Abstract:
A power combining apparatus includes a waveguide structure and a plurality of antenna elements arranged in the waveguide structure, wherein each of the antenna elements comprises a center planar antenna layer, two outer planar antenna layers arranged on opposite sides of the center planar antenna layer, a non-conductive layer between the center planar antenna layer and one of the outer planar antenna layers, and another non-conductive layer between the center planar antenna and the other one of the outer planar antenna layers. The power combining apparatus includes a waveguide structure having an input, an output, and a plurality of antenna elements arranged in the waveguide structure, wherein each antenna element is configured to transform an electric field direction of an electromagnetic field by substantially 90 degrees rotation about a longitudinal axis of the waveguide structure, wherein a bandwidth of the antenna is less than, equal to, or greater than a decade of frequency range.

Description:
FIELD 
     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 antenna elements provided within a coaxial waveguide cavity with matched impedance for reduced insertion loss, wherein the antennas have tri-planar microstrip to stripline to balanced antipodal finline exponential taper. 
     BACKGROUND 
     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 number 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. 
     SUMMARY 
     In accordance with the invention, a power combining device includes a waveguide structure, and a plurality of antenna elements arranged in the waveguide structure, wherein each of the antenna elements comprises a center planar antenna layer, two outer planar antenna layers arranged on opposite sides of the center planar antenna layer a non-conductive layer between the center planar antenna layer and one of the outer planar antenna layers, and another non-conductive layer between the center planar antenna and the other one of the outer planar antenna layers. 
     In a further aspect of the disclosure, a power combining apparatus includes a waveguide structure having an input and an output, and a plurality of antenna elements arranged in the waveguide structure, wherein each antenna element is configured to transform an electric field vector direction of an electromagnetic field by substantially 90 degrees rotation about a longitudinal axis of the waveguide structure, and wherein a bandwidth of the power combining apparatus is equal to or greater than a decade of frequency bandwidth. 
     In a further aspect of the disclosure, a power combining apparatus includes an output waveguide section having a central longitudinal axis, and inner and outer coaxial conductors, wherein an outer surface of the inner conductor and an inner surface of the outer conductor have a substantially constant ratio of radial dimension along the central longitudinal axis, a center waveguide section having an input, an output, and a plurality of antenna elements, the output of the center waveguide section being coupled to the output waveguide section and an input waveguide section coupled to the input of the center waveguide section. 
     In a further aspect of the disclosure, a power combining apparatus includes an output waveguide section having a central longitudinal axis, and inner and outer coaxial conductors configured to maintain a substantially constant characteristic impedance along the central longitudinal axis, a center waveguide section having an input, an output, and a plurality of antenna elements, the output of the center waveguide section being coupled to the output waveguide section, and an input waveguide section coupled to the input of the center waveguide section. 
    
    
     
       BRIEF DESCRIPTION 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 a 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; and 
         FIG. 6  is a view of a tri-plane antenna in accordance with the invention. 
         FIG. 7A  is a left end cross-section of a tri-plane antenna in accordance with the invention. 
         FIG. 7B  is a middle cross-section of a tri-plane antenna in accordance with the invention. 
         FIG. 7C  is a right end cross-section of a tri-plane antenna in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the accompanying drawings is intended as a description of various embodiments of the invention and is not intended to represent the only embodiments in which the invention may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention may be practiced without these specific details. In some instances, well known structures and components are shown in block diagram form in order to avoid obscuring the concepts of the invention. 
     In accordance with the invention, a broadband spatial power combining device has an input waveguide section, an output waveguide section, and a center waveguide section. The center waveguide section is provided with longitudinally parallel, stacked wedge shaped trays. Antenna elements are mounted on each tray. When the trays are stacked together to form a coaxial waveguide, the antenna elements are disposed into the waveguide and form a dividing array at the input and a combining array at the output. One or more active elements may be arranged between an antenna element of the input array and an antenna element of the output array. With the use of antenna elements inside the coaxial waveguide for power dividing and combining, a broadband frequency response covering approximately a decade of bandwidth (such as, for example 2 to 20 GHz, or 4 to 40 GHz) may be realized. The design may be optimized for smaller or larger bandwidths to optimize performance according to varying criteria, such as bandwidth versus insertion loss, for example. The antenna element is easy to manufacture using conventional printed circuit board (PCB) processes. It also enables easy integration with commercial off-the-shelf (COTS) millimeter wave integrated circuits (MMICs). Further, the division of a coaxial waveguide into wedge-shaped trays enables simplified DC biasing and enables good thermal management. 
     As illustrated in  FIG. 1 , in the spatial power combining device  2  of the invention, an electromagnetic (EM) 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 an embodiment, the outer surface of inner conductor  20  and the inner surface of the outer conductor  16  have profiles to obtain a transformation of waveguide impedance. 
     In a 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 antenna element  48 , may include at least one active element  56 , an output antenna element  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 antenna elements on carriers  54  is such that the antenna elements 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 antenna elements  48 ,  50 , active elements  56  and DC circuitry  58 . When in position in a first carrier  54 , the back edges of antenna elements  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 antenna elements respectively face cut-out regions  38 ,  40  of that first tray. Contact between the back faces  48   b  and  50   b  of antenna elements  48 ,  50  and the corresponding recessed edges  38   a ,  40   a  of the carrier  54  provides grounding to the antenna elements. 
     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 microstrip lines on the antenna elements 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  30  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. 
     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. 
       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 a balanced antipodal finline antenna element  70  of the invention are disclosed. Antenna element  48 ,  50  is formed on a substrate  70  that includes two outer planar antenna layers  74 A,  74 B arranged on opposite sides of a center planar antenna layer  72 , a non-conductive layer  76  between the center planar antenna layer  72  and one of the outer planar antenna layers  74 A, and another non-conductive layer  76  between the center planar antenna  72  and the other one of the outer planar antenna layers  74 B. 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 ) center planar antenna layer  72  and outer planar antenna layers  74 A AND  74 B (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  70 . In Section 2 (between lines b and c) center planar antenna layer  72  narrows to a strip  75 , while outer planar antenna layers  74 A,  74 B expand to form a wider ground that has the same width as the substrate  71 . Section 3 has a straight stripline line in the center plane, and two outer conductors as ground. The stripline line transitions to a microstrip line using known methods. This arrangement offers good compatibility with COTS MMICs. Through superposition, the electric fields are effectively rotated through 90 degrees for improved radiation efficiency and coupling between the input/output waveguide sections and the active element. The 3-section antenna elements  48 ,  50  are referred to herein as a balanced antipodal finline taper antenna elements. In the preferred embodiment, the overall length of an antipodal finline taper is about 2.4 inches. 
     The number of trays  30 , and corresponding number of antenna elements  48 ,  50 , may be related to the impedance of the active elements  56  coupled to the antenna elements  48 ,  50 . The receive and transmit antenna elements  48 ,  50  couple to the EM field at the input/output waveguide sections  12 ,  14 . For example, where the output wave guide section  14  has a characteristic impedance of 50 ohms, the center waveguide section  24  includes 10 trays  30 , where each tray  30  includes a transmit antenna element  50  that may have, e.g., a characteristic output impedance of 480 ohms, where the transmit antennas  50  are effectively in electrical parallel. The characteristic impedance of the array of 10 transmit antenna elements  50  is then effectively 48 ohms. Therefore, 10 may be the preferred number of trays, where each tray includes a single transmit antenna element  50  and a single receive antenna element  48 . The output impedance of the transmit antenna element array is then said to be substantially matched to the output waveguide section, i.e., 48 ohms˜50 ohms. The characteristic impedance of the transmit antenna element  50  is determined at least by the dielectric constant, thickness and planar dimensions of the substrate material of the transmit antenna element  50 . Similarly, the input waveguide section  12  and the receive antenna elements  48  may be substantially impedance matched by the judicious design of the input waveguide section  12 , receive antenna elements  48  and the number of trays  30  forming the center waveguide section  24  according to the description above for transmit antenna element  50  impedance matching. However, because design principals allow for the input/output waveguide sections and center waveguide section to have different impedances, the number of trays and corresponding number of antenna elements  48 ,  50  may vary. 
     Typically, an active element impedance may be about 50 ohms, but other impedance levels are possible. A profile of the conductive patterns of the center planar antenna layer  72  and outer planar antenna layers  74 A AND  74 B on the antenna elements  48 ,  50  may be designed by well know principals, e.g., small reflection theory, to minimize reflection of the traveling EM wave. The profile of conductive patterns on the antenna elements  48 ,  50  is judiciously chosen to avoid exciting multimode resonance at higher frequency (i.e., cutoff) and response deterioration at lower frequency. 
     The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects. Thus, the claims are not intended to be limited to the aspects shown herein, but is to be accorded the full scope consistent with the language of the claims, wherein reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more.” All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. §112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.”