Patent Publication Number: US-10770775-B2

Title: Radial combiner

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a radial combiner, and more specifically to a radial combiner lacking internal dielectric, metallic or resistive components, within the radial cavity. 
     2. Introduction 
     Microwave dividers receive a microwave signal, divide the signal into N parts, and forward each of those divided parts to a power amplifier or other signal manipulation tool. Microwave combiners perform the same function in reverse, taking multiple signals, amplifying the multiple signals, then combining the amplified signals into a single output. In certain configurations, a single device can be used as either a divider or a combiner, depending on the source of the device input(s). When the N inputs/outputs are arranged in a circle, these devices are called radial combiners, radial dividers, or radial combiner/dividers. 
     With a few exceptions, conventional radial dividers/combiners fall into one of two categories, namely a “stripline waveguide” configuration or a “peripheral rectangular waveguide” configuration. The most prevalent category, the stripline waveguide configuration, consists of a circular metallic conductor at the center of a radial waveguide, the radial waveguide connecting to a series of radial microstrip or stripline conductors circumferentially spaced and each connecting to a peripheral coax connector. The peripheral rectangular waveguide configuration consists of a series of peripherally spaced co-planar rectangular waveguides directed toward the center of the structure, where the waveguide ends join contiguously in a central circular region to excite a radial TEM mode that couples to a coaxial waveguide at the center. 
     SUMMARY 
     Additional features and advantages of the disclosure will be set forth in the description which follows, and in part will be obvious from the description, or can be learned by practice of the herein disclosed principles. The features and advantages of the disclosure can be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the disclosure will become more fully apparent from the following description and appended claims, or can be learned by the practice of the principles set forth herein. 
     An exemplary embodiment of radial combiner as disclosed herein can include a radial waveguide cavity defined by: a top plate having a circular cross-section and a metallic composition; a bottom plate having a circular cross-section, a metallic composition, and being situated substantially parallel to the top plate; an outside wall connecting the top plate and the bottom plate along a circumference of at least one of the top plate and the bottom plate; and an interior of the radial waveguide cavity located between the top plate, the bottom plate, and the outside wall with a substantially uniform height throughout. The radial combiner can also include a plurality of monopole radiators located within the interior of the radial waveguide cavity and a plurality of coaxial ports mounted on one of the top plate or the bottom plate, mounted exterior to the radial waveguide cavity, and substantially equally spaced towards a periphery of the radial waveguide cavity. Each coaxial port in the plurality of coaxial ports can be mounted normal to the circular cross-section of the top plate or the bottom plate, and having a coaxial center conductor which extends through a hole in one of the top plate and the bottom plate, the coaxial center conductor of each coaxial port being electrically connected to one of the respective monopole radiators for each coaxial port within the interior of the radial waveguide cavity. The radial combiner can also have a center conductor, wherein the center conductor is located substantially at a center of the interior of the radial waveguide cavity, is normal to the circular cross-section of the top plate or the bottom plate, extends within the interior of the radial waveguide cavity between the top plate and the bottom plate, transitions, on a first side of the interior, to a coaxial waveguide located exterior to the radial waveguide cavity, and terminates on a second side of the interior which is opposite the first side. 
     An exemplary method embodiment as disclosed herein can include: receiving a plurality of first signals at a plurality of coaxial ports equally spaced towards a periphery of a radial waveguide, and combining the plurality of first signals using a microwave combiner. The microwave combiner can include a radial waveguide cavity defined by: a top plate having a circular cross-section and a metallic composition; a bottom plate having a circular cross-section, a metallic composition, and being situated substantially parallel to the top plate; an outside wall connecting the top plate and the bottom plate along a circumference of at least one of the top plate and the bottom plate; and an interior of the radial waveguide cavity located between the top plate, the bottom plate, and the outside wall with a substantially uniform height throughout. The microwave combiner can have a plurality of monopole radiators located within the interior of the radial waveguide cavity, a plurality of coaxial ports mounted on one of the top plate or the bottom plate, mounted exterior to the radial waveguide cavity, and substantially equally spaced towards a periphery of the radial waveguide cavity. Each coaxial port in the plurality of coaxial ports can be mounted normal to the circular cross-section of the top plate or the bottom plate and have a coaxial center conductor which extends through a hole in one of the top plate and the bottom plate, the coaxial center conductor of each coaxial port being electrically connected to one of the respective monopole radiators for each coaxial port within the interior of the radial waveguide cavity. The microwave combiner can also have a center conductor which is located substantially at a center of the interior of the radial waveguide cavity, is normal to the circular cross-section of the top plate or the bottom plate, extends within the interior of the radial waveguide cavity between the top plate and the bottom plate, transitions, on a first side of the interior, to a coaxial waveguide located exterior to the radial waveguide cavity, and terminates on a second side of the interior which is opposite the first side. 
     Another exemplary embodiment of a radial combiner as disclosed herein can include a hollow cylindrical cavity, wherein the hollow cylindrical cavity: is metallic; is defined by a first plate as a top of the hollow cylindrical cavity, a second plate as a bottom of the hollow cylindrical cavity, and an outside wall as a side of the hollow cylindrical cavity, the first plate being substantially parallel to the second plate; and has a substantially uniform height between the first plate and the second plate. The radial combiner can also include a plurality of coaxial ports mounted exterior to the hollow cylindrical cavity and on either one of the first plate or the second plate, and a plurality of monopole radiators substantially equally spaced around a circle and attached to an interior of one of the first plate or the second plate, each monopole radiator being electrically connected respectively to one of the plurality of coaxial ports using a coaxial center conductor extending through a hole in the one of the first plate or the second plate. The radial combiner can include a center conductor, where the center conductor is located substantially at a center of an interior of the hollow cylindrical waveguide cavity, is normal to the circular cross-section of the first plate or the second plate, extends within the interior of the hollow cylindrical cavity between the first plate and the second plate, transitions, on a first side of the interior, to a coaxial waveguide located exterior to the hollow cylindrical cavity, and terminates, on a second side of the interior which is opposite the first side. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an isometric view of an exemplary radial combiner; 
         FIG. 2  illustrates a lateral sectional view of the exemplary radial combiner of  FIG. 1 ; 
         FIG. 3  illustrates an isometric view of a top plate of the exemplary radial combiner of  FIG. 1 ; 
         FIG. 4  illustrates an isometric view of a bottom plate of the exemplary radial combiner of  FIG. 1 ; 
         FIG. 5  illustrates a lateral sectional view of the exemplary radial combiner of  FIG. 2  with a different transition and a waveguide; 
         FIG. 6  illustrates an isometric view of the waveguide of  FIG. 5 ; and 
         FIG. 7  illustrates an exemplary method of using the exemplary radial combiner of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments of the disclosure are described in detail below. While specific implementations are described, it should be understood that this is done for illustration purposes only. Other components and configurations may be used without parting from the spirit and scope of the disclosure. 
     The principles disclosed herein apply equally to radial combiners and radial dividers. For example, the concepts and disclosure contained herein directed to combining signals using a radial combiner can also be applied to a radial divider dividing a signal. Unless otherwise stated, examples provided using the term “radial combiner” should be interpreted to also apply to radial dividers. 
     As realized by the inventor, stripline waveguide configurations and peripheral rectangular waveguide configurations for radial combiners can suffer deficiencies, specifically regarding the resources required for their construction and the resulting outputs of such configurations. For example, aspects such as dielectrics within the radial combiner, use of incorporated air, positioning of the peripheral coaxial ports, etc., can increase the time, costs, and materials needed to manufacture a radial combiner. 
     In contrast, as discovered by the inventor, radial combiners as disclosed herein can be constructed in complete absence of any internal dielectrics, resistors, or additional metallic structures. This absence can minimize the internal propagation loss while accommodating high microwave power levels. Preferably, radial combiners built according to this disclosure can provide peripheral coaxial ports which are normal to one of the parallel radial waveguide walls, rather than co-planar with the parallel radial waveguide walls (and normal to an outside wall connecting the parallel radial waveguide walls). Likewise, to improve reliability while minimizing the cost of fabrication, radial waveguides constructed according to this disclosure can employ a substantially uniform height which simplifies the design, improves the reliability, and minimizes cost of fabrication. Moreover, the radial waveguide disclosed herein can use a coax conductor which extends (using a coaxial center conductor) through a plate into a monopole probe, where the coaxial center conductor to monopole probe connection yields an insertion loss lower than the loss of an end-on coax launcher. The monopole probes for a radial combiner are monopole radiators, where a signal is input from the coax center conductor, then radiated within the central cavity from the corresponding monopole radiator. If, instead of a radial combiner the embodiment is acting as a radial divider, the monopole probes will act as monopole collectors, not radiators. 
     The peripheral coax ports are capable of receiving/transmitting signals, and can include any RF (Radio Frequency) capable connector, for example SMA (SubMiniature version A), TNC (Threaded Neill-Concelman), and Type-N peripheral connectors, with the choice depending on the microwave power level and Solid-State Module interface requirements. For any frequency band, EM (Electromagnetic) simulations using the disclosed concepts and designs predict the same insertion loss and active return loss for any of the connectors selected. If a larger sized coax connector is selected, the only corresponding design difference would be an increase in the radius of the radial waveguide to accommodate the peripheral spacing of the larger sized coax connectors. 
     Consider the example of a 32-way radial combiner for C-band frequencies. Such an exemplary radial combiner can be formed by two metallic, cylindrical plates having parallel (or substantially parallel) planes, with a coaxial waveguide port at the center, and having a peripheral wall bridging the two cylindrical plates. Around the periphery of the radial waveguide are equally spaced holes in one of the two parallel plates. These holes are offset a fraction of a wavelength toward the center from the peripheral wall bridging the two cylindrical plates. Coax connectors are located at each hole with the center conductor of the coax connector extending into the space between the two plates, forming a monopole probe within the cavity between the two plates. The number of peripheral coax connectors in this example is thirty two. While this number is not restricted to any particular number, the number of connectors is typically between 10 and 100, and is based on the ratio of the total desired divider/combiner power to the power of an individual coaxial port. For efficient power combining, the individual port signals are all required to be essentially equi-amplitude and equi-phase (i.e., within a threshold range). 
     Continuing this example, the distance (or height) between the two parallel circular metallic plates is preferably less than one-half a wavelength at the uppermost frequency of operation. Of the radial waveguide modes, only the dominant TEM (Transverse Electromagnetic) mode is supported. The hole, or opening, at the center of one cylindrical plate can provide a transition to a coaxial waveguide that can end in either a flanged coax joint or a coax transition to rectangular waveguide. It is noted that the location of the hole should be centered to obtain the same electrical path length between the peripheral coax ports and the central coaxial transmission line. The center conductor of this coaxial waveguide can extend across the space between the parallel plates and terminate in a version of, what has historically been called, a “doorknob” transition that contacts the opposite wall. 
     Within the radial cavity created by the two plates and the peripheral wall, a doorknob transition (hereafter called a doorknob collector when associated with combining signals, and a doorknob emitter when radiating signals) can be located in the center, next to the center conductor, and can have a conical shape with the height and base radius chosen to center the impedance plot of the radial combiner on a Smith Chart over a predetermined range. For example, the range of the impedance plot may be 3.82 GHz to 5.98 GHz. 
     The peripheral coax ports can be, for example, type-N flange mount jack receptacles with the center conductor of each coaxial port forming a monopole radiator. These connectors may be individually removed or replaced without any disassembly beyond the screws that attach a particular connector to the cylindrical plate to which they are connected. Each monopole radiator can be shaped to provide an impedance match to the radial TEM mode of the signal radiating within the cavity. 
     Below are simulation results for two exemplary 32-way radial dividers, with a WR187 waveguide common port, are shown below. These examples include the loss of aluminum materials and use a Teflon connector. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Peak Power (W) 
                   
                   
                   
                   
               
               
                   
                   
                 (safety factor 4:1)  
                   
                   
                   
                   
               
               
                   
                   
                 At sea level 
                   
                   
                   
                   
               
               
                   
                   
                 atmospheric air 
                   
                   
                   
                   
               
               
                 Frequency 
                 Frequency 
                 pressure 
                 Maximum 
                   
                 Amplitude 
                 Phase 
               
               
                 Range 
                 Bandwidth 
                 (standard 
                 Insertion Loss 
                 Maximum 
                 Imbalance 
                 Imbalance 
               
               
                 (GHz) 
                 (%) 
                 atmosphere) 
                 (dB) 
                 VSWR 
                 (dB) 
                 (deg.) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 4.4-6.0 
                 30 
                 125,000 
                 0.25 
                 1.30:1 
                 ±0.20 
                 ±2.0 
               
               
                 5.3-6.0 
                 12 
                 150,000 
                 0.10 
                 1.10:1 
                 ±0.10 
                 ±1.0 
               
               
                   
               
            
           
         
       
     
     Having provided a general description of the invention, the disclosure now turns to the specific examples illustrated by the figures. 
     An exemplary radial combiner is discussed with respect to  FIGS. 1-6 . 
       FIG. 1  illustrates an isometric view of an exemplary radial combiner  100 . The radial combiner  100  as illustrated has a top plate  108  and an outside wall  110 . The top plate  108  is circular cross-section and has a metallic composition. Attached to the top plate  108  are coaxial ports  102 . In this example, each of the coaxial ports is attached to the top plate  108  with a connector plate  104 , though in some configurations the connector plate  104  may not be necessary. A base  112  of a transition to a waveguide is provided at the bottom of the radial combiner  100 . 
       FIG. 2  illustrates a lateral sectional view of the exemplary radial combiner  100 . In this example, the top plate  108 , the outside wall  110  (also known as the peripheral wall), and a bottom plate  204  form a radial cavity  206 . The bottom plate  204  has a circular cross-section, a metallic composition, and being situated substantially parallel to the top plate  108 . The outside wall  110  connects to the top plate  108  and the bottom plate  204 , with a circumference of at least one of the top plate  108  or the bottom plate  204 . Above the top plate  108  are the coaxial ports  102 , with the accompanying connector plates  104 . The coaxial ports  102  are mounted normal with respect to the top plate  108 . For each of the coaxial ports  102 , there is a hole through the top plate  108  connecting each coaxial port  102  to a corresponding monopole radiator  208  within the radial cavity  206 , the connection occurring via a coaxial central conductor (such as a wire) connecting the coaxial port  102  with the monopole radiator  208 . In other embodiments, the monopole radiators  208  can be replaced with distinct types of radiators, such as dipole antennas. In some embodiments, the monopole radiators  208  are all a single type of monopole antenna (such as a whip, rubber ducky, helical, random wire, umbrella, etc.) whereas in other embodiments the monopole radiators  208  are of mixed type (such as some whip monopole radiators, some rubber duckies, etc.). 
       FIG. 2  further illustrates a “doorknob” collector  210  located at the center of the top plate  108 , next to the center conductor which descends into a transition  202  to a waveguide. The doorknob collector  210  is referred to as a “doorknob” collector  210  due to its shape resembling half section of a common doorknob, namely a wide base angling, or curving, to a distal height. The doorknob collector  210  may be conical in shape and approximately centered below the center of the top plate  108 . Instead of a “doorknob” collector  210 , the transition  210  may be an “L transition” or other device designed to provide impedance matching. More specifically, any collector  210  may be used which matches the characteristic impedance of the center conductor  212  to the output over the useful bandwidth of radiated signal while minimizing discontinuities arising from the geometry of the transition device  210 . In other words, the doorknob collector  210  may be used to keep the voltage standing wave ratio at a constant minimum over a specified bandwidth in both the radial combiner cavity  206  and in the center conductor  212 . 
     When being used as a radial combiner  100 , the multiple coaxial ports  102  can be simultaneously excited by individual sold state amplifiers, which in turn will excite a radial TEM mode within the cavity  206 . The radial TEM mode will propagate toward the center of the cavity  206  and couple into the center conductor  212 , which preferably is a coaxial transmission line. The center conductor  212  is part of a transition  202  linking the radial combiner  100  to an output, such as a waveguide or coaxial output. 
     In some embodiments, the doorknob collector  210  can extend through an entirety of the height of the cavity  206 , whereas in other embodiments the doorknob collector  210  will only extend partially through the cavity  206 . The center conductor  212  physically connects to the doorknob collector  210  regardless of which configuration is used, then continues into the transition  202  to an output as described below. The center conductor  212  is normal to the circular cross-section of the top plate or the bottom plate, and extends within the interior of the radial waveguide cavity  206  between the top plate  108  and the bottom plate  204 . The center conductor  212  transitions, on a first side of the interior, to a coaxial waveguide  202  located exterior to the radial waveguide cavity  206 , and terminates, on a second side of the interior which is opposite the first side, in a conically shaped dome (aka the doorknob collector  210 ). 
       FIG. 3  illustrates an isometric view of the top plate  108  of the exemplary radial combiner  100 . This view emphasizes the interior  302  of the top plate  108 , with the coaxial ports  102  barely visible. The monopole radiators  208  are located around the periphery of the top plate  108 . In an exemplary embodiment, the monopole radiators  208  may be spaced around a circle inset from the perimeter of the top plate  108 . Further, the monopole radiators  208  may be approximately equally spaced around a circle approximately equally inset from the perimeter of the top plate  108 . The doorknob collector  210  and the center conductor  212  are also visible in this view. 
       FIG. 4  illustrates an isometric view of the bottom plate  204  of the exemplary radial combiner  100 . This view emphasizes the interior  402  of the bottom plate  204 . The bottom plate  402  includes a hole  404  through which the center conductor  212  can pass. The doorknob collector  210  cannot pass through the hole  404  of the bottom plate  204 . As illustrated, the bottom plate  402  is shown attached to the outside wall  110 . However, how the bottom plate  402 , the outside wall  110 , and the top plate  108  are formed and combined does not affect functionality. For example, the bottom plate  402 , the outside wall  110 , and the top plate  108  can all be individually formed, then combined during manufacture of the radial combiner  100 . In another case, the bottom plate  402  and the outside wall  110  can be formed as one integrated unit, then combined with the top plate  108 . In other cases the top plate  108  and the outside wall  110  may be formed jointly, then combined with the bottom plate  402 , or the bottom plate  402 , the outside wall  110 , and the top plate  108  may all be formed together as a single integrated unit. 
       FIG. 5  illustrates a lateral sectional view of the exemplary radial combiner  100  illustrated in  FIG. 2 , further including an extended transition  502  and a rectangular waveguide  506 . The extended transition  502  replaces the transition  202  in  FIG. 2 , and contains both the center conductor  212  as well as an output conductor  516 . The transition  502  functions as a coaxial waveguide conveying the combined signal from the radial waveguide cavity  206  to an output. Exemplary outputs can include a coaxial output or a rectangular waveguide output  506 . 
     The transition  502  is located substantially at a center of the radial waveguide cavity  206 , and is normal to the circular cross-section of the top plate  108  or the bottom plate  204 . The transition  502  contains the center conductor  212  and the output conductor  516 , which together form a transition center conductor  212 ,  516 . The center conductor  212  extends across the interior of the radial waveguide cavity  206  between the top plate  108  and the bottom plate  204 , and can be at least partially surrounded by the doorknob collector  210  before entering the transition  502  portion. Likewise, the output conductor  516  illustrated extends into the rectangular waveguide  506 , and can be at least partially surrounded by the doorknob emitter  504 . 
     As described above, the multiple coaxial ports  102  can be simultaneously excited by individual sold state amplifiers, which in turn will excite a radial TEM mode within the cavity  206 . The radial TEM mode will propagate toward the center, then be combined into the center conductor  212  for transmission to the rectangular waveguide  506 . To reach the rectangular waveguide  506  from the radial cavity  206 , the center conductor  212  carries the combined signal from the radial cavity  206  to an output conductor  516 . In embodiments where the combined signal output is a coaxial signal, the rectangular waveguide  506  will not be present, and the output conductor  516  will conduct the combined signal to an output location. In the illustrated embodiment, the output is a radiated signal through the rectangular waveguide  506 , so the output conductor  516  connects to a doorknob emitter  504 . That is, the combined signal propagates down the output conductor  516 , then radiates from the output conductor  516  into the rectangular waveguide  506 . 
     Because the center conductor  212  joins with the output conductor  516 , there can be an impedance mismatch if the center conductor  212  and the output conductor  516  are not properly selected and formed. As is typical in coaxial waveguide connections, the center conductor of the cavity probe and the center conductor of the waveguide probe can be joined by a coaxial “button”. Each end of the button is inserted into a hole in the ends of the coaxial center conductors. 
     As stated above, in some embodiments the output can be a coax line carrying the combined signal, such as the output conductor  516 . However, as illustrated in  FIG. 5 , the signal is radiated out of a rectangular waveguide  506 . More specifically, the output conductor  516  extends from the center conductor  212  and into the rectangular waveguide  506 . In this example, the output conductor  516  terminates in a doorknob emitter  504 , and the output conductor  516  radiates the combined signal into the rectangular waveguide cavity  514 . Radiation from the second coaxial transmission line  516  propagates into the waveguide  506 . To ensure proper impedance matching for a desired frequency range, the doorknob emitter  504  and/or a series of steps  508 ,  510 , and  512  can be included in the waveguide  506 , gradually opening into a wider cavity  514 . Although three steps  508 ,  510 , and  512  are illustrated, more or less than three steps may be included in the waveguide  506  depending on the specific frequency range of the combined signal. The resulting electromagnetic wave may then exit the waveguide  506 . 
       FIG. 6  illustrates an isometric view of the rectangular waveguide  506  of  FIG. 5 . As illustrated, the output conductor  516  may connect to the doorknob emitter  504 , and the combined signal can radiate from the output conductor into the air within the rectangular waveguide  506 , with steps  508 ,  510 , and  512  providing impedance matching. A top portion  602  of the rectangular waveguide  506  may include a hole  604  through which the output conductor  516  may pass through. 
       FIG. 7  illustrates an exemplary method of using the exemplary radial combiner  100  of  FIGS. 1-6 , or any radial combiner configured according to the concepts disclosed herein. In this example, the radial combiner  100  receives a plurality of first signals at a plurality of coaxial ports  102  equally spaced towards a periphery of a radial waveguide ( 702 ). The radial combiner  100  combines the plurality of first signals using a microwave combiner, to obtain a combined signal ( 704 ), and transmits the combined signal to a waveguide  506  ( 706 ). 
     With the invention, a dielectric is not needed within the radial combiner cavity  206 . For example, the cavity  206  of the radial waveguide  100  lacks a dielectric, such as one or more internal walls, baffles, or other additional metallic material. In other words, the interior of the radial waveguide  100  lacks a dielectric and also lacks incorporated air (such as air which is sealed within the radial waveguide  100 ), and instead uses atmospheric air. For example, a typical embodiment can include a “weep hole” at the bottom of a vertically mounted waveguide. 
     In some embodiments, the interior cavity  206  of the radial waveguide  100  can have a substantially uniform height, whereas in other embodiments the interior can vary in height. In those modes with a substantially uniform height, the substantially uniform height of the interior of the radial waveguide cavity  206  may only support a dominant Transverse Electromagnetic (TEM) radial mode for a band of interest. In such embodiments, each respective monopole radiator  208  may be shaped to match an impedance required for the dominant TEM radial mode. The shape of the monopole radiators  208  can be determined, for example, by using Electromagnetic (EM) simulation software to provide an impedance match to the dominant TEM radial mode when all coaxial ports are equally excited. This matching can include the mutually coupled energy from all neighboring monopulse radiators. 
     In some embodiments, upon being radiated, the rectangular waveguide  506  receiving the radiated combined signal can use impedance matching steps  508 ,  510 , and  512 . In other embodiments, there may be no steps or transitions into the coaxial waveguide  506  from where the doorknob emitter  504  is located. 
     The coaxial ports  102  are mounted normally to at least one of the top plate  108  or the bottom plate  204 . In some embodiments, all of the coaxial ports  102  and all of the monopole radiators  208  are mounted on either the top plate  108  or the bottom plate  204 . In other configurations, there can be some coaxial ports  102  and corresponding monopole radiators  208  on both the top plate  108  and the bottom plate  204 , provided the 180 degree phase difference between the top plate  108  and the bottom plate  204  is accounted for. Likewise, the doorknob collector  210  may be mounted on the top 108 or bottom plate  204 , depending on the configuration. However, in such a case, the doorknob collector  210  is located opposite the hole  404  through which the center conductor  212  passes. 
     In some embodiments, all of the monopole radiators  208  are mounted on either the top plate  108  or the bottom plate  204 , and the other one of the top plate  108  or the bottom plate  204  includes a hole  404  located substantially at a center of the top plate  108  or the bottom plate  204 . In such configurations, the transition center conductor  212  can extend through the hole  404  to an exterior of the radial waveguide cavity. 
     In some embodiments, the rectangular waveguide  506  may include a plurality of steps  508 ,  510 , and  512 , each subsequent step in the plurality of steps  508 ,  510 , and  512  expanding the cavity  514  of the rectangular waveguide  506  when compared to a previous step in the plurality of steps. 
     Use of language such as “at least one of X, Y, or Z” or “at least one or more of X, Y, or Z” are intended to convey a single item (just X, or just Y, or just Z) or multiple items (i.e., {X and Y}, {Y and Z}, or {X, Y, and Z}). “At least one of” is not intended to convey a requirement that each possible item must be present. 
     The various embodiments described above are provided by way of illustration only and should not be construed to limit the scope of the disclosure. Various modifications and changes may be made to the principles described herein without following the example embodiments and applications illustrated and described herein, and without departing from the spirit and scope of the disclosure.