Short coaxial transmission line and method for use thereof

[0002] This invention relates to electromagnetic transmission lines, and more particularly to short coaxial transmission lines, and methods for using such transmission lines to interconnect RF ports on mutually parallel printed-circuit boards.

[0003] FIG. 1 is a simplified perspective or isometric view of a combination 10 of modules, including a main module mated with a plurality of other modules. In FIG. 1, a rectangular, somewhat planar housing 12

[0004] contains at least one printed-circuit board lying in the plane parallel to dash line 14, which is to say parallel to that one of the two broad faces, namely near face 12fn. A second housing designated 16ais one of a set 16 of additional housings, illustrated in phantom, which are adapted to be coupled to housing 12 for the flow of electromagnetic energy therebetween by way of coaxial transmission-line structures.

[0005] As is well known to those skilled in the art, a coaxial transmission line includes a center conductor lying within, and concentric with, a cylindrical outer conductor. Coaxial transmission lines take many forms, but all are characterized by the flow of electromagnetic energy in the region or space between the center and outer conductors. The generally accepted meaning of a transmission line, as opposed to simple “wires,” is that the surge impedance or characteristic impedance of the transmission line tends to be constant along its length, or if the impedance changes along its length, the change is effected in a controlled manner which tends to minimize overall reflection of energy, so that energy introduced into one end of the transmission line is “transmitted” to the remote end with a minimum of loss attributable to mismatch. While coaxial transmission lines may be made to have almost any characteristic impedance, two such impedances are most common. The nominal impedance of 50 ohms is theoretically capable of handling the highest power without voltage breakdown, while the nominal impedance of 75 ohms theoretically provides the least loss per unit length. Military equipments such as radar and other transmitting devices requiring high power thus tend to use 50-ohm transmission lines and associated connectors, while some commercial users such as cable television have, at least in the past, operated at 75 ohms.

[0006] In the arrangement of FIG. 1, the coupling between module 16aand a portion of module 12 is provided by coaxial connectors of a set 18aof such connectors, not entirely illustrated in FIG. 1. In FIG. 1, portions of four connectors 18a1, 18a2, 18a3, and 18a4 associated with module 16aare illustrated. It should be understood that this number, and their layout, are illustrative only, and that there may be more or fewer such connectors associated with each module of set 16 of modules.

[0007] In order to minimize losses in interconnecting transmission lines, and to minimize the volume occupied by the combined modules 10 of FIG. 1, it is desirable to have each module 16xof set 16 of modules immediately adjacent the near or facing surface 12fnof module 12. In order to accomplish a close spacing, it is possible to use as connectors type GPO interconnects, manufactured by Gilbert Engineering, whose address is 5310 W Camelback Road, Glendale, Ariz. 85301. These connectors are characterized from DC to 40 GHz.

[0008] FIG. 2ais a simplified, exploded view of a particular version of the GPO connector, adapted for connecting conductive traces on a printed circuit board to conductive traces on another such board, and FIG. 2brepresents a cross-section of a portion of the arrangement of FIG. 1 in its assembled state. For specificity, FIGS. 2aand 2billustrates connector 18a1 of FIG. 1. In FIG. 2a, 214arepresents a portion of the printed circuit board lying within housing 12 in plane 14, and 214brepresents another printed circuit board lying within housing 16a. In this particular layout, the two printed circuit boards 214aand 214blie mutually parallel.

[0009] In the arrangement of FIGS. 2aand 2b, both printed-circuit boards are “stripline” boards, in that the transmission lines within each board lie between ground planes. In FIGS. 2aand 2b, printed-circuit board 214aincludes a near ground plane 214agnand a remote or far ground plane 214agf.Lying between ground planes 214agnand 214agfare two layers of dielectric or insulating material, a near layer 214amand a far layer 214aif.Lying between insulating layers 214amand 214aifis plane 14 of FIG. 1, and lying “in” the plane are a plurality of conductive traces, one of which is illustrated as 214act.It should be understood that the “plane” 14 is conceptual, as a plane is dimensionless and therefore cannot contain a three-dimensional object such as trace 214act,however thin.

[0010] In FIGS. 2aand 2b, a circular aperture 214agais defined in ground plane 214agn,thereby exposing the near surface 214ainsof near dielectric or insulating layer 214ain.Near the center of aperture 214aga,a conductive pad 214acpis affixed to the near surface 214ainsof near dielectric layer 214ain,and is electrically coupled, as by a plated-through via 214av,to an end of the underlying conductive trace 214act.

[0011] Also in FIG. 2ais another printed circuit board 214b, which is similar in its construction to printed circuit board 214a, but which appears somewhat different, since it is seen from the rear rather than from the front. Elements of circuit board 214bcorresponding to those of 214aare designated by the same reference numerals, but in the “214b” series rather than in the “214a” series. Thus, printed-circuit board 214bshows a near ground plane 214bgn, a far ground plane 214bgf, a near dielectric layer 214bin, a far dielectric layer 214bif, with a conductive trace 214bctextending therebetween, and ending at the center of an aperture 214bgain near ground plane 214bgn.

[0012] The GPO connector 18aof FIG. 2aincludes two surface connector portions 210aand 210b, together with a “bullet” portion 216, some of which is also visible in the cross-section of FIG. 2b. Surface connector portion 210ais illustrated as including three portions exploded away from each other. Surface connector portion 210aincludes an outer conductor or flange portion 210aoc, an inner conductor portion 210aic, and a cylindrical insulator portion 210ai. As illustrated, outer conductor 210aocincludes a tubular body portion 210aocb, a lower flange 210aoclf, and a strengthening flange 210aocsf. Body 210aocbof outer conductor 210aocdefines an inner bore 210aocbbwhich is dimensioned to accept the outer diameter of cylindrical insulator portion 210ai. Cylindrical insulator portion 210aicontains an axial bore dimensioned to accommodate the body portion of center or inner conductor portion 210aic. Assembly of surface connector portion 210ato printed circuit board 214ais accomplished by soldering or otherwise metallurgically connecting an end of center conductor portion 210aicto the center pad 214acp. The bore of insulator 210aiis then fitted over the protruding center conductor, the outer conductor 210aocis fitted over and around insulator 210ai, and the lower flange 210aoclfis then soldered to that portion of the near ground plane 214agnsurrounding aperture 214agadefined by dashed circle 214ac.

[0013] Connector portion 210bof connector 18aof FIG. 2ais similar to portion 210a, and is similarly affixed to printed circuit board 214b. More particularly, the end of inner conductor 210bicis soldered or otherwise metallurgically connected to a pad (not designated) connected to the end of conductive trace 214bctso that it projects away from the surface of the board 214b. Dielectric insulator 210biis fitted over protruding inner conductor 210bic. Outer conductor 210bocis then fitted over insulator 210biwith its lower flange abutting that portion of near ground plane 214bgnsurrounding aperture 214bga.

[0014] With both surface connector portions 210aand 210bof connector 18afastened to their respective printed circuit boards 214aand 214b, the surface connector portions are aligned with axis 8, and brought together with bullet portion 216 of connector 18atherebetween. Bullet portion 216 includes a plurality of elongated conductive strips 216oc₁, 216oc₂, . . . fastened about a generally cylindrical dielectric body 216 with their axes of elongation coaxial with the local axis (which in the illustration is axis 8) and held in place by a spring ring 216ocr. The inner conductor is generally similar to the outer conductor. When the bullet portion 216 of connector 18ais pressed against the assembled surface portions 210aand 210b, the ends of the center conductor of bullet portion 216 spreads slightly to fit over the two center pins 210aicand 210bic, and the two ends of the outer conductor made up of pieces 216oc₁, 216oc₂, . . . contract slightly to fit within bore 210aocbbof the strengthening flange portions 210aocsfand of the equivalent bore associated with strengthening flange 210bocsf. In this fashion, the GPO connector set 18aof FIGS. 2aand 2bprovides for a suitable transmission line interconnecting the two mutually parallel printed circuit boards 214aand 214b.

[0015] It will be understood that each printed-circuit board 214aand 214bof FIG. 2amay be mounted within its own protective housing or module, with the surface mounting portions 210aand 210bprojecting through one or more sides of the housing in the manner suggested in FIG. 1, and with the bullet portion of each connector as a separate item which is assembled to one or both sides of the module pair to be interconnected.

[0016] While the GPO connectors provide good connection between conductors on mutually parallel boards. they have disadvantages including relatively high cost. The assembly of each connector imposes an additional cost. A connector arrangement with the potential for lower cost would be advantageous.

[0017] An assemblage according to an aspect of the invention includes a first stripline board including first and second ground planes, and a circuit trace lying between the first and second ground planes. The first ground plane defines a circular first electromagnetic coupling aperture, and a conductive center trace lies at the center of the first electromagnetic coupling aperture, to thereby define, in cross-section, a first coaxial transmission line structure. The center trace of the first coupling aperture connects by way of a plated-through via to an end of the circuit trace of the first stripline board. A second stripline board includes first and second ground planes, and a circuit trace lying between the first and second ground planes of the second stripline board. The first ground plane of the second stripline board defines a circular second electromagnetic coupling aperture and a conductive center trace at the center of the second electromagnetic coupling aperture, to thereby define, in cross-section, a second coaxial transmission line essentially identical to the first coaxial transmission line. The center trace of the second coupling aperture connects by way of a plated-through via to an end of the circuit trace of the second stripline board. A separator defines two parallel planar surfaces spaced from each other by a predetermined distance. The separator defines a circular through aperture extending between the parallel planar surfaces. The through aperture is larger in diameter than the first and second coupling apertures, and the separator is physically fastened to the first and second stripline boards so that the through aperture is concentric with the first and second electromagnetic coupling apertures. An axially compliant third coaxial transmission line insert includes a center conductor coaxial with an outer conductor. At least one of the center conductor and the outer conductor of the third coaxial transmission line is in the form of a conductive ladder-shaped spring structure defining a plurality of rungs lying between elongated parallel members, which elongated members are formed into curved shapes resembling circles, or at least portions thereof, so that the rungs lie generally parallel with each other and with the axis of the other one of the center conductor and the outer conductor. The axially compliant third coaxial transmission line lies within the through aperture with a first end of the center conductor abutting the center trace of the first coaxial transmission line and a second end of the center conductor abutting the center trace of the second coaxial transmission line, and with a first end of the outer conductor of the coaxial third transmission line abutting the first ground plane of the first stripline board in a region surrounding the through aperture of the first stripline board, and with a second end of the outer conductor of the coaxial third transmission line abutting the first ground plane of the second stripline board in a region surrounding the through aperture of the second stripline board.

[0018] In a particularly advantageous avatar of the invention, both the center conductor and the outer conductor of the third coaxial transmission line are in the form of a conductive ladder-shaped spring structure defining a plurality of rungs lying between elongated parallel members, with the elongated members formed into curved structures so that the rungs lie generally parallel with each other and with the axis of the other one of the center conductor and the outer conductor. In such an arrangement, the elongated portions of the planar structures which are formed into circles may overlap so that more than a complete “turn” of structure is formed, or may not quite overlap, so that a “gap” lacking one or more rungs is left in one side of the structure. In this particularly advantageous avatar, the rungs of either the center conductor or the outer conductor of the third coaxial transmission line may bulge away from the axis of third coaxial transmission line at regions remote from the elongated members, to thereby form a somewhat barrel shape.

[0019] In a particular version of the structure, a generally barrel-shaped dielectric form may occupy the interstice between the center and outer conductors, and in a mode of making the connections, the dielectric form may have the ladder of the outer conductor wound thereabout. This dielectric form is a solid, but may be an elastomeric solid. The conductors of the ladders may comprise beryllium copper.

[0020] A method according to an aspect of the invention is for interconnecting transmission paths defined on the surface of first and second separated stripline boards, where the transmission paths are in the form of a circular aperture defined in a conductive ground plane of the associated stripline board, together with a conductive trace located at the center of the aperture. The method includes the step of making a center conductor having a nominal diameter by forming a first electrically conductive planar ladder-shaped spring structure defining two elongated parallel elements and a plurality of rungs extending therebetween into a tube-like form defined about an axis, so that the rungs lie parallel with the axis. The method includes the forming of a second electrically conductive planar ladder-shaped spring structure defining two elongated parallel elements and a plurality of rungs extending therebetween into a second tube-like form defined about an axis, so that the rungs lie parallel with the axis, to thereby define an outer conductor having a nominal diameter greater than the nominal diameter of the center conductor. The center conductor is placed coaxially within the outer conductor to thereby form a coaxial transmission line. The first and second stripline boards are juxtaposed with the circular apertures facing each other, and with the center and outer conductors of one end of the coaxial transmission line abutting the central trace and that portion of the ground plane surrounding the circular aperture, respectively, of the transmission path of the first stripline board, and with the center and outer conductors of the other end of the coaxial transmission line abutting the central trace and that portion of the ground plane surrounding the circular aperture, respectively, of the transmission path of the second stripline board, to thereby establish a coaxial transmission path between the transmission paths of the first and second stripline boards. The first and second stripline boards are then fastened together, including by way of intermediary structures, with the coaxial transmission path in place.

[0021] FIG. 1 is a simplified exploded view of a first housing adapted to be connected to a plurality of second housings;

[0022] FIG. 2ais an exploded view of two printed-circuit stripline boards, each of which may be in a separate housing of FIG. 1, together with an interconnecting transmission line connector arrangement such as may be used in the prior art, and FIG. 2bis a cross-section of a portion of the arrangement of FIG. 2ain its assembled form;

[0023] FIGS. 3aand 3bare perspective or isometric views of planar “ladder” structures which are a starting condition for inner and outer conductors, respectively, of a connector according to an aspect of the invention, and FIG. 3cis a perspective or isometric view of a dielectric support piece for the connector;

[0024] FIG. 4 is an exploded view of the ladders of FIGS. 3aand 3bfashioned into center and outer conductors, respectively, exploded away from the support dielectric of FIG. 3c;

[0025] FIG. 5ais a simplified perspective or isometric view of the arrangement of FIG. 4, with the center and outer conductors assembled to the support dielectric without overlap, and FIG. 5billustrates a portion of the arrangement of FIG. 5awith overlap;

[0026] FIG. 6ais a side view of one rung of either the inner or outer conductor of FIG. 5a, and FIG. 6bis a cross-section of the rung;

[0027] FIG. 7 is a simplified exploded view of a structure in which a bullet connector according to an aspect of the invention can be used, partially cut away to reveal interior details;

[0028] FIG. 8 is a cross-section of the arrangement of FIG. 7, illustrating details of the structure; and

[0029] FIG. 9 illustrates a planar ladder structure suited for use with a dielectric piece such as that shown in FIGS. 3cand 4.

[0030] FIG. 3ais a simplified perspective or isometric view of an electrically conductive planar spring structure 310 in the general form of a thin plate or foil. The plate 310 defines a set of apertures 312 including a plurality of elongated mutually parallel apertures, some of which are designated 312a, 312b, . . . , 312c, and 312d. These apertures can be formed by selective etching. Each of the apertures has rounded ends, such as rounded ends 312b₁and 312b₂. The plurality of apertures of set 312 forms plate 310 into a structure somewhat resembling a ladder, including elongated “sides” 314aand 314band a set 316 including a plurality of elongated rungs 316a, 316b, . . . , 316c, and 316d. The overall length of the “ladder” of FIG. 3a, as described below, is dimensioned to equal the circumference of an inner conductor, so is designated as dimension C_i. The other major dimension of plate 310 is defined as dimension “L.”

[0031] FIG. 3bis a simplified perspective or isometric view of an electrically conductive planar spring structure 340 in the general form of a thin plate or foil. The plate 340 defines a set 342 including a plurality of elongated mutually parallel apertures, some of which are designated 342a, 342b, . . . , 342c, and 342d. Each of the apertures has rounded ends, such as rounded ends 342b₁and 342b₂. The plurality of apertures of set 342 forms plate 340 into a structure somewhat resembling a ladder, including elongated “sides” 344aand 344band a set 346 including a plurality of elongated rungs 346a, 346b, . . . , 346c, and 346d. The overall length of the “ladder” of FIG. 3b, as described below, is dimensioned to equal the circumference of an outer conductor, so the length of the ladder between first and second ends 344a₁and 344a₂of side 344a, and between first and second ends 344b₁and 344b₂of side 344b, is designated as dimension C_o. The other major dimension of plate 340 is defined as dimension “L.”

[0032] FIG. 3cis a perspective or isometric view of a dielectric, preferably elastomeric piece 360 having the general external shape of a barrel including a flat top 362 defining a radius R, a similar and parallel flat bottom 364, and a somewhat bulging or convex circumferential side 366. In one embodiment, dielectric piece 360 is solid, and defines an axial bore 368 having a diameter r. When used with the ladders 310 and 340 of FIGS. 3aand 3b, respectively, the larger radius R of the dielectric piece 360 is approximately equal to C_o/2π, and the smaller radius r of the axial bore 368 is equal to C_i/2π. Radius R is about 10% greater than radius r.

[0033] With the dimensions C_o/2π and C_i/2π for radii R and r of dielectric piece 360 of FIG. 3c, the ladders 310 and 340 of FIGS. 3aand 3b, respectively, can be formed into roughly cylindrical shapes and applied to the dielectric piece 360. FIG. 4 is a simplified perspective or isometric view of a bullet connector 400 according to an aspect of the invention, in which the ladders of FIGS. 3aand 3brolled or formed into inner and outer conductors, respectively, exploded away from the dielectric piece of FIG. 3c. In FIG. 4, the inner conductor ladder sides 314aand 314bare longitudinally disposed relative to axis 8, and are formed into circles surrounding the axis. The rungs of set 312 of rungs extend generally parallel with axis 8. Similarly, the outer conductor ladder sides 344aand 344bare longitudinally disposed relative to axis 8, and are formed into circles surrounding the axis. The rungs of set 342 of rungs extend generally parallel with axis 8. As is indicated by the break in ladder sides 344aand 344bnear a plane 410 which includes the axis 8, the circle made by the ladder sides 344aand 344bneed not fully completed. As an alternative, the ends of ladder sides 344aand 344bmay overlap somewhat. As illustrated in FIG. 4, the dielectric piece 360 may define a right-circular-cylindric portion 422aadjacent the top, and another right-circular-cylindric portion 422bat the bottom of the dielectric piece, to accommodate the sides 344aand 344bof the ladder.

[0034] FIG. 5ais a simplified perspective or isometric view of an assembled “bullet” connector 400 according to an aspect of the invention. In FIG. 5a, the completed structure has an axial length L, and the inner conductor 310 lies axially within outer conductor 340. The rolled-up or cylindrical inner conductor 310 can compress slightly in diameter in order to be accommodated within the axial bore 368, and the rolled-up outer conductor 340 can expand slightly, due to the fact that the ends of the rolled-up structures are not physically fastened together. Thus, the rolled structures have a certain amount of spring, which allows the conductors to be affixed to the dielectric support to form the complete bullet 400. FIG. 5billustrates the elongated member 344aof FIG. 5aalone, with only portions of the rungs attached thereto. The two ends of the elongated structure overlap in a region 570.

[0035] The bullet connector 400 of FIG. 5acan be axially compliant. In order for the bullet to be axially compliant, the individual longitudinally-oriented rungs of the ladder structure must also be compliant. The shape taken by the rung members (of either the inner or outer conductors) is illustrated in FIGS. 6aand 6b. In FIG. 6a, the rung-plus-elongated-support is illustrated as 610. These rungs are etched such that they are individual metal members oriented parallel to the axis of the connector. Each rung is connected to the other rungs at the ends, but they are independent of other rungs along the middle portion of their lengths. Each individual rung, then, acts as a long, slender column when subjected to an axial force P. A long slender column has the ability to withstand large axial deflection when subject to a constant axial force P. In other words, the force versus deflection ratio is not constant. The force P must be sufficiently large that it meets or exceeds the “buckling” force of the column, or the force required to cause the buckling. Taking the material of the conductive rung as beryllium-copper, a compressive member must meet the following relationship in order to be considered as a long slender column $\begin{array}{cc}\frac{L_e}{r_n}\ge \sqrt{2\frac{{\pi }^{2}E}{S_{yp}}}& 1\end{array}$

[0036] where:

[0037] L_eis the equivalent length=0.5 for a column clamped at both ends;

[0038] r_n=radius of gyration;

[0039] E=Young's modulus=20 (10⁶) for beryllium copper;

[0040] S_yp=yield strength=100,000 psi for heat-treated beryllium copper.

[0041] The radius of gyration is determined as $\begin{array}{cc}{r}_n=\sqrt{\frac{I}{A}}=\frac{\sqrt{\frac{1}{12}{b}_h^3}}{bh}=\sqrt{\frac{h^2}{12}}=\frac{h}{\sqrt{12}}& 2\end{array}$

[0042] where:

[0043] I=moment of inertia;

[0044] A=cross-sectional area;

[0045] b=width of rung as in FIG. 6b; and

[0046] h=thickness of plate as in FIG. 6b. From equations (1) and (2) $\begin{array}{cc}\frac{L_e}{r_n}=\frac{\frac{L}{2}}{\frac{h}{12}}\ge \sqrt{\frac{2{\pi }^2\left(20\times {10}^6\right)}{100,000}}& 3\end{array}$

[0047] which yields

[0048] 199.2≧62.8

[0049] thereby indicating that the beryllium copper rung members can be treated as long slender columns.

[0050] The critical load P_CRfor one rung member is determined by $\begin{array}{cc}{P}_{\mathrm{CR}}=\frac{{\pi }^2\mathrm{EI}}{L_e^2}=\left(\frac{1}{12}\right)\left(\frac{{\pi }^2{\mathrm{Eb}}_h^3}{L_e^2}\right)& 4\end{array}$

[0051] where b is b_icfor the inner conductor, and b_ocfor the outer conductor, as determined by $\begin{array}{cc}{P}_{{\mathrm{CR}}_{\mathrm{oc}}}=\frac{1}{12}\frac{{\pi }^2\left(20\times {10}^6\right)\left(0.021\right){\left(0.002\right)}^3}{{\left(\frac{0.230}{2}\right)}^2}=0.209\mathrm{lbs}\text{/}\mathrm{member}& 5\end{array}$

[0052] and $\begin{array}{cc}{P}_{{\mathrm{CR}}_{\mathrm{oc}}}=\left(\frac{1}{12}\right)\frac{{\pi }^2\left(20\times {10}^6\right)\left(0.0058\right){\left(0.002\right)}^3}{{\left(0·\frac{230}{2}\right)}^2}=0.58\mathrm{lbs}\text{/}\mathrm{member}& 6\end{array}$

[0053] so the total force to buckle a beryllium copper structure having sixteen rungs is

[0054] P_T=16P_CR  7

[0055] where:

[0056] P_T=Total compressive force to buckle either the inner or outer conductor member;

[0057] 16=total number of beryllium copper rungs in each member;

[0058] P_CR=critical load for either the inner or outer conductor.

[0059] The total amount of compressive force required to buckle the inner conductor is 0.93 lb, while the total force required to buckle the outer conductor is 3.344 lb. Thus, the total force required to adequately compress the bullet interconnect is the sum of 0.93 plus 3.44 or about 4.37 lb, plus any force required to distort the elastomeric dielectric material, which will depend upon the material and exact structure. When subjected to this force, the column members buckle, such that a wide range of axial deflections can be accommodated while maintaining a substantially constant axial force. The buckling force can be selected over a wide range, depending upon parameters such as the number of rungs, conductor thickness, axial length, and the mechanical properties of the dielectric support.

[0060] FIG. 7 is a simplified exploded view of a structure in which a bullet connector according to an aspect of the invention can be used, partially cut away to reveal interior details. In FIG. 7, exposed center conductive traces on a beamformer dielectric board 710 are designated 712aand 712b, and exposed outer conductor traces are designated 714aand 714b, respectively. These traces define electromagnetic ports having a coaxial cross-sectional shape: center trace 712aand outer trace 714adefine an “A” RF port, while center trace 712band outer trace 714btogether define a “B” RF port. A cold plate 716 has a flat lower face for abutting at least portions of the upper surface 710usof beamformer 710. Cold plate 716 also defines a through aperture extending between its upper surface 716US and a lower surface 716LS and centered on each coaxial port defined by center and outer conductive traces. More particularly, an aperture 722A is registered with RF port A of beamformer 710, and an aperture 722B is registered with RF port B of beamformer 710. Each aperture 722 is dimensioned to clear the associated bullet connector. A TR module, which represents one of many such modules to be thermally coupled to the cold plate and electrically coupled to the beamformer, is represented by 718. TR module 718 has a locating depression which is registered with each of the RF ports of the beamformer, which in the case of FIG. 7 is the A and B RF ports. More particularly, a locating depression 720A is registered with the A RF port of beamformer 710, and a locating depression 720B is registered with the B RF port of beamformer 710. Within each depression 720A and 720B of TR module 718 lies a set of RF port metallizations corresponding to the inner and outer conductor metallizations of ports A and B of beamformer 710. A plurality of bullet connectors, only one of which is illustrated in FIG. 7, are provided for assembly with the structure. As illustrated, the bullet connector is connector 400 of FIG. 4. During assembly, the cold plate 716 is assembled to either the beamformer 710 or to the TR module 718, and a bullet connector is dropped into each aperture 722. The presence of the beamformer prevents the bullet connector from simply falling through the aperture 722. The next assembly, in this case the TR module, is affixed on the other side of the cold plate, effectively stopping up the apertures 722, so that the bullet connectors are captivated therein. The dimensions of the bullet connector are selected in conjunction with the thickness of the cold plate to cause the rungs of the ladder structures to act as long slender columns, so that they deflect to tend to maintain a substantially constant axial force.

[0061] This axial force, in turn, tends to keep the conductors of the bullet connector in intimate contact with the metallizations of the various RF ports with which they are associated.

[0062] FIG. 8 is a cross-section of the arrangement of FIG. 7, illustrating details of the structure. In FIG. 8, a lower module board 810 is spaced away from an upper module board 830. Each board 810 and 830 is a stripline board similar to that described in conjunction with FIG. 2b, and each includes at least one conductive lead 818; 838 spaced by dielectric layers from ground planes 814, 816;834, 836. Each module board 810 and 830 also includes an RF port 820; 840 defined by a circular aperture in a ground plane 814; 834 together with a conductive pad 822; 842 centrally located within the associated aperture 820; 840. The conductive pad 822; 842 is connected by way of a plated-through via 824; 844 to the conductive trace 818; 838. The bullet connector 400 including the inner conductor 310, outer conductor 340, and dielectric elastomer 360, as described in conjunction with FIGS. 3a, 3b, 3c, and 4, lies in through aperture 852 defined in cold plate 850, and its center conductor 310 makes electrical contact with pads 822 and 842. The ends of outer conductor 340 make contact with ground plane 814 in a region around aperture 820 and with ground plane 834 in a region around aperture 840. The structure as a whole is held together by means including a screw or bolt 754.

[0063] The planar ladder of FIG. 9 is equivalent to that of FIG. 3b, but its shape is adjusted or distorted so that the circumference represented by length C_MIDDLErepresents the circumference of the barrel-shaped dielectric 360 of FIG. 3caround its middle, remote from either end, while the circumference represented by length C_ENDrepresents the lesser circumference at either end of the dielectric 360.

[0064] Other embodiments of the invention will be apparent to those skilled in the art. For example, the exposed portion of the outer conductor traces 714aand 714bmay merely be a portion of a ground plane covering the near side of the beamformer board 710 of FIG. 7. While two RF connections have been illustrated in FIG. 7, those skilled in the art know that one or many such connections may be appropriate, and that the RF connections need not be associated with TR modules and beamformers, but may be any structure requiring RF connections between mutually parallel printed circuit boards, regardless of the number of layers of the boards.

[0065] Thus, an assemblage (800) according to an aspect of the invention includes a first stripline board (810) including first (814) and second (816) ground planes, and a circuit trace (818) lying between the first (814) and second (816) ground planes. The first ground plane (814) defines a circular first electromagnetic coupling aperture (820), and a conductive center trace (822) lies at the center of the first electromagnetic coupling aperture (820), to thereby define, in cross-section, a first coaxial transmission line structure (A). The center trace (822) of the first coupling aperture (820) connects by way of a plated-through via (824) to an end of the circuit trace (818) of the first stripline board (810). A second stripline board (830) includes first (834) and second (836) ground planes, and a circuit trace (838) lying between the first (834) and second (836) ground planes of the second stripline board (830). The first ground plane (834) of the second stripline board (830) defines a circular second electromagnetic coupling aperture (840), and a conductive center trace (842) lies at the center of the second electromagnetic coupling aperture (840), to thereby define, in cross-section, a second coaxial transmission line essentially identical to the first coaxial transmission line. The center trace (842) of the second coupling aperture (840) connects by way of a plated-through via (844) to an end of the circuit trace (838) of the second stripline board (830). A separator or spacer (850) defines two parallel planar surfaces (850US, 850LS) spaced from each other by a predetermined distance (DS). The separator (850) defines a circular through aperture (852) extending between the parallel planar surfaces (850US, 850LS). The through aperture (852) is larger in diameter than the first (820) and second (840) coupling apertures, and the separator (850) is physically fastened (bolt 754) to the first (810) and second (830) stripline boards so that the through aperture (852) is concentric with the first (820) and second (840) electromagnetic coupling apertures. An axially compliant third coaxial transmission line insert (400) includes a center conductor (310) coaxial with an outer conductor (340). At least one of the center conductor (310) and the outer conductor (340) of the third coaxial transmission line is in the form of a conductive ladder-shaped spring structure (FIGS. 3a, 3b) defining a plurality of rungs (312, 342) lying between elongated parallel members (314), which elongated members (314) are formed (FIG. 4) into curved shapes resembling circles, or at least portions thereof, so that the rungs (312, 342) lie generally parallel with each other and with the axis (8) of the other one of the center conductor (310) and the outer conductor (340).

[0066] The axially compliant third coaxial transmission line (400) lies within the through aperture (852) with a first end of the center conductor (310) abutting the center trace (822) of the first coaxial transmission line (A) and a second end of the center conductor (310) abutting the center trace (842) of the second coaxial transmission line (B of FIG. 8), and with a first end of the outer conductor (340) of the coaxial third transmission line (400) abutting the first ground plane (714a; 814) of the first stripline board (810) in a region surrounding the through aperture (820) of the first stripline board (810), and with a second end of the outer conductor (340) of the coaxial third transmission line (400) abutting the first ground plane (834) of the second stripline board (830) in a region surrounding the through aperture (840) of the second stripline board (830).

[0067] In a particularly advantageous avatar of the invention, both the center (310) conductor and the outer conductor (340) of the third coaxial transmission line (400) are in the form of a conductive ladder-shaped spring structure (FIGS. 3a, 3b) defining a plurality of rungs (sets 312, 342) lying between elongated parallel members (314a, 314b; 344a, 344b), with the elongated members formed into curved structures (FIG. 4) so that the rungs (312, 342) lie generally parallel with each other and with the axis (8) of the other one of the center conductor and the outer conductor. In such an arrangement, the elongated portions of the planar structures (FIGS. 3a, 3b) which are formed into circles may overlap (570) so that more than a complete “turn” of structure is formed, or may not quite overlap (370), so that a “gap” is left in one side of the structure. In this particularly advantageous avatar, the rungs (312, 342) of either the center conductor (310) or the outer conductor (340) of the third coaxial transmission line (400) may bulge away from the axis (8) of third coaxial transmission line (400) at regions remote from, or between, the elongated members (314a, 314b; 344a, 344b), to thereby form a somewhat barrel shape.

[0068] In a particular version of the structure, a generally barrel-shaped dielectric (360) form may occupy the interstice between the center (310) and outer (340) conductors, and, in a mode of making the connections, the dielectric form (360) may have the ladder of the outer conductor (340) wound thereabout. This dielectric form (360) is of a solid material, but may be an elastomeric solid material, and may be hollow if desired, so long as there is sufficient purchase at the through aperture to grasp the ends of the center conductor. The conductors of the ladders may comprise beryllium copper.

[0069] A method according to an aspect of the invention is for interconnecting transmission paths (A, B of FIG. 8) defined on the surface of first and second separated stripline boards (810, 830), where the transmission paths are in the form of a circular aperture (820, 840) defined in a conductive ground plane (814, 834) of the associated stripline board (810, 830), together with a conductive trace (822, 842) located at the center of the aperture (820, 840). The method includes the step of making a center conductor (310) having a nominal diameter by forming a first electrically conductive planar ladder-shaped spring structure (FIG. 3a) defining two elongated parallel elements (314a, 314b) and a plurality of rungs (312) extending therebetween into a tube-like form (FIG. 4) defined about an axis (8), so that the rungs (312) lie parallel with the axis (8). The method includes the forming of a second electrically conductive planar ladder-shaped spring structure (FIG. 3b) defining two elongated parallel elements (344a, 344b) and a plurality of rungs (342) extending therebetween into a second tube-like form (FIG. 4) defined about an axis (8), so that the rungs (342) lie parallel with the axis, to thereby define an outer conductor having a nominal diameter greater than the nominal diameter of the center conductor. The center conductor (310) is placed coaxially within the outer conductor (340) to thereby form a coaxial transmission line (400). The first (810) and second (830) stripline boards are juxtaposed with the circular apertures (820, 840) facing each other, and with the center (310) and outer (340) conductors of one end of the coaxial transmission line abutting the central trace (822) and a portion of the ground plane (814) surrounding the circular aperture (820), respectively, of the transmission path (A) of the first stripline board (810), and with the center (310) and outer (340) conductors of the other end of the coaxial transmission line (400) abutting the central trace (842) and a portion of the ground plane (834) surrounding the circular aperture (840), respectively, of the transmission path (B of FIG. 8) of the second stripline board (830), to thereby establish a coaxial electromagnetic transmission path between the transmission paths (A, B) of the first (810) and second (830) stripline boards. The first (810) and second (830) stripline boards are then fastened (754) together, including by way of intermediary structures (850), with the coaxial transmission path in place.