  308  of the center-conductor connection  316   c  of port  490  of the HDI circuit  10  are coaxial with the axis  1308  of the center-conductor connection  1332  of the port  1331  of the beamformer or second circuit  430 , and with the axes  1408 ,  1210   cca , and  1432   ca  of the center conductors of the first compliant interconnect  610   a , the rigid transmission line  1210 , and the second compliant interconnect  610   b , and the compliant interconnects are of sufficient length, an electrically continuous path will be made between the two center conductor contacts. Similarly, with the center conductors and center conductor contacts coaxial, all that is required to guarantee that the outer conductors make corresponding contact is that they have the same number and be equally spaced about the center conductors, and that one of the outer conductors or outer conductor contacts in each piece lie in a common plane with the common axes of the center conductors. When any one of the eight outer conductors or contacts of any one of the interconnection elements is aligned with the corresponding others, all of the outer conductors or outer conductor contacts is also aligned with its corresponding elements. 
     In the particular embodiment of the invention illustrated in FIGS. 12,  13 ,  14 , and  15 , the separation plate  510  consists of a stiffener plate  510   a  which is adhesively or otherwise held to the otherwise flexible plastic HDI circuit  12 , and the second portion  510   b  of separator plate  510  is a cold plate, which includes interior chambers (not illustrated) into which chilled water or other coolant may be introduced by pipes illustrated as  1230   a  and  1230   b . In a particular embodiment of the invention, the planar plastic HDI circuit (only a portion illustrated) defines an antenna array, and the MMIC (not illustrated in FIGS. 12,  13 ,  14 , and  15 ) associated with the planar plastic HDI circuit include chips operated as active amplifiers for the antenna elements. The second circuit  430  is part of a beamformer which supplies signals to, and receives signals from, the MMIC associated with the planar plastic HDI circuit  12 . 
     Other embodiments of the invention will be apparent to those skilled in the art. For example, while the described flat antenna structure lies in a plane, it may be curved to conform to the outer contour of a vehicle such as an airplane, so that the flat antenna structure takes on a three-dimensional curvature. It should be understood that an active antenna array may, for cost or other reasons, define element locations which are not filled by actual antenna elements, such an array is termed “thinned.” The term “RF” has been used to indicate frequencies which may make use of the desirable characteristics of coaxial transmission lines; this term is meant to include all frequencies, ranging from a few hundred kHz to at least the lower infrared frequencies, about 10 13  Hz., or even higher if the physical structures can be made sufficiently exactly. While the short transmission line illustrated in FIGS. 3 a ,  3   b , and  3   c  has eight outer conductors, the number may greater or lesser. The dielectric constant of the dielectric conductor holder of the short transmission lines is selected to provide the proper impedance, whereas the specified ranges are suitable for 50 ohms. While the cold plate has been described as being for carrying away heat generated by chips in the first planar circuit  10 , it will also carry away heat from the distribution beamformer. While the diameters of the center and outer conductors have been illustrated as being equal, the center conductor may have a different diameter or taper than the outer conductors, and the outer conductors may even have different diameters among themselves. 
     Thus, a short-circuited transmission line ( 900 ) according to an aspect of the invention includes a monolithic, electrically conductive structure including (a) a solid center conductor ( 916 ) having a circular cross-section about a central axis ( 908 ). The center conductor ( 916 ) terminates at a first plane ( 901 ) and has a first diameter (d 1 ) at the first plane ( 901 ) in a direction transverse to the central axis ( 908 ), and a second diameter (d 2 ), greater than the first diameter (d 1 ), at a second plane ( 902 ) parallel to the first plane ( 901 ). The diameter of the center conductor ( 916 ) tapers monotonically between the first (d 1 ) and second (d 2 ) diameters. The length of the center conductor ( 916 ) is defined by the separation of the first ( 901 ) and second ( 902 ) planes. The monolithic structure further includes (b) a plurality (eight, in the illustrated embodiment) of mutually identical solid outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ). Each one of the outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) has a circular cross-section about a longitudinal axis (such as axis  910   aa ). The longitudinal axes (such as  918   aa ) of the outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) are parallel with the central axis ( 908 ) of the center conductor ( 916 ). Each of the outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) terminates at the first plane ( 901 ), and has a third diameter (d 3 ) at the first plane ( 901 ), and a fourth diameter (d 4 ), greater than the third diameter (d 3 ), at the second plane ( 902 ). The diameter of each of the outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) tapers monotonically between the first (d 1 ) and second (d 2 ) diameters. The outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) have their longitudinal axes (such as  918   aa ) equally spaced from each other at radii ( 920 ) from the center axis ( 908 ) which make equal angles (45° in the case of eight outer conductors) with adjacent radii ( 920 ). The monolithic structure also includes (c) a solid short-circuiting plate ( 907 ) interconnecting the center conductor ( 916 ) and the outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) at the second plane ( 902 ). 
     In one embodiment of the short-circuited transmission line, a disk-like dielectric insulator ( 911 ) encapsulates the monolithic structure. The insulator ( 911 ) defines a central axis ( 908 ) coincident with the central axis ( 908 ) of the center conductor ( 916 ), and also defines a thickness (t 2 ) sufficient to enclose that portion of the center and outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) lying between the first ( 901 ) and second ( 902 ) planes, and further defines a periphery ( 911   p ), at least in part, by a radius ( 998 ) from the central axis ( 908 ) sufficient to encapsulate the sides of the outer conductors at their largest diameter, which is a radius ( 998 ) which is greater than the sum of (a) one of the radii ( 920 ) plus (b) half of the greater of (i) the second diameter (d 2 ) and (ii) the fourth diameter (d 4 ). 
     In one embodiment of the invention, the second (d 2 ) and fourth diameters (d 4 ) are equal, so the radius ( 998 ) of the encapsulating insulator ( 911 ) is equal to the radius of the circle ( 921 ) on which the axes (such as  918   aa ) of the outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) lie, plus half the diameter (d 2 ) of a conductor at the second plane ( 902 ). The insulator ( 911 ) surrounds at least portions of the center ( 916 ) and outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ), for insulating the center conductor ( 916 ) from the outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) and the outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) from each other, except at the short-circuiting plate ( 907 ). In a particular embodiment of the invention, the third diameter (d 3 ) equals the first diameter (d), and the fourth diameter (d 4 ) equals the second diameter (d 2 ), and the taper of the diameters of the center and outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) is linear. In another embodiment of the invention, the short-circuiting plate ( 907 ) has a thickness (t) no greater than the length of the center conductor ( 916 ). The periphery of the short-circuiting plate ( 907 ) may be the same as that of the insulator ( 911 ), which is to say that it is defined by a radius measured from the central axis ( 908 ) of the center conductor ( 916 ), which radius is equal to the sum of (a) one of the radii ( 920 ) plus (b) half of the greater of (i) the second diameter (d 2 ) and (ii) the fourth diameter (d 4 ). In yet another embodiment, the length of the center conductor ( 916 ) is no greater than the diameter of the dielectric insulator ( 911 ). The dielectric insulator ( 911 ), if used, may be either rigid or deformable, as for example an elastomer. 
     A method for producing a flat circuit structure, which may be an antenna array, according to another aspect of the invention, includes the step of affixing a plurality of microwave integrated-circuit chips ( 14 ) to a planar support ( 410 ), with connections ( 14   1 ,  14   2 ) of the chips ( 14 ) adjacent to the support ( 410 ). A short-circuited electrical transmission-line is procured. The short-circuited electrical transmission-line includes 
     (i) a monolithic, electrically conductive structure ( 900 ) which includes 
     (a) a solid center conductor ( 916 ) having a circular cross-section about a central axis ( 908 ), and terminating in a first end ( 916   e ) at a first plane ( 901 ). The center conductor ( 916 ) has a first diameter (d 1 ) at a first plane ( 901 ) transverse to the central axis ( 908 ), and a second diameter (d 2 ), greater than the first diameter (d 1 ), at a second plane ( 902 ) parallel to the first plane ( 901 ). The diameter of the center conductor ( 916 ) tapers monotonically between the first (d 1 ) and second (d 2 ) diameters. The length of the center conductor ( 916 ) is defined by the separation of the first ( 901 ) and second ( 902 ) planes, 
     (b) a plurality (eight) of mutually identical solid outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ). Each one of the outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) has a circular cross-section about a longitudinal axis (such as axis  918   aa ), and the longitudinal axes (such as axis  918   aa ) of the outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) lie parallel with the central axis ( 908 ) of the center conductor ( 916 ). Each of the outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) terminates at a first end at the first plane ( 901 ), and the first ends of each of the outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) have a third diameter (d 3 ) at the first plane ( 901 ). The outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) have a fourth diameter (d 4 ), greater than the third diameter (d 3 ), at the second plane ( 902 ). The diameter of the outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) tapers monotonically between the third (d 3 ) and fourth (d 4 ) diameters. The outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) have their longitudinal axes (such as axis  918   aa ) equally spaced from each other at radii ( 920 ) which make equal angles (45° for the case of eight outer conductors) with adjacent radii ( 920 ). 
     (c) a solid short-circuiting plate ( 907 ) interconnecting the center conductor ( 916 ) and the outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) at the second plane ( 902 ). The electrical transmission line ( 900 ) may also include 
     (ii) a disk-like dielectric insulator ( 911 ) defining a central axis coincident with the central axis ( 908 ) of the center conductor ( 916 ), and a thickness sufficient to enclose that portion of the center and outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) lying between the first ( 901 ) and second ( 902 ) planes. The periphery ( 911   p ) of the disk-like dielectric insulator ( 911 ) is defined, at least in part, by a radius ( 998 ) from the central axis ( 908 ) which is sufficient to enclose the all the outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) at their greatest diameter. This radius ( 998 ) is no less than the sum of (a) one of the radii ( 920 ) plus (b) half of the greater of (i) the third diameter (d 3 ) and (ii) the fourth diameter (d 4 ). The insulator ( 911 ) surrounds at least portions of the center and outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) in the axial direction, for electrically insulating the center conductor ( 916 ) from the outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) and the outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) from each other, but no electrical isolation between the conductors exists at the short-circuiting plate ( 907 ). 
     The method includes the step of applying the short-circuited transmission line ( 900 ) to the support ( 410 ) with the first ends ( 916   e ,  918   xe , where x ranges from a to h) of the center ( 916 ) and outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) adjacent the support ( 410 ), and encapsulating the chips ( 14 ) and the short-circuited transmission line ( 900 ) in rigid dielectric material ( 412 ), to thereby produce a structure (FIG. 4 c ) including an encapsulated chip ( 14 ) and transmission line ( 900 ). At least portions of the support ( 410 ) are removed from the encapsulated chip ( 14 ) and transmission line ( 900 ), to thereby expose at least portions of the first side of the encapsulated chip ( 14 ) and transmission line ( 900 ), including at least the connections ( 14   1 ,  14   2 ) of the chips ( 14 ) and the first ends ( 916   e ,  918   xe ) of the center ( 916 ) and outer ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) conductors of the short-circuited transmission line ( 900 ). At least one of the connections ( 14   1 ,  14   2 ) of at least one of the chips ( 14 ) is interconnected (by way of paths  32   1  and vias  36 ) with the first end ( 916   e ) of the center conductor ( 916 ) of the short-circuited transmission line ( 900 ), and at least one other of the connections ( 14   1 ,  14   2 ) of the one of the chips ( 14 ) is interconnected to the first ends ( 918   xe ) of all of the outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) of the transmission-line, by way of conductive traces and through vias, to thereby produce a first-side-connected encapsulated arrangement (FIG. 4 d ,  10   a ). This may be accomplished by connecting the traces of a layer ( 424 ) of flexible dielectric sheet carrying a plurality of electrically conductive traces ( 32   1 ,  322 ) applied to the first side of the encapsulated chip ( 14 ) and short-circuited transmission line ( 900 ). At least so much material ( 1010 ) is removed (FIG. 10 a ,  10   b ) from that side of the first-side-connected encapsulated arrangement (FIG. 4 d ) remote from the first side as will expose separated second ends ( 1016   e ,  1018   xe ), remote from the first ends ( 916   e ,  918   xe ), of the center ( 916 ) and outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) of the transmission line, and to thereby eliminate the short-circuit, to thereby produce a first planar arrangement ( 1050  of FIG. 10 b ) having exposed second ends ( 1016   e ,  1018   xe ) of the center ( 916 ) and outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) of the transmission line. A planar conductor arrangement ( 430 ) including a plurality of individual electrical connections ( 432 ) is applied over the first planar arrangement ( 1050 ), adjacent the side of the first planar arrangement with exposed second ends ( 1016   e ,  1018   xe ) of the center ( 916 ) and outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ). The electrical connections ( 432 ) of the planar conductor arrangement ( 430 ) are selected so that, when the planar conductor arrangement ( 430 ) is registered with the first planar arrangement ( 1050 ), the electrical connections ( 432 ) are registered with the center ( 916 ) and outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) of the transmission line. The planar conductor arrangement ( 430 ) is registered with the first planar arrangement ( 1050 ), and electrical connections ( 450 ) are made between the second ends ( 1016   e ,  1018   xe ) of the center ( 916 ) and outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) of the transmission line ( 900 ) of the first planar arrangement ( 1050 ) and the connections ( 432 ) of the planar conductor arrangement ( 430 ). 
     In a particular method according to an aspect of the invention, the step of making electrical connections includes the steps of placing a compressible floccule (fuzz buttons) of electrically conductive material between the second ends ( 1016   e ,  1018   xe ) of each of the center ( 916 ) and outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) of the transmission line of the first planar arrangement ( 1050 ) and the registered ones of the electrical connections ( 432 ) of the planar conductor arrangement ( 430 ), and compressing the compressible floccule of electrically conductive material between the second ends ( 1016   e ,  1018   xe ) of the center ( 916 ) and outer conductors ( 918   a ,  918   b ,  918   c ,  918   d ,  918   e ,  918   f ,  918   g , and  918   h ) of the transmission line of the first planar arrangement ( 1050 ) and the registered ones of the electrical connections ( 432 ) of the planar conductor arrangement ( 430 ), to thereby establish the electrical connections and to aid in holding the compressible floccules ( 450 ) in place. In a preferred embodiment of the invention, the method encapsulates the chips ( 14 ) and the short-circuited transmission line ( 900 ) in the same dielectric material ( 412 ) used in the dielectric disk ( 911 ).