Patent Publication Number: US-7215220-B1

Title: Broadband power combining device using antipodal finline structure

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a device for spatially dividing and combining power of an EM wave using a plurality of longitudinally parallel trays. More particularly, the invention relates to a device for dividing and combining the EM wave by antipodal finline arrays provided within a coaxial waveguide cavity. 
   2. Description of the Related Art 
   The traveling wave tube amplifier (TWTA) has become a key element in broadband microwave power amplification for radar and satellite communication. One advantage of the TWTA is the very high output power it provides. However, several drawbacks are associated with TWTAs, including short life-time, poor linearity, high cost, large size and weight, and the requirement of a high voltage drive, imposing high voltage risks. 
   Solid state amplifiers are superior to TWTAs in several aspects, such as cost, size, life-time and linearity. However, currently, the best available broadband solid state amplifiers can only offer output power in a watt range covering about 2 to 20 GHz frequency band. A high power solid state amplifier can be realized using power combining techniques. A typical corporate combining technique can lead to very high combining loss when integrating a large amount of amplifiers. Spatial power combining techniques are implemented with the goal of combining a large quantity of solid-state amplifiers efficiently and improving the output power level so as be competitive with TWTAs. 
   U.S. Pat. No. 5,736,908, issued to Alexanian et al., discloses a power combining device using a slotline array within rectangular waveguides. In an embodiment shown in  FIG. 7  of that patent, a circular waveguide is shown, but the slotline array is arranged with elements that are disposed in parallel within the waveguide. 
   In N. S. Cheng, Pengcheng Jia, D. B. Rensch and R. A. York, “A 120-Watt X-Band Spatially Combined Solid-State Amplifier”, IEEE Trans. Microwave Theory and Tech., vol. 47, (no. 12), IEEE, December 1999. p. 2557–61, a working active combiner unit using a slotline array inside an X band rectangular waveguide is disclosed. The bandwidth of the combiners is limited by the bandwidth of the rectangular waveguide, which has an fmax:fmin (maximum operational frequency over minimum operational frequency ratio) of less than 2. Since the dominant mode inside the rectangular waveguide is TE10 mode, the combiners also have a dispersion problem over the whole waveguide band. 
   In another reference, Jinho Jeong, Youngwoo Kwon, Sunyoung Lee, Changyul Cheon, Sovero EA. “A 1.6 W Power Amplifier Module At 24 Ghz Using New Waveguide-Based Power Combining Structures,” 2000 IEEE MTT-S International Microwave Symposium Digest (Cat. No.00CH37017), IEEE, Part vol. 2, 2000, pp. 817–20 vol. 2. Piscataway, N.J., USA, there is proposed an antipodal finline structure with double antipodal finlines inside a rectangular waveguide. The antipodal finline provides no-bond-wire transition from waveguide finline to microstrip line. It simplifies the connection with commercial off-the-shelf (COTS) microwave monolithic integrated circuits (MMIC) which predominantly use microstrip lines. However, as in U.S. Pat. No. 5,736,908 and other prior art, the bandwidth of the system is limited by the rectangular waveguide used. 
   U.S. Pat. No. 5,920,240, issued to Alexanian et al., discloses a coaxial waveguide power combiner/splitter, which inserts slotline cards into the coaxial waveguide for power distribution and combining. In the combiner/splitter, power devices are mounted on the slotline cards and then slid into the waveguide. This arrangement suffers from serious heat dissipation issues, as it is difficult to remove heat effectively from the power devices to an outside heat sink since the heat spreads to the slotline card first, then conducts to the waveguide through the sliding contacts between the slotline card and the waveguide. Because the combiner is mainly used for high power amplifier design and active devices are mostly high power amplifiers, the amount of heat generated is considerably high. The heat increases the operation temperature and decreases the lifetime of the amplifiers dramatically. Moreover, it is difficult to connect outside DC bias into the active devices on the slotline cards, and to access the slotline cards generally, as these are disposed inside an enclosed waveguide structure. 
   Two other references (Pengcheng Jia, R. A. York, “Multi-Octave Spatial Power Combining in Oversized Coaxial Waveguide”, IEEE Trans. Microwave Theory and Tech, vol. 50, (no. 5), IEEE, May 2002. p. 1355–60) and (Pengcheng Jia, Lee-Yin Chen, Alexanian A, York R A. “Broad-Band High-Power Amplifier Using Spatial Power-Combining Technique.” IEEE Transactions on Microwave Theory &amp; Techniques, vol. 51, no. 12, December 2003, pp. 2469–75. Publisher: IEEE, USA) propose a stacked tray approach for power combining inside a coaxial waveguide. A plurality of identical wedge-shaped trays are stacked to form a coaxial waveguide, providing DC paths in the middle of the tray. In the first reference, active devices are mounted on the slotline card and directly connected to the end of the slotlines. Even though a metal tray is added underneath the slotline card, the thermal resistance caused by many layers of material and junctions remains problematic when high power devices are used. Since bonding wires are used to connect from slotline to MMIC which is not on the same layer, the parasitic effect will deteriorate the performance at higher frequency band. Further, assembly complications and costs are high. 
   In the second reference, an improved design enables easy assembly with COTS MMICs by integrating slotline to microstrip baluns to the end of slotlines. This provides improved thermal management since the active devices are directly mounted on to the metal wedge shaped trays. However, the balun has a slotline stub at the end of the narrow slotline on the backside of the substrate and a microstrip line stub on the top side of the substrate. The centers of the two stubs require alignment on the same axis perpendicular to the surface of the substrate. The accurate back side-to-top side alignment requirement significantly complicates the manufacturing process. The balun also takes considerable surface area. The size of the balun depends on the lower cutoff frequency of the system. The lower the cutoff frequency, the bigger the balun is. Since the surface area on the slotline circuit is limited, the maximum operational frequency range demonstrated by an arrangement of this second reference is only from 6 to 18 GHz, a 3:1 f max :f min  ratio. 
   The slotline card design without slotline to microstrip balun disclosed in U.S. Pat. No. 5,920,240, shows a broader bandwidth ratio. However, if the end of the slotline is mounted on metal trays, then its dominant mode is TE mode, a non-TEM mode and dispersive over broad bandwidth. To achieve broad bandwidth response, the slotline needs to match with standard MMIC input/output impedance, 50 Ohm. Since the slotline tends to have high characteristic impedance, the gap of the slotline will be as narrow as 1 to 2 mil. The slotline cards thus require high accuracy photo-lithography instead of the conventional PCB (printed circuit board) processes which can normally achieve a best gap width of 4 to 6 mil. For this reason, the slotline cards used in real systems shown in the above-cited references are all built on ceramics with highly accurate lithography. This increases costs dramatically, and since the ceramics are fragile, it raises significant reliability issues. 
   BRIEF SUMMARY OF THE INVENTION 
   In accordance with the invention, a broadband power combining device uses antipodal finline arrays disposed inside a coaxial waveguide to spatially divide and combine a TEM (transverse electromagnetic) wave. The antipodal finline structures, each of which is part of a wedge shaped tray, are transformed into an array inside the waveguide by stacking the wedge shaped tray to form a coaxial waveguide. 
   The device includes an input port, an input waveguide section, a center waveguide section formed by stacked wedge shaped trays, an output waveguide section, and an output port. Each tray comprises a wedge shaped metal carrier, an input antipodal finline structure, one or more active elements, an output antipodal finline structure and necessary biasing circuitry. The wedge shaped metal carriers have a predetermined wedge angle and predetermined cut-out regions. The inside/outside surfaces of the metal carrier and surfaces of the cut-out regions all preferably have cylindrical curvatures. When the trays are stacked together, a cylinder is formed with a coaxial waveguide opening inside. The antipodal finline structures form input and output arrays. An incident wave is passed through the input port and the first waveguide section, distributed by the input antipodal finline array to the active elements, combined again by the output antipodal finline array, then passed to the output waveguide section and output port. 
   The broadband power combining device spatially divides and combines waves. It has the high combining efficiency when combining a large quantity of active elements. 
   The wedge shaped carriers in the device provide a DC bias path and good thermal management. Slots or holes are machined in the middle of the metal carrier for DC lines. When the trays are stacked together, DC bias lines will be connected to inside active elements through those slots or holes. Active elements are eutectically attached to the center of the metal carrier. It will minimize the thermal resistance from active element to the outside heat sink. 
   The antipodal finline is disposed on a soft board substrate material and can be manufactured by a conventional PCB process. The antipodal finline has a tapered conductor on the top side of the substrate and a tapered conductor on the back side. The top side conductor tapers to about half of the board width, then tapers to a narrow strip, which becomes a microstrip line. The back side conductor tapers to about half of the board width, then tapers to the full board width which will become the ground for the top side microstrip line. Since the tolerance for back side to top side alignment is not tight and all the dimensions are large enough, it is much easier to manufacture as compared with circuits using a slotline to microstrip balun and still offers good compatibility with COTS MMIC&#39;s. 
   The antipodal finline tapers disposed inside a coaxial waveguide can achieve broadband frequency response since the waveguide system is a Quasi transverse Electromagnetic (TEM) structure. The dominant mode propagating inside the coaxial waveguide is TEM mode, which means the electromagnetic (EM) field is perpendicular to the propagation direction. The antipodal finline disposed inside the coaxial waveguide has electric field points from one conductor to the other conductor. Its magnetic field is in the tangent direction on the cross section plane and perpendicular to both the electric field and propagation direction. The antipodal finline inside coaxial waveguide is a balanced transmission line. When the antipodal finline tapers down and begins to overlap, either side can be selected to become the microstrip line. When the balance waveguide finline tapers to an unbalanced planar microstrip line, which is a quasi-TEM transmission line, the EM field is still transverse. The whole antipodal finline structure is a Quasi-TEM structure and has very small dispersion over broad bandwidth. 
   By using antipodal finlines, the invention achieves the broadest bandwidth that has ever been practically achieved by a spatial power combiner. Moreover, the antipodal finline design makes it possible to fabricate the circuit with a PCB process. It simplifies the assembly process and dramatically reduces the cost for manufacturing. 
   In the aforementioned prior art, MMICs (monolithic microwave integrated circuits) in the bare die form are used. However, many military applications require hermetic sealing. It is difficult to seal the whole waveguide structure since many wedge trays are stacked together with many mechanical connections. Heretofore, there has been no solution yet addressing the hermetic seal problem for spatial waveguide combiners using stacked trays, not only in coaxial waveguide combiners, but also in rectangular waveguide combiners. 
   In the presently claimed invention, individually packaged MMICs are used in the combining device. The packages are hermetically sealed. Since all the other elements are passive, the whole structure is considered hermetically sealed. This will significantly reduce the complexity of the system and make it accessible for easy repair. 
   The packages of the invention are also surface mountable and have a metal base which is soldered to the metal tray. RF input/output ports are soldered to the microstrip line of the antipodal finline structure. The soldering connections will minimize both thermal resistance from chip to carrier and RF parasitic noise. 
   In another aspect of the invention, there is provided an innovative biasing scheme to maximize the combining efficiency for spatial waveguide power combining devices. Since MMIC&#39;s are used as active elements, the maximum combining efficiency will be achieved when all the MMIC&#39;s have uniform performance. Loss can be caused by amplitude and phase variation among the elements. The current semiconductor integrated circuits still have considerable variations from die to die. In most of the amplifier MMIC&#39;s, the semiconductor devices are GaAs HEMTs (high electron mobility transistor) which use gate voltage to control the output current. To insure each element is putting out the same amount of power, a feedback circuit is used to sense the drain current and lock it to a fixed value by adjusting gate voltage. Since the load for each active element is the same, for a fixed drain current, the output power will be the same too. This scheme helps to improve the power combining efficiency for spatial waveguide power combining devices. 
   Further in accordance with the invention, there is disclosed a novel thermal management scheme for spatial waveguide power combining devices. A heat sink is machined with a cylindrical cavity. The heat sink further operates as a clamp, holding the center trays tightly and providing good thermal and mechanical contact therewith, thereby conducting heat effectively away from the trays to the fins of the heat sink for dissipation from the device. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     Many advantages of the present invention will be apparent to those skilled in the art with a reading of this specification in conjunction with the attached drawings, wherein like reference numerals are applied to like elements, and wherein: 
       FIG. 1  is a perspective view of the power combining system in accordance with the invention; 
       FIG. 2  is perspective view of a wedge shaped tray; 
       FIG. 3  is the cross section of the wedge shaped metal carrier; 
       FIG. 4  is back side view of the wedge shaped metal carrier; 
       FIG. 4A  is the cross section of center waveguide structure which has a plurality of planar surfaces; 
       FIG. 4B  is the cross section of center waveguide structure which has a rectangular outside profile and a rectangular coaxial waveguide opening; 
       FIGS. 5A and 5B  are longitudinal cross sections of the input/output waveguide section; 
       FIG. 6  is a schematic view of an antipodal finline structure; 
       FIG. 6A  is a schematic view of an antipodal finline structure with double finline tapers; 
       FIG. 6B  is a schematic view of another antipodal finline structure with double finline tapers; 
       FIG. 6C  is a schematic perspective view of a pair of antipodal finline structures in which each antipodal finline taper is connected to more than one active element by a multi-way planar divider and combiner; 
       FIG. 7  is a schematic view of the cross sections of the antipodal finline structure; 
       FIG. 8  is an assembly diagram of an active element; 
       FIG. 8A  is a back side view of the active element of  FIG. 8 ; 
       FIG. 9  is the assembly diagram with another active element which is in a flat surface mount package; 
       FIG. 9A  is a back side view of the active element of  FIG. 9 ; 
       FIG. 10  is schematic diagram of a DC controlling circuit used to achieve unified output power from each active element in accordance with the invention; 
       FIG. 11  is the perspective view of a thermal management scheme in accordance with the invention; and 
       FIG. 12  is a diagram of the s parameters of a broadband power combining device using antipodal finline structures. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In accordance with the invention, a broadband spatial power combining device using longitudinally parallel, stacked wedge shaped trays is provided. Antipodal finline structures are mounted on each tray. When the trays are stacked together to form a coaxial waveguide, the antipodal finline structures are disposed into the waveguide and form a dividing array at the input and a combining array at the output. With the use of antipodal finline arrays inside the coaxial waveguide for power dividing and combining, a broadband frequency response covering the range of about 2 to 20 GHz is realized. The antipodal finline structure is easy to manufacture using conventional printed circuit board (PCB) processes. It also enables easy integration with COTS (commercial off-the-shelf) MMICs. Further, the division of a coaxial waveguide into wedge-shaped trays enables simplified DC biasing and provides good thermal management. 
   As illustrated in  FIG. 1 , in the spatial power combining device  2  of the invention, an EM (electromagnetic) wave is launched from an input port  4  to an input coaxial waveguide section  12 , then the EM wave is collected through an output coaxial waveguide section  14  to an output port  6 . The input/output waveguide sections  12  and  14  provide broadband transitions from the input/output ports  4  and  6  to a center waveguide section  24 . The outer surfaces of inner conductors  20  and  22  and the inner surfaces of outer conductors  16  and  18  all have gradually changed profiles. The profiles are determined to minimize the impedance mismatch from the input/output ports  4  and  6  to the center waveguide section  24 . 
   In the preferred embodiment, the input/output ports  4  and  6  are field replaceable SMA (Subminiature A) connectors. The flanges of the input/output port  4  and  6  are screwed to the outer conductors  16  and  18  with four screws each, although that number is not crucial, and other types of fasteners may be used. Pins  8  and  10  are used to connect between centers of the input/output port  4  and  6  and inner conductors  20  and  22 . In other embodiments, the input/output ports may be super SMA connectors, type N connectors, K connectors or any other suitable connectors. The pins  8  and  10  can also be omitted, if the input/output ports already have center pins that can be mounted into inner conductors  20  and  22 . 
   The center waveguide section  24  comprises a plurality of trays  30  and a cylinder post  32  whose major longitudinal axis is coincident with a central longitudinal axis of the center waveguide section. The plurality of trays  30  are stacked circumferentially around the post  32 . Each tray  30  includes a carrier  54  ( FIG. 2 ) having a predetermined wedge angle α ( FIG. 3 ), an arcuate inner surface  36  conforming to the outer shape of post  32 , and arcuate outer surface  34 . When the trays  30  are assembled together, they form a cylinder with a cylindrical central cavity defined by inner surfaces  36  which accommodates the post  32 . Post  32  connects with inner conductors  20  and  22  of input/output waveguide sections  12  and  14  by way of screws  26  and  28  on opposite ends of the post. Post  32  is provided for simplifying mechanical connections, and may have other than a cylindrical shape, or be omitted altogether. 
   As detailed in  FIG. 2 , each tray  30  also includes an input antipodal finline structure  48 , at least one active element  56 , an output antipodal finline structure  50 , and attendant DC circuitry  58 . The metal carrier  54  has an input cut-out region  38  and an output cut-out region  40 . The input and output cut-out regions are separated by a bridge  46 . Opposing major surfaces  42  and  44  of the regions  38  and  40  are arcuate in shape. When the trays  30  are stacked together, the regions  38  and  40  form a coaxial waveguide opening defined by circular outer and inner surfaces corresponding to arcuate major surfaces  42  and  44 , and the arrangement of the input and output finline structures on carriers  54  is such that the finline structures lie radially about the central longitudinal axis of center waveguide section  24 . Alternatively, major surfaces  42  and  44  can be planar, rather than arcuate, such that the coaxial waveguide opening, in cross-section, will be defined by polygonal outer and inner boundaries corresponding to planar major surfaces  42  and  44 . 
   The top surface  54   a  of metal carrier  54  is provided with recessed edges  38   a  and  40   a  in the periphery of cut-out regions  38  and  40 , and is recessed at bridge  46 , in order to accommodate the edges of antipodal finline structures  48 ,  50 , active elements  56  and DC circuitry  58 . When in position in a first carrier  54 , the back edges of antipodal finline structures  48 ,  50  rest in the corresponding recessed edges  38   a ,  40   a  of the carrier  54 , and back faces  48   b  and  50   b  of the finline structures respectively face cut-out regions  38 ,  40  of that first tray. Contact between the back faces  48   b  and  50   b  of antipodal finline structures  48 ,  50  and the corresponding recessed edges  38   a ,  40   a  of the carrier  54  provides grounding to the finline structures. 
   The back side of each carrier  54  has a cavity  62  as shown in  FIG. 4 , such that when the trays are stacked together, the cavity  62  will provide enough space to accommodate the active elements on the abutting tray and carrier. In the preferred embodiment, the cavity  62  is provided with channels  64  and  66  to avoid electrical contact with the microstrip lines of the finline structures of the abutting tray and carrier. 
     FIG. 3  shows a cross section at the middle of a carrier  54 . Outer surface  34  of the carrier is arcuate in shape such that when assembled together, the trays provide the center coaxial waveguide section  24  with a substantially circular cross-sectional shape. It is contemplated that other outer surface shapes, such as planar shapes, can be used, in which case the outer cross-sectional shape of the center coaxial waveguide section  24  becomes polygonal (see  FIG. 4A ). Further, as mentioned above, the carrier has a predetermined wedge angle α. Preferably, 16 trays are used, with the wedge angle α being 22.5°. 
   While it is preferred that the outside surfaces  34 ,  36  of each carrier  54 , along with the inside surfaces  42 ,  44  of the cut-out regions all be arcuate in shape so as to provide for circular cross-sections, it is possible to use straight edges for some or all of these surfaces, or even other shapes instead, with the assembled product thereby approximating cylindrical shapes depending on how many trays  30  are used.  FIG. 4A  shows an embodiment in which a cross section of the center waveguide shows that the outside surfaces and inside coaxial waveguide openings are all approximated by straight planes. A polygonal cross-sectional shape results, but if a sufficient number of trays are used, a circular cross section is approximated. 
   In the preferred embodiment, the wedge shaped trays  30  are radially oriented when stacked together to form a circular coaxial waveguide, as seen schematically in  FIG. 4A . However, the trays can have other shapes, which may be different from one another, and a non-cylindrical coaxial waveguide can thus result.  FIG. 4B  shows such an arrangement, resulting in a rectangular (square) coaxial waveguide. In  FIGS. 4A and 4B , the bold solid lines represent the finline structures. The dashed lines represent the inter-tray boundaries. 
   Returning to  FIG. 2 , it can be seen that at least one active element  56  is disposed on bridge  46 , between the antipodal finline structures  48  and  50 . DC bias circuitry  58  is also disposed on the tray. Holes  60  are provided for the DC bias connection (not shown) to circuitry  58 , which then passes to active element  56  as described below. In the preferred embodiment, input/output antipodal finline structure  48 ,  50  and DC bias circuitry  58  are disposed on separate boards. Alternatively, they may be disposed on the same board. 
   When the trays  30  are stacked together, the cut-out regions  38 ,  40  cumulatively form a coaxial waveguide opening. The antipodal finline structures  48 ,  50  form input and output antenna arrays in the coaxial waveguide opening. The input array couples the incoming signal, which enters from the input port  4  through input waveguide section  12 , from the stacked tray-formed waveguide opening, distributing the energy substantially evenly to each tray  30 , and passing it to the active elements for processing. Then the processed signal is combined by the output antipodal finline array inside the output coaxial waveguide opening, and propagated through the output waveguide section  14  to the output port  6 . 
     FIGS. 5A and 5B  shows a longitudinal cross-sectional view of the output coaxial waveguide section  14 . The waveguide section provides a smooth mechanical transition from a smaller input/output port (at Zp) to a flared center section  17 . Electrically, the waveguide section provides broadband impedance matching from the input/output port impedance Zp to the center section waveguide impedance Zc. The profiles of the inner conductors and outer conductors are determined by both optimum mechanical and electrical transition in a known fashion. 
   With reference to  FIGS. 6 and 7 , details of the antipodal finline structure  70  of the invention are disclosed. Three sections (Sections  1 ,  2 , and  3 , demarcated by lines a, b, and c), are delineated in the drawing figures for ease of explanation and discussed separately, with the understanding that these sections are not separate but are actually part of one unitary component. In Section  1 , lying between lines a and b, top side (corresponding to side  48   a  of  FIG. 2 ) metal conductor  72  and back side metal conductor  74  (corresponding to side  48   b  of  FIG. 2 ) are shown to expand in area outward respectively from the lower and upper edges of the substrate  76 . In Section  2  (between lines b and c) top side conductor  72  narrows to a strip  75 , while back side conductor  74  expands to a wider ground that has the same width as the substrate. Section  3  has a straight microstrip line on the top side, and a back side conductor as ground. This arrangement is easier to manufacture by eliminating a conventional balun as is know in the prior art, while still offering good compatibility with COTS MMICs. The tapered 3-section antipodal finline is referred to herein as an antipodal finline taper. In the preferred embodiment, the overall length of an antipodal finline taper is about 2.4 inches. 
     FIG. 7  shows the cross sections of the antipodal finline taper taken along lines a, b and c. The top side conductor  72  and back side conductor  74  are preferably disposed on a soft PTFE based substrate  76 . The substrate can also be any other suitable material, such as ceramic, or non-PTFE substrate. The cross sections of  FIG. 7  show the gradual changes of the top and back side metal conductors from left side to the right side. The top side conductor  72  becomes wider first and then narrower as a microstrip line. The back side conductor  74  becomes wider, then a ground plane. 
   The described antipodal finline structures provide broadband transitions from a waveguide impedance Zfw to a microstrip impedance Zfm. The Section  1  of the antipodal finline is determined for minimizing the reflection between Zfw and Zfm. Small reflection theory is used to synthesize the profile of the taper shape. The Section  2  in the antipodal finline transits the balanced finline to an unbalanced microstrip line. The top side connector  72  is tapered to the center of the structure, away from the waveguide wall. The back side conductor  74  is extended to the other side of the waveguide wall to form a full ground plane. At the overlapping area, a cavity area  78  in the substrate is formed. The length of Section  2  must be judiciously chosen, with the caveats that if the section is too long, the cavity will excite resonance at higher frequency, while if it too short, then the shortened distance from the center microstrip to the waveguide wall will deteriorate the lower frequency response. 
   As described above, a single antipodal finline taper is included in each antipodal finline structure. The input taper connects to one active element, which then connects to one output taper. However, more antipodal finline tapers can be added in each antipodal finline structure and more active elements can be added as well. Examples of such arrangements can be seen in  FIGS. 6A and 6B , wherein arrangements for more than one antipodal finline taper, disposed parallel to each other are shown. Each input antipodal finline taper in these arrangements connects to a single active element (not show), to which an output antipodal finline taper is then connected. In  FIG. 6A , the top side conductors T 1  and T 2  are shown to taper from the edge of the waveguide to the microstrip lines L 1  and L 2 ; in  FIG. 6B , they taper from the center to the microstrip lines. It is also contemplated that at least one antipodal finline taper is included in each finline structure, but with each antipodal finline taper being connected to more than one active element by a multi-way planar divider and combiner. One example is shown in  FIG. 6C . L 3  and L 4  are 2-way planar divider/combiners. 2 active elements can be further combined by the divider and combiner. Multi-way divider/combiners with more than 2 channels can also be used for combining more active elements to each finline taper. 
     FIG. 12  shows the frequency response of the broadband power combining device using antipodal finline structures of the invention. It can be seen that a broadband frequency response from 2 to 20 GHz is achieved. Broadband amplifiers are used as active elements. Hence, a 14 dB gain across the band was observed. 
     FIG. 8  shows details of a packaged form, surface-mountable active element  56  assembled between the input/output antipodal finline structures  48  and  50 . Alternatively, a bare die form active element can be used, although in most circumstances a packaged form active element is preferred. A hermetically sealed packaged active element more easily meets more stringent hermiticity requirements, for example for military applications, since it is more difficult to hermetically seal the whole system. Both surface-mountable or leaded packages can be used in the system. However, a surface-mountable package is preferred for less parasitic effects at higher frequencies. Active elements typically require good thermal management, and packages with good heat dissipation are desirable. As seen in  FIG. 8 , a highly thermal conductive base  86  is included in the package. The base  86  is directly mounted on the wedge-shaped metal tray  30 . The backside of the package, detailing pad layout, is shown in  FIG. 8A . Pads  89  matching the package pad layout are disposed on the input/output finline structure  48  and  50  and make electrical contact therewith at assembly. As will be appreciated, use of a hermetically sealed active element package is not limited to an antipodal finline structure, but includes any type of antenna structure, such as a slotline structure. Further, the hermetically sealed active element package can be used with antennas used in a coaxial or rectangular type waveguide combiner. 
   In another embodiment illustrated in  FIG. 9 , a surface mount package  90  is directly mounted on a board  88  which also includes an input/output finline structures, all arranged as one unitary component. The back side view of package  90  is shown in  FIG. 9A . A center ground area  91  is disposed on the package for both RF grounding and heat dissipation. A via-filled area  92  is provided on the board  88 . The via holes provide good heat dissipation for the active elements of package  90 . Pads  93  provided on board  88  are for RF, DC and ground connections matching the pad layout of package  90 . 
     FIG. 10  shows a schematic diagram of a spatial power combining device. Element  94  is an exemplary active element in the system. In power combining applications, maximum power combining efficiency is achieved when the active elements all output the same amount of power at the same phase. However, variations are inevitable for semiconductor devices used in the active elements. A DC control circuit  96  is therefore added to each active element to equalize the output power from each element. In the preferred embodiment, a field effect transistor (FET) (not shown) is used as an active element. A feedback network from drain current to gate voltage is used as a DC control circuit. The drain current is used to determine the maximum output power capacity from each active element. The feedback circuit is used to adjust the gate voltage to maintain a fixed drain current, and hence a fixed output power. The AM to PM distortion will thus be similar for each element, and the phase difference can also be minimized. In another embodiment, a power sensor is added at the output of active elements. A feedback circuit is provided from the output of the power sensor to the gate voltage. By sensing the output power, the feedback circuit will lock it to a fixed value. Then the combining efficiency can be maximized. 
   It will be appreciated that the active elements are not limited to FETs. They can be bipolar transistors (BJT) or HBTs (Heterjuntion BJTs). Further, the feedback DC control circuit is not limited to gate voltage controlling. It can control the base current, drain or collector voltage, and drain or collector current. In accordance with one embodiment, BJTs are used as active elements. A feedback circuit can be added to sense the output current, voltage or power and adjust the base current to control the output current, voltage or power. It will equalize the output power from the active elements and minimize the phase difference to achieve the maximum combining efficiency. 
     FIG. 11  shows a thermal management scheme for the power combining device. A heat sink  100  is comprised of two sections, with each section having a pair of separable halves defining a cavity  101  therebetween, the cavity having a shape which conforms to the outer shape of center waveguide section  24 , which in the illustrated case is cylindrical. The halves delineated  102  and  104  are assembled together; and the halves delineated  106  and  108  assembled together. Flanges  105  are provided through which screws  107  or other fastening means pass to tighten the halves together. When mated together, each pair of halves defines a cylindrical or other shaped cavity conforming to the outer shape of center waveguide section  24 . The heat sink is provided with fins  109 , preferably formed in a machined manner. The height of the fins  109 , along with the length of the heat sink, is determined by the amount of heat to be dissipated. The heat sink also operates to clamp the stacked trays together, making for a robust device even when significant vibration or other insult are encountered. A gap  111  between the two sections of the heat sink is provided for DC connections through holes  60  as discussed above. Thermal grease can be used to fill the gaps between the two pairs of separable halves of the heat sink. It will be appreciated that the heat sinks are not limited to two sections of two halves each; rather, more or less than two sections, each having more or subparts, can be used. Other connections of the subparts and different manufacturing techniques can be used. 
   Further, it will be appreciated that the teachings of the invention, including the hermetic sealing scheme, the power controlling scheme and the thermal management scheme can, can be applied to any known spatial power combining devices. These include a grid amplifier, an active array spatial power combiner, and all waveguide power combining devices using finline structure arrays. The finline structures include both slotline structures with necessary baluns and antipodal finline structures. 
   The length of the power combining device for broadband applications of the invention is mainly determined by the lower cut-off frequency of the operation frequency band. However, the teachings of the invention also apply for narrower bandwidth applications. The dimensions of the power combining device are changeable for different impedance matching levels and different frequency bandwidths. In the preferred embodiment, the input/output waveguide sections are about 2 inches in length. The wedge shaped trays  30  are each about 6 inches in length. However, it will be appreciated that other dimensions can be used, depending on desired frequency response and impedance matching level. 
   The above are exemplary modes of carrying out the invention and are not intended to be limiting. It will be apparent to those of ordinary skill in the art that modifications thereto can be made without departure from the spirit and scope of the invention as set forth in the following claims.