Abstract:
A microwave stripline signal distribution system utilizes a central conductor that is supported on a substrate, which is itself captured and supported between two foam layers. External ground plane shielding is provided on each side of the foam/central-conductor-substrate sandwich. The metallic central conductor is thin, as are the other structural elements, leading to a lightweight signal distribution system. The central conductor may be patterned to provide a large number of feeds. The approach allows: the inexpensive fabrication of lightweight signal distribution boards, whose inputs and outputs may be combined to provide single-channel or multi-channel combination or division of microwave signals. In a typical application, there is combination or division of microwave signals in an antenna involving hundreds of individual feed horns.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to microwave distribution systems, and, more particularly, to a stripline structure used in a microwave system. 
     Microwave energy is employed to transmit communications signals because of its high frequency and the consequent ability to convey a large amount of information, and because it may be amplified to high power levels. For example, extremely high frequency (EHF) energy in the 15-40 GHz (gigahertz) range is used in many communications applications. The communications signals conveyed through communications satellites are transmitted from an earth ground station through free space to the satellite in geosynchronous orbit. The signals are there amplified by an on-board amplifier and retransmitted through free space to another earth ground station. 
     When the microwave signals are being amplified and otherwise processed on board the satellite, they are conveyed in waveguides and/or on thin metallic substrates termed striplines. At some points in the distribution system, waveguides are too large in physical dimensions too heavy, and too complex to be practical. For example, microwave signals conveyed from and to the segmented antennas of the satellite must be combined when received and divided when transmitted. A waveguide system may be used for these purposes, but it is large, heavy, and complex. A stripline system is much smaller, lighter, cheaper, and less complex, but it exhibits a higher signal attenuation than the waveguide. 
     There is therefore a tradeoff between the two approaches. The stripline system would be more attractive in applications such as antenna systems if it could be built to be lighter and less costly than possible with presently available approaches. Accordingly, there is a need for a better approach to microwave stripline structures which are particularly suited for packing a large number of stripline conductors into a small space. The present invention fulfills this need, and further provides related advantages. 
     SUMMARY OF THE INVENTION 
     The present invention provides a stripline structure suitable for conducting microwave signals. The stripline structure is compact and extremely light in weight. It is constructed from available, space-qualified materials, and may be readily fabricated. Its radio frequency attenuation is acceptable, while maintaining the mechanical rigidity for use in spacecraft. The stripline structure may be sized to be suitable for use with a wide range of microwave frequencies, including the 15-40 Gigahertz extremely high frequency range that is desirable for communications satellites. The stripline structure is designed for efficient scale-up to a multichannel form that accommodates a large number of signals on individual stripline conductors. The stripline structure is thus particularly useful for combiner/divider applications such as those used to carry signals from and to microwave antennas. 
     In accordance with the invention, a microwave stripline structure includes a first-layer conductor structure comprising a nonmetallic central conductor substrate having a first side and a second side,band a first ground plane layer spaced apart from the first side of the central conductor substrate. The first ground plane layer includes a first ground plane substrate, preferably comprising a nonmetallic material, and a first metallic layer structure contacting at least one side of the first ground plane substrate. The stripline structure further includes a first foam layer disposed in contact with the first side of the central conductor substrate and with the first ground plane layer. The first foam layer optionally but preferably has a first channel therethrough with the first foam layer, the central conductor substrate, and the ground plane layer bounding the first channel. An elongated metallic central conductor is present on the first side of the central conductor substrate, within the first channel in the embodiments having the first channel. 
     The stripline structure further includes a second ground plane layer spaced apart from the second side of the central conductor substrate. The second ground plane layer includes a second ground plane substrate, preferably comprising a nonmetallic material, and a second metallic layer structure contacting at least one side of the second ground plane substrate. A second foam layer may be disposed in contact with the second side of the central conductor substrate and with the second ground plane layer. The second foam layer optionally but preferably has a second channel therethrough in registry with the first channel, with the second foam layer, the central conductor substrate, and the ground plane layer bounding the second channel. It is preferred that the central conductor substrate, the first ground plane layer, and the second ground plane layer are substantially planar and parallel to each other to a tolerance of within +/−0.001 inch. 
     In a form particularly suitable for a multichannel, stacked arrangement, a microwave stripline structure includes a first-layer conductor structure comprising a substantially planar nonmetallic central conductor substrate having a first side and a second side. The central conductor substrate preferably comprises a composite material of fibers embedded in a cured resin. There is a substantially planar first ground plane layer spaced apart from the first side of the central conductor substrate. The first ground plane layer includes a first ground plane substrate, preferably comprising a nonmetallic material, and a first metallic layer structure on the first ground plane substrate. The first metallic layer structure includes a first metallic inner layer facing the central conductor substrate and a first metallic outer layer disposed remotely from the central conductor substrate. There is a substantially planar first foam layer in contact with the first side of the central conductor substrate and with the first ground plane layer. The first foam layer has a first channel therethrough with the first foam layer, the central conductor substrate, and the ground plane layer bounding the first channel. An elongated metallic central conductor is positioned on the first side of the central conductor substrate within the first channel. There is a substantially planar second ground plane layer spaced apart from the second side of the central conductor substrate. The second ground plane layer includes a second ground plane substrate, preferably comprising a nonmetallic ;material, and a second metallic layer structure on the second ground plane substrate. The second metallic layer structure includes a second metallic inner layer facing the central conductor substrate and a second metallic outer layer disposed remotely from the central conductor substrate. There is additionally a substantially planar second foam layer disposed in contact with the second side of the central conductor substrate and with the second ground plane layer. The second foam layer has a second channel therethrough, in registry with the first channel, with the second foam layer, the central conductor substrate, and the ground plane layer bounding the second channel. Optionally, a nonmetallic post may extend through the central conductor substrate, the first ground plane layer, the first foam layer, the second ground plane layer, and the second foam layer. 
     Stated alternatively, a microwave stripline structure includes a first-layer conductor structure having a first suspended stripline conductor comprising a planar nonmetallic central conductor substrate having a first side and a second side, an elongated metallic central conductor on a first side of the central conductor substrate, and two planar ground plane, layers. One ground plane layer is in facing-but-spaced apart relation to each side of the central conductor substrate. Each ground plane layer comprises a ground plane substrate, preferably made of a nonmetallic material, and a metallic layer structure contacting at least one side of the ground plane substrate. The stripline structure further includes two planar foam layers. Each foam layer contacts one side of the central conductor substrate and one of the ground plane layers. Each foam layer has a channel therethrough in registry with the channel of the other foam layer, with the elongated metallic central conductor lying within one of the channels. The respective foam layer, the central conductor substrate, and the respective ground plane layer bound each channel. 
     In any of these embodiments, the basic stripline structure may be readily expanded to a multichannel form. In one approach involving an in-plane expansion, the first-layer conductor structure has at least one additional stripline conductor, with each additional stripline conductor having a structure substantially identical to the first stripline conductor. In a second approach involving a parallel-plane expansion, a second-layer conductor structure is in facing relation to the first-layer conductor structure. The second-layer conductor structure has the same structure as the first-layer conductor structure. 
     Typically, the ground plane substrates in the various embodiments comprise a flexible absorber material having an electrical resistance of about that of free space (i.e., about 377 ohms). The ground plane substrates are each preferably a layer of semi-conductive, absorptive fibers. The central conductor substrate comprises a composite material of quartz fibers embedded in a cured cyanate ester resin. The foam layers comprise an electrically nonconductive, closed-cell foam such as polymethacrylimide foam. 
     A feature of the preferred form of the invention is that it contains no polytetrafluoroethylene (sometimes known as Teflon™) polymer. This material is difficult to bond and usually requires a housing to mechanically position it. Further, it has a tendency to cold flow in a space environment. The presently preferred approach uses no polytetrafluoroethylene. 
     The present approach provides a light weight, strong, readily manufactured stripline structure. The basic design may be expanded to a large number of applications using in-plane or parallel-plane arrangements. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic sectional view of a stripline structure according to the invention; 
     FIG. 2 is a schematic sectional view of a stripline structure having multiple striplines in a single plane; 
     FIG. 3 is a schematic sectional view of a stripline structure having multiple striplines in parallel planes; 
     FIG. 4 is a plan view of the central conductor in one embodiment of the invention; 
     FIG. 5 is a schematic exploded plan view of a flat stripline/coaxial/coaxial/stripline connector; 
     FIG. 6 is a schematic exploded perspective view of a single channel signal distribution system; 
     FIG. 7 is a schematic exploded perspective view of a multiple (in this case, three) channel signal distribution system; and 
     FIG. 8 is a schematic sectional view of another embodiment of the stripline structure according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 depicts a stripline structure  20  according to a preferred embodiment of the invention. The stripline structure  20  includes a first-layer conductor structure  22 , which comprises a nonmetallic central conductor substrate  24  having a first side  26  and a second side  28 . The central conductor substrate  24  is preferably, but not necessarily, substantially planar. The stripline structure  20  further includes a first ground plane layer  30 , which is preferably but not necessarily planar, spaced apart from the first side  26  of the central conductor substrate  24 . The first ground plane layer  30  includes a first ground plane substrate  32 , preferably made of a nonmetallic material, and a first metallic layer structure  36  on the first ground plane substrate  32 . The first metallic layer structure  36  includes a first metallic inner layer  38  facing the central conductor substrate  24 , and, optionally, a first metallic outer layer  40  disposed remotely from the central conductor substrate  24 . 
     A first foam layer  42 , which is preferably but not necessarily planar, is disposed in contact with and between the first side  26  of the central conductor substrate  24  and the first ground plane layer  30 . The first foam layer  42  preferably has a first channel  44  therethrough. The first foam layer  42 , the central conductor substrate  24 , and the first ground plane layer  30  bound the first channel  44  and form its sides, top, and bottom. 
     An elongated metallic central conductor  46  is disposed on the first side  26  of the central conductor substrate  24  within the first channel  44 . 
     A second ground plane layer  48 , which is preferably but not necessarily planar, is spaced apart from the second side  28  of the central conductor substrate  24 . The second ground plane layer  48  includes a second ground plane substrate  50 , preferably made of a nonmetallic material, and a second metallic layer structure  54  on the second ground plane substrate  50 . The second metallic layer structure includes a second metallic inner layer  56  facing the central conductor substrate  24  and, optionally, a second metallic outer layer  58  disposed remotely from the central conductor substrate  24 . 
     A second foam layer  60 , which is preferably but not necessarily planar, is disposed in contact with and between the second side  28  of the central conductor substrate  24  and with the second ground plane layer  48 . The second foam layer  60  preferably has a second channel  62  therethrough, in registry with the first channel  44 . (As used herein, the term “registry” means that elements of structure are spatially aligned with each other, vertically in the view of FIG. 1.) The second foam layer  60 , the central conductor substrate  24 , and the second ground plane layer  48  bound the second channel  62  and form its sides, top, and bottom. 
     Thus, in this embodiment, the central conductor substrate  24  is sandwiched and captured between the two foam layers  42  and  60 , which are in turn sandwiched and captured between the ground plane layers  30  and  48 . The ground plane layers  30  and  48 , with their respective metallic layer structures  36  and  54 , together with the foam layers  42  and  60 , define the pair of hollow channels  44  and  62  in which the metallic central conductor  46  is suspended on the central conductor substrate  24 . The presence of the hollow channels  44  and  62  minimizes the attenuation of the microwave signal propagated on the metallic central conductor  46 , as there is no structure contacting the metallic central conductor  46  which would load it and change its electrical properties, and there is no structural foam present in the vicinity of the primary electrical fields emanating from the central conductor  46 . The metallic layer structures  36  and  54 , together with the foam layers  42  and  60 , confine the primary electrical fields to the unloaded, material-free zones or channels  44  and  62 . 
     The channels  44  and  62  need not be present, and the metallic central conductor  46  may be sandwiched directly between the foam layers  42  and  60 . This embodiment is illustrated in FIG.  8 . This embodiment is operable but is not preferred, because the portions of the foam layers  42  and  60  adjacent to the metallic central conductor  46  add about 0.25 dB per foot of loss to the stripline structure at a frequency of about 20 GHz. 
     Optionally, nonmetallic posts  64  may extend through the central conductor substrate  24 , the first ground plane layer  30 , the first foam layer  42 , the second ground plane layer  48 , and the second foam layer  60 . These posts, which are positioned on each side of the channels  44  and  62 , enhance the isolation between horizontally (laterally) adjacent and vertically stacked adjacent metallic central conductors  46  as will be discussed in relation to FIGS. 2 and 3. 
     Any operable materials of construction and dimensions may be used in the construction of the stripline structure  20 . The preferred materials of construction and dimensions were selected for the construction of a stripline structure  20  for use in a communications satellite with propagated microwave signals in the 20-30 gigahertz range. These materials and dimensions were selected for operability as well as for considerations of cost, fabricability, and both short-term and long-term stability in a space environment. Additionally, the materials desirably meet NASA Specification SP-R-0022A and are therefore qualified for use in a spacecraft application. This specification requires that the total mass loss (TML) not exceed 1.0 percent and the proportion of collected volatile condensable material (CVCM) be not more than 0.10 percent, when tested by the method set forth in ASTM E595. This testing process is discussed in W. Campbell, Jr. and R. Marriott,  Outgassing Data for Selected Spacecraft Materials,  NASA Reference Publication 1124 Revised (1987), pages 1-3. 
     The central conductor substrate  24  is an electrical nonconductor that has sufficient mechanical strength to support the metallic central conductor  46 , is light in weight, and is stable. The preferred material for use in the central conductor substrate  24  is a composite material of quartz fibers embedded in a cyanate ester resin. Other types of electrically nonconductive fibers and resins may be used as well. The central conductor substrate  24  is preferably from about 0.004 to about 0.006 inches thick. 
     The metallic central conductor  46  is a thin layer of a metal such as copper or aluminum. It may be applied onto the central conductor substrate  24  by any operable technique, such as screen printing, vapor deposition and etching, bonding, or the like. The metallic central conductor  46  is preferably from about 0.0007 to about 0.0014 inches thick. 
     The foam layers  42  and  60  are preferably made of a material that is light in weight and electrically invisible, most preferably a closed-cell, nonmetallic structural foam. A preferred foam material is a polymethacrylimide closed-cell foam available commercially as Rohacell foam from Richmond Aircraft Products. The foam layers  42  and  60  are preferably about 0.025±/−0.001 inch thick. The first-layer stripline conductor structure  22  is therefore about 0.050 inch thick. The foam material may optionally be doped to have a resistivity of about 377 ohms, the resistivity of free space, to aid in the control of spurious energy. 
     The ground plane substrates  32  and  50  are each preferably a free space absorber that attenuates spurious energy. The preferred material is an open-weave of carbon fibers sometimes termed “space cloth”. This material is light in weight, aids in achieving inter-channel isolation, and supports the metallic layer structures  36  and  54 . The ground plane substrates  32  and  50  are each preferably from about 0.004 to about 0.006 inches thick. 
     The metallic layer structures  36  and  54  include thin metallizations that form the layers  38 ,  40 ,  56 , and  58 . The layers  38 ,  40 ,  56 , and  58  may be any electrically conductive metal, such as copper, silver, gold, and the like. The layers  38 ,  40 ,  56 , and  58  are typically from about 0.0007 to about 0.0014 inches thick, and are deposited by plating or other operable deposition approach. 
     The support posts  64  are preferably made of carbon fiber composite material, and are about 0.10 inch in diameter. 
     The elements of the structure are joined by any operable approach. The metallic layers  38 ,  40 ,  46 ,  56 , and  68  are typically deposited upon their respective substrates as discussed earlier. The layers  22 ,  30 ,  42 ,  48 , and  60  may be collated and joined as they are collated using any operable adhesive. Vertically stacked stripline structures, such as shown in FIGS. 2 and 3, may also be joined by adhesive. Other approaches may be used to reduce the adhesive weight that is used. For example, the layers  22 ,  30 ,  42 ,  48 , and  60  may be joined together by stitching (sewing) using an appropriate thread such as a polymeric thread and a surface washer. In yet another approach, the layers may be first appropriately punched and then assembled onto the posts  64 , which thereby serve as guides for the assembly and alignment of the layers. Stitches or other appropriate caps are added to the ends of the posts  64  to hold the assembled layers in place. The posts  64  thereby serve both the electrical function and the structural function. 
     The stripline structure  20  desirably does not contain any polytetrafluoroethylene, a material often termed “Teflon™” polymer Polytetrafluoroethylene is widely used in other stripline structures, but it has the disadvantages that it is difficult to bond and that it tends to cold flow in a space environment. An extra housing is therefore required to confine the polytetrafluoroethylene, adding to the weight; of the structure. The preferred present approach avoids the use of polytetrafluoroethylene, reducing manufacturing difficulties and improving the life expectancy and reliability of the stripline structure. 
     The first-layer conductor structure  22  may be handled for assembly into larger structures, and processed by many conventional techniques such as drilling, fastening, cutting, and finishing. It is about, ⅓ the weight of conventional structures that accomplish the same function. 
     The first-layer conductor structure  22  of FIG. 1 carries a single signal on a single metallic central conductor  46 . However, the layered approach depicted in FIG. 1 allows the construction of more-complex structures capable of carrying multiple signals. One approach is depicted in FIG. 2, in which there are multiple metallic central conductors  46   a,    46   b,    46   c,  and  46   d  in a single layer conductor structure  22   a.  A number of the structures illustrated in FIG. 1 are thus arranged horizontally (laterally) in a side-by-side fashion within a single layer in this approach. Another approach is depicted in FIG. 3, in which there are multiple metallic central conductors  46   a,    46   b,  and  46   c  in the first-layer conductor structure  22   a,  as in FIG.  2 . Additionally, there are multiple metallic central conductors  46   e,    46   f,  and  46   g  in a second-layer conductor structure  22   b.  The central conductors  46   e,    46   f,  and  46   g  are arranged vertically in registry with the respective central conductors  46   a,    46   b,  and  46   c  of the first-layer conductor structure  22   a.  The various conductors are shielded from each other by the respective first ground plane layers  30   a,    30   b,  and  30   c.  The construction of the stripline structures of FIGS. 2 and 3 is otherwise as described in relation to FIG. 1, and that discussion is incorporated here. 
     Any or all of the central conductors  46  may be etched or otherwise formed into complex shapes, when viewed in a plan view, as may be required for a specific conductor requirement. FIG. 4 illustrates one such form of the central conductor  46  used for signal distribution in an antenna system. This form of central conductor  46  allows the input of a signal on one conductor or set of conductors and output on another conductor or set of conductors. This form of central conductor may be provided as a compact, lightweight board structure, much like a planar circuit board, for distributing microwave signals as will be illustrated in relation to FIGS. 6-7. 
     The present approach provides for the mechanical interconnection of the stripline central conductors  46  using an interconnect structure  69 , illustrated in FIG.  5 . The metallic central conductors  46 ′ and  46 ″ are each provided with metallic wire-like extensions on their ends to permit their interconnection. These extensions, termed coaxial extensions, include a male extension  70  and a female extension  72  with a receptacle therein. The extensions  70  and  72  may be coaxially connected to each other. The embodiment of FIG. 5 provides for coplanar connection in a single plane, but the connection may be non-coplanar as well. The ability to interconnect the stripline conductors may be implemented in a wide variety of approaches, and examples are shown in FIGS. 6 and 7. 
     The embodiment of FIG. 6 illustrates the interconnection of a single first signal feed  80  with a vertical distribution board  82  and thence with horizontal distribution boards  84 , and thence into a large number of second signal feeds  86 . The distribution boards  82  and  84  are structured in the manner illustrated in FIG.  4 . In all cases, the interconnections may be accomplished by the use of the interconnect structure  69  such as shown in FIG.  5 . This embodiment of FIG. 6 achieves combination or division of a microwave signal in a single channel (i.e., a single feed  80  and multiple feeds  86 ). The energy flow may be from feed  80  to feeds  86 , resulting in signal division, or from feeds  86  to feed  80 , resulting in signal combination. The drawing illustrates three horizontal distribution boards, but there may be many more horizontal distribution boards as required for specific applications. In an application of interest to the inventors, there is the single first signal feed  80  and  48  horizontal distribution boards  84 , and each of the horizontal distribution boards  84  has  48  second signal feeds  86 . The device thus accomplishes either 2304-way combination or 2304-way division of microwave signals in a square aperture application, or approximately 1750-way combination or 1750-way division in a circular aperture application. 
     The embodiment of FIG. 7 illustrates the interconnection of multiple independent channels, here first signal feeds  88   a,    88   b,  and  88   c,  with respective vertical distribution boards  90   a,    90   b,  and  90   c,  thence with multiple horizontal distribution boards  92   a,    92   b,  and  92   c  and thence into a large number of second signal feeds  94   a,    94   b,  and  94   c  communicating with the same output horn. This embodiment of FIG. 7 achieves combination or division of microwave signals in multiple (here, three) channels. That is, there are three independent communication paths (i.e., channels) between the feeds  88   a,    88   b,  and  88   c  and all of the feeds  94   a,    94   b,  and  94   c  that feed the output horn. In all cases, the interconnections may be accomplished by the use of the interconnect structure  69  such as shown in FIG.  5 . The drawing illustrates only three independent first signal feeds  88   a,    88   b,  and  88   c  and three horizontal distribution boards  92   a,    92   b,  and  92   c,  but there may be many more first signal feeds and horizontal distribution boards as required for specific applications, limited only by the space available for the output horn aperture dimensions. 
     Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.