Patent Publication Number: US-2004041661-A1

Title: Dielectric filter, communication apparatus, and method of controlling resonance frequency

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to a dielectric filter that is used in a mobile communication base station such as a cellular phone, a broadcast radio wave transmission station and the like, and a communication apparatus using the dielectric filter.  
       [0003] 2. Related Art of the Invention  
       [0004] In recent years, high-sensitivity send/receive characteristics and satisfactory speech quality have become essential in cellular phone systems, and low-loss passage characteristics with almost no degradation of signal components and steep-attenuation characteristics capable of reliably removing undesired disturbing wave components are required for filters in the base station. Filters satisfying such demands include a dielectric filter using a dielectric resonator having a high Q value (e.g. see James K. Plourde, Application of Dielectric Resonators in Microwave Components, “IEEE TRANSACTION ON MICROWAVE THEORY AND TECHNIQUES”, IEEE, August, 1981 vol. MTT-29, No. 8, p.754-p. 769.). The entire disclosure of the above document are incorporated herein by reference by its entirety.  
       [0005] One example of a conventional dielectric filter will be described with reference to the drawings. FIG. 13 shows a dielectric filter  1000  in which TE 01 δ resonance mode dielectric resonators are four-stage connected (the TE 01 δ resonance mode represents a basic resonance mode). In this dielectric filter  1000 , four cavities (spaces) are formed by a metal case  1001  forming a shield box, a metal partition plate  1002  and a metal lid  1003 , and dielectric resonance elements  1005   a  to  1005   d  are bonded on the bottom surface of the case  1001  through a support  1004  with each dielectric resonance element located at almost the center of each cavity. The support  1004  is made of dielectric material of low radio frequency wave loss such as alumina.  
       [0006] A coupling window  1010  formed by providing a space between the partition plate  1002  and the case  1001  is provided between the partition plate  1002  and the side face of the case  1001 . Input/output terminals  1007   a  and  1007   b  are attached to the both ends of the cavities communicating through the coupling window  1010 , and input/output probes  1008   a  and  1008   b  for electromagnetic field coupling with dielectric resonance elements  1005   a  and  1005   d  are provided for the internal conductors of the input/output terminals  1007   a  and  1007   b , respectively. The dielectric resonance elements  1005   a  to  1005   d  are electromagnetic field-coupled via the coupling window  1010 . The magnitude of this coupling is dependent on the size of the coupling window  1010 , and is finely adjusted by moving toward or away from the partition plate  1002   a  coupling adjusting screw  1009  extending to the coupling window of each partition plate  1002 . Also, tuning means  1012  constituted by a metal bolt  1006  and a metal plate  1007  for adjusting the resonance frequency is provided in the lid  1003  in correspondence with the positions of the dielectric resonance elements  1005   a  to  1005   d.    
       [0007] When a signal is inputted from the input/output terminal  1007   a , the input/output probe  1008   a  and the dielectric resonance element  1005   a  are first electromagnetic field-coupled. Then, the dielectric resonance element  1005   a  and the dielectric resonance element  1005   b  placed in the adjacent cavity are electromagnetic field-coupled via the coupling window  1010 , and the dielectric resonance element  1005   b  and the dielectric resonance element  1005   c , the dielectric resonance element  1005   c  and the dielectric resonance element  1005   d , and the dielectric resonance element  1005   d  and the input/output probe  1008   b  are electromagnetic field-coupled, respectively, and the signal is outputted from the input/output terminal  1007   b . By adjusting the strength of each respective electromagnetic coupling, and adjusting the space between the plate  1007  of each tuning means  1012  and the upper face of each of the dielectric resonance elements  1005   a  to  1005   d , the resonance frequency of each of the dielectric resonance elements  1005   a  to  1005   d , and thus desired characteristics of the dielectric filter  1000  as a bandpass filter are achieved.  
       [0008]FIG. 16 is a perspective view of a conventional single TE 01 δ resonance mode dielectric filter  1100 . It has a structure in which a cavity is formed by a metal case  1101  and a metal lid  1102 , and a dielectric resonance element  1104  is bonded to the case  1101  through a support  1103  with the dielectric resonance element  1104  located at almost the center of the cavity as in the case of the four-stage filter. The resonance frequency of the dielectric resonance element  1104  is adjusted by tuning means  1012 . When a signal is inputted from an input/output terminal  1106   a , an input/output probe  1107   a  and a dielectric resonance element  1104  are electromagnetic field-coupled. Then, the dielectric resonance element  1104  and an input/output probe  1107   b  are electromagnetic field-coupled, and the signal is outputted from an input/output terminal  1106   b.    
       [0009] However, the configuration described above has a disadvantage that an undesired resonance mode (spurious) other than a desired resonance mode (TE 01 δ resonance mode) occurs on the high-pass side of the filter pass band, thus allowing an undesired signal to pass through.  
       [0010] In particular, the spurious due to insertion of tuning means  1012  appears near the TE 01 δ resonance mode. For example, FIG. 17 shows how the resonance frequency of the TE 01 δ resonance mode and the resonance frequency of the spurious are changed when the tuning means  1012  is moved down and inserted into the case  1101  in a single resonance filter  1100  (FIG. 16).  
       [0011] The single resonance filter  1100  shown in FIG. 16 is constituted by a metal case  1101 , a metal lid  1102 , input/output terminals  1106   a  and  1106   b , input/output probes  1107   a  and  1107   b , a dielectric resonance element  1104 , a support  1103  and tuning means  1012 .  
       [0012] As apparent from FIG. 17, a conventional dielectric filter  1000  has a disadvantage that when a plate  1007  is brought close to the dielectric resonance element  1104  with the tuning means  1012  inserted therein (i.e. the insertion length of a bolt  1006  is increased), in particular, the spurious is close or identical to the resonance frequency of the TE 01 δ resonance mode, thus making it impossible to achieve desired filter characteristics.  
       [0013] Also, the conventional dielectric filter  1000  has a disadvantage that the spurious cannot be sufficiently shielded in the electromagnetic coupling between adjacent dielectric resonance elements with a coupling window  1010  formed by providing a space between a partition plate  1002  and the side face of the case  1001 .  
       [0014] In addition, there is a disadvantage that the influence of the spurious also becomes significant in the electromagnetic coupling between dielectric resonance elements as a coupling adjusting screw  1009  is inserted into the coupling window  1010 .  
       SUMMARY OF THE INVENTION  
       [0015] In view of the problems described above, the present invention has as its object the provision of a dielectric filter of low-spurious characteristics capable of securing a, sufficient amount of attenuation on the high-pass side of the passband.  
       [0016] The 1 st  aspect of the present invention is a dielectric filter comprising:  
       [0017] a metal case having an opening in the upper part;  
       [0018] a metal lid of closing said opening;  
       [0019] a dielectric resonance element placed on the internal bottom face of said case through a support;  
       [0020] insertion means made of dielectric material inserted in a position in said lid corresponding to said dielectric resonance element; and  
       [0021] a metal plate placed at the end of said insertion means substantially in parallel with the upper face of said dielectric resonance element,  
       [0022] wherein the position of said insertion means is adjusted to change the space between said dielectric resonance element and said plate, whereby the resonance frequency is controlled.  
       [0023] The 2 nd  aspect of the present invention is the dielectric filter according to the 1 st  aspect, wherein said insertion means and said plate are fixed to each other using a screw made of dielectric material.  
       [0024] The 3 rd  aspect of the present invention is the dielectric filter according to the 2 nd  aspect, wherein said screw is a screw protrusion placed on said plate.  
       [0025] The 4 th  aspect of the present invention is the dielectric filter according to the 1 st  aspect, wherein said plate is fixed to said insertion means by bonding.  
       [0026] The 5 th  aspect of the present invention is a dielectric filter comprising:  
       [0027] a metal case;  
       [0028] at least one metal partition wall of partitioning the inside of said case into a plurality of spaces; and  
       [0029] dielectric resonance elements each placed on the bottom of each of said plurality of partitioned spaces through a support,  
       [0030] wherein for at least one of said partition walls partitioning adjacent spaces, a notch is provided in an area other that the area facing the side face of said case to form a coupling window of coupling said adjacent spaces.  
       [0031] The 6 th  aspect of the present invention is the dielectric filter according to the 5 th  aspect, further comprising a metal plate placed above said dielectric resonance element,  
       [0032] wherein said notch is provided on the side of said partition wall where said support is placed.  
       [0033] The 7 th  aspect of the present invention is the dielectric filter according to the 5 th  aspect, a metal coupling adjusting member of adjusting the strength of coupling between said adjacent dielectric resonance elements is inserted in said coupling window from said side face, and said coupling adjusting member is insulated from said side face.  
       [0034] The 8 th  aspect of the present invention is the dielectric filter according to the 5 th  aspect, said coupling window is rectangular.  
       [0035] The 9 th  aspect of the present invention is a dielectric filter comprising:  
       [0036] a metal case having an opening in the upper part;  
       [0037] a metal lid of closing said opening;  
       [0038] at least one metal partition wall of partitioning the inside of said case into a plurality of spaces; and  
       [0039] dielectric resonance elements each placed on the bottom of each of said plurality of partitioned spaces-through a support,  
       [0040] wherein a notch formed by providing a space between the side face of said case and at least part of said partition wall is formed in at least one of said partition walls partitioning adjacent spaces, and  
       [0041] a metal coupling adjusting member of adjusting the strength of coupling between said adjacent dielectric resonance elements is inserted in a position on the side face of said case corresponding to said notch, and said coupling adjusting member is insulated from said side face.  
       [0042] The 10 th  aspect of the present invention is the dielectric filter according to the 5 th  aspect, further comprising:  
       [0043] insertion means made of dielectric material inserted in a position located in the upper part of said metal case and corresponding to said dielectric resonance element; and  
       [0044] a metal plate placed at the end of said insertion means substantially in parallel with the upper face of said dielectric resonance element,  
       [0045] wherein said insertion means is adjusted to change a space between said dielectric resonance element and said plate, whereby the resonance frequency is controlled.  
       [0046] The 11 th  aspect of the present invention is a dielectric filter comprising:  
       [0047] a metal case;  
       [0048] a dielectric resonance element placed on the internal bottom face of said case through a support;  
       [0049] insertion means inserted in a position located in the upper part of said metal case and corresponding to said dielectric resonance element;  
       [0050] a metal plate placed at the end of said insertion means substantially in parallel with the upper face of said dielectric resonance element; and  
       [0051] a metal ring-shaped material run through said insertion means between the face of said metal case in which said insertion means is inserted and said plate.  
       [0052] The 12 th  aspect of the present invention is the dielectric filter according to the 1 st  aspect, wherein said dielectric material has a relative dielectric constant of 10 or smaller.  
       [0053] The 13 th  aspect of the present invention is a communication apparatus comprising a transmission apparatus and a reception apparatus,  
       [0054] wherein at least one of said transmission apparatus and reception apparatus comprises the dielectric filter according to any of the 1 st , 5 th , 9 th  and 11 th  aspects.  
       [0055] The 14 th  aspect of the present invention is a method of controlling a resonance frequency using a dielectric filter comprising:  
       [0056] a metal case having an opening in the upper part;  
       [0057] a metal lid of closing said opening;  
       [0058] a dielectric resonance element placed on the internal bottom face of said case through a support;  
       [0059] insertion means made of dielectric material inserted in a position in said lid corresponding to said dielectric resonance element; and  
       [0060] a metal plate placed at the end of said insertion means substantially in parallel with the upper face of said dielectric resonance element,  
       [0061] wherein the position of said insertion means is adjusted to change the space between said dielectric resonance element and said plate, whereby the resonance frequency is controlled. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0062]FIG. 1 is an exploded translucent perspective view of a dielectric filter of Embodiment 1 of the present invention.  
     [0063]FIG. 2 is an exploded translucent perspective view of the dielectric filter of Embodiment 2 of the present invention.  
     [0064]FIG. 3 is an enlarged translucent perspective view of a coupling adjusting screw of the dielectric filter of the embodiment of the present invention.  
     [0065]FIG. 4 is an enlarged translucent perspective view of a coupling window of the dielectric filter of the embodiment of the present invention.  
     [0066]FIG. 5 shows pass characteristics of the dielectric filter of the embodiment of the present invention and the conventional dielectric filter.  
     [0067]FIG. 6 is an exploded perspective view of tuning means of the dielectric filter of the embodiment of the present invention.  
     [0068]FIG. 7 shows changes in TE 01 δ resonance mode and spurious resonance frequencies of the dielectric filter of Embodiment 1 of the present invention.  
     [0069]FIG. 8 shows changes in TE 01 δ resonance mode and spurious resonance frequencies of the dielectric filter of the embodiment of the present invention.  
     [0070]FIG. 9 shows changes in TE 01 δ resonance mode and spurious resonance frequencies of the dielectric filter of the embodiment of the present invention.  
     [0071]FIG. 10 is an exploded perspective view of tuning means of the dielectric filter of the embodiment of the present invention.  
     [0072]FIG. 11 is an enlarged view of tuning means of the dielectric filter of the embodiment of the present invention.  
     [0073]FIG. 12 is a characteristic view of a single resonator of the dielectric filter of the embodiment of the present invention.  
     [0074]FIG. 13 is an exploded translucent perspective view of the conventional dielectric filter.  
     [0075]FIG. 14 is a translucent plan view of the dielectric filter of the present invention.  
     [0076]FIG. 15 is an exploded translucent perspective view of the dielectric filter of the present invention.  
     [0077]FIG. 16 is an exploded translucent perspective view of a single dielectric resonator of the conventional dielectric filter.  
     [0078]FIG. 17 shows the tuning means insertion length and changes in TE 01 δ resonance mode and spurious resonance frequencies of the single dielectric resonator of the conventional dielectric filter.  
     [0079]FIG. 18 illustrates the operation principle of the dielectric filter of the present invention.  
     [0080]FIG. 19 illustrates the operation principle of the conventional dielectric filter.  
     [0081]FIG. 20 is a block diagram showing an outlined configuration of a communication apparatus of the present invention. 
    
    
     DESCRIPTION OF SYMBOLS  
     [0082] 101 ,  1001 ,  1101  Case  
     [0083] 102 ,  1002  Partition plate  
     [0084] 103 ,  1003 ,  1102  Lid  
     [0085] 104 ,  1004 ,  1103  Support  
     [0086] 105 ,  1005 ,  1104  Dielectric resonance element  
     [0087] 106 ,  1006  Bolt  
     [0088] 107 ,  701  Plate  
     [0089] 108   a ,  108   b ,  1007   a ,  1007   b ,  1106   a ,  1106   b  Input/output terminal  
     [0090] 109   a ,  109   b ,  1008   a ,  1008   b ,  1107   a ,  1107   b  Input/output probe  
     [0091] 110 ,  1009  Coupling adjusting screw  
     [0092] 111  Notch  
     [0093] 112 ,  1012  Tuning means  
     [0094] 113 ,  1010  Coupling window  
     [0095] 201  Bushing  
     [0096] 501  Screw  
     [0097] 801  Nut  
     PREFERRED EMBODIMENTS OF THE INVENTION  
     [0098] (Embodiment 1)  
     [0099] A dielectric filter of Embodiment 1 of the present invention will be described with reference to the drawings.  
     [0100]FIG. 1 is an exploded translucent perspective view of a five-stage TE 01 δ resonance mode bandpass filter (five-stage dielectric filter)  100  in Embodiment 1 of the present invention. In FIG. 1, the five-stage dielectric filter  100  comprises a case  101  being one example of the metal case of the present invention, a partition plate  102  being one example of the partition wall of the present invention, a lid  103  being one example of the metal lid of the present invention, a support  104 , dielectric resonance elements  105   a  to  105   e  being one example of the dielectric resonance element of the present invention, tuning means  112 , input/output terminals  108   a  and  108   b  (not shown), input/output probes  109   a  and  109   b , and a coupling adjusting screw  110  being one example of the metal coupling adjusting member of the present invention.  
     [0101] The tuning means  112  has a bolt  106  being one example of insertion means of the present invention and a plate  107  being one example of the metal plate of the present invention. Five cavities (spaces) are formed by the case  101 , the partition plate  102  placed in the case  101  and the lid  103 , and the dielectric resonance elements  105   a  to  105   e  having flat upper faces are each mounted on the bottom face of the case  101  through the support  104  to be located at almost the center of each cavity.  
     [0102] Materials constituting the plate  107 , the case  101  and the lid  103  are preferably those of high conductivity such as aluminum, copper, brass, silver, aluminum plated with silver, brass plated with silver and iron plated with silver.  
     [0103] The cavities partitioned by the case  101 , the partition plate  102  and the lid  103  communicate through a coupling window  113  formed in the partition plate  102 , and the input/output terminals  108   a  and  108   b  are attached to the both ends of these cavities. The input/output probes  109   a  and  109   b  for electromagnetic coupling with the dielectric resonance elements  105   a  and  105   e  are connected to the internal conductors of the input/output terminals  108   a  and  108   b , respectively. The input/output probes  109   a  and  109   b  are placed in the close proximity of the dielectric resonance elements  105   a  and  105   e.    
     [0104] In the partition plate  102 , a notch is provided in such a manner that it contacts part of the bottom face of the case  101  to form the coupling window  113  of coupling electromagnetically adjacent dielectric resonance elements together. The coupling window  113  is formed in such a manner that it does not contact the side face of the case  101 , and the size of the coupling window  113  is dependent on the required magnitude of the coupling between dielectric resonance elements.  
     [0105] On the side face of the case  101 , a hole  114  is formed in a position corresponding to the partition plate  102 , the coupling adjusting screw  110  (e.g. having a diameter of M 2 ) of finely adjusting the strength of coupling between dielectric resonance elements is inserted through the hole  114 , and the coupling adjusting screw  110  penetrates through part of the partition plate  102  to protrude into the coupling window  113 .  
     [0106] In the lid  103 , holes  115  each provided in its inner face with a thread are formed in correspondence to the positions of dielectric resonance elements  105   a  to  105   e , and the bolt  106  made of polycarbonate being one example of insertion means of the present invention, which is engaged with the thread, is inserted in the hole  115 . The plate  107  is provided at one end of each bolt  106  substantially in parallel with the upper face of each dielectric resonance element. In this way, each bolt  106  and plate  107  constitutes the tuning means  112  of adjusting the resonance frequency.  
     [0107]FIG. 6 is an exploded perspective view of the tuning means  112  of FIG. 1. A recess  502  internally provided with a thread for engagement with a screw  501  made of polycarbonate is formed at the end of the bolt  106 , and a hole  503  for insertion of the screw  501  is formed at the center of the plate  107 . The screw  501  is squeezed into the recess  502  of the bolt  106  through the hole  503  formed in the plate  107 , whereby the bolt  106  and the plate  107  are fixed together to fabricate the tuning means  112 .  
     [0108] As one example of the dimension of the tuning means  112 , the bolt  106  made of polycarbonate has an outer face provided with a thread having a diameter of 4 mm and a pitch of 0.7 mm, and an inner face provided in the recess  502  with a thread having a diameter of 2 mm, a pitch of 0.4 mm and a depth of about 5 mm with the axis shared with the thread with the diameter of 4 mm. The plate  107  is a copper plate having an outer diameter of 10 mm and a thickness of 0.5 mm, and the diameter of the hole  503  is 2.2 mm. These are only one example of the dimension and material, the dimension and material of the tuning means  112  is not limited to those described above.  
     [0109] Operations of the five-stage dielectric filter  100  configured as described above will now be described.  
     [0110] When a signal is inputted from the input/output terminal  108   a , the input/output probe  109   a  and the dielectric resonance element  105   a  are electromagnetic field-coupled. Then, the dielectric resonance element  105   a  and the dielectric resonance element  105   b  placed in the cavity adjacent the cavity in which the dielectric resonance element  105   a  are electromagnetic field-coupled through the coupling window  113 , and the dielectric resonance element  105   b  and the dielectric resonance element  105   c , the dielectric resonance element  105   c  and the dielectric resonance element  105   d , the dielectric resonance element  105   d  and the dielectric resonance element  105   e , and the dielectric resonance element  105   e  and the input/output probe  109   b  are electromagnetic field-coupled, respectively, and the signal is outputted from the input/output probe  109   b.    
     [0111] At this time, the coupling adjusting screw  110  is adjusted for finely adjusting the strength of each electromagnetic field coupling, and the bolt  106  of the tuning means  112  is rotated to adjust the length of its insertion into the case  101  (i.e. space between the plate  107  and each of the dielectric resonance elements  105   a  to  105   e ) to control the resonance frequency.  
     [0112] When the tuning means  112  is adjusted and thereby the plate  107  approaches each dielectric resonance element, the electromagnetic field of TE 01 δ resonance mode formed in each cavity is depressed, resulting in a situation equivalent to a decrease in the size of the cavity. The resonance frequency is controlled in this way, and thus desired bandpass filter characteristics are achieved.  
     [0113] Since the coupling window  113  is formed in such a manner that it contacts the bottom face of the case  101 , the electromagnetic field coupling between adjacent cavities is not significantly weakened even if the electromagnetic field of TE 01 δ resonance mode is depressed by the plate  107 .  
     [0114] The principle of how the spurious is reduced by the dielectric filter of this embodiment will now be described. FIG. 18 shows a resonator called a semi-coaxial resonator. This semi-coaxial resonator has a metal box  300  and a metal axis unit  301 , and the length of the metal axis unit  301  is one quarter of the working wavelength. According to this semi-coaxial resonator, resonance occurs at a frequency having a wavelength equal to one quarter of the working wavelength or an odd multiple thereof.  
     [0115]FIG. 19 shows a cross section of the side face of a conventional dielectric filter  1100 . In the conventional dielectric filter  1100 , it can be considered that because the bolt  1006  is made of metal, a semi-coaxial resonator or its altered mode dictated by the insertion length of the bolt  1006 , and together with the plate  1007 , thus spurious components appear as shown by the dashed line in FIG. 19. According to the dielectric filter  100  of this embodiment, however, the bolt  106  made of dielectric material is used instead of the metal bolt  1006 , and therefore the semi-axial resonator as shown by the dashed line in FIG. 19 is less likely formed. Also, the coupling window  113  is formed in such a manner that it contacts the bottom face of the case  101 . In other words, the coupling window  113  is formed at a location away from the plate  107  on the partition plate  102 . Thus, the rate at which the spurious is transferred to the adjacent cavities is low even if the spurious is generated. In this way, in general, the ratio of the undesired spurious to the desired TE 01 δ resonance mode is reduced.  
     [0116] Also, in the dielectric filter of this embodiment, the bolt  106  and the plate  107  are coupled together by the screw  501  made of polycarbonate, and therefore there is no possibility that the spurious is generated due to the influence of the screw  501 . That is, if the screw  501  is made of metal, a semi-coaxial resonator is formed by the metal screw  501  and the plate  107 , and the spurious associated with the insertion length of the metal screw  501  is generated, but such a spurious is not generated in the dielectric filter of the present invention.  
     [0117]FIG. 7 shows how the TE 01 δ resonance mode resonance frequency and the spurious resonance frequency change with the insertion length of the bolt  106  when the tuning means  1012  is changed to the tuning means  112  described above in the single resonance filter shown in FIG. 16. As apparent from FIG. 7, according to the single resonance filter using the tuning means  112 , the spurious resonance frequency almost never approaches the TE 01 δ resonance mode resonance frequency even if the bolt is inserted so that the tuning means  112  is brought into close proximity to the dielectric resonance element  1104 . The dielectric resonance filter  100  of this embodiment has these single resonance filters stacked in four stages, and therefore the same effect can be obtained in the dielectric resonance filter  100 .  
     [0118]FIG. 5 shows a comparison of the frequency pass characteristics of the dielectric filter  100  of this embodiment and the frequency pass characteristics of the dielectric filter having no partition plate  102  shown in FIG. 15. Other conditions are the same. In FIG. 5, the narrow line shows the frequency pass characteristics of the dielectric filter of this embodiment, and the wide line shows the frequency pass characteristics of the dielectric filter having no partition plate  102 . As apparent from FIG. 5, a large amount of spurious occurs in the frequency pass characteristics in which no partition plate  102  is used, while the spurious is considerably reduced when the partition plate  102  is used. That is, it can be understood that the spurious can be effectively reduced by forming the coupling window  113  in such a manner that it does not contact the side face of the case  101 .  
     [0119] As described above, according to the dielectric filter of this embodiment, the coupling window  113  dictating the coupling between dielectric resonance elements is formed on the partition plate  102  in such a manner that it does not contact the side face of the case  101 , whereby a resonance mode (spurious) other than a desired resonance mode can be reduced.  
     [0120] Also, according to the dielectric filter of this embodiment, the pole (bolt  106 ) of the tuning means  112  of adjusting the resonance frequency of the dielectric resonator is used as a dielectric (insulator), whereby the spurious appearing when the tuning means  112  is moved down is prevented from being brought into close proximity to a desired resonance mode.  
     [0121] Furthermore, in the description of the above embodiment, the coupling window  113  is formed in such a manner that it contacts part of the bottom face of the case  101  and does not contact the side face of the case  101 , but the coupling window  113  may be formed in such a manner that it contacts the entire bottom face, or may be formed in a different form. For example, the coupling window  113  may be formed in such a manner that it contacts the lid  103  and does not contact the side face of the case  101 , or may be formed in such a manner that it contacts another partition plate  102 . Also, the coupling window  113  may be enclosed on the partition plate  102  as shown in FIG. 4. That is, any form of coupling window can bring about the same effect as described above as long as the coupling window  113  is formed in such a manner that it does not contact the side face of the case  101 .  
     [0122] Also, in the embodiment described above, the dielectric filter has as its components the bolt  106  made of dielectric material and the coupling window  113  formed in such a manner that it does not contact the side face of the case  101 , but the dielectric filter may have only one of these components.  
     [0123] That is, only the bolt  1006  of the tuning means  1012  in the dielectric filter  1000  of the prior art may be changed to the bolt  106  of the dielectric filter  100  in the embodiment described above, or only the partition plate  1002  having the coupling window  1010  in the dielectric filter  1000  of the prior art may be changed to the partition plate  102  having the coupling window  113  in the embodiment described above. The same effect as described above can be obtained even when this dielectric filter is used.  
     [0124] (Embodiment 2)  
     [0125]FIG. 2 is an exploded translucent perspective view of a four-stage TE 01 δ resonance mode bandpass filter (four-stage dielectric filter)  200  in Embodiment 2. In FIG. 2, components same as those of the five-stage dielectric filter  100  of Embodiment 1 are given like reference symbols, and the description thereof is omitted. The four-stage dielectric filter  200  of this embodiment comprises dielectric resonance elements  105   a  to  105   d  being one example of the dielectric resonance element of the present invention. In the four-stage dielectric filter of this embodiment, four cavities (spaces) are formed in such a manner that each cavity adjoins other two cavities by a case  101 , a partition plate  102  placed in the case  101  and a lid  103 , and the dielectric resonance elements  105   a  to  105   d  having flat upper faces are each mounted on the bottom face of the case  101  through a support  104  to be located at almost the center of each cavity.  
     [0126] In the partition plate  102 , a notch  111  is formed in such a manner that it contacts the side face of the case  101 . However, the notch  111  is not formed in the area of the partition plate  102  between the dielectric resonance element  105   a  connected to an input/output terminal  108   a  and the dielectric resonance element  105   d  connected to an input/output terminal  108   b . The notch  111  is formed in such a manner that it contacts the side face of the case  101 .  
     [0127] A hole  114  is formed in a position on the side face of the case  101  corresponding to the notch  111 , and a coupling adjusting screw  110  (e.g. having a diameter of M 2 ) of adjusting the strength of coupling between dielectric resonance elements is inserted through the hole  114 . A bushing  201  made of polycarbonate provided with a thread matched with the coupling adjusting screw  110  is inserted in the hole  114  (e.g. having a diameter of M 4 ) as shown in FIG. 3, and the coupling adjusting screw  110  is thereby supported. That is, the metal case  101  and the metal coupling adjusting screw  110  are electrically insulated from each other by the bushing  201 .  
     [0128] Operations of the four-stage dielectric filter configured as described above will now be described.  
     [0129] When a signal is inputted from the input/output terminal  108   a , the input/output probe  109   a  and the dielectric resonance element  105   a  are electromagnetic field-coupled. Then, the dielectric resonance element  105   a  and the dielectric resonance element  105   b  placed in the cavity adjacent to the cavity in which the dielectric resonance element  105   a  are electromagnetic field-coupled through the notch  111 , and the dielectric resonance element  105   b  and the dielectric resonance element  105   c , the dielectric resonance element  105   c  and the dielectric resonance element  105   d , and the dielectric resonance element  105   d  and the input/output probe  109   b  are electromagnetic field-coupled, respectively, and the signal is outputted from the input/output probe  108   b.    
     [0130] At this time, the coupling adjusting screw  110  is adjusted for finely adjusting the strength of each electromagnetic field coupling, and a bolt  106  of tuning means  112  is rotated to adjust the insertion length into the case  101  (i.e. space between the plates  107  and each of the dielectric resonance elements  105   a  to  105   d ) for controlling the resonance frequency. In this way, desired bandpass filter characteristics are achieved.  
     [0131]FIG. 9 shows a comparison of the frequency pass characteristics when the coupling adjusting screw  110  is not supported by the bushing  201  made of dielectric material but supported directly on the metal case  101  and the frequency pass characteristics when the coupling adjusting screw  110  is supported by the bushing  201 . This data shows characteristics for existence of the bushing  201  versus characteristics for nonexistence of the bushing  201  when the two-stage dielectric filter shown in FIG. 14 is used, the lengths of input/output probes  109   a  and  109   b  are decreased to reduce the coupling, and the coupling adjusting screw  110  is inserted to the depth of 9 mm. Same components as those of the dielectric filter shown in FIG. 2 are used. Also, for the coupling adjusting screw  110 , a copper screw is used, and it is inserted into the metal case  130  to the depth of 9 mm to make measurements.  
     [0132] As apparent from FIG. 9, occurrence of the undesired spurious. (e.g. area shown as a pole in FIG. 9) can be inhibited more effectively when the bushing  201  is used. That is, the spurious is more effectively inhibited when the coupling adjusting screw  110  is electrically insulated from the metal case  101 .  
     [0133] As described above, according to the dielectric filter of the present invention, the metal coupling adjusting screw  110  of adjusting inter-stage coupling between dielectric resonance elements is connected to the metal case through the bushing  201  made of polycarbonate, whereby the spurious appearing when the coupling adjusting screw  110  is inserted in the case  101  can be prevented being brought into close proximity to a desired resonance mode.  
     [0134] Furthermore, in this embodiment, the material of the bushing  201  is not limited to polycarbonate, and any other materials having good high-frequency characteristics may be used.  
     [0135] Also, in the embodiment described above, the dielectric filter has as its components the bolt  106  made of dielectric material, and the bushing  201  made of dielectric material for supporting the coupling adjusting screw  110  on the side face of the case  101 , but it may have a configuration in which the bushing  201  made of dielectric material for supporting the coupling adjusting screw is used in the dielectric filter  1000  of the prior art.  
     [0136] Also, in the above description, the tuning means  112  is such that the plate  107  is screw-fixed to the bolt  106  by the screw  501 , but as shown in FIG. 10, it may have a structure in which a screw protrusion  702  is attached to the central portion of a plate  701  by bonding or the like. In this way, the number of parts can be reduced.  
     [0137] The bolt  106  may be bonded to the plate  107  with no recess provided in the bolt  106  and with no hole provided in the plate  107 . In this way, the number of parts can further be reduced, thus making it possible to achieve a cost reduction.  
     [0138] Also, in the above description, polycarbonate is used as a material of the bolt  106 , but the same effect can be obtained even if any other nonmetal dielectric material having good high-frequency characteristics (e.g. dielectric loss tangent of 0.001 or smaller) such as syndiotactic polystyrene is used. FIG. 8 shows how the TE 01 δ resonance mode resonance frequency and the spurious resonance frequency change with the insertion length of the bolt  106  when a polyphenylene sulfide resin is used as the material of the bolt  106 . As apparent from FIG. 8, the TE 01 δ resonance mode resonance frequency and the spurious resonance frequency are almost no longer closer to each other even in the high-frequency range.  
     [0139] Also, in the above description, the tuning means  112  is constituted by the bolt  106  and the plate  107 , and is inserted in the hole  115  formed in the lid  103 , and by rotating the bolt  106 , the distance between the plate  107  and each dielectric resonance element is adjusted to control the resonance frequency of each dielectric resonance element, but it is also conceivable that an axis member provided with no thread is used instead of the bolt  106 . In this case, the insertion length of this axis member into the case  101  may be adjusted using, for example, a vernier gear.  
     [0140] Also, in the above description, examples in which the dielectric filters  100 ,  200  of the embodiments of the present invention have a structure of cavities formed in series in the case  101  by the partition plates  102 , or a structure of four cavities partitioned by the intersecting partition plates  102  in the case  101  have been described, but the present invention is not limited to the examples, but cavities may be provided in other forms, and any structure may be used as long as dielectric resonance elements placed in cavities are electromagnetically coupled through the coupling window (or notch) formed in the partition plate.  
     [0141] Also, needless to say, the number of cavities partitioned by the partition plates  102  in the case  101  is not limited to the number described in the above example, but the case  101  may be partitioned into any number of cavities.  
     [0142] Also, in the above description, the case  101  of the dielectric filter is rectangular, but the case  101  may have any shape such as, for example, a cylindrical shape as long as it is capable of placing therein dielectric resonance elements, tuning means, coupling adjusting screws and the like.  
     [0143] Also, there may be cases where a metal nut  801  is inserted between the plate  1007  and the lid  1003  as shown in FIG. 11 when conventional tuning means  1012  is used. By inserting the metal nut  801  in this way, generation of the spurious by a semi-coaxial resonator formed by the bolt  1006  and the plate  1007  is inhibited. The characteristics of the single resonance filter at this time are shown in FIG. 12. The narrow line shows the frequency pass characteristics when the nut  801  does not exist, and the wide line shows the frequency pass characteristics when the nut exists. As apparent from FIG. 12, the spurious frequency is shifted to the high-pass range in the pass characteristics of the single resonance filter when the nut  801  is inserted. In this way, the spurious can easily be isolated from the passband.  
     [0144] Furthermore, one or more nuts  801  may be used as necessary. The same effect can be obtained even if a metal body such as a ring-shaped metal is used instead of the nut.  
     [0145] Also, in the above description, the dielectric filter of the present invention comprises the tuning means  112  and the coupling adjusting screw  110 , but the filter may have a configuration in which the tuning means  112  or the coupling adjusting screw  110  is absent. In this dielectric filter, the same effect as described above can be obtained.  
     [0146] Also, in the above description, the dielectric filter of the present invention comprises the metal case  101  having an opening in the upper part and the metal lid  103  of closing the opening, and the tuning means  112  is inserted into the lid  103 , but the tuning means  112  may be inserted into the upper face of the case with the upper part closed. In this case, the same effect as described above can be obtained.  
     [0147] Also, in the above description, the dielectric filter of the present invention has the partition plate  102  used as a partition wall, but the present invention is not limited to this configuration, and cavities may be arranged in other forms. For example, the dielectric filter may have a configuration in which a plurality of single resonance filters shown in FIG. 16 are arranged, and the side face of the case  1101  of each single resonance filter acts as a partition wall. In this case, the coupling window described above may be formed on each side face.  
     [0148] In addition, the dielectric filter itself has been described above, but the present invention also include a communication apparatus  1204  comprising a transmission apparatus  1202  and a reception apparatus  1201 , wherein at least one of the transmission apparatus  1202  and the reception apparatus  1201  comprises any of the dielectric filters described above. A schematic block configuration of the communication apparatus  1204  is shown in FIG. 20.  
     [0149] According to the present invention, a dielectric filter of low-spurious characteristics capable of securing a sufficient amount of attenuation on the high-pass side of the passband can be provided.