Patent Publication Number: US-9887442-B2

Title: RF filter for adjusting coupling amount or transmission zero

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. application Ser. No. 13/434,679, filed Mar. 29, 2012, all of which are hereby expressly incorporated by reference in its entirety for all purposes. 
    
    
     TECHNICAL FIELD 
     Example embodiment of the present invention relates to an RF filter, e.g. RF cavity filter for adjusting cross-coupling amount or transmission zero. 
     BACKGROUND ART 
     An RF cavity filter includes plural cavities formed therein to pass only use frequency band of a signal, and is employed generally at a base station, etc. using comparative high power of a frequency signal. 
       FIG. 1  is a plan view illustration structure of a common RF cavity filter. 
     In  FIG. 1 , the RF cavity filter includes an input connector  100 , an output connector  102 , a housing member  104 , cavities  110 ,  112 ,  114 ,  116 ,  118  and  120  defined by the housing member  104  and a wall, resonators  130 ,  132 ,  134 ,  136 ,  138  and  140  in each of the cavities  110 ,  112 ,  114 ,  116 ,  118  and  120 , and a coupling bar  160 . 
     An RF signal inputted through the input connector  100  is provided to the cavity  110 . Each of the cavities  110 ,  112 ,  114 ,  116 ,  118  and  120  and corresponding resonators  130 ,  132 ,  134 ,  136 ,  138  and  140  function as LC resonance elements, respectively. 
     The RF signal is delivered from one cavity to another cavity through a coupling window  150 . 
     A resonance frequency of the filter is determined by size of the cavities and size of the resonators. A user may tune finely characteristics of the filter using tuning bolts which are not shown. 
     The skirt characteristic which means a slope of a boundary band in a pass band characteristic curve is important in view of the filter, and preferably should be formed sharply. 
     The skirt characteristic is improved according as order of the filter increases, i.e. the number of the cavities and the resonators increases. However, the skirt characteristic has trade-off relation with an insertion loss. That is, as the number of the cavities and the resonators increases, the skirt characteristic is enhanced but the insertion loss augments. 
     The filter forms a notch using cross coupling to improve the skirt characteristic with maintaining constant insertion loss. 
     The cross coupling means coupling between resonators which are not adjacent, e.g. coupling between a second resonator  132  and a fifth resonator  138 . The cross coupling is realized generally through the coupling bar  160 . 
     The coupling bar  160  is formed through a wall between the second cavity  122  and the fifth cavity  128 , and is made up of a metal. The coupling bar  160  functions to deliver for example a signal of the second resonator  132  to the fifth resonator  138 . 
     A hole for the coupling bar  160  is formed on the wall between the second cavity  122  and the fifth cavity  128 , a dielectric layer is formed on an inner surface of the wall corresponding to the hole, and so the coupling bar  160  is not connected electrically to the wall. 
     However, this cross coupling structure may not adjust coupling amount between resonators. The coupling amount is determined by size of the coupling bar  160 , and thus the coupling bar  160  should be replaced by new coupling bar having different size in case that desired coupling amount is not realized. In addition, the coupling amount may not be adjusted under the condition that the coupling bar  160  is set on the wall. 
     Moreover, the filter may not adjust transmission zero related to the skirt characteristic. 
     DISCLOSURE 
     Technical Problem 
     Example embodiment of the present invention provides an RF filter, for example RF cavity filter for adjusting cross coupling amount or transmission zero. 
     Technical Solution 
     An RF filter according to one embodiment of the present invention includes a housing member in which cavities are defined by walls; resonators located in the cavities; a cover combined with an upper surface of the housing member; a first tuning element inserted into a first cavity of the cavities through the cover; and a second tuning element inserted into a second cavity of the cavities through the cover. Here, the first tuning element and the second tuning element are connected electrically. 
     An RF filter according to another embodiment of the present invention includes a housing member; a cover combined with an upper surface of the housing member; a first tuning area formed on one surface of the cover; a second tuning area formed on the one surface of the cover with separated from the first tuning area; a dielectric area formed between the first tuning area and the second tuning area on the one surface of the cover; and a third tuning element disposed on the dielectric area. Here, the first tuning area and the second tuning area are conductive areas. 
     An RF filter according to still another embodiment of the present invention includes a housing member; a cover combined with an upper surface of the housing member; a first tuning area formed on one surface of the cover; a second tuning area formed on the one surface of the cover with separated from the first tuning area; a dielectric area formed between the first tuning area and the second tuning area on the one surface of the cover; a first tuning element inserted into the housing member through the first tuning area of the cover; a second tuning element inserted into the housing member through the second tuning element of the cover; and a tuning sliding member disposed on the one surface of the cover. Here, a part of the tuning sliding member overlaps with the dielectric area. 
     Advantageous Effects 
     An RF filter according to the present invention controls tuning elements inserted in corresponding cavities through a cover, thereby adjusting cross coupling amount or transmission zero. Specially, variable range of coupling coefficient and transmission zero may be considerably wide. 
     The RF filter according to the present invention adjusts capacitance between the tuning elements using a lumped element or a tuning sliding member under the condition that the tuning elements are inserted into corresponding cavities through the cover, and so cross coupling amount between corresponding resonators or transmission zero may be adjusted. Specially, a user may control the lumped element and the tuning sliding member outside to tune easily characteristics of the filter. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Example embodiments of the present invention will become more apparent by describing in detail example embodiments of the present invention with reference to the accompanying drawings, in which: 
         FIG. 1  is a plan view illustration structure of a common RF cavity filter; 
         FIG. 2  is a perspective view illustrating an RF filter according to a first embodiment of the present invention; 
         FIG. 3(A)  and  FIG. 3(B)  are views illustrating schematically a part of the RF filter according to one embodiment of the present invention and  FIG. 3(C)  is a view illustrating a rotation tool according to one embodiment of the present invention; 
         FIG. 4(A)  and  FIG. 4(B)  are views illustrating structure of a cover and connection structure of tuning elements according to one embodiment of the present invention; 
         FIG. 5(A)  and  FIG. 5(B)  are views illustrating structure of a cover and connection structure of tuning elements according to another embodiment of the present invention; 
         FIG. 6(A)  and  FIG. 6(B)  are sectional views illustrating various tuning elements according to one embodiment of the present invention; 
         FIG. 7  is a view illustrating experimental result of coupling coefficient of the RF filter according to one embodiment of the present invention; 
         FIG. 8  is a view illustrating experimental result of transmission zero of the RF filter according to one embodiment of the present invention; 
         FIG. 9  is a view illustrating schematically structure of an RF filter according to a second embodiment of the present invention; 
         FIG. 10  is a perspective view illustrating an RF filter according to a third embodiment of the present invention; 
         FIG. 11(A)  and  FIG. 11(B)  are views illustrating structure of an RF filter according to a third embodiment of the present invention; 
         FIG. 12(A) ,  FIG. 12(B) , and  FIG. 12(C)  are top views illustrating a cover according to another embodiment of the present invention; 
         FIG. 13  is a perspective view illustrating an RF filter according to a fourth embodiment of the present invention; 
         FIG. 14(A) ,  FIG. 14(B) , and  FIG. 14(C)  are top views illustrating a cover of the RF filter in  FIG. 13  according to one embodiment of the present invention and  FIG. 14(D)  is a view illustrating a tuning sliding member of the RF filter in  FIG. 13  according to one embodiment of the present invention; and 
         FIG. 15(A) ,  FIG. 15(B) , and  FIG. 15(C)  are views illustrating a cover of an RF filter according to a fifth embodiment of the present invention and  FIG. 15  (D) is a view illustrating a tuning sliding member of the cover in  FIG. 15  (C) according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to accompanying drawings. 
       FIG. 2  is a perspective view illustrating an RF filter according to a first embodiment of the present invention. 
     In  FIG. 2 , the RF filter of the present invention is for example an RF cavity filter, and includes a housing member  200 , a cover  202 , an input connector  204 , an output connector  206 , cavities  208 , resonators  210 , walls  212  and tuning elements  214 . 
     The housing member  200  protects elements in the RF filter, and blocks an electromagnetic wave. The housing member  200  may be formed by coating silver having high conductivity on an aluminum material. 
     The cover  202  is combined with an upper surface of the housing member  200 , for example may be combined with the upper surface of the housing member  200  through a bolt, etc. The cover  202  may be formed by for example coating silver on aluminum material, and functions as a ground. 
     An RF signal is inputted through the input connector  204  and is outputted through the output connector  206 . Here, the RF signal propagates through coupling windows formed in each of the cavities  208 . Resonance of the RF signal is generated by the cavities  208  and the resonators  210 , and the RF signal is filtered by the resonance. 
     The cavities  208  are defined by the walls  212 , and each of the resonators  210  is formed in corresponding cavity  208 . As the number of the resonator  210  and the cavity  208  increases, the skirt characteristic of the RF filter is enhanced but the insertion loss is deteriorated. Accordingly, the number of the resonator  210  and the cavity  208  are determined according to desired skirt characteristic and the insertion loss. 
     The resonator  210  may be a cylindrical resonator as shown in  FIG. 2 , but various resonators such as a disk type resonator, etc. may be employed as the resonator  210 . The resonator  210  may be made up of a metal or dielectric member according to mode of the RF filter, i.e. TE mode or TM mode. 
     The tuning elements  214  are made up of for example a metal, are used to adjust cross coupling amount or transmission zero, and are inserted in corresponding cavity  208  under the condition that it combines with the cover  202 . It is desirable that two tuning elements  214  face on the basis of the wall  212 , i.e. a first tuning element is inserted in the cavity  208  located in the left of the wall  212  as shown in  FIG. 2  and a second tuning element  214  is inserted in the cavity  208  located in the right of the wall  212 . 
     In one embodiment of the present invention, the tuning elements  214  are tuning bolts, and are connected electrically each other under the condition that they combine with the cover  202 . However, the tuning elements  214  are not connected electrically to the cover  202 . 
     A user may adjust cross coupling amount or transmission zero by moving up and down the tuning element  214 . 
     Hereinafter, a process of adjusting coupling amount or transmission zero using the tuning elements  214  and disposition of the tuning elements  214  will be described in detail with reference to accompanying drawings. 
       FIG. 3  is a view illustrating schematically a part of the RF filter according to one embodiment of the present invention, and  FIG. 4  is a view illustrating structure of a cover and connection structure of tuning elements according to one embodiment of the present invention.  FIG. 5  is a view illustrating structure of a cover and connection structure of tuning elements according to another embodiment of the present invention, and  FIG. 6  is a sectional view illustrating various tuning elements according to one embodiment of the present invention. 
     In  FIG. 3(A) , a first resonator  210   a  is disposed in a first cavity  208   a , a second resonator  210   b  is located in a second cavity  208   b , and the wall  212  is disposed between the cavities  208   a  and  208   b.    
     A first tuning element  214   a  is inserted into the first cavity  208   a  through the cover  202 , and combines with the cover  202  through a first nut  216   a . A screw thread is formed on the first tuning element  214   a , and so the first tuning element  214   a  moves up and down with combined with the cover  202  as shown in  FIG. 3(B)  in case that the first tuning element  214   a  rotates. It is desirable that a groove in which a rotation tool such as a driver, etc. is inserted is formed on an upper surface of the first tuning element  214   a  as shown in  FIG. 3(C) . The user may move the first tuning element  214   a  up and down by rotating the first tuning element  214   a  after the user inserts the rotation tool into the groove. As a result, insertion depth of the first tuning element  214   a  to the first cavity  208   a  is changed, and thus cross coupling amount or transmission zero may be adjusted. 
     A second tuning element  214   b  is inserted into a second cavity  208   b  through the cover  202 , faces to the first tuning element  214   a  on the basis of the wall  212 , and combines with the cover  202  through a second nut  216   b . A screw thread is formed on the second tuning element  214   b , and so the second tuning element  214   b  moves up and down with combined with the cover  202  as shown in  FIG. 3(B)  in case that the second tuning element  214   b  rotates. As a result, cross coupling amount or transmission zero is adjusted. 
     The tuning elements  214   a  and  214   b  are connected electrically. In one embodiment of the present invention, a first tuning area  402  and holes  404   a  and  404   b  may be formed on an upper surface  202   a  of the cover  202  as shown in  FIG. 4(A) , and a second tuning area  406 , a third tuning area  408  and the holes  404   a  and  404   b  may be formed on a rear surface of the cover  202  as shown in  FIG. 4(B) . The first tuning element  214   a  is inserted into the first cavity  208   a  through the first hole  404   a , and the second tuning element  214   b  is inserted into the second cavity  208   b  through the second hole  404   b . Here, the tuning areas  402 ,  406  and  408  are conductive areas, and thus the tuning elements  214   a  and  214   b  are connected electrically through the first tuning area  402  formed on the upper surface  202   a  of the cover  202 . 
     In case of etching the upper surface and the rear surface of the cover  202  formed by coating conductive material on a dielectric member according to the tuning areas  402 ,  406  and  408 , the etching areas  400   a ,  400   b  and  400   c  are formed. No conductive material exists in the etching areas  400   a ,  400   b  and  400   c . As a result, the tuning areas  402 ,  406  and  408  are separated electrically from the conductive material of the cover  202  by the etching areas  400   a ,  400   b  and  400   c . Accordingly, the tuning elements  214   a  and  214   b  are connected electrically through the first tuning area  402 , but are not connected electrically to the coating material of the cover  202 , i.e. are not connected to the ground. 
     In another embodiment of the present invention, a second tuning area, a third tuning area and holes may be formed on the upper surface  202   a  of the cover  202  as shown in  FIG. 5(A) , and a first tuning area and the holes may be formed on the rear surface  202   b  of the cover  202  as shown in  FIG. 5(B) . The tuning elements  214   a  and  214   b  are connected electrically through the first tuning area formed on the rear surface  202   b  of the cover  202 . 
     In still another embodiment of the present invention, a tuning area and holes may be formed on the upper surface  202   a  of the cover  202  as shown in  FIG. 4(A) , and a tuning area and holes may be formed on the rear surface  202   b  of the cover  202  as shown in  FIG. 5(B) . The tuning elements  214   a  and  214   b  are connected electrically by the tuning areas. 
     In still another embodiment of the present invention, a groove  220  is formed on an upper surface of the wall  212  to prevent electrical connection of the conductive material in the tuning area formed on the rear surface  202   b  of the cover  202  and the wall  212 . 
     The user may adjust the cross coupling amount or the transmission zero in the above RF filter by changing the insertion depths of the tuning elements  214   a  and  214   b  to the cavities  208   a  and  208   b . Here, insertion depths of the tuning elements  214   a  and  214   b  are the same or different. In addition, tuning of the tuning elements  214   a  and  214   b  when the coupling amount is adjusted may be different from that of the tuning elements  214   a  and  214   b  when the transmission zero is adjusted. 
     On the other hand, length, thickness and shape, etc. of the tuning elements  21   a  and  214   b  may be different. For example, the first tuning element  214   a  may have greater thickness than the second tuning element  214   b  as shown in  FIG. 6(A) , or the first tuning element  214   a  may have different shape and thickness compared with the second tuning element  214   b . That is, the tuning elements  214   a  and  214   b  may be variously modified as long as the cross coupling amount or the transmission zero is adjusted. 
     Hereinafter, experimental result of coupling characteristics of the RF filter according to the present invention will be described in detail. 
       FIG. 7  is a view illustrating experimental result of coupling coefficient of the RF filter according to one embodiment of the present invention, and  FIG. 8  is a view illustrating experimental result of transmission zero of the RF filter according to one embodiment of the present invention. 
     Referring to  FIG. 7 , it is verified that the coupling coefficient between the resonators  210  is changed by 0.019 (changed from 0.0191 to 0.0382). Coupling coefficient of the resonators in convention RF filter is changed by 0.0039. In other words, the RF filter of the present invention may be changed by approximately three times compared with the conventional RF filter. Accordingly, the RF filter of the present invention may tune the cross coupling amount between the resonators  210  in wide range. 
     In  FIG. 8 , it is verified that the transmission zero is changed from 1.805 GHz to 1.907 GHz. That is, the transmission zero is changed by 102 MHz. However, the transmission zero may be changed by above 102 MHz. Accordingly, the RF filter of the present invention may adjust skirt characteristic, etc. according to spec required for the RF filter. 
       FIG. 9  is a view illustrating schematically structure of an RF filter according to a second embodiment of the present invention.  FIG. 9  does not show a housing member, resonators, etc. 
     In  FIG. 9 , the RF filter of the present embodiment includes a cover  200 , a supporting member  900 , tuning elements  902   a  and  902   b  and nuts  904   a  and  904   b.    
     The supporting member  900  is formed on an upper surface of the cover  200 , and may be formed by coating conductive material on a dielectric member. 
     A first tuning element  902   a  is inserted into corresponding cavity through the supporting member  900  and the cover  200 , and a second tuning element  902   b  is inserted into corresponding cavity through the supporting member  900  and the cover  200 . Since an upper surface of the supporting member  900  is coated with conductive material, the tuning elements  902   a  and  902   b  are connected electrically. However, the tuning elements  902   a  and  902   b  are not connected electrically to the cover  202  because a base of the supporting member  900  is made up of dielectric member. 
     The tuning elements  902   a  and  902   b  are fixed to the supporting member  900  by the nuts  904   a  and  904   b.    
     In brief, in the RF filter, the supporting member  900  is formed on the cover  200 , and the tuning elements  902   a  and  902   b  are inserted into corresponding cavities through the supporting member  900  and the cover  200 . Accordingly, unlike the RF filter in the first embodiment where the upper surface of the cover is etched, the upper surface of the cover may not be etched in the RF filter of the present embodiment. 
       FIG. 10  is a perspective view illustrating an RF filter according to a third embodiment of the present invention. 
     In  FIG. 10 , the RF filter of the present invention is for example an RF cavity filter, and includes a housing member  1000 , a cover  1002 , an input connector  1004 , an output connector  1006 , cavities  1008 , resonators  1010 , walls  1012  and a third tuning element  1030 . 
     Since the housing member  1000 , the cover  1002 , the input connector  1004 , the output connector  1006 , the cavities  1008 , the resonators  1010  and the walls  1012  are the same in  FIG. 2 , any further description concerning the same elements will be omitted. 
     The third tuning element  1030  is for example a metal, is used for adjusting cross coupling amount or transmission zero, and is disposed on an upper surface of the cover  1002 . 
     In one embodiment of the present invention, the third tuning element  1030  may be a lumped element such as a capacitor, an inductor, etc. 
     Hereinafter, a process of adjusting coupling amount or transmission zero using the third tuning element  1030  and structure of the RF filter will be described in detail with reference to accompanying drawings. 
       FIG. 11  is a view illustrating structure of an RF filter according to a third embodiment of the present invention, and  FIG. 12  is a top view illustrating a cover according to another embodiment of the present invention. 
     In  FIG. 12(A) , an etching area  1200   a , a first tuning area  1202   a , a second tuning area  1202   b  and a dielectric area  1210  are formed on an upper surface  1002   a  of the cover  1002 . 
     In  FIG. 12(B) , etching areas  1200   b  and  1200   c , a third tuning area  1206  and a fourth tuning area  1208  are formed on a rear surface  1002   b  of the cover  1002 . 
     The tuning areas  1202   a ,  1202   b ,  1206  and  1208  are conductive areas, for example are coated by conductive material. 
     A first hole  1204   a  is formed in the first tuning area  1202   a  and the third tuning area  1206 , and a second hole  1204   b  is formed in the second tuning area  1202   b  and the fourth tuning area  1208 . A first tuning element  1014   a  as for example a conductor is inserted into a first cavity  1008   a  through the first tuning area  1202   a  and the third tuning area  1206  of the cover  1002  as shown in  FIG. 11 . A second tuning element  1014   b  as for example a conductor is inserted into a second cavity  1008   b  through the second tuning area  1202   b  and the fourth tuning area  1208  of the cover  1002 . 
     In one embodiment of the present invention, screw thread is formed on outer surfaces of the first tuning element  1014   a  and the second tuning element  1014   b . Accordingly, the first tuning element  1014   a  or the second tuning element  1014   b  may move up and down with supported by the cover  1002  in case that the first tuning element  1014   a  or the second tuning element  1014   b  rotates. 
     The first tuning element  1014   a  is fixed to the upper surface  1002   a  of the cover  1002  by a first nut  1016   a , and the second tuning element  1014   b  is fixed to the upper surface  1002   a  of the cover  1002  by a second nut  1016   b.    
     The dielectric area  1210  locates between the first tuning area  1202   a  and the second tuning area  1202   b  in the first etching area  1200   a . Here, the first tuning area  1200   a  and the second tuning area  1200   b  are separated physically, but coupling is generated between the first tuning area  1200   a  and the second tuning area  1200   b  by the dielectric area  1210 . That is, certain capacitance is formed between the first tuning area  1202   a  and the second tuning area  1202   b . Accordingly, the first tuning element  1014   a  and the second tuning element  1014   b  are connected electrically through a coupling method, and so cross coupling generates between the resonators  1010   a  and  1010   b  in the cavities  1008   a  and  1008   b  where the tuning elements  1014   a  and  1014   b  are inserted. 
     In one embodiment of the present invention, the third tuning element  1030  as a lumped element is disposed in the dielectric area  1210 . As a result, the capacitance between the first tuning area  1202   a  and the second tuning area  1202   b  is changed by the third tuning element  1030 , e.g. a capacitor, and thus cross coupling amount between the resonators  1010   a  and  1010   b  or transmission zero is changed. In other words, the cross coupling amount between the resonators  1010   a  and  1010   b  or the transmission zero may vary depending on the third tuning element  1030  disposed on the dielectric area  1210  as shown in  FIG. 12(C) . Accordingly, the user may select properly the third tuning element  1030  to realize desired cross coupling amount or transmission. 
     In one embodiment of the present invention, to adjust the cross coupling amount or the transmission zero, the user may change only the third tuning element  1030  under the condition that he fixes the first tuning element  1014   a  and the second tuning element  1014   b , or change the first tuning element  1014   a  or the second tuning element  1014   b  as well as the third tuning element  1030 . Here, the first tuning element  1014   a  or the second tuning element  1014   b  moves up and down. 
     In another embodiment of the present invention, a groove  1020  may be formed on an upper surface of the wall  1012  to prevent electrical connection of conductive material of the tuning area  1206  and  1208  formed on the rear surface  1002   b  of the cover  1002  and the wall  1012 . 
     The dielectric area  1210  may be formed by removing coating material of the cover  1002 , i.e. the dielectric member of the cover  1002  is exposed. 
     In short, the RF filter of the present embodiment may adjust the cross coupling amount or the transmission zero by using the third tuning element  1030 . 
       FIG. 13  is a perspective view illustrating an RF filter according to a fourth embodiment of the present invention, and  FIG. 14  is a top view illustrating a cover of the RF filter in  FIG. 13  according to one embodiment of the present invention. 
     In  FIG. 13 , the RF filter of the present embodiment includes a housing member  1000 , a cover  1002 , a first tuning element  1014   a , a second tuning element  1014   b , a first nut  1016   a , a second nut  1016   b  and a tuning sliding member  1300  as a dielectric member. 
     In  FIG. 14 , an etching area  1400   a , a first tuning area  1402   a , a second tuning area  1402   b  and a dielectric area  1302  are formed on an upper surface  1002   a  of the cover  1002 . 
     In  FIG. 14(B) , etching areas  1400   b  and  1400   c , a third tuning area  1406  and a fourth tuning area  1408  are formed on a rear surface  1002   b  of the cover  1002 . 
     The tuning areas  1402   a ,  1402   b ,  1406  and  1408  are conductive areas, for example are coated with conductive material. 
     A first tuning element  1014   a  is inserted into a first cavity  1008   a  through the first tuning area  1402   a  and the third tuning area  1406  of the cover  1002 , and a second tuning element  1014   b  is inserted into a second cavity  1008   b  through the second tuning area  1402   b  and the fourth tuning area  1408  of the cover  1002 . 
     The first tuning element  1014   a  is fixed to the upper surface of the cover  1002  by the first nut  1016   a , and the second tuning element  1014   b  is fixed to the upper surface  1002   a  of the cover  1002  by the second nut  1016   b.    
     The dielectric area  1302  locates between the first tuning area  1402   a  and the second tuning area  1402   b  in the first etching area  1400   a . The first tuning area  1400   a  and the second tuning area  1400   b  are separated physically, but coupling generates between the first tuning area  1400   a  and the second tuning area  1400   b.    
     In one embodiment of the present invention, two holes  1410  and  1412  may be formed on the tuning sliding member  1300  as shown in  FIG. 14(D) . The tuning sliding member  1300  is fixed by the first tuning element  1014   a  inserted into the first cavity  1008   a  through the first hole  1410  and the cover  1002  or the first nut  1016   a , and may shift left and right as shown in  FIG. 14(C)  under the condition that it is fixed by the first tuning element  1014   a  or the first nut  1016   a . An end part of the tuning sliding member  1300  overlaps on the dielectric area  1302 , and capacitance between the tuning elements  1402   a  and  1402   b  is varied according to the overlap area. As a result, cross coupling amount between corresponding resonators or transmission zero may be changed. The tuning sliding member  1300  may shift front and rear direction. The tuning sliding member  1300  may be fixed through various methods after it is shifted to desired position. 
     In brief, the RF filter of the present invention may adjust the cross coupling amount between corresponding resonators or the transmission zero by controlling the overlap area of the tuning sliding member  1300  disposed on the upper surface  1002   a  of the cover  1002  and the dielectric area  1302 . 
     In above description, the tuning sliding member  1300  has rectangular shape, but may have variously shapes as long as it is overlapped on the dielectric area  1302  to change the capacitance between the tuning elements  1402   a  and  1402   b.    
     The tuning sliding member  1300  is disposed on the position corresponding to the first tuning element  1014   a  in  FIG. 14 , but may be disposed on the position corresponding to the second tuning element  1014   b.    
     The tuning elements  1014   a  and  1014   b  may move up and down through their rotation. 
       FIG. 15  is a view illustrating a cover of an RF filter according to a fifth embodiment of the present invention. 
     In  FIG. 15 , a tuning sliding member  1500  is disposed on an upper surface  1002   a  of a cover  1002  in the RF filter of the present embodiment. Unlike the fourth embodiment where the tuning sliding member  1300  is supported by the first tuning element  1014   a , the tuning sliding member  1500  is disposed on the first tuning element  1014   a  as shown in  FIG. 15(C) . The tuning sliding member  1500  overlaps on a dielectric area  1502 , and so cross coupling amount between corresponding resonators or transmission zero is changed. 
     A hole  1510  may be formed on the tuning sliding member  1500  as shown in  FIG. 15(D) . 
     Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.