Abstract:
A waveguide circulator comprising at least three waveguide arms intersecting at a junction, at least one ferrite element positioned within the junction, an impedance transformer and a recessed transformer. At least a portion of each of the at least three waveguide arms and the junction define a first wall and a second wall that are positioned in an opposing relationship. The impedance transformer is positioned in proximity to the at least one ferrite element and projects from the first wall. The recessed transformer is positioned in proximity to the impedance transformer and is recessed within the first wall.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of passive microwave components, and specifically to a waveguide circulator that includes at least one recessed transformer for improving the bandwidth handling capabilities of the waveguide circulator. 
       BACKGROUND OF THE INVENTION 
       [0002]    Waveguide circulators are known in the art for handling RF waves. Typically, waveguide circulators include three ports (although more or less ports are possible) and are used for transferring wave energy in a non-reciprocal manner, such that when wave energy is fed into one port, it is transferred to the next port only. A common use for waveguide circulators is to transmit energy from a transmitter to an antenna during transmitting operations, and to transmit energy from an antenna to a receiver during receiving operations. 
         [0003]    In order to enable the non-reciprocal energy transfer, the waveguide circulators include ferrite resonators to which are applied a magnetic field via one or more magnets or electromagnets. In order to match the impedance of the ferrite gyrator (which includes the ferrite resonators and their mounting posts) to the input waveguides, a matching network is inserted between them. However, in practice, a conventional circulator with a ferrite gyrator coupled to a ¼ wavelength transformer produces a frequency response of about 21 dB return loss over a 26% frequency bandwidth. This is not the desired handling of the circulator. 
         [0004]    In light of the above, there is a need in the industry for an improved waveguide circulator that alleviates, at least in part, the deficiencies with existing waveguide circulators. 
       SUMMARY OF THE INVENTION 
       [0005]    In accordance with a first broad aspect, the present invention provides a circulator comprising at least three waveguide arms intersecting at a junction, at least one ferrite element positioned within the junction, an impedance transforner and a recessed transformer. At least a portion of each of the at least three waveguide arms and the junction define a first wall and a second wall that are positioned in an opposing relationship. The impedance transformer is positioned in proximity to the at least one ferrite element and projects from the first wall. The recessed transformer is positioned in proximity to the impedance transformer and is recessed within the first wall. 
         [0006]    In accordance with a second broad aspect, the present invention provides a waveguide circulator comprising three waveguide arms each comprising a first wall and a second wall. The three waveguide arms intersect at a junction that includes at least one ferrite element therein. Each of the three waveguide arms further comprises an impedance transformer projecting from the first wall and a recessed transformer that is recessed within the second wall. 
         [0007]    In accordance with a third broad aspect, the present invention provides a method. The method comprises providing a circulator with at least three waveguide arms intersecting at a junction and a pair of ferrite elements positioned in a spaced-apart opposing relationship within the junction. Each of the at least three waveguide arms and the junction define a first wall and a second wall that are positioned in an opposing relationship. The method further comprises providing at least one impedance transformer within the circulator in proximity to at least one of the pair of ferrite elements. The impedance transformer projects from the first wall. The method further comprises providing at least one recessed transformer in proximity to the at least one impedance transformer. The recessed transformer is recessed within the first wall of the circulator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    In the accompanying drawings: 
           [0009]      FIG. 1  shows a perspective view of a waveguide circulator in accordance with a non-limiting example of implementation of the present invention; 
           [0010]      FIG. 2  shows a side plan view of the waveguide circulator of  FIG. 1 , exposing the interior of a waveguide arm; 
           [0011]      FIG. 3  shows a top perspective view of the waveguide circulator of  FIG. 1  with a top portion cut away in order to expose an interior of the waveguide circulator; 
           [0012]      FIG. 4  shows a top plan view of the portion of the waveguide circulator shown in  FIG. 3 ; 
           [0013]      FIG. 5  shows a cross-sectional view of the waveguide circulator of  FIG. 1 , taken along lines A-A; 
           [0014]      FIG. 6  shows a non-limiting flow diagram of a process for manufacturing a waveguide circulator in accordance with the present invention; and 
           [0015]      FIG. 7  shows a graph of Return Loss vs. Frequency for two waveguide circulators; 
       
    
    
       [0016]    Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
       DETAILED DESCRIPTION 
       [0017]    Shown in  FIGS. 1 through 5  is a waveguide circulator  10  in accordance with a non-limiting example of implementation of the present invention. As shown, the waveguide circulator  10  comprises three waveguide arms  12 ,  14  and  16  that meet at a common junction  18 . In the embodiment shown, the three waveguide arms  12 ,  14  and  16  are evenly spaced at 120° angles in relation to each other. Although three evenly spaced waveguide arms  12 ,  14  and  16  are shown in the Figures, it is within the scope of the present invention for the waveguide circulator to include more or less than three waveguide arms, as well as waveguide arms that are not evenly spaced. 
         [0018]    Positioned within the junction  18  of the waveguide circulator  10  are a pair of gyromagnetic members  20 , which are typically made of a ferrite material. The gyromagnetic members  20  are positioned within the junction  18  in a spaced-apart, opposing relationship, such that they are centrally disposed, and arranged symmetrically, with respect to the three waveguide arms  12 ,  14  and  16 . The gyromagnetic members  20  can be of a variety of shapes and/or sizes, depending on the desired characteristics of the waveguide circulator. For example, the gyromagnetic members can be of a triangular shape or a cylindrical disk shape, as shown in the Figures. For the remainder of the present description, the gyromagnetic members  20  will be referred to as ferrite elements  20 . 
         [0019]    During operation, the ferrite elements  20  are subjected to the influence of a magnetic field that is generated by one or more magnets or an electromagnet (not shown), which can be positioned above and below the ferrite elements  20 . The magnetic field that is generated is a unidirectional magnetic field, represented by arrow  22  in  FIG. 1 , such that wave energy entering each waveguide arm  12 ,  14  and  16  will move in a counter-clockwise direction towards it&#39;s neighboring waveguide arm. For example, wave energy from waveguide arm  12  propagates to waveguide arm  16 . Likewise, wave energy from waveguide arm  16  propagates to waveguide arm  14  and wave energy entering waveguide arm  14  propagates to waveguide arm  12 . In this manner, wave energy is always propagated in a single direction. As such, the waveguide circulator  10  is a non-reciprocal transmitter of electromagnetic wave energy propagating in the waveguide arms. By changing the direction of the magnetic field, it is possible for the wave energy to propagate in the opposite, clockwise, direction. However, regardless of the direction in which the wave energy is propagated, it can only ever travel in one direction at a time. 
         [0020]    Shown in  FIG. 2  is a side view of the waveguide circulator  10  with a view into waveguide arm  16 . The waveguide arms  12 ,  14  and  16  have a substantially rectangular cross section, defined by a base wall  30 , an upper wall  32  and two side walls  33 . The ratio of the sides of the rectangular waveguide arms  12 ,  14  and  16  (known as the aspect ratio) is generally in the order of 2:1, such that the waveguide arms can propagate wave energy in the transverse electric (TE10) mode. Although the waveguide arms shown are of a generally rectangular cross section, it should be appreciated that waveguide arms of other cross sections (such as oval, or circular) are also included within the scope of the present invention. 
         [0021]    As mentioned above, the ferrite elements  20  depicted in the Figures are of a cylindrical disk shape. The ferrite element  20  can be solid or small pieces tiled to form a disk shape, triangular shape or hexagonal shape. In addition, the two ferrite elements  20  shown are identical in diameter and thickness. In the non-limiting embodiment shown, the diameter is approximately a half wavelength at a selected frequency in the operational band of the circulator. The space between the ferrite elements  20  can vary, which will affect what is referred to as the “filling factor”. The separation between the ferrite elements  20  can be used to adjust the gain bandwidth, and the peak power handling of the design. The size of the ferrite elements  20  is dictated by the desired frequency of the circulator. 
         [0022]    Each of the ferrite elements  20  is mounted to a mounting post  24 , which in turn is mounted to a respective impedance transformer  26 . The mounting posts  24  hold each of the respective ferrite elements  20  in place, and form an electrical wall by making contact with the ferrite elements  20 . This arrangement provides a resonator with both a top and bottom electrical wall and a magnetic wall all around the formed effective resonator. 
         [0023]    The ferrite elements  20  have an intrinsic impedance that is different from the impedance of the feeding waveguide arms. As such, the two impedance transformers  26  are included within the waveguide circulator  10  in order to reduce the impedance of the waveguide circulator  10  at the location of the ferrite elements  20 . This acts to maximize the power transfer between the input and the output of the circulator as well as to minimize the internal reflection within the circulator. Therefore, the impedance transformers  26  are included in order to match the impedance of a gyrator (which is the combination of the ferrite elements  20  and the mounting posts  24 ) to the waveguide arms  12 ,  14  and  16 . This smooth transition of the impedance is performed by reducing the effective height of the waveguide circulator  10  in the region of the junction  18 . By reducing the height within the junction  18 , the impedance of this section is reduced to a certain value between the impedance of the circulator arms  12 ,  14  and  16  and that of the gyrator. 
         [0024]    As shown, the impedance transformers  26  are essentially formed from metal plates that reduce the height within the junction  18  of the waveguide circulator  10 . Depending on the operating frequency band, the impedance transformers  26  will have a different length and height (which translates into a different wavelength). In accordance with a non-limiting example of implementation, the impedance transformers  26  are ¼λ transformers. In accordance with a non-limiting example of implementation, this is achieved by reducing the aspect ratio within the waveguide arms to 4:1. However, it should be appreciated that any dimension and shape could be used without affecting the end result significantly. It should, however, be appreciated that the dimension of the waveguide is related to frequency. 
         [0025]      FIG. 3  shows a perspective view of the waveguide circulator  10  with the top portion removed, such that only the ferrite element  20 , the mounting post  24  and the impedance transformer  26  positioned on the base wall  30  are shown.  FIG. 4  shows a top plan view of the waveguide circulator  10  with the top portion removed. In accordance with the non-limiting embodiment shown, the impedance transformer  26  is in the shape of a single Y-shaped plate having three arms  26   a ,  26   b  and  26   c . The impedance transformer  26  may be positioned entirely within the junction  18 , or as shown in  FIGS. 3 and 4 , the three arms  26   a ,  26   b  and  26   c  of the impedance transformer  26  may extend into each waveguide arm  12 ,  14  and  16 , respectively. As such, the single impedance transformer  26  acts as three transformers  26   a ,  26   b  and  26   c , that are each respectively associated with one of the waveguide arms  12 ,  14  and  16 . In an alternative embodiment, it is possible to have three separate impedance transformers  26   a ,  26   b  and  26   c  that are not attached together, with each one of the impedance transformers corresponding to a respective one of the waveguide arms  12 ,  14  and  16 . 
         [0026]    Referring back to  FIG. 2 , the impedance transformers  26  project respectively from the base wall  30  and the upper wall  32  of the waveguide arms  12 ,  14 ,  16  and the junction  18 . In accordance with a non-limiting example of implementation, the impedance transformers  26  are integrally formed with the base wall  30  and upper wall  32  of the waveguide circulator  10 . However, it should be appreciated that the impedance transformers  26  could be formed as separate plates that are secured to the base wall  30  and the upper wall  32 , respectively, during the manufacturing process. 
         [0027]    As mentioned above, the inclusion of the impedance transformers  26  within the junction  18  of the waveguide circulator  10  reduces the height separating the ferrite elements  20 . This reduction in height, while working towards normalizing the impedance of the waveguide circulator  10 , reduces the power handling capabilities and the bandwidth handling capabilities of the waveguide circulator  10 . 
         [0028]    In order to increase the power and bandwidth handling capabilities, the waveguide circulator  10  in accordance with the present invention includes a set of three transformers  28   a ,  28   b  and  28   c  that are recessed within the base wall  30  of the waveguide arms  12 ,  14  and  16 , and a set of three transformers  28   a ,  28   b  and  28   c  that are recessed within the upper wall  32  of the waveguide arms  12 ,  14  and  16 . These transformers  28   a ,  28   b  and  28   c  enable the waveguide circulator  10  to be able to handle increased power and bandwidth. 
         [0029]    As shown with reference to  FIG. 5 , the three transformers  28   a ,  28   b  and  28   c  that are recessed within the base wall  30  and the three transformers  28   a ,  28   b  and  28   c  that are recessed within the upper wall  32 , are positioned in a substantially opposing relationship. As such, the height within the waveguide arms  12 ,  14  and  16  in the region where the transformers  28   a ,  28   b  and  28   c  are positioned opposite each other, is greater than the height in the remaining portions of the waveguide arms  12 ,  14  and  16 . More specifically, the waveguide circulator  10  of the present invention includes three different heights “h I” “h 2 ” and “h 3 ” within the three waveguide arms  12 ,  14 ,  16  and the junction  18 . The height “h 1 ” is the height between the two impedance transformers  26 , the height “h 2 ” is the height between the base wall  30  and the upper wall  32 , and the height “h 3 ” is the height between the two recessed transformers  28   c . As shown, the height “h 1 ” between the surfaces of the two impedance transformers  26  is less than the height “h 2 ” between the base wall  30  and the upper wall  32 , which is less than the height “h 3 ” between the surfaces of the two opposing recessed transformers  28   c.    
         [0030]    Referring back to  FIG. 4 , the transformers  28   a ,  28   b  and  28   c  each have a width “w 1 ” that is equivalent to the width of each of the impedance transformer arms  26   a ,  26   b  and  26   c . In accordance with the present invention, the transformers  28   a ,  28   b  and  28   c  are centered between the two side walls  33  of each waveguide arm  12 ,  14  and  16 , and have a width “w 1 ” of approximately 30-70% of the width “w 2 ” of the base wall  30  and upper wall  32 . As such, the transformers  28   a ,  28   b  and  28   c  are not as wide as the base wall  30  and the upper wall  32 , thereby leaving a small gap between each of the side walls  33  and the transformers  28   a ,  28   b  and  28   c . This gap provides a lower cut off frequency for the waveguide circulator, and the optimal width for this gap can be determined empirically for each waveguide circulator. 
         [0031]    The transformers  28   a ,  28   b  and  28   c  are positioned within the waveguide arms  12 ,  14  and  16 , respectively, at a further radial distance from the ferrite elements  20  than the impedance transformers  26 . In general, the transformers  28   a ,  28   b  and  28   c  are positioned at the characteristic plane of the gyrator, which may be determined by locating the position of the short circuit plane at one port with another one terminated in a short circuit piston. 
         [0032]    Each of the transformers  28   a ,  28   b  and  28   c  is recessed within the base wall  30  and the upper wall  32  of the waveguide arms  12 ,  14  and  16 . In accordance with a non-limiting example of implementation, the transformers  28   a ,  28   b  and  28   c  are recessed to a depth “d” in such a way to lower the impedance of this section between 5-15% from the normalized impedance of the circulator waveguide arm. In the case where the impedance transformers  26  are ¼λ transformers, it has been found that in order to satisfactorily improve the waveguide circulator&#39;s  10  power handling capabilities and bandwidth handling capabilities, the transformers  28   a ,  28   b  and  28   c  should be ½λ transformers. It should be appreciated that the transformers  28   a ,  28   b  and  28   c  could also be 1/12λ, ¼λ and ½λ, without departing from the spirit of the invention. The selection of recessed transformer wavelength can depend on a variety of factors, such as the size and wavelength of the impedance transformers  26  that are included within the waveguide circulator  10 . 
         [0033]    By adding the recessed transformers  28   a ,  28   b  and  28   c  to the waveguide circulator  10 , the passband return loss of the waveguide circulator  10  remains low, which results in a low reflected power. The reflected power must be kept below an acceptable level (so as not to damage the input source), and therefore, by maintaining the reflected power low, more power can be input into the waveguide circulator  10 , thus improving the waveguide circulator&#39;s power handling abilities. In addition to maintaining the return loss at a relatively low level, the recessed transformers  28   a ,  28   b  and  28   c  enable an increase in the bandwidth that can be handled by the waveguide circulator  10 . This will be described in more detail below with respect to the graph shown in  FIG. 7 . 
         [0034]    As mentioned above, in accordance with a non-limiting example of implementation, the impedance transformers  26  are ¼λ transformers and the recessed transformers  28   a ,  28   b  and  28   c  are ½λ transformers. Shown in  FIG. 7  is a graph that plots the “Return Loss (dB) vs Frequency” for a waveguide circulator that has only impedance transformers  26  (the dashed line  70 ), and a waveguide circulator  10 , in accordance with the present invention, that includes both impedance transformers  26  and recessed transformers  28   a ,  28   b  and  28   c  (the solid line  72 ). As shown, the bandwidth handling capabilities of the waveguide circulator  10  of the present invention are greater than those of the waveguide circulator that does not include the recessed transformers. More specifically, in accordance with the non-limiting example shown, the waveguide circulator that does not include the recessed transformers displays a bandwidth range (at a return loss of −23) of approximately 6.1-8.2 GHz, which translates into a percentage bandwidth value of approximately 26-30%. However, the waveguide circulator  10  in accordance with the present invention, displays a bandwidth range of approximately 5.5-8.8 GHz, which translates into a percentage bandwidth of approximately 42-48%. The percentage bandwidth value is calculated via the following formula: 
         [0000]    
       
         
           
             
               % 
                
               
                   
               
                
               bandwidth 
             
             = 
             
               
                 
                   
                     frequency 
                      
                     
                       ( 
                       high 
                       ) 
                     
                   
                   - 
                   
                     frequency 
                      
                     
                       ( 
                       low 
                       ) 
                     
                   
                 
                 
                   ( 
                   
                     
                       frequency 
                        
                       
                         ( 
                         high 
                         ) 
                       
                     
                     + 
                     
                       frequency 
                        
                       
                         ( 
                         low 
                         ) 
                       
                     
                   
                   ) 
                 
               
               / 
               2 
             
           
         
       
     
         [0035]    An advantage of having the ½λ transformers  28   a ,  28   b  and  28   c  be recessed within the base wall  30  and within the upper wall  32  is that the recessed transformers do not create additional impedance within the waveguide circulator  10 . In the case where a waveguide circulator includes ½λ transformers that project from the base wall  30  and the upper wall  32  of the waveguide arms (instead of being recessed within these walls), it is impossible for the ¼λ transformers to properly normalize the impedance created. As such, in the case where the ½λ transformers project within the waveguide arms  12 ,  14  and  16 , the ¼λ transformers are considered to be non-optimum transformers. Whereas, by recessing the ½% transformers within the base wall  30  and the upper wall  32 , this deficiency is eliminated, such that the ¼λ transformers can adequately match the impedance of the gyrator (which is a combination of the ferrite elements  20  and the mounting posts  24 ) and the ½λ transformers. 
         [0036]    In the case where only ferrite elements  20  are included within a waveguide circulator, the ferrite elements act as a resonator, such that the waveguide circulator displays a degree-1 response (1-pole). In the case where a waveguide circulator includes both the ferrite elements  20  and impedance transformers  26 , the waveguide circulator displays a degree-2 response (2-pole). The waveguide circulator  10  in accordance with the present invention includes both impedance transformers  26  and recessed transformers  28   a ,  28   b  and  28   c , thus displaying a degree-3 response (3-pole). This can be seen in the graph of  FIG. 7 , wherein the waveguide circulator with only the impedance transformers (line  70 ) that it is a 2-pole waveguide circulator has two peaks, and the waveguide circulator  10  with the impedance transformers  26  and the recessed transformers  28   a ,  28   b  and  28   c  that is a 3-pole waveguide circulator  10  has three peaks. 
         [0037]    Waveguide circulators  10  in accordance with the present invention can be manufactured via molding, casting, or machining, among other possible manufacturing techniques. Generally speaking, the waveguide circulators  10  are made in two separate portions; namely a bottom portion and an upper portion, that are then coupled together in order to form the complete waveguide circulator  10 . The bottom portion and the top portion can be coupled together via welding, bolts, rivets, or any other type of mechanical fastener known in the art. 
         [0038]    In accordance with a non-limiting example of implementation, the waveguide circulators  10  of the present invention are made of aluminum. However, it should be appreciated that the waveguide circulator  10  could be made of any suitable material, such as copper or brass, among other possibilities. 
         [0039]    In the case where the portions of the waveguide circulator  10  are manufactured via molding or casting, then the recessed transformers  28   a ,  28   b  and  28   c  can be created at the same time as the impedance transformers. However, in the case where the waveguide circulator  10  is made via machining, the recessed transformers  28   a ,  28   b  and  28   c  may be machined into the base wall  30  and the upper wall  32  of the waveguide arms  12 ,  14  and  16 , after the impedance transformers have been formed. Typically, the ferrite elements  20  are the final components to be included within the two portions of the waveguide circulator  10 . In some embodiments, a piece of dielectric can be inserted between the ferrite elements  20  (thus filling the gap between the ferrite elements  20 ) to increase the peak power handling of the waveguide circulator  10 . 
         [0040]    An advantage of recessed transformers  26   a ,  26   b  and  26   c  is that they can be added to existing waveguide circulator designs in order to improve the power handling capabilities and bandwidth handling capabilities of existing waveguide circulators  10 . More specifically, in order to add a recessed transformer to existing waveguide circulators, the waveguide circulator is taken apart, such that the base wall  30  and the upper wall  32  of the waveguides are exposed. The recessed transformers  28   a ,  28   b  and  28   c  can then be machined into the surfaces of these two walls. 
         [0041]    Shown in  FIG. 6  is a non-limiting flow diagram of a method of retrofitting a waveguide circulator  10  with recessed transformers  28   a ,  28   b  and  28   c . Firstly, at step  60 , the method involves providing a waveguide circulator  10  that comprises at least three waveguide arms  12 ,  14  and  16 , at least one ferrite element  20  (which is preferably in the shape of a cylindrical disk) and at least one impedance transformer  26 . Although the ferrite elements are included in the flow chart of  FIG. 6 , it is possible that the ferrite elements are added following step  62  of the method. At step  62 , once the waveguide circulator has been taken apart so as to expose the base wall  30  and the upper wall  32 , the method involves providing at least one recessed transformer within one of the waveguide arms  12 ,  14  and  16 . 
         [0042]    In the case where the recessed transformers  28   a ,  28   b  and  28   c  are retro-fit into existing waveguide circulators  10 , they are generally machined into the base wall  30  and upper walls  32  of the waveguide arms  12 ,  14  and  16 , once the waveguide transformer has been taken apart. 
         [0043]    In the above description, only three ports (waveguide arms  12 ,  14  and  16 ) have been shown and discussed. It should however be appreciated that the recessed transformers  28   a - c  shown and described herein could be equally applied to T-junction circulators, four-port circulators, or circulators having any number of ports. 
         [0044]    Although the present invention has been described in considerable detail with reference to certain preferred embodiments thereof, variations and refinements are possible without departing from the spirit of the invention, Therefore, the scope of the invention should be limited only by the appended claims and their equivalents.