Patent Publication Number: US-7719391-B2

Title: Dielectric resonator circuits

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention pertains to dielectric resonator circuits. More particularly, the invention pertains to dielectric resonator circuits comprising housings adapted to prevent cross coupling between non-adjacent resonators. 
     2. Background 
     Dielectric resonators are used in many circuits, particularly microwave circuits, for concentrating electric fields. They can be used to form filters, oscillators, triplexers, and other circuits. The higher the dielectric constant of the material out of which the resonator is formed, the smaller the space within which the electric fields are concentrated. Suitable dielectric materials for fabricating dielectric resonators are available today with dielectric constants ranging from approximately 10 to approximately 150 (relative to air). These dielectric materials generally have a magnetic constant of 1, i.e., they are transparent to magnetic fields. 
     Generally, as the dielectric constant of the material of the resonators increases, higher center frequencies of the given circuit can be achieved. 
       FIG. 1  is a perspective view of a typical dielectric resonator of the prior art. As can be seen, the resonator  10  is formed as a cylinder  12  of dielectric material with a circular, longitudinal through hole  14 . Individual resonators are commonly called “pucks” in the relevant trades. While dielectric resonators have many uses, their primary use is in connection with microwaves and, particularly, in microwave communication systems and networks. 
     As is well known in the art, dielectric resonators and resonator filters have multiple modes of electrical fields and magnetic fields concentrated at different center frequencies. A mode is a field configuration corresponding to a resonant frequency of the system as determined by Maxwell&#39;s equations. In a dielectric resonator, the fundamental resonant mode frequency, i.e., the lowest frequency, is the transverse electric field mode, TE 01δ  (or TE, hereafter). The second mode is commonly termed the hybrid mode, H 11δ  (or H 11 , hereafter). The H 11  mode is excited from the dielectric resonator, but a considerable amount of electric field lays outside the resonator and, therefore, is strongly affected by the cavity. The H 11  mode is the result of an interaction of the dielectric resonator and the cavity within which it is positioned. The H 11  mode field is orthogonal to the TE mode field. There also are additional higher modes. 
     Typically, it is the fundamental TE mode that is the desired mode of the circuit or system into which the resonator is incorporated. However, other modes, and particularly the H 11  mode, often are used in the proper circumstances, such as dual mode filters. Typically, all of the modes other than the mode of interest, e.g., the TE mode, are undesired and constitute interference. 
       FIG. 2  is a perspective view of a dielectric resonator filter  20  of the prior art employing a plurality of dielectric resonators  10   a ,  10   b ,  10   c ,  10   d . The resonators  10   a ,  10   b ,  10   c ,  10   d  are arranged in the cavity  22  of a conductive enclosure  24 . The conductive enclosure  24  typically is rectangular, as shown in  FIG. 2 . Microwave energy is introduced into the cavity via an input coupler  28  coupled to a cable, such as a coaxial cable. The energy may then be coupled to a first resonator (such as resonator  10   a ) using a coupling loop. 
     The high dielectric constant of the material out of which the resonators are formed concentrates the electrical fields within the resonators. However, most dielectric resonators have a magnetic constant of 1, i.e., they are transparent to the magnetic fields. Accordingly, the magnetic fields exist mostly outside of the resonator bodies. The electromagnetic coupling between the resonators that occurs in multi resonator circuits such as illustrated in  FIG. 2  is magnetic field coupling. As is well known, the magnetic fields are orthogonal to their associated electrical fields. 
     Conductive separating walls  32  separate the resonators from each other and block (partially or wholly) magnetic field coupling between physically adjacent resonators  10   a ,  10   b ,  10   c ,  10   d . Particularly, irises  30   a ,  30   b ,  30   c  in walls  32   a ,  32   b ,  32   c ,  32   d  control the coupling between adjacent resonators  10   a ,  10   b ,  10   c ,  10   d . Conductive walls without irises generally prevent any coupling between the resonators separated by the walls, while walls with irises allow some coupling between these resonators. Specifically, conductive material within the electric field of a resonator essentially absorbs the ohmic component of the field coincident with the material and turns it into a current in the conductive material. In other words, conductive materials within the electric fields cause losses in the circuit. 
     Conductive adjusting screws (not shown) in conductive contact with the enclosure may be placed in the irises to further affect the coupling of the fields between adjacent resonators and provide adjustability of the coupling between the resonators, but are not used in the example of  FIG. 2 . When positioned within an iris, a conductive screw partially blocks the coupling between adjacent resonators permitted by the iris between them. Inserting more of the conductive screw into the iris reduces electric coupling between the resonators while withdrawing the conductive screw from the iris increases electric coupling between the resonators. 
     By way of example, the field of resonator  10   a  couples to the field of resonator  10   b  through iris  30   a , the field of resonator  10   b  further couples to the field of resonator  10   c  through iris  30   b , and the field of resonator  10   c  further couples to the field of resonator  10   d  through iris  30   c.    
     Wall  32   a , which does not have an iris or a cross-coupler, entirely prevents the field of resonator  10   a  from coupling with the physically adjacent resonator  10   d  on the other side of the wall  32   a . Furthermore, resonator  10   a  does not appreciably couple with resonator  10   c  and resonator  10   b  does not appreciably couple with resonator l 0   d  because of 1) the various blocking walls  32   a ,  32   b ,  32   c ,  32   d  and 2) the significant distance between the resonators that the field lines would have to traverse in order to get around those walls to couple with each other. 
     One or more metal plates  42  may be positioned adjacent each resonator to affect the field of the resonator to set the center frequency of the filter. Particularly, plate  42  may be mounted on a screw  44  passing through a top surface (not shown) of the enclosure  24 . The screw  44  may be rotated to vary the spacing between the plate  42  and the resonator  10   a ,  10   b ,  10   c , or  10   d  to adjust the center frequency of the resonator. A coupling loop connected to an output coupler  40  is positioned adjacent the last resonator  10   d  to couple the microwave energy out of the filter  20 . Signals also may be coupled into and out of a dielectric resonator circuit by other methods, such as microstrips positioned on the bottom surface  44  of the enclosure  24  adjacent the resonators. 
     The sizes of the resonators  10   a ,  10   b ,  10   c ,  10   d , their relative spacing, the number of resonators, the size of the cavity  22 , the size of the irises  30   a ,  30   b ,  30   c , and the size and position of the metal plates  42  all need to be precisely controlled to set the desired center frequency of the filter, the bandwidth of the filter, and the rejection in the stop band of the filter. More specifically, the bandwidth of the filter is controlled primarily by the amount of coupling of the electric and magnetic fields between the resonators. Generally, the closer the resonators are to each other, the more coupling between them and the wider the bandwidth of the filter. On the other hand, the center frequency of the filter is controlled in large part by the size of the resonator and the size and the spacing of the metal plates  42  from the corresponding resonators  10   a ,  10   b ,  10   c , or  10   d.    
     Thus, while the presence of separating walls such as walls  32   a ,  32   b ,  32   c ,  32   d  may be desirable in order to control the coupling between the adjacent resonators to the desired level, they generally lower the quality factor Q of the circuit. Q essentially is an efficiency rating of the system and, more particularly, is the ratio of stored energy to lost energy in the system. Parts of the fields generated by the resonators pass through all of the conductive components of the system, such as the enclosure, separating walls tuning plates, and adjusting screws and inherently generate currents in those conductive elements. Those currents essentially comprise energy that is lost to the system. 
     Occasionally, controlled cross coupling between non-adjacent resonators is desirable and can be provided by the incorporation of cross coupling mechanisms. For instance, U.S. Pat. No. 7,057,480 issued Jun. 6, 2006, which is incorporated fully herein by reference, discloses various mechanisms for cross-coupling a non-adjacent resonators in a resonator circuit. 
     However, in the majority of dielectric resonators filters and other circuits, cross coupling between non-electrically adjacent resonators is not desired. 
     Therefore, it is an object of the present invention to provide an improved dielectric resonator circuit. 
     It is a further object of the present invention to provide a dielectric resonator circuit having improved coupling isolation between non-adjacent resonators. 
     SUMMARY OF THE INVENTION 
     The invention is a dielectric resonator circuit comprising a housing and first, second, and third resonators positioned substantially in a row within the housing with the second resonator positioned between the first and third resonators, wherein the resonators are positioned relative to each other such that a field generated in each resonator couples to an adjacent resonator, wherein the housing encloses the resonators and has a separating wall positioned between the first and third resonators in order to control electromagnetic coupling between the first and third resonators; and wherein the first separating wall comprises a first end and a second end along a length thereof and wherein the separating wall defines an iris at the first end, the wall comprising a main wall portion positioned substantially between the first and third resonators and an extension wall portion at the first end that extends at an angle from the main wall portion of the wall. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a cylindrical dielectric resonator of the prior art. 
         FIG. 2  is a perspective view of an exemplary cross-coupled dielectric resonator filter of the prior art. 
         FIG. 3  is a perspective view of a conical dielectric resonator. 
         FIG. 4A  is a top view of a dielectric resonator filter comprising a plurality of resonators arranged in a single row. 
         FIG. 4B  is a perspective view of the filter of  FIG. 4A . 
         FIG. 5  is a top view of a dielectric resonator circuit comprising a plurality of resonators arranged in a single row and incorporating the principles of the present invention. 
         FIG. 6A  is a drawing illustrating the interaction of magnetic fields of the two non adjacent resonators in a dielectric resonator circuit of the prior art. 
         FIG. 6B  is a drawing illustrating the interaction of magnetic fields of the two non adjacent resonators in a dielectric resonator circuit in accordance with the present invention. 
         FIG. 7  is a top plan view of a dielectric resonator circuit in accordance with an alternate embodiment of the present invention. 
         FIG. 8  is a top plan view of a dielectric resonator circuit in accordance with a third embodiment of the present invention. 
         FIG. 9  is a top plan view of a dielectric resonator circuit in accordance with a fourth embodiment of the present invention. 
         FIG. 10  is a top plan view of a dielectric resonator circuit in accordance with a fifth alternate embodiment of the present invention. 
         FIG. 11  is a top plan view of a dielectric resonator circuit in accordance with a sixth alternate embodiment of the present invention. 
         FIG. 12  is a top plan view of a dielectric resonator circuit in accordance with a seventh alternate embodiment of the present invention. 
         FIG. 13  is a top plan view of a dielectric resonator circuit in accordance with an eighth alternate embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is an improved dielectric resonator housing and dielectric resonator circuit in which the separating walls between non-adjacent resonators that define the irises for permitting adjacent resonators to electromagnetically couple are designed to include a first wall portion substantially parallel to the longitudinal axes of those non-adjacent resonators and an extension wall portion extending at an angle from the first wall portion. The extension wall portion preferably comprises two halves that are mirror images of each other about the plane defined by the first wall portion. Specific separating wall shapes include Y-shaped and T-shaped walls. In a preferred embodiment, each separating wall actually comprises two completely separate walls that define an open space there between, that open space having a length running along the longitudinal axis of a resonator that is intended to electromagnetically couple to the resonators on either side thereof. These separating walls permit essentially unfettered coupling between the adjacent resonator pairs, but substantially block electromagnetic coupling between the non-adjacent resonator pairs. 
     In the dielectric resonator circuit illustrated in  FIG. 2  having a square housing with four resonator pucks and separating walls, the non-adjacent resonators are quite well isolated from each other in order to prevent cross coupling between non-adjacent resonators. However, in a resonator circuit in which three or more resonators are arranged in a row, it is more difficult to provide isolation between the non-adjacent resonators in the line. Particularly, when three or more resonators are in a row, there is a relatively direct path for electromagnetic coupling between the two non-adjacent resonators through the two irises or other openings that permit the adjacent resonators to couple with each other. 
     U.S. Pat. No. 7,310,037, issued Dec. 18, 2007, entitled Dielectric Resonators And Circuits Made Therefrom which is fully incorporated herein by reference, discloses new dielectric resonators as well as circuits using such resonators. One of the key features of the new resonators disclosed in the aforementioned patent application is that the field strength of the TE mode field outside of and adjacent the resonator varies along the longitudinal dimension of the resonator. As disclosed in the aforementioned patent application, a key feature of these new resonators that helps achieve this goal is that the cross-sectional area of the resonator measured parallel to the electric field lines of the TE mode varies along the longitude of the resonator, i.e., perpendicularly to the TE mode electric field lines. In one embodiment, the cross-section varies monotonically as a function of the longitudinal dimension of the resonator, i.e., the cross-section of the resonator changes in only one direction (or remains the same) as a function of height. In one preferred embodiment, the resonator is conical, as discussed in more detail below. Preferably, the cone is a truncated cone. 
       FIG. 3  is a perspective view of an exemplary embodiment of a dielectric resonator disclosed in the aforementioned patent application. As shown, the resonator  300  is formed in the shape of a truncated cone  301  with a central, longitudinal through hole  302 . This design has many advantages over conventional, cylindrical dielectric resonators, including physical separation of the H 11  mode from the TE mode and/or almost complete elimination of the H 11  mode. Specifically, the TE mode electric field tends to concentrate in the base  303  of the resonator while the H 11  mode electric field tends to concentrate at the top  305  (narrow portion) of the resonator. The longitudinal displacement of these two modes improves performance of the resonator (or circuit employing such a resonator) because the conical dielectric resonators can be positioned adjacent other microwave devices (such as other resonators, microstrips, tuning plates, and input/output coupling loops) so that their respective TE mode electric fields are close to each other and therefore strongly couple, whereas their respective H 11  mode electric fields remain further apart from each other and, therefore, do not couple to each other nearly as strongly, if at all. Accordingly, the H 11  mode would not couple to the adjacent microwave device nearly as much as in cylindrical resonators, where the TE mode and the H 11  mode are physically located much closer to each other. 
     In addition, the mode separation (i.e., frequency spacing between the modes) is increased in a conical resonator. Even further, the top of the resonator may be truncated or the through hole may be counterbored with a larger diameter near the top to eliminate much of the portion of the resonator in which the H 11  mode field would be concentrated, thereby substantially attenuating the strength of the H11 mode. 
     Some of the concepts of the present invention are particularly useful when used in connection with conical resonators such as disclosed in U.S. Pat. No. 7,310,031, but also are applicable to more conventional cylindrical resonators, such as illustrated in  FIG. 1 . 
       FIGS. 4A and 4B  depict a top view and a perspective view, respectively, of a dielectric resonator filter  400  in which a plurality of conical resonators  402   a ,  402   b ,  402   c ,  402   d ,  402   e ,  402   f ,  402   g ,  402   h  are disposed in a single row running in a first direction as illustrated by arrow  405 . The filter  400  comprises an enclosure or housing  401  having a bottom  401   a , side walls  401   b , end walls  401   c  and a top wall (not shown in order to permit viewing of the components inside of the housing) to form a complete enclosure. Resonators  402   a ,  402   b ,  402   c ,  402   d ,  402   e ,  402   f ,  402   g ,  402   h  are positioned within the enclosure  401  for processing a field received within the cavity of the filter  400 . 
     A field may be coupled into the filter  400  through any reasonable means, including by forming microstrips on a surface of the enclosure or by use of coupling loops as described in the background section of this specification. In one embodiment, a field supplied from a coaxial cable is coupled to an input coupling loop  408  ( FIG. 4A ) positioned near the first resonator  402   a  and passed at an output coupling loop  410  positioned near the last resonator  402   h.    
     The plurality of resonators  402   a ,  402   b ,  402   c ,  402   d ,  402   e ,  402   f ,  402   g ,  402   h  are arranged within the enclosure in any configuration suitable to achieve the performance goals of the filter. In the illustrated embodiment, the resonators  402   a ,  402   b ,  402   c ,  402   d ,  402   e ,  402   f ,  402   g ,  402   h  are positioned in a row as previously mentioned. Specifically, the resonators  402   a ,  402   b ,  402   c ,  402   d ,  402   e ,  402   f ,  402   g,    402   h  are positioned with their longitudinal axes  403  ( FIG. 4A ) parallel and coplanar with each other (that plane being the plane of the page in  FIG. 4A ). However, their longitudinal axes are not collinear, i.e., they are not stacked longitudinally, but, rather, are positioned generally laterally (side-by-side) with each other. As will be described in detail below, the resonators  402   a ,  402   b ,  402   c ,  402   d ,  402   e ,  402   f ,  402   g ,  402   h  may be moved along their longitudinal axes  403  for tuning purposes (i.e., to adjust the bandwidth of the filter). The resonators  402   a ,  402   b ,  402   c ,  402   d ,  402   e ,  402   f ,  402   g ,  402   h  are positioned to permit electromagnetic field coupling between adjacent resonators, i.e., resonators having longitudinal axes that are closest in a linear direction substantially perpendicular to their longitudinal axes (e.g., resonator pairs  402   a  and  402   b , resonator pairs  402   b  and  402   c , etc.). Cross coupling between non-adjacent resonators is not desired in this particular circuit design, i.e., resonators having longitudinal axes that are on opposite sides of the longitudinal axis of another resonator in a linear direction substantially perpendicular to their longitudinal axes (e.g., resonators  402   a ,  402   c ). In at least one preferred embodiment, the resonators have a dielectric constant of at least 45 and are formed of barium tetratitanate. 
     Preferably, each resonator  402   a ,  402   b ,  402   c ,  402   d ,  402   e ,  402   f ,  402   g ,  402   h  is longitudinally inverted relative to its adjacent resonator or resonators. Thus, resonator  402   a  is right side up, resonator  402   b  is upside down, resonator  402   c  is right side up, etc. This arrangement permits the resonators to be placed in closer proximity to one another than in the prior art, thus smaller enclosures  401  are obtainable. 
     In order to prevent cross coupling between non-adjacent resonators, the housing includes separating walls  430  intermediate non-adjacent resonators in direction  405 . Each separating wall  430   b ,  430   c ,  430   e ,  430   f ,  430   g  is parallel to and in the same plane as the longitudinal axis of one of the resonators  402   a ,  402   b ,  402   c ,  402   d ,  402   e ,  402   f ,  402   g ,  402   h  and is substantially perpendicular to direction  405  such that each resonator  402   b - 402   g  has an associated separating wall  430   b - 430   g  that essentially is intended to block coupling between the two resonators on either side of that wall. Thus, for example, separating wall  430   b  helps prevent cross coupling between non-adjacent resonators  402   a  and  402   c  while substantially permitting coupling between the associated resonator  402   b  and its adjacent resonators  402   a  and  402   c . Likewise, separating wall  430   c  helps prevent cross coupling between non-adjacent resonators  402   b  and  402   d , while substantially permitting coupling between adjacent resonators  402   b  and  402   c  as well as  402   c  and  402   d . The first and last resonator  402   a  and  402   h  do not have associated separating walls for obvious reasons. However, including separating walls associated with the first and last resonators would have little or no impact on circuit performance. Such separating walls may be included due to practical fabrication reasons. Particularly, the housing is designed to be extremely flexible so as to permit the construction of many different filters with different numbers of resonators and different sized resonators with different resonator spacings while using a single generic housing design. For instance, if fewer or more resonators than shown in these figures are desired, if separating walls are provided associated with all of the resonator mounting positions, including the first and last, then the housing can simply be shortened or lengthened without changing any other design specification of the housing to accommodate any number of resonators. 
     A tuning plate  440  is positioned opposite the bottom surface  406   a  of each resonator  402   a ,  402   b ,  402   c ,  402   d ,  402   e ,  402   f ,  402   g ,  402   h  in a through hole  444  in the side wall  401   b  of the housing  401 . Alternately, the tuning plate may be placed adjacent the top surface  406   b  of the resonator. The tuning plate can be used to tune the center frequency of each resonator as described above in connection with  FIG. 2 . The tuning plate may be externally threaded and positioned within a matingly threaded through hole  444  in the housing so as to permit it to be moved longitudinally closer or farther from the associated resonator in order to effect tuning. 
     Each resonator  402   a ,  402   b ,  402   c ,  402   d ,  402   e ,  402   f ,  402   g ,  402   h  is coupled to the enclosure  401  via a mounting member, such as mounting post  414 . The mounting post  414  is parallel to the longitudinal axis of the resonator  402   a ,  402   b ,  402   c ,  402   d ,  402   e ,  402   f ,  402   g ,  402   h  it mounts and, preferably, is coaxial thereto. The mounting post  414  in the illustrated embodiment is adjustable to position the resonator  402   a ,  402   b ,  402   c ,  402   d ,  402   e ,  402   f ,  402   g ,  402   h  for tuning and, preferably, is non-conductive to prevent interference with the coupling between the adjacent and alternate resonators. 
     In the illustrated embodiment, the displacement of the resonators  402   a ,  402   b ,  402   c ,  402   d ,  402   e ,  402   f ,  402   g ,  402   h  relative to each other is fixed in the transverse direction upon assembly, but is adjustable in the longitudinal direction after assembly. Particularly, in one embodiment, the mounting posts  414  are screwed into threaded holes, such as threaded hole  416  in the side wall;  401   b  of the enclosure  401 . The resonators  402   a ,  402   b ,  402   c ,  402   d ,  402   e ,  402   f ,  402   g ,  402   h  also may be adjustably mounted on the mounting posts  414 . Particularly, the through holes  404  in the resonators  402   a ,  402   b ,  402   c ,  402   d ,  402   e ,  402   f ,  402   g ,  402   h  may also be threaded to mate with the threads of the mounting posts  414 . Accordingly, by rotating the mounting cylinder relative to the holes in the enclosure  401  and/or the through holes in the resonators  402   a ,  402   b ,  402   c ,  402   d ,  402   e ,  402   f ,  402   g ,  402   h , the longitudinal positions of the resonators relative to each other and to the enclosure  401  can be adjusted easily. 
     The mounting posts  414  pass through the separating walls  430   b ,  430   c ,  430   e ,  430   f ,  430   g  associated with the corresponding resonator. 
     In a preferred embodiment, the holes  416  in the enclosure are through holes, i.e., they pass completely through the separating walls, and the mounting posts  414  are long enough to pass completely through the length of the separating walls  430  and to the outside of the enclosure  401 . This enables the resonator spacing, and thus the bandwidth of the filter, to be adjusted by rotating the mounting cylinders that protrude from the enclosure without even opening the enclosure  401 . 
     The design shown in  FIGS. 4A and 4B  is extremely flexible and permits the construction of a wide variety of filters having different center frequencies and bandwidths with a single basic design. Some of the features of this design that enable such flexibility are the threaded adjustable mounting posts, the separating walls between the non-adjacent resonators, the longitudinally adjustable tuning plates, and the fact that the resonators are positioned in a single row. 
     Aforementioned U.S. Pat. No. 7,057,480 issued Jun. 6, 2006 discloses a very similar looking circuit, but in which cross-coupling between non-adjacent resonators, is encouraged. In U.S. Pat. No. 7,057,480, cross-coupling is induced by designing the mounting posts as hollow cylinders having internal threads and placing a conductive cylinder having external threads for mating with the internal threads of the hollow mounting post inside of the hollow resonator mounting post. By turning the conductive cylinder within the hollow mounting cylinder, the position of the conductive cylinder is altered such that more or less of the conductive cylinder is inserted between the resonators on either side of the conductive cylinder, thereby affecting the cross-coupling between the alternate resonators separated by the conductive cylinder. Since the conductive member is isolated from the enclosure (which is grounded) by the non-conductive mounting member, generated charges in the conductive member do not flow to ground. Instead, the charges are stored in the conductive member to produce capacitive cross-coupling between the non-adjacent resonators. 
     The circuit of  FIGS. 4A and 4B  of the present application is a more conventional circuit in which there is to be no cross-coupling between non-adjacent resonators. Accordingly, as noted above, the mounting posts  414  are non-conductive and do not contain any conductive inserts or cores. Accordingly, cross-coupling between non-adjacent resonators is not encouraged. 
     Nevertheless, because, in this single row design, there is a relatively direct path for electromagnetic coupling between two non-adjacent resonators through the irises or other openings that permit the adjacent resonators to couple with each other, as illustrated by arrow  439  ( FIG. 4A ), non-negligible cross-coupling between non-adjacent resonators can occur. This would adversely affect the desired operation of the circuit. 
     Generally, undesired cross-coupling between non-adjacent resonators is not appreciable when the dielectric resonators of the circuit have a relatively high dielectric constant, approximately 45 or greater. Also, if the horizontal spacing between the resonators is large enough, cross-coupling between non-adjacent resonators also is not appreciable. 
     However, many circuit designs call for, or at least utilize, dielectric resonators with dielectric constants lower than about 45. For instance, providing very high quality factor, Q, is often a key concern in dielectric resonator circuit design. Generally, higher Q can be provided by using lower dielectric constant materials for the dielectric resonators. Furthermore, generally, lower dielectric constant materials are used in circuits with lower center frequencies. 
     The lower the dielectric constant of the resonator material, the less concentrated the electric field is within the resonator. The concentration of the magnetic fields (i.e., the fields that actually couple between separate resonators) are proportional to their corresponding electrical field. Accordingly, the lower the dielectric constant of the resonator material, the more spread out the magnetic field. Hence, the lower the dielectric constant of the resonator material, the closer the horizontal spacing between the resonators that will be necessary to achieve a given circuit&#39;s objectives. Accordingly, in circuits utilizing dielectric resonators with dielectric constants of less than about 45, undesired cross coupling between non-adjacent resonators can be problematic. 
     Cross-coupling between non-adjacent resonators can be reduced or even eliminated by making the separating walls longer (and, consequently, the irises smaller). However, making the separating walls longer has several adverse effects. Most notably, it will decrease the Q of the circuit because it will place more metal closer to the resonators. Furthermore, although less of an concern than the effect on the Q of the circuit, it also will decrease coupling between the adjacent resonators. 
       FIG. 5  is a plan view of a dielectric resonator circuit similar to the circuit shown in  FIGS. 4A and 4B , but incorporating technology in accordance with an embodiment of the present invention that reduces or entirely eliminates cross-coupling between the nonadjacent resonators. In order not to obfuscate the invention,  FIG. 5  shows only the housing, including the separating walls, and the resonators  502 . All other structure, such as mounting posts and input and output couplers, are not shown for sake of clarity. The separating walls  430   b ,  430   c ,  430   e ,  430   f ,  430   g  of  FIGS. 4A and 4B  have been replaced with separating walls  530  that terminate (at the end of the wall adjacent the iris), with an extension wall portion that extends at least in part at an angle (other than 0 degrees) from the main wall portion. Preferably, the extension wall portion comprises two halves that are mirror images of each other, the plane of reflection being the plane of the main wall portion. Also preferably, each half of the extension comprises a surface that extends in a direction parallel to the side wall of the resonator that it is intended to prevent cross coupling from. In the particular embodiment illustrated in  FIG. 5 , the separating walls  530  include two mirror image legs,  530   a  and  530   b , each extending at an angle to each other and at an angle to the main wall portion  530   c . Overall, the separating wall  530  in this particular embodiment is Y-shaped. In a preferred embodiment of the invention, the two legs  530   a  and  530   b  extend at angles from the main wall portion  530   c  such that their sides are parallel to the sides of the resonators  502  that are to the corresponding side of the wall  530 . 
     Particularly, this is an advantageous angle for at least two reasons. First, this helps maximize the portion of the magnetic field that might otherwise extend all the way to the next non-adjacent resonator that instead intersects the separating wall  530  (and, therefore, essentially is lost and, hence, cannot cross couple with another resonator). Second, the inside planar surfaces  532  of the legs  530   a ,  530   b  define a space  533  generally between leg  530   a , leg  530   b , and the top surface of the associated resonator. This open space is advantageous because metal near the top of the resonator body would substantially reduce the Q of the circuit. 
       FIGS. 6A and 6B  helps illustrate the effectiveness of the present invention.  FIG. 6A  illustrates the magnetic field coupling between two nonadjacent resonators  602   a  and  602   c  in a dielectric resonator circuit utilizing a housing having conventional straight separating walls  630  like those illustrated in  FIGS. 4A and 4B . The middle resonator has been removed so as not to obfuscate the illustration. The first resonator has been excited with a field at a center frequency of 2.5135 GHz.  FIG. 6A  shows that there is coupling of this field to the non-adjacent resonator  602   c  around the separating walls  630 . The path is illustrated generally by path  654 . Simulations demonstrate that coupling is about 0.6 MHz. 
       FIG. 6B  illustrates the magnetic field coupling between two nonadjacent resonators  602   a ′ and  602   c ′ in a dielectric resonator circuit utilizing a housing having Y shaped separating walls  630 ′ like those illustrated in the embodiment of the invention shown in  FIG. 5 . As can be seen, the Y-shaped wall  630 ′ in the middle, and particularly the extending legs  630   b ′,  630   c ′ that are parallel to the side surfaces of the resonator  602   a ′,  602   c ′, respectively, substantially intersect that portion of the magnetic field lines of resonator  602   a ′ that might otherwise couple to non-adjacent resonator  602   c ′. On the other hand, wall  630 ′ substantially does not block the portion of the magnetic field lines that pass through and, therefore, would couple with the adjacent resonator (not shown). Path  655  illustrates the changes to the field lines relative to the  FIG. 6A  simulation. There is essentially no coupling between the non-adjacent resonators  602   a ′ and  602   c ′.    
     Further, note that the area between the legs  630   b ′,  630   c ′ define an open space  633  near the top of the intermediate resonator (which is not shown in  FIG. 6B  so as not to obfuscate the illustration) so as not to substantially reduce the Q of the resonator  602   b  or the overall circuit (because there is no metal near the resonator). 
     The Y-shaped wall configuration, while particularly advantageous, especially in connection with conical resonators, is merely exemplary. Other wall configurations are possible. Particularly,  FIG. 7  is a plan view of a dielectric resonator circuit illustrating an alternative embodiment of the invention in which the separating walls  730  are T-shaped, comprising main wall portion  730   c  and perpendicular extensions  730   a ,  730   b . A T-shaped separating wall is particularly suitable for use in dielectric resonator circuits employing cylindrical resonators. Again, this particular T-shaped configuration will be especially effective in blocking the portion of the magnetic field lines of the resonators  702  that would otherwise couple to non-adjacent resonators, while substantially not blocking the portion of the magnetic field that couples to the adjacent resonators.  FIG. 8  illustrates yet another embodiment of the invention. In this embodiment, the separating walls  830  includes two parallel straight portions  830   a ,  830   b  defining a space  833  there between. The two wall portions  830   a ,  830   b  are substantially parallel to the longitudinal axis of the associated resonator and on opposing sides thereof. The longitudinal axis of the associated resonator runs down the middle of the gap. 
     Note that this embodiment provides open space  833  above the longitudinal end of the middle resonator along and surrounding the longitudinal axis of that resonator, while simultaneously providing conducting surfaces near the resonators on either side of the middle resonator. Furthermore, in the case of cylindrical resonators, these wall portions  830   a ,  830   b  are parallel to the side walls of those side resonators. This particular separating wall shape, however, is also highly effective in connection with conical resonators. 
       FIG. 9  illustrates an even further embodiment of the invention in which each separating wall  930  comprises two L-shaped portions  930   a ,  930   b  defining a space  933  there between. This is quite similar to the T-shaped embodiment of  FIG. 7 , except in addition, it provides additional open space along and adjacent the longitudinal axis of the middle resonator. 
       FIG. 10  shows another embodiment similar to the Y-shaped wall embodiment of  FIG. 5 , except, like the embodiment of  FIG. 9 , the separating wall  1030  comprises two mirror image, half Y portions  1030   a  and  1030   b  providing open space  1033  there between. This is similar to the embodiment of  FIG. 5  comprising Y-shaped walls  530 , except that it provides even more open space along the longitudinal axis of the middle resonator. Specifically, in addition to providing triangular open space  1033  similar to the triangular open space  533  in the  FIG. 5  embodiment, it also provides additional open space  1034  along and adjacent the longitudinal axis of the middle resonator further away from the end of the resonator. Like the embodiment of  FIG. 5 , this embodiment is particularly effective in circuits having conical resonators and particularly, if the extensions are parallel to the side walls of the resonators. 
       FIG. 11  illustrates yet a further embodiment of the invention in which each separating wall  1130  generally has a shape similar to that of an American football goalpost. In the particular embodiment illustrated by  FIG. 11 , it comprising two wall portions  1130   a ,  1130   b , each comprising three segments  1151 ,  1152 ,  1153 , as shown. This embodiment is suitable for use in connection with both cylindrical resonators and conical resonators. Note that it provides substantial open space  1133  along and adjacent the longitudinal axis of the middle resonator. Specifically, immediately adjacent the longitudinal end of the middle resonator, the portion of the open space  1133   a  between the two walls has a width of x, whereas, further away from the resonator, the portion of the open space  1133   b  has a smaller width y. In an alternative embodiment, the separating wall  1130  may be a single wall, e.g., main walls  1151  of wall portions  1130   a  and  1130   b  may be a single wall (thus eliminating the portion of space  1130  having width y, but leaving the portion of space  1133  having width x. 
     Another alternative shape is a separating wall  1230  that terminates in a U-shaped projection comprised of extension halves  1230   a ,  1230   b , as shown in  FIG. 12 . This is similar to the embodiment of  FIG. 5 , except that the extensions comprises curved walls  1230   a ,  1230   b , rather than straight walls  530   a ,  530   b . Alternatively, as illustrated in  FIG. 13 , a separating wall with U shaped extensions very similar to that illustrated in  FIG. 12  can be split into two separate walls  1330   1 ,  1330   2  providing a long space  1376  there between aligned with the longitudinal axis of the associated resonator  1302 . 
     Although a filter is depicted and described in the various embodiments mentioned above, the present invention is applicable to other types of dielectric resonator circuits, including by way of example, but not limited to, oscillators, triplexers, antennas, etc. 
     Having thus described a few particular embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. For example, the mounting members may mount the resonators in a fixed position with tuning being fixed upon assembly or adjusted through the use of tuning plates and/or conductive members. Such alterations, modifications and improvements as are made obvious by this disclosure are intended to be part of this description though not expressly stated herein, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and not limiting. The invention is limited only as defined in the following claims and equivalents thereto.