Abstract:
A novel method for producing improved RF transmission lines for satellite beamforming networks and printed circuit antenna(s) comprising the steps of (1) bonding together (a) a central conductor strip or trace comprising a dielectric layer or circuit board having on one or both surfaces thereof conductive strip circuitry; (b) upper and lower core layers of lightweight closed-cell plastic foam bonded to (a), and (c) upper and lower surface layers or faceskins of conductive metal foil or of dielectric material bonded to metal foil layers inside and/or outside, to form ground planes, bonded to the surfaces of the adjacent foam core layers. The next step involves boring a plurality of holes or vias through the ground plane layers and the core layers, and plating the bores or vias with an electroconductive metal to provide a plurality of electroconductive connections between the ground planes, for parallel plate propagating mode suppression, and to connect independent assemblies by using plated through via interconnects and quarter wavelength overlapping line interconnects to electrically connect numerous central conductor strip along with a two step bonding process, or to form a multilayer package.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method for producing radio frequency transmission line beamforming networks and antennas having improved electrical properties and increased strength-to-weight or mass ratios, for use in beamforming networks for satellite antennas.  
           [0003]    2. State of the Art  
           [0004]    Satellite communications multibeam antennas or phased arrays have been developed to provide precisely tailored beams to cover multiple designated coverage areas on the earth without wasting antenna beam coverage or radiated power on regions where there are no users of interest.  
           [0005]    Space-borne antennas were individually designed and assembled for a particular satellite, usually launched for a specific purpose. Each element of the many elements of the antenna had to be individually fabricated and assembled. Thus, the antenna was very expensive to fabricate and assemble.  
           [0006]    Commonly-assigned U.S. Pat. No. 5,539,415 discloses an inexpensive, small, compact, light weight, easy to assemble, multibeam or phased array device useful as a direct radiating array or as a feed for a reflector or lens antenna. The device employs an array of planar radiators coupled to radio frequency (R.F.) transmission lines to form individual feed or antenna strips. The feed or antenna strips are coupled into a filter to pass the desired band of frequencies and reject undesirable bands of frequencies. The filters are coupled to Monolithic Microwave Integrated Circuit (MMIC) amplifiers that contain N amplifiers with an integral isolator. Amplifiers are Solid State Power Amplifiers (SSPA&#39;s) or Low Noise Amplifiers (LNA&#39;s). SSPA&#39;s are used for the transmit mode and LNA&#39;s are used for the receive mode. Amplifiers are utilized to amplify the aforementioned RF signals.  
           [0007]    Previously-known RF strip transmission lines for use in antenna(s), antenna feed(s) and beamforming network(s) of the types disclosed by U.S. Pat. No. 5,539,415 have a wide variety of disadvantages.  
           [0008]    A microwave transmission line configuration known as bonded strip transmission line is formed from two solid dielectric substrates with metal foil (usually copper) bonded or plated to one or both flat sides of each substrate. The substrates are bonded together under pressure and at elevated temperature to form a sealed package. The foil is configured (usually by photo etching) to have two metal ground planes with one or more metal strip conductors in the middle to form a strip transmission line. The middle conductor can be formed (usually by photo etching) to produce various microwave circuits. Channelization may be used to prevent unwanted parallel plate modes.  
           [0009]    Other previously-known microwave transmission lines are referred to as bar line and suspended air strip transmission lines. They are composed of one or more metal bars or thin dielectric supported strips located midway between two metal ground planes forming a microwave transmission line. The middle bar in bar lines is suspended between the ground planes using low dielectric constant honeycomb or foam. The package for bar line and for suspended air strip transmission lines is held together by mechanically clamping, usually with bolts or screw fasteners.  
           [0010]    The previously-known structures for microwave transmission lines of the aforementioned types have important disadvantages with respect to their electrical and/or their physical properties such as strength and weight. The barline and suspended air strip transmission lines are composed of unbonded layers, mechanically fastened together, requiring thicker structural panels and numerous mechanical fasteners to provide a good degree of mechanical performance under sheer and tension forces. The resulting barline and suspended air strip transmission line configurations are heavy and have a high weight to strength ratio. Also, they are composed of individual components: machined ground planes, conducting bars for the barline and photo etched strips for the suspended air strip transmission line, and foam or honeycomb cores mechanically held together with fasteners requiring substantial assembly time and labor intensive methods used to fabricate and assemble the barline and suspended air strip transmission line packages.  
           [0011]    Also, the prior known bonded strip transmission line beamforming networks suffer high RF energy insertion loss.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention provides a novel method for producing improved RF transmission lines for satellite antennas and beamforming networks, comprising the steps of (1) bonding together (a) a central conductor strip or trace strip comprising a dielectric layer or circuit board having on one or both surfaces thereof conductive strip circuitry; (b) upper and lower core layers of lightweight closed-cell plastic foam bonded to (a), and (c) upper and lower surface layers or faceskins of conductive metal foil or of dielectric material bonded to metal foil layers inside and/or outside, or solid metal plates, to form ground planes, bonded to the surfaces of the adjacent foam core layers, (2) boring holes or vias through the ground plane or through the central conductor strips, core layers and ground plane, layers and the core layers, and (3) plating the bores or vias with an electroconductive metal, usually copper, to provide a plurality of electroconductive connections between the ground planes, for parallel plate propagating mode suppression, or between central conductor strips to connect independent assemblies. 
       
    
    
     THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a cross-sectional view of a bonded air strip transmission line according to an embodiment of the present invention;  
         [0014]    [0014]FIG. 2 is a cross-sectional view of a present bonded air strip transmission line assembly implemented with a side launch connector, according to an embodiment of the present invention;  
         [0015]    [0015]FIG. 3 is a view of the component of FIG. 2 taken along the line  3 - 3  thereof;  
         [0016]    [0016]FIG. 4 is a cross-sectional view of a present bonded air strip transmission line assembly implemented with an end launch connector;  
         [0017]    [0017]FIG. 5 is a view of the component of FIG. 4 taken along the line  5 - 5  thereof;  
         [0018]    [0018]FIG. 6 is a cross-sectional view of an air strip transmission line comprising normally-independent circuits electrically-connected to each other without external connectors or internal solder joints;  
         [0019]    [0019]FIG. 7 is a view taken along the line  7 - 7  of FIG. 6;  
         [0020]    [0020]FIG. 8 is a cross-sectional view of an air strip transmission line component incorporating a quarter wavelength overlapping line interconnection according to another embodiment of the invention;  
         [0021]    [0021]FIG. 9 is a view taken along the line  9 - 9  of FIG. 8;  
         [0022]    [0022]FIGS. 10 and 11 are cross-sectional views of first and second sub-assembly strip transmission line components incorporating a quarter wave line segment, to be united by resinous adhesive to produce a multi-layer assembly; and  
         [0023]    [0023]FIG. 12 is a cross-sectional view of a multilayer strip transmission line assembly produced by the bonding together of the sub-assemblies of FIGS. 10 and 11. 
     
    
     DETAILED DESCRIPTION  
       [0024]    Referring to FIG. 1, the strip transmission line (STL) component  10  thereof is a bonded composite assembly of a center conductor strip  11  comprising a dielectric support  12  having bonded to one or both sides thereof (as shown) narrow elongate metal layers or foils  13 . The center strip  11  is bonded between lightweight plastic foam core layers  14  and  15  by means of a resinous adhesive at elevated temperature and pressure, and thin faceskins  16  and  17  are similarly adhesive-bonded to the upper and lower surfaces of the core layers  14  and  15 , respectively.  
         [0025]    The illustrated faceskins  16  and  17  each comprise a dielectric or metal support  18  or  19  and an inner conductive metal foil  20  or  21  bonded thereto, adjacent the foam core layer  14  or  15 . The conductive metal foils  20  and  21  form ground planes on the composite assembly. For parallel plate propagating mode suppression the ground planes are electrically united by conductive connections. Such connections are produced in the present STL assemblies by forming bores  22  or vias through the faceskins  16 ,  17  and foam core layers  14 ,  15  and the dielectric support  12  of the center strip  11 , and coating or plating the inner walls of the bores  22  with a continuous layer  23  of conductive material such as electroless copper as used in the printed circuit board art. The conductive layer  23  in each bore  22  connects electrically with the conductive metal foil layers  20  and  21  on the opposed faceskins  16  and  17  to produce electrical conductivity between the faceskins for mode suppression.  
         [0026]    It should be understood that the faceskins  16  and  17  may have the foil layers  20  and  21  on either the inner or outer surface of the dielectric layer  18  or  19 , or on both surfaces. Also it is possible to use conductive metal foil or metal plate, per se, without the need for the dielectric support layer  18  or  19 .  
         [0027]    The bonded composite assembly of FIG. 1 is integral, strong and lightweight and derives its strength from the fact that the various layers thereof are bonded to one another and cannot slip or slide under sheer and tension forces relative to one another. Also, it is fabricated using high speed, mass production printed circuit board (PCB) techniques including photo-etching, bonding, routing, drilling and plating or via-filling, resulting in a unitary fabricated unit requiring minimum assembly.  
         [0028]    For applications requiring low values of radio frequency energy insertion loss the present invention is advantageous for the following reasons:  
         [0029]    a. Plastic foam core layers have a much higher content of air than solid dielectric material resulting in a lower dielectric constant with a lower loss tangent, and therefore, a lower radio frequency energy loss in the dielectric material.  
         [0030]    b. Due to the lower dielectric constant, the width of the strip conductor will be wider for the same ground plane spacing and characteristic impedance. A wider strip width results in lower radio frequency copper loss and less sensitivity to etching tolerances.  
         [0031]    c. Since the foam is physically lighter than the solid dielectric material, the ground plane spacing can be greater for the same weight resulting in a wider strip conductor and, therefore, a lower radio frequency copper loss for the same weight or mass.  
         [0032]    For applications requiring high impedance lines this invention is advantageous for the following reason:  
         [0033]    a. Due to the lower dielectric constant, the width of the strip conductor will be wider for high impedance characteristic and, therefore, less sensitive to etching tolerances.  
         [0034]    For applications requiring low weight or mass, this invention is advantageous for the following reason:  
         [0035]    a. The foam core is much lighter than solid dielectric material resulting in a lighter configuration.  
         [0036]    For applications requiring high strength to weight or mass ratios, this invention is more advantageous.  
         [0037]    It should be noted that the present assemblies are sandwich constructions of face plates and core material, and the strength and stiffness are determined primarily by the thickness of the core material (ground plane spacing). Therefore, the invention is advantageous for the following reasons:  
         [0038]    a. The foam core configuration is much lighter for the same thickness than a solid core configuration.  
         [0039]    b. The foam core configuration can be made much thicker than the solid core configuration for the same weight resulting in a higher strength to weight or mass ratio.  
         [0040]    Referring to the assembly  25  of FIGS. 2 and 3, which is a bonded air strip transmission line similar to that of FIG. 1, comprising a center metal conductor strip  26  or trace on a dielectric substrate NM, core layers  27  and  28  and ground plane faceskins  29  and  30 , all adhesively-bonded together to form a strong, lightweight integral assembly. Access to the center strip  26  or trace is provided by drilling away a section of the upper faceskin  29  and core layer  27  to expose the strip  26 , and inserting and soldering or otherwise conductively bonding a side terminal  31  to said strip  26  at  32  to produce a side launch connection to the center trace.  
         [0041]    A dielectric bead or retaining piece  33  is inserted into the bore surrounding the terminal  31 , as illustrated, and an outer shroud  34 , shown by means of broken lines, may be bonded to the faceskin  29 , to complete the side access connector, The shroud  34  is in conductive contact with the conductive plate or via fill  23  lining a bore  22  electrically connecting with the faceskin ground planes  29  and  30  in numerous places. In the illustrated embodiment, the core material  27   a  and  28   a  in the connector areas is a higher density foam than  27  and  28  or is a solid dielectric, to provide increased mechanical support for the connector. However this is optional, and the core foam  27  and  28  may be used throughout. Any commercially-available strip transmission connector may be used to provide the terminal  31 , dielectric bead  33  and outer shroud  34 .  
         [0042]    In the embodiment shown in FIGS. 4 and 5 an end launch connector  35  is integrated with and mechanically-fastened to a unitary bonded air strip transmission line assembly  36  according to the present invention. The assembly  36  is similar to that of FIGS. 2 and 3 except that the edge or end to which the conventional strip transmission connector  35  is attached is strengthened or reinforced by the use of higher density foam or by the use of solid dielectric material  37   a , whereas lower density lightweight plastic foam is used for core layers  37  and  38 . The use of higher density foam or solid dielectric material, however, is optional and the core foam  37  and  38  may be used throughout. The STL assembly is similar to that of FIG. 1 in that it comprises a center conductor strip or trace  39  on a thin dielectric board or mechanical support  39   a  bonded between foam core layers  37  and  38  to which are bonded outer faceskin ground planes  40 . A plurality of bores or vias  41  are drilled through the assembly, from one ground plane  40  to the other, and each is filled or wall-coated with conductive metal, usually electroless copper,  42 . The trace  39 , such as copper, is formed by etching the unwanted part from the support  39   a.    
         [0043]    The connector  35  is a commercially-available STL component comprising a metal housing  45  having a U-shaped attachment section  46  and a receptacle section  47  surrounding a terminal  48  which is electrically connected with the center trace  39  by being mechanically pressed or soldered thereto at area  49 . The connector housing section  46  is mechanically fastened to the unitary STL component by means of bolts or fasteners  50  through bores drilled through the foam or solid dielectric material  37   a  at the connector end of the STL assembly  36 .  
         [0044]    [0044]FIGS. 6 and 7 illustrate integrations or interconnections between otherwise independent different STL members to form a unitary strong assembly, without the need for conventional connectors.  
         [0045]    In the embodiment of FIGS. 6 and 7, a two step bonding process is used to interconnect two separate assemblies without the need for external connectors. Subassembly  6 a is formed by bonding together the center dielectric layers or circuit boards having metal foil  51  and  52 , their respective foam core layers  54  and the central ground planes  56 . Two metal ground planes are shown, however, a single plane may be used. The desired circuitry is then formed on metal foil  51  and  52 , usually by photo-etching. The subassembly  6   a  is then drilled and plated through hole or via  57  is formed by plating metal, usually electroless copper, to the edges of the via  58 , electrically connecting the circuits on  51  and  52  to form a RF via.  
         [0046]    The second bonding step is performed by aligning the top and bottom ground planes  55 , and the core layers  53  to the subassembly  6   a , forming an integral package. The last step is to form the parallel plate propagating mode suppression holes or vias by drilling hole(s)  59  through the assembly and plating the edges of the hole(s) with metal  60  to form a mode suppression via.  
         [0047]    In the embodiment of FIGS. 8 and 9 a unitary assembly is produced which incorporates overlapping quarter wavelength center conductors or traces. As illustrated, a first sub-assembly  9   a  and a second sub-assembly  9   b  are independently formed by bonding together a ground plane layer  60 , a foam core layer  61  and a circuit layer or trace  62 ,  63  having a narrow end termination  62   a  and  63   a , respectively. The sub-assemblies are superimposed with the circuit layers  62  and  63  facing each other and the terminations  62   a  and  63   a  overlapping from different directions, and are adhesively bonded together to form a unitary assembly incorporating a quarter wavelength overlapping line interconnect.  
         [0048]    By combining the plated through interconnect and the quarter wavelength overlapping line interconnect shown in FIGS. 8 and 9, and using a multi-step bonding process, any number of assemblies or layers may be combined and bonded to form a single integral, composite structure. This approach produces a mechanically strong, low cost, multilayer assembly with no internal connectors or solder joints for increased reliability in a space-borne environment. FIGS. 10 and 11 present a procedure for using the plated through and quarter wavelength overlapping line interconnect to form a single, integral, composite structure containing, in this example, three otherwise independent assemblies without the use of connectors or solder joints. For simplicity, three assemblies containing three circuit layers and a two step bonding process are illustrated, however, the procedure can be extended to any desired number of circuit layers and bonding steps.  
         [0049]    The first step in the two step bonding process is illustrated schematically in FIG. 10. Circuit layer  70 , a foam dielectric layer  71 , ground plane layer  72 , another foam dielectric layer  73  and half of a quarter wavelength overlapping line on layer  74  are bonded together in a single bonding step to form subassembly  10   a . Similarly, the other part of the quarter wavelength overlapping line on layer  74 , two foam substrate layers  71 ,  73 , ground plane layer  75  and circuit layer  76  are bonded together to form subassembly  11   a . After bonding, circuit layer  70  and  74  on subassembly  10   a  and circuit layers  76  and  74  on subassembly  11   a  are interconnected with plated through holes  77 . As shown in the figures circuit layer  74  on subassembly  10   a  contains one half of a quarter wavelength overlapping interconnect and circuit layer  74  on subassembly  11   a  contains the other half.  
         [0050]    The second step in the two step bonding process is illustrated schematically in FIG. 12. Subassembly  10   a  and subassembly  11   a  are properly aligned, separated by a thin layer of bonding film and assembled together. Ground plane  80 , a layer of bonding film, a foam dielectric layer  81  and another layer of bonding film are aligned and assembled to circuit layer  70 . Ground plane  82 , a layer of bonding film, a foam dielectric layer  83  and another layer of bonding film are aligned and assembled to circuit layer  76 . The completed assembly is then bonded together under elevated temperature and pressure to form a single, integral composite structure  78 .  
         [0051]    The present assemblies accommodate all strip transmission line circuits and antennas formed by photo etching any or all of the foil surfaces and include configurations utilizing resistive film between the copper foil and the dielectric substrate.  
         [0052]    It will be apparent to those skilled in the art, in the light of the present disclosure that the central conductor strip or trace of the present assemblies can be used to form all of the strip transmission line circuits, usually by photoetching, used in beam forming networks, including resistive elements, and that the center trace or strip can be used to form electromagnetically coupled probe(s), slot(s) or plated through vias to connect to any printed circuit slot or patch type antennas(s) formed into one or both outside ground planes, usually by photoetching.  
         [0053]    Also, any printed circuit patch-type antenna(s) can be formed, usually by photoetching, into one or more of the outermost circuit layers with the outermost ground planes omitted. The patch-type antenna(s) can be fed directly from circuitry on the same layer or can be connected to internal circuitry with electromagnetically-coupled probe(s), slot(s) or plated-through vias(s).  
         [0054]    This enables plated-through vias and quarter wavelength overlapping line interconnections to electrically interconnect conductor strips on numerous different layers and produce a multi-layer package requiring no other internal or separate connectors or solder joints.  
         [0055]    All circuit layers can be extensive and contain any desired strip transmission line circuits. Circuit layer  74  of FIG. 12, for example, can have a long trace length between either or both plated through interconnects  77  and the quarter wavelength overlapping lines.  
         [0056]    Multistep drilling and plating will accommodate any required channelization for unwanted parallel plate modes in any or all of the layers including any necessary joining together electrically of configurations using metal foil on the top and bottom of thin substrate material.  
         [0057]    To increase the edge strength of the completed assemblies, higher density foam, solid dielectric material or metal can be bonded into the external edges replacing the foam cores.  
         [0058]    After final bonding the entire assembly may be edge plated with metal to suppress electromagnetic interferences and intermodulation products while providing a hermetically sealed package.  
         [0059]    Side launch connectors can be mounted to the outside ground planes, or with the proper edge extensions of ground planes and center conductor traces, end launch connectors may be used to electrically connect to the assembly.  
         [0060]    Any printed circuit slot type antenna element(s) can be etched into one or both outside ground planes and connected to internal circuitry with electromagnetic coupled probe(s) or with a plated through hole(s). Alternately, any printed circuit patch type antenna element(s) can be etched into one or more of the outermost circuit layer(s) with the outermost ground planes omitted. The patches can be fed directly from circuitry on the same layer or connected to internal circuitry with electromagnetic coupling probe(s), slot(s), or plated through holes.  
         [0061]    While preferred embodiments of the invention have been disclosed in detail, it should be understood by those skilled in the art that various other modifications may be made to the illustrated embodiments without departing from the scope and spirit of the invention as described in the specification and defined in the appended claims.