Abstract:
An iris-less combline filter and a method of providing the iris-less combline filter are disclosed. The filter includes a conductive housing, first and second resonators disposed in the housing, and at least one capacitive coupling element disposed between the first and second resonators, wherein there is no decoupling iris between the first and second resonators.

Description:
RELATED APPLICATION  
       [0001]    The present application claims the priority benefit based on U.S. Provisional Application No. 60/305,050 (Attorney Docket No. 17546L), filed on Jul. 13, 2001, entitled “Iris-Less Combline Filter Having Capacitive Coupling Elements”, assigned to the assignee of the present application, which is herein fully incorporated by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to electronic filters and, more particularly, to an iris-less combline filter with improved performance characteristics and low manufacturing costs.  
         BACKGROUND OF THE INVENTION  
         [0003]    Conventional combline filters typically are used in front-end transmit/receive filters and diplexers of communication systems such as Personal Communication System (PCS), and Global System for Mobile communications (GSM). The combline filters are configured to pass only certain frequency bands of electromagnetic waves as needed by the communication systems. The combline filters can include uniform resonators rods or stepped resonator rods having steps.  
           [0004]    [0004]FIG. 1 is a perspective view of a conventional combline filter  10  (with a cover removed therefrom) having uniform resonator rods. As shown, the combline filter  10  includes a plurality of uniform resonator rods  6  disposed within a metal housing  2 , input and output terminals  12  and  14  disposed on the outer surface of the metal housing  2 , and loops  16   a  and  16   b  for inductively coupling electromagnetic signals to and from the input and output terminals  12  and  14 . The metal housing  2  is provided with a plurality of cavities  4  separated by dividing walls  4   a . Certain dividing walls  4   a  have a well-known structure called a decoupling “iris”  8  having an opening  8   a . The dividing wall  4   a  having the iris  8  is used to control the amount of coupling between two adjacent resonator rods  6 , which controls the bandwidth of the filter. The resonator rods  6  resonate at particular frequencies to filter or selectively pass certain frequencies of signals inductively applied thereto. Particularly, input signals from the input terminal  12  of the combline filter  10  are inductively transmitted to the first resonator rod  6  through the loop  16   a  and are filtered through the resonance of the resonator rods  6 . The filtered signals are then outputted at the output terminal  14  of the combline filter  10  through the loop  16   b.    
           [0005]    A combline filter having stepped resonator rods is also known in the art. In such a filter, resonator rods having steps are used in lieu of the uniform resonator rods. The structure of this filter would be identical to that of the filter  10  shown in FIG. 1, except that the uniform resonator rods  6  are replaced with stepped resonator rods and different dimensions may be used. This type of filter also has the decoupling irises and multiple dividing walls to control the coupling coefficients between the stepped resonator rods.  
           [0006]    In all these conventional combline filters, the passing frequency range of the filter is selectively varied by changing the lengths or dimensions of the resonator rods whether they be uniform rods or stepped rods. The operational bandwidth of the filter is selectively varied by changing the electromagnetic (EM) coupling coefficients between the resonator rods. The EM coupling coefficient represents the strength of EM coupling between two adjacent resonator rods and equals the difference between the magnetic coupling coefficient and the electric coupling coefficient between the two resonator rods. The magnetic coupling coefficient represents the magnetic coupling strength between the two resonator rods, whereas the electric coupling coefficient represents the electric coupling strength between the two resonator rods. Usually, the magnetic coupling coefficient is larger than the electric coupling coefficient.  
           [0007]    To vary the EM coupling (i.e., EM coupling coefficient) between two resonator rods, the size of the iris opening disposed between the two resonator rods is varied. The larger the iris between the two resonator rods, the higher the EM coupling between the two resonator rods. This results in a wide bandwidth operation of the filter. In contrast, if the iris  8  has a smaller opening, a lower EM coupling between the resonator rods is effected, resulting in a narrow bandwidth operation of the filter.  
           [0008]    Although effective, conventional combline filters with decoupling irises have a number of problems or drawbacks. For instance, the cavities, dividing walls and decoupling irises in the metal housing must be formed very precisely. Thus, the conventional combline filters require sophisticated milling, which increases costs and decreases throughput. Further, the plurality of dividing walls erected between the resonator rods of the filter significantly increases the signal loss known as “insertion loss”. Moreover, if different bandwidth characteristics are desired for the combline filter, the metal housing of the filter must be re-machined to change the size of the iris openings. In this respect, the milling of the metal housing only allows the iris openings to be enlarged (e.g., by removing a portion of the dividing wall), but does not allow a reduction in the size of the iris openings. Thus, if a decrease in the coupling coefficient between the resonator rods is desired, the metal housing cannot be re-machined and the entire filter housing must be replaced to provide the desired coupling coefficient. Conventional combline filters are therefore restricted in applicability and adaptability.  
           [0009]    Accordingly, there is a need for an improved combline filter which overcomes the above-described problems and other problems that are associated with conventional combline filters.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention presents an innovative approach for controlling the EM coupling between resonators (resonator rods) which overcomes problems that are associated with conventional combline filters. Particularly, the present invention eliminates the use of decoupling irises and instead utilizes a capacitive coupling element to enhance electric coupling between resonators to control the overall EM coupling between the resonators. In one embodiment, the capacitive coupling element is a conductive rod supported by a non-conductive support member and disposed between two adjacent resonators. The capacitive coupling element is placed between the resonators, without contacting the resonators, where the electrical field is dominant, which improves the electric coupling between the resonators. In another embodiment, the capacitive coupling element is a conductive rod attached to one of two adjacent resonators, and is placed between the two resonators where the electrical field is dominant, which improves the electric coupling between the resonators. An increase in the electric coupling decreases the overall EM coupling between the resonators. Then, by selectively varying the dimensions of the capacitive coupling element which varies the amount of electric coupling present between the two resonators, the present invention controls the overall EM coupling between the two resonators without the use of decoupling irises. The use of capacitive coupling elements according to the present invention provides many advantages over conventional combline filters having decoupling irises. For example, a capacitive coupling element is more configurable than a decoupling iris. To modify the size of the iris openings to vary the EM coupling between the resonators, the entire metal housing needs to be re-machined. In contrast, in the present invention, only the capacitive coupling element needs to be reconfigured. Reconfiguration of the capacitive coupling element may involve trimming the ends of the capacitive coupling element, which can be easily accomplished, or replacing the capacitive coupling element with a new capacitive coupling element having different dimensions and/or configurations, which also can be easily accomplished. For instance, If less EM coupling is desired between two resonators, the existing capacitive coupling rod can be replaced with a longer capacitive coupling rod or a thicker capacitive coupling rod, or the height of the coupling rod can be increased. Thus, by merely varying the length, thickness, diameter, and/or height of the capacitive coupling elements and without requiring re-machining or replacement of the metal housing as in the conventional combline filters, the present invention permits easy modifications to EM coupling between the resonators.  
           [0011]    Furthermore, the present invention eliminates the use of de-coupling irises and thereby reduces the number of dividing walls needed in the filter. This feature reduces the milling cost associated with manufacturing the filter, thereby greatly decreasing the manufacturing cost and time for the filter. This feature also reduces the insertion loss for the filter, which is typically caused by dividing walls, and thereby improves the performance characteristics of the filter. Moreover, the use of the capacitive coupling elements in conjunction with the resonators allows for signal attenuation zeros close to the passband of the filter, thereby providing high selectivity for the filter.  
           [0012]    In one embodiment, the present invention is directed to a filter comprising a conductive housing, first and second resonators disposed in the housing, and at least one capacitive coupling element disposed between the first and second resonators, wherein there is no decoupling iris between the first and second resonators.  
           [0013]    In another embodiment, the present invention is directed to a method of providing a filter, the method comprising the steps of providing a conductive housing, disposing first and second resonators in the housing, and disposing at least one capacitive coupling element between the first and second resonators, no decoupling iris existing between the first and second resonators.  
           [0014]    In yet another embodiment, the present invention is directed to a method of providing a filter, comprising the steps of providing a conductive housing; disposing an integrated unit in the housing, the integrated unit including a resonator section and a capacitive coupling element section extending directly from the resonator section; and disposing a resonator in the housing a predetermined distance from the integrated unit, wherein there is no decoupling iris between the integrated unit and the resonator, and the capacitive coupling element section of the integrated unit controls coupling between the resonator and the resonator section of the integrated unit. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    In the drawings, the same reference numerals are used to indicate the same elements.  
         [0016]    [0016]FIG. 1 is a perspective view of a conventional combline filter with a cover removed therefrom.  
         [0017]    [0017]FIG. 2 is a perspective view of a combline filter with a cover removed therefrom according to one embodiment of the present invention.  
         [0018]    [0018]FIG. 3A is a diagram of a support member coupled to a capacitive coupling element usable in the combline filter of FIG. 2 according to one embodiment of the present invention.  
         [0019]    [0019]FIG. 3B is a diagram of a support member coupled to a pair of capacitive coupling elements usable in the combline filter of FIG. 2 according to another embodiment of the present invention.  
         [0020]    [0020]FIG. 3C is a support member coupled to a pair of capacitive coupling elements usable in the combline filter of FIG. 2 according to still another embodiment of the present invention.  
         [0021]    [0021]FIG. 4 is a perspective view of the combline filter of FIG. 2 that is assembled with a cover according to one embodiment of the present invention.  
         [0022]    [0022]FIG. 5A is a top plan view of a combline filter with a cover removed therefrom according to another embodiment of the present invention.  
         [0023]    [0023]FIG. 5B is a longitudinal sectional view of the combline filter of FIG. 5A.  
         [0024]    [0024]FIG. 6 is a perspective top plan view of a combline filter according to still another embodiment of the present invention.  
         [0025]    [0025]FIG. 7 is a cross-sectional view of an example of a stepped resonator rod usable in the combline filter shown in FIGS. 2, 5A and  6  according to one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    [0026]FIG. 2 is a perspective view of a combline filter  50  with a cover removed therefrom according to one embodiment of the present invention. As shown in FIG. 2, the combline filter  50  includes a metal housing  52  having a meandering cavity  54  and two dividing walls  51  and  53 , a plurality of resonators  6   a - 6   f  disposed within the cavity  54  of the metal housing  52 , and a plurality of capacitive coupling elements  56   a - 56   e  each disposed between two of the resonators  6   a - 6   f . The combline filter  50  further includes input and output terminals  12  and  14  disposed on the outer surface of the metal housing  52 , and loops  16   a  and  16   b  for inductively coupling electromagnetic signals to and from the input and output terminals  12  and  14 .  
         [0027]    The resonators  6   a - 6   f  are uniform rods and can be mounted on the bottom inner surface of the metal housing  52  using known techniques. For example, threads can be provided on the ends of the resonators  6   a - 6   f  and the resonators  6   a - 6   f  can be screwed into the bottom inner surface of the metal housing  52  configured to receive the resonators  6   a - 6   f . The metal housing  52  and the resonators  6   a - 6   f  are made with conductive materials. For instance, the housing  52  can be made with aluminum plated with silver.  
         [0028]    The capacitive coupling elements  56   a - 56   e  are conductive rods for selectively limiting the EM coupling between two adjacent resonators. Each of the first through fifth coupling elements  56   a - 56   e  is perpendicularly disposed with respect to the projection direction of the two resonators and does not contact the corresponding two resonators. Each of the first through fifth coupling elements  56   a - 56   e  extends through the space between two of the resonators  6   a - 6   f  without directly contacting the resonators  6   a - 6   f , so that it is aligned with the middle portions of the corresponding two resonators where the electric field is dominant or the strongest, i.e., along a line intersecting the longitudinal axes of the two adjacent resonators. The position of the capacitive coupling elements  56   a - 56   e  increases the electric coupling between the two resonators. Since the overall EM coupling coefficient between the two resonators is the difference between the magnetic coupling coefficient and the electric coupling coefficient, the increase in the electric coupling coefficient decreases the overall EM coupling coefficient between the two resonators. Thus, by selectively varying the dimensions of the capacitive coupling elements  56   a - 56   e , which improve the electric coupling between the resonators, the overall EM coupling between the resonators can be selectively varied.  
         [0029]    The capacitive coupling elements  56   a - 56   e  are positioned between the resonators by using support members  58   a - 58   e  which are made with a non-conductive material providing low signal loss, such as Teflon®. Each of the capacitive coupling elements  56   a - 56   e  is supported by the corresponding support member  58  provided between two of the resonators. For instance, the first capacitive coupling element  56   a  is supported between the first and second resonators  6   a  and  6   b  by the first support member  58   a , the second capacitive coupling element  56   b  is supported between the second and third resonators  6   b  and  6   c  by the second support member  58   b , and so on.  
         [0030]    One skilled in the art would readily understand that, according to well-established electromagnetic theories and filter characteristics, dividing walls may be needed between certain resonator rods. For example, in the six-section (six resonator rods) filter of FIG. 2, the dividing wall  51  is needed between the first and fourth resonators  6   a  and  6   d  in order to prevent EM coupling therebetween, which might otherwise occur because they are physically adjacent each other. Specifically, the intended operation of filter  50  is for the signal input at terminal  12  to be coupled from resonator  6   a  to resonator  6   b  and then from resonator  6   b  to resonator  6   c  and then from resonator  6   c  to resonator  6   d  and so on. Accordingly, EM coupling directly between resonator  6   a  and  6   d  should be prevented (by means of wall  51 ).  
         [0031]    When electromagnetic signals are applied to the input terminal  12 , these input signals are inductively coupled to the first resonator  6   a  by the loop  16   a . Then the resonators  6   a - 6   f  resonate to pass certain frequencies of the input signals. The capacitive coupling elements  56   a - 56   e  selectively couple the signals between the resonator  6   a  to control the bandwidth at which the filtering process occurs. Then the filtered signals are outputted from the sixth resonator  6   f  and inductively coupled to the output terminal  14  by the loop  16   b . As a result, the filtered signals are outputted at the output terminal  14  of the filter.  
         [0032]    FIGS.  3 A- 3 C show three different examples  58 ,  58 ′,  58 ″ of a support member  58   a ,  58   b ,  58   c ,  58   d , or  58   e  for supporting at least one capacitive coupling element for use in the filter  50  according to different embodiments of the present invention. These examples are provided only to demonstrate that a variety of different schemes can be used to provide support members for the capacitive coupling elements of the present invention. Obviously, other examples are possible and contemplated as part of the present invention.  
         [0033]    As shown in FIG. 3A, in accordance with one embodiment, the support member  58  is a non-conductive rod having a threaded portion  62  at one end portion thereof and a transverse through hole  60  near the other end portion thereof. A capacitive coupling element  56  ( 56   a ,  56   b ,  56   c ,  56   d  or  56   e ) having a rod configuration is inserted through the hole  60  so that equal portions of the coupling element  56  project from the support member  58 . If desired, known fastening techniques such as glue, tape, mating threads, screws, etc., can be used to secure the position of the coupling element  56  within the hole  60  of the support member  58 . The threaded portion  62  of the support member  58  is screwed into a threaded hole located at the bottom inner surface of the metal housing  52 . The height of the coupling element  56  with respect to the corresponding two resonators (i.e., the distance between the coupling element  56  and the bottom inner surface of the housing  52 ) can be changed easily by varying the degree in which the support member  58  is screwed into the metal housing  52 . The height of the coupling element  56  affects the coupling coefficient between the corresponding two resonators.  
         [0034]    In another embodiment, the hole  60  can be made large so that the coupling elements  56  of different diameters or thicknesses can be interchangeably positioned within the hole  60 . In such cases, additional fastening techniques may be used to securely position the coupling element  56  in the hole  60 .  
         [0035]    The structures discussed in connection with FIG. 3A allow easy modification of coupling coefficients between the resonators because it allows the size, shape and/or configuration of the capacitive coupling element  56  to be changed easily, which controls the EM coupling coefficients between the resonators. For instance, to increase the EM coupling between the resonators, the length of the coupling element  56  can be easily reduced by either trimming the end portion(s) of the coupling element  56  or replacing the coupling element  56  with a new capacitive coupling element having a shorter length or cross-section. On the other hand, to decrease the EM coupling between the resonators, the length of the coupling element  56  can be increased easily by replacing the coupling element  56  with a new capacitive coupling element having a greater length and/or cross-section.  
         [0036]    In accordance with another embodiment as shown in FIG. 3B, a pair of capacitive coupling elements  56 ′ and  56 ″ are used in lieu of one capacitive coupling element  56 . The support member  58 ′ includes two blind holes  60 ′ and  60 ″ disposed on the opposite sides of the support member  58 ′ for receiving the pair of capacitive coupling elements  56 ′ and  56 ″. Alternately, a single through hole as in the FIG. 3A embodiment still may be used. The support member  58 ′ also includes the threaded portion  62  for selectively varying the height of the coupling elements  56 ′ and  56 ″ as discussed above. If desired, the coupling elements  56 ′ and  56 ″ can be further secured in the holes  60 ′ and  60 ″ of the support member  58 ′ by using any existing fastening techniques. For instance, the holes  60 ′ and  60 ″ and one end of each of the coupling elements  56 ′ and  56 ″ can be threaded so that the coupling elements  56 ′ and  56 ″ can be screwed into the holes  60 ′ and  60 ″ respectively. Glue, tape, screws or any other fastener can also be used to secure the coupling elements  56 ′ and  56 ″ in the holes  60 ′ and  60 ″ of the support member  58 ′. With the configurations discussed in connection with FIG. 3B, the EM coupling coefficient between the resonators can be varied easily because the size, shape and/or configuration of the coupling elements  56 ′ and  56 ″ can be varied easily, e.g., by trimming or replacing the coupling elements  56 ′ and  56 ″.  
         [0037]    In another embodiment, as an alternative to having the holes  60 ′ and  60 ″ in the support member  58 ′, the capacitive coupling elements  56 ′ and  56 ″ can be attached to the outer surface of the support member  58 ′ using known fasteners such as glue, tape, screws, etc.  
         [0038]    In accordance with still another embodiment as shown in FIG. 3C, the support member  58 ″ is disposed on a base member  64 . The base member  64  will be disposed underneath the bottom surface of the metal housing  52  so that only the support member  58 ″ projects from the bottom surface of the metal housing  52 . The support member  58 ″ can be mounted on the base member  64  in any known manner. The pair of coupling elements  56 ′ and  56 ″ are attached to the outer surface of the support member  58 ″ by a fastener such as glue, tape, screws, etc. However, one or two capacitive coupling elements can be coupled to the support member  58 ″ in any matter, such as by using the holes  60 ,  60 ′,  60 ″ as discussed above.  
         [0039]    In other embodiments, if desired, the capacitive coupling elements may be securely positioned between the resonators by a support structure that is coupled to any other part of the metal housing and/or the cover. For instance, if needed, the support members may be supported by the cover, rather than by the bottom surface of the metal housing, so that the support members hang from the cover.  
         [0040]    In addition to the above described examples, the present invention contemplates as part of the invention a variety of different schemes that can be used to provide capacitive coupling element(s) between two resonators without contacting the resonators. Any scheme for extending the capacitive coupling element(s) between corresponding two resonators can be used as long as the capacitive coupling element(s) can be positioned in the space gap between the corresponding two resonators where the electric field is dominant or strongest. For instance, the support member and at least one capacitive coupling element of the present invention can be integrally formed as one unit. In this case, the EM coupling coefficient control may be achieved by replacing the integrated unit with a different integrated unit providing the desired coupling coefficient. In another example, instead of having the capacitive coupling element  56  within the hole  60  of the support member  58  as in FIG. 3A, the coupling element  56  may be attached to a side or top surface of the support member  58 . In still another example, multiple support members can be used to support the capacitive coupling element(s) between corresponding two resonators.  
         [0041]    Although the support members (e.g., members  58 ,  58 ′,  58 ″) have been described above as having rod configurations, they can have any different configurations, sizes, cross-sections and/or shapes. For example, the support member can be a square rod, an oval rod, a cone shaped member, etc.  
         [0042]    [0042]FIG. 4 is a perspective view of the combline filter  50  assembled with a cover  66  according to one embodiment of the invention. The cover  66  is made with a conductive material such as aluminum and is fastened to the housing  52  by using a known fastener such as screws  68 . The combline filter  50  may include, if needed, a plurality of tuning screws  70  for fine tuning the filter characteristics of the filter  50  according to known techniques. Typically, these tuning screws  70  are positioned above the resonators  6   a - 6   f.    
         [0043]    [0043]FIG. 5A is a top plan view of a combline filter with a cover removed therefrom according to another embodiment of the present invention and FIG. 5B is a longitudinal sectional view of the combline filter of FIG. 5A. As shown in FIGS. 5A and 5B, the combline filter  150  includes a metal housing  152  having a cavity  154 , a pair of resonators  160  disposed within the cavity  154  of the metal housing  152 , and a capacitive coupling element  156  attached to each one of two adjacent resonators  160 . Each of the resonators  160  is a stepped resonator rod having stepped cross-section(s) and can be mounted on the bottom inner surface of the metal housing  152  using known techniques such as screws or threaded stepped resonator rods. The metal housing  152  and the resonators  160  are made with conductive materials. For instance, the housing  152  may be formed of aluminum plated with silver.  
         [0044]    The combline filter  150  further includes input and output terminals  12  and  14  disposed on the outer surface of the metal housing  152 , loops  16   a  and  16   b  for inductively coupling electromagnetic signals to and from the input and output terminals  12  and  14 , and tuning screws  70  (optionally provided) for fine tuning the filter characteristics of the filter  150  according to known techniques. Typically, the tuning screws  70  are positioned above the resonators  160 .  
         [0045]    The capacitive coupling element  156  is a conductive rod for selectively controlling the EM coupling between the two resonators  160 . The capacitive coupling element  156  is perpendicularly disposed with respect to the projection direction of the two resonators  160  and directly contacts one of the two resonators  160 . The capacitive coupling element  156  extends preferably through the space between the two resonators  160  so that it is aligned with the middle portions or any portions of the two resonators  160  where the electric field is the strongest or dominant. This allows the capacitive coupling element  156  to increase the electric coupling between the two resonators  160 . Since the overall EM coupling coefficient between the two resonators is the difference between the magnetic coupling coefficient and the electric coupling coefficient, the increase in the electric coupling coefficient decreases the overall EM coupling coefficient between the two resonators. Thus, by selectively varying the dimensions of the capacitive coupling element  156 , the overall EM coupling between the two resonators  160  can be selectively varied.  
         [0046]    [0046]FIG. 6 is a perspective top plan view of a combline filter having stepped resonators according to another embodiment of the present invention. As shown in FIG. 6, a combline filter  100  according to this embodiment includes a metal housing  104  with multiple cavities and that is separated into a receive filter section  101  and a transmit filter section  102 . The receive filter section  101  includes eight resonators  160  and seven capacitive coupling elements  156  arranged in a particular manner. Each capacitive coupling element  156  is attached to first through seventh resonators  160  so that a capacitive coupling element exists between two adjacent resonators. The receive filter section  101  further includes a dividing wall  57  for separating the resonators  160  according to known techniques. The transmit filter section  102  includes six resonators  160  and five capacitive coupling elements  156  arranged in a particular manner. Each capacitive coupling element  156  in the transmit filter section  102  is attached to first through fifth resonators  160  so that a capacitive coupling element exists between every pair of adjacent resonators. The transmit filter section  102  further includes dividing walls  57  that are well known in the art.  
         [0047]    When electromagnetic signals are applied to the input terminal of the filter  150  or  100 , these input signals are inductively coupled to the first resonator and the resonators  160  resonate to pass certain frequencies of the input signals. The capacitive coupling elements control the bandwidth in which the filter operates by controlling the EM couplings between the resonators  160 . Filtered signals are then inductively coupled from the last resonator to the output terminal of the filter by the corresponding coupling loop so that the filtered signals are output at the output terminal of the filter.  
         [0048]    One skilled in the art would readily understand that, according to well-established electromagnetic theories and filter characteristics, certain dividing walls such as the dividing walls  57  may be needed between resonators for successful operation of the filter.  
         [0049]    [0049]FIG. 7 shows one example of a stepped resonator  160  that may be used in the present invention according to one embodiment of the present invention. In accordance with one embodiment, the resonator  160  is a non-conductive stepped rod having an upper portion  160   a  and a lower portion  160   b  extending from the upper portion  160   a . The diameter or cross-section of the upper portion  160   a  is greater than the diameter or cross-section of the lower portion  160   b . The resonator  160  further includes a transverse through hole  161  disposed in one wall of the upper portion  160   a . The capacitive coupling element  156  which may have a rod configuration or other shape is inserted into the hole  161  so that the capacitive coupling element  156  projects horizontally from the side of the upper portion  160   a  of the resonator  160 . If desired, known fastening techniques such as glue, tape, screws, etc., can be used to secure the position of the capacitive coupling element  156  within the hole  161  of the resonator  160 .  
         [0050]    In another embodiment, the hole  161  does not completely pass through the wall of the upper portion  160   a . In still another embodiment, the hole  161  and one end of the capacitive coupling element  156  are matingly threaded so that the capacitive coupling element  156  can be rotatably inserted into the hole  161 . This feature can enhance the interchangeability of capacitive coupling elements in and out of the hole  161  without requiring re-machining or other costly processes. For instance, all the capacitive coupling elements of varying thickness and/or length can have the same-sized end that is threaded. To change the EM coupling coefficient between the resonators, all that is required is to remove the existing capacitive coupling element from the hole  161  (e.g., by unscrewing it from the hole  161 ) and insert a new capacitive coupling element of appropriate thickness and/or length into the hole  161  (e.g., by screwing it into the hole  161 ).  
         [0051]    In another embodiment, the hole  161  can be made large so that the capacitive coupling elements of different diameters or thicknesses can be interchangeably positioned within the hole  161 . In such cases, known fasteners may be used to secure the position of the capacitive coupling element in the hole  161 . In still another embodiment, as an alternative to having the hole  161  in the upper portion  160   a  of the resonator  160 , the capacitive coupling element  156  can be attached directly onto the outer surface of the upper portion  160   a  so that the capacitive coupling element  156  points to the adjacent capacitive coupling element. This can be accomplished using known fasteners such as screws, etc.  
         [0052]    The resonator  160  can be fastened to the inner bottom surface of the metal housing (e.g., housing  152 ) using any known techniques. For example, a screw  163  or other fasteners can be used. In another example, threads can be provided at the end of the lower portion  160   b  of the resonator  160  and a corresponding threaded hole can be provided at the bottom inner surface of the metal housing. Then the threaded portion of the resonator  160  is rotatably inserted into the threaded hole located at the bottom inner surface of the metal housing. By having the threaded portion at the resonator  160 , the height of the capacitive coupling element  156  (i.e., distance between the capacitive coupling element and the bottom inner surface of the metal housing) can be changed easily by varying the degree to which the resonator  160  is screwed into the metal housing. The height of the capacitive coupling element  156  affects the EM coupling coefficient between the corresponding two resonators.  
         [0053]    In addition to the above described examples, the present invention contemplates as part of the invention a variety of different schemes that can be used to provide the capacitive coupling element between two resonators. Any scheme for attaching or directly connecting a capacitive coupling element to one of the corresponding two resonators and extending it between the two resonators can be used. For instance, the resonator and the capacitive coupling element of the present invention can be integrally formed as one unit so that the integrated unit is composed of a resonator section and a capacitive coupling element section directly extending from the resonator section. In this case, the EM coupling coefficient control may be achieved by replacing the integrated unit with a different integrated unit providing the desired coupling coefficient.  
         [0054]    According to the present invention, although a certain number of resonators are illustrated in the drawings, the filter of the present invention can have any number of resonators depending on the application. The number of capacitive coupling elements and support members (if needed) present in the filter will then vary appropriately depending on the number of resonators in the filter. Moreover, the shape, size and/or configuration of the filter housing (e.g., housing  52  or  152 ) can vary depending on the application. For example, the filter housing can be in the shape of a square box or a rectangular box depending on the number of resonators needed in the filter housing. Further, although the resonators of the present invention are shown in the drawings as round resonator rods, they can have other shapes, sizes and/or configurations such as square-faced resonator rods, etc. Moreover, other types of resonators can be used in the filters of the present invention. For example, in the filter  50  shown in FIG. 2, the resonators  6   a - 6   f  can be replaced with stepped resonators such as the resonator  160  shown in FIGS. 5A and 7. In another example, the stepped resonators  160  in the filters  100  and  150  shown in FIGS. 5A and 6 can be replaced with the resonators  6   a - 6   f  shown in FIG. 2. In these cases, appropriate dimensions for the components of the filter can be selected according to the present invention to achieve the desired or optimal performance characteristics.  
         [0055]    It should be noted that the combline filters of the present invention are without any decoupling iris for controlling EM coupling between resonators. Instead, the present invention utilizes a capacitive coupling element disposed between resonators to control the EM coupling coefficient between the two adjacent resonators. By varying the size, length, thickness, and/or “height” (distance from the bottom surface of the filter housing) of the capacitive coupling elements, the EM coupling coefficients between the resonators can be easily varied. The use of the capacitive coupling elements also replaces some of the dividing walls, thereby reducing a number of dividing walls needed in the filter. As a consequence, the size of the overall housing for the filter can be reduced significantly, thereby producing a more compact combline filter.  
         [0056]    The present invention is applicable to any system requiring filters such as communication systems, e.g., Personal Communication System (PCS), Digital Communication System (DCS), Global System for Mobile communications (GSM), and 3G. The size, shape and/or configurations of the capacitive coupling elements, support members and/or resonators can be selected appropriately to achieve the desired filter characteristics in an optimal manner. Further, the concept of utilizing capacitive coupling elements to control EM coupling coefficients between resonators without the use of decoupling irises and/or decoupling walls according to the present invention is equally applicable to other types of filters such as interdigital filters, dielectric filters which employ combline or interdigital geometries, etc.  
         [0057]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.