Abstract:
A dielectric filter includes two dielectric resonators each having a dielectric base body including first, second, third and fourth side surfaces and first and second end surfaces. A through hole is provided from the first end surface to the second end surface to define an inner surface. An outer conductor is located on and partially covers the four side surfaces. Uncovered portions of the second and third side surfaces are provided that are adjacent to each other and bounded by the outer conductor in three directions. An uncovered portion of the fourth side surface is adjacent to an uncovered portion of the first side surface and the first end surface, and is separated from the third side surface by the outer conductor. An inner conductor is located on the inner surface. A connection conductor is also located on the second end surface and connects the inner conductor to the outer conductor. An interstage coupling electrode is located on the uncovered portion of the first side surface. An area of the uncovered portion of the first side surface extends in at least three directions from the interstage coupling electrode and separates the interstage coupling electrode from the outer conductor. An input/output coupling electrode is located on the uncovered portion of the second and third side surfaces. Areas of the uncovered portions of the second and third side surfaces extend in three directions from the input/output coupling electrode and separate the input/output coupling electrode from the outer conductor. The the outer conductor of a first dielectric resonator is electrically connected to the outer conductor of a second dielectric resonator.

Description:
This is a Division of application Ser. No. 08/202,073 filed Feb. 25, 1994, now issued as U.S. Pat. No. 5,499,004. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a dielectric filter installed on a communication apparatus, such as a portable telephone and a wireless telephone. 
     2. Description of the Prior Art 
     A conventional dielectric filter will be explained hereinafter. 
     FIG. 27 is a perspective view showing a conventional dielectric filter. Reference numerals 1 and 2 represent respective dielectric resonators comprising: dielectric base bodies 1a, 2a made of dielectric material, with respective centrally axially extending through holes 1b, 2b; respective outer conductors 1c, 2c surrounding respective outer surfaces of the dielectric base bodies 1a, 2a; respective inner conductors 1d, 2d provided along respective inner surfaces of the through holes 1b, 2b; and respective connecting conductors 1e and 2e connecting the outer conductors 1c, 2c and the inner conductors 1d, 2d. A reference numeral 5 represents a coupling substrate 5 comprising conductors 5a, 5b (as shown in FIG. 28A) and 5c, 5d (as shown in FIG. 28B) formed on a dielectric substrate (alumina substrate or the like). This connecting substrate 5 realizes input/output coupling capacitances (one is between conductors 5a and 5c, the other between conductors 5b and 5d) and an interstage coupling capacitance (between 5a and 5b). FIG. 28(A) is a detailed front view showing the coupling substrate 5 of the conventional dielectric filter, and FIG. 28(B) is a detailed rear view showing the coupling substrate 5 of the conventional dielectric filter. Reference numerals 6 and 7 represent central conductors connecting the inner conductors 1d, 2d and the coupling substrate 5 electrically and mechanically. Reference numerals 8 and 9 represent input and output terminals which are connected to the conductors 5c, 5d of the coupling substrate 5 electrically and mechanically by means of solder or the like. 
     However, this kind of conventional dielectric filter inherently requires numerous components, such as the coupling substrate 5, central conductors 6, 7, and input/output terminals 8, 9. This encounters with the difficulty in reducing the size of a dielectric filter. Furthermore, mass-productivity is not good due to a large number of parts and time-consuming production process. 
     SUMMARY OF THE INVENTION 
     Accordingly, in view of above-described problems encountered in the prior art, an object of the present invention is to provide a dielectric filter capable of reducing the size and the number of parts and having excellent mass productivity. 
     In order to accomplish the above object, a first aspect of the invention provides a dielectric filter comprising: at least two dielectric resonators each including: a respective dielectric base body including corresponding first, second, third and fourth side surfaces and first and second end surfaces, wherein a respective through hole is provided from the first end surface to the second end surface along a central axis of the corresponding dielectric base body to define a respective inner surface thereof; a respective outer conductor located on and partially covering the first, second, third and fourth side surfaces of the corresponding dielectric base body, wherein the outer conductor provides an uncovered portion of the first side surface that extends across a width of the respective first side surface and is adjacent to the corresponding first end surface, uncovered portions of the respective second and third side surfaces that are adjacent to each other and bounded by the corresponding outer conductor in three directions, and uncovered portion of the respective fourth side surface that is adjacent to the uncovered portion of the corresponding first side surface and the corresponding first end surface and is separated from the corresponding third side surface by the respective outer conductor; a respective inner conductor located on the inner surface of the corresponding dielectric base body; a respective connection conductor located on the second end surface of the corresponding dielectric base body, wherein the respective connection conductor connects the inner conductor to the outer conductor on the corresponding dielectric base body; a respective interstage coupling electrode located on the uncovered portion of the corresponding first side surface, wherein a respective area of the uncovered portion of the corresponding first side surface extends in at least three directions from the respective interstage coupling electrode and separates the interstage coupling electrode from the corresponding outer conductor; and a respective input/output coupling electrode located on the uncovered portion of the second and third side surfaces of the corresponding dielectric base body, wherein respective areas of the uncovered portions of the corresponding second and third side surfaces extend in three directions from the input/output coupling electrode and separate the respective input/output coupling electrode from the corresponding outer conductor; wherein the respective dielectric resonators are connected with each other in such a manner that the outer conductor of a first one of the dielectric resonators is electrically connected to the outer conductor of a second one of the dielectric resonators. 
     In accordance with a second aspect of the invention, the interstage coupling electrode of the first one of the dielectric resonators is electrically connected to the interstage coupling electrode of the second one of the dielectric resonators with respect to the first aspect of the invention discussed above. 
     In accordance with a third aspect of the invention, a dielectric filter is provided that comprises: at least two dielectric resonators each including: a respective dielectric base body including corresponding first, second, third and fourth side surfaces and first and second end surfaces, wherein a respective through hole is provided from the first end surface to the second end surface along a central axis of the corresponding dielectric base body to define a respective inner surface thereof; a respective outer conductor located on the first, second, third and fourth side surfaces of the corresponding dielectric base body, wherein the respective outer conductor covers the fourth side surface and leaves an uncovered portion of the first side surface that is adjacent to the first end surface and bounded in three directions by the corresponding outer conductor, and uncovered portions of the respective second and third side surfaces that are adjacent to each other and bounded by the corresponding outer conductor in three directions; a respective inner conductor located on the inner surface of the corresponding dielectric base body; a respective connection conductor located on the second end surface of the corresponding dielectric base body, wherein the respective connection conductor connects the inner conductor to the outer conductor on the corresponding dielectric base body; a respective interstage coupling electrode located on the uncovered portion of the corresponding first side surface, wherein a respective area of the uncovered portion of the corresponding first side surface extends in three directions from the respective interstage coupling electrode and separates the respective interstage coupling electrode from the corresponding outer conductor; and a respective input/output coupling electrode located on the uncovered portion of the second and third side surfaces of the corresponding dielectric body, wherein a respective area of the uncovered portions of the corresponding second and third side surfaces extend in three directions from the respective input/output coupling electrode and separate the respective input/output coupling electrode from the corresponding outer conductor; wherein the respective dielectric resonators are connected with each other in such a manner that the outer conductor of a first one of the dielectric resonators is electrically connected to the outer conductor of a second one of the dielectric resonators. 
     In accordance with a fourth aspect of the invention, the interstage coupling electrode of the first one of the dielectric resonators is electrically connected to the interstage coupling electrode of the second one of the dielectric resonators with respect to the third aspect of the invention discussed above. 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description which is to be read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing a dielectric filter in accordance with a first embodiment of the present invention; 
     FIG. 2 is an exploded perspective view showing the dielectric filter in accordance with the first embodiment of the present invention; 
     FIG. 3 is a circuit diagram showing an equivalent circuit of the dielectric filter in accordance with the first embodiment of the present invention; 
     FIG. 4 is a perspective view showing the dielectric filter mounted on a substrate in accordance with the first embodiment of the present invention; 
     FIG. 5 is a graph showing comparison of damping characteristics between the first embodiment and a prior art filter; 
     FIG. 6 is a perspective view showing a process of one manufacturing method of a dielectric resonator in accordance with the first embodiment of the present invention; 
     FIG. 7 is a perspective view showing another process of the one manufacturing method of the dielectric resonator in accordance with the first embodiment of the present invention; 
     FIG. 8 is a perspective view showing still another process of the one manufacturing method of the dielectric resonator in accordance with the first embodiment of the present invention; 
     FIG. 9 is a perspective view showing still another process of the one manufacturing method of the dielectric resonator in accordance with the first embodiment of the present invention; 
     FIG. 10 is a perspective view showing a modified embodiment of the first embodiment of the present invention; 
     FIG. 11 is an exploded perspective view showing the modified embodiment of the first embodiment of the present invention; 
     FIG. 12 is a perspective view showing a dielectric filter in accordance with a second embodiment of the present invention; 
     FIG. 13 is an exploded perspective view showing the dielectric filter in accordance with the second embodiment of the present invention; 
     FIG. 14 is a perspective view showing the dielectric filter mounted on a substrate in accordance with the second embodiment of the present invention; 
     FIG. 15 is a perspective view showing a modified embodiment of the second embodiment of the present invention; 
     FIG. 16 is an exploded perspective view showing the modified embodiment of the second embodiment of the present invention; 
     FIG. 17 is a perspective view showing a dielectric filter in accordance with a third embodiment of the present invention; 
     FIG. 18 is an exploded perspective view showing the dielectric filter in accordance with the third embodiment of the present invention; 
     FIG. 19 is a perspective view showing the dielectric filter mounted on a substrate in accordance with the third embodiment of the present invention; 
     FIG. 20 is a perspective view showing the dielectric filter mounted on a substrate in accordance with the third embodiment of the present invention; 
     FIG. 21 is a graph showing the comparison of damping characteristics outside the band with respect to frequencies between the first embodiment (represented by curve B) and third embodiment (represented by curve A); 
     FIG. 22 is a perspective view showing a dielectric filter in accordance with a fourth embodiment of the present invention; 
     FIG. 23 is an exploded perspective view showing the dielectric filter in accordance with the fourth embodiment of the present invention; 
     FIG. 24 is a perspective view showing a modified dielectric filter of the first embodiment, wherein three dielectric resonators are included; 
     FIG. 25 is a perspective view showing a modified dielectric filter of the second embodiment, wherein three dielectric resonators are included; 
     FIG. 26 is a perspective view showing a modified dielectric filter of the third embodiment, wherein three dielectric resonators are included; 
     FIG. 27 is a perspective view showing a conventional dielectric filter; 
     FIG. 28(A) is a front view showing a coupling substrate of the conventional dielectric filter; 
     FIG. 28(B) is a rear view showing a coupling substrate of the conventional dielectric filter; 
     FIG. 29 is a perspective view showing a dielectric filter in accordance with a fifth embodiment of the present invention; 
     FIG. 30 is an exploded perspective view showing the dielectric filter in accordance with the fifth embodiment of the present invention; 
     FIG. 31 is a graph showing a relationship between an area S of a cutout portion and an interstage coupling degree K in accordance with the fifth embodiment of the present invention; 
     FIG. 32 is a perspective view showing a dielectric filter in accordance with a sixth embodiment of the present invention; 
     FIG. 33 is an exploded perspective view showing the dielectric filter in accordance with the sixth embodiment of the present invention; 
     FIG. 34 is a perspective view showing a modified dielectric filter of the sixth embodiment, wherein three dielectric resonators are included; 
     FIG. 35 is a rear view showing one dielectric resonator of the dielectric filter in accordance with the first embodiment of the present invention; and 
     FIG. 36 is a cross-sectional view showing the dielectric resonator, taken along a line X--X of FIG. 35. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be explained in detail with reference to accompanying drawings. 
     First Embodiment 
     FIGS. 1 and 2 are perspective and exploded views showing a dielectric filter of the first embodiment of the present invention. In FIGS. 1 and 2, reference numerals 11 and 12 represent dielectric resonators. Hereinafter, taking the dielectric resonator 11 of these two dielectric resonators, a construction of the dielectric resonator will be explained. A dielectric base body 11a made of dielectric material, such as BaO--TiO 2  --Nd 2  O 3 , BaO--TiO 2 , ZrO 2  --SnO 2  --TiO 2 , BaO--Sm 2  O 3  --TiO 2 , is formed with a centrally axially extending through hole 11b. The dielectric base body 11a has an outer configuration of rectangular parallelepiped having a square cross section, while the through hole 11b has a circular cross section. 
     An outer conductors 11c is provided on the outer side surface of the dielectric base body 11a so as to surround it. An inner conductor 11d is provided along the inner side surface of the through hole 11b. The outer conductor 11c and the inner conductor 11d are connected with each other via a connecting conductor 11e. This connecting conductor 11e is provided on a closed end surface, i.e. a base of the rectangular parallelepiped, of the dielectric base body 11a as shown in FIGS. 35 and 36. Of four outer side surfaces of the dielectric base body 11a, two side surfaces 11i and 11h are partly cut off so that the dielectric base body 11a is exposed near the open end 11j. Namely, a cutout 11k (see FIG. 2) of the outer conductor 11c bridging both the two side surfaces 11i and 11h is formed near the open end of the dielectric base body 11a. 
     Furthermore, an interstage coupling electrode 11f is provided within the region of the cutout 11k on the outer side surface 11i, so that this interstage coupling electrode 11f does not contact with other conductors. In the same manner, an input/output coupling electrode 11g is provided within the region of the cutout 11k on the outer side surface 11h, so that this input/output coupling electrode 11g does not contact with other conductors. 
     Likewise, the other dielectric resonator 12 comprises: a dielectric base body 12a, a through hole 12b, an outer conductor 12c, an inner conductor 12d, a connecting conductor 12e, an interstage coupling electrode 12f, an input/output coupling electrode 12g, outer side surfaces 12h and 12i, an open end 12j, and a cutout 12k (see FIG. 2). 
     The like parts between the dielectric resonators 11 and 12 are denoted by the same reference alphabet throughout views. (For example, the dielectric base body 11a is substantially identical with the dielectric base body 12a) However, as apparent from FIG. 2, the relationship between the dielectric resonator 11 and the dielectric resonator 12 are mirror symmetry. Therefore, in production of each dielectric resonator, it is necessary to pay attention to the positional relationship between the electrodes 11f, 11g and 12f, 12g. 
     More specifically, the dielectric resonator 11 and the dielectric resonator 12 are connected by means of, for example, cream solder in such a manner that the electrodes 11f and 12f confront and contact with each other and the electrodes 11g and 12g are placed on the same plane. 
     Furthermore, the outer conductors 11c, 12c, inner conductors 11d, 12d, connecting conductors 11e, 12e, electrodes 11f, 12f, and electrodes 11g, 12g (hereinafter, these components respectively are referred to as conductor film) are basically thin film made of conductive material such as copper and silver. The thickness of the film is approximately 5 μm. Although the conductor film is a single layer in this embodiment, it is needless to say that two or more layer structure can be allowed. Regarding the film thickness of approximately 5 μm, this value should be adequately changed depending on the condition of the dielectric filter in service. 
     Although the electrodes 11g, 12g and 11f, 12f are rectangular in this first embodiment, the configuration of these electrodes 11g, 12g and 11f, 12f can be any other shape, such as circle, ellipse, and polygon. 
     FIG. 3 is a circuit diagram showing an equivalent circuit of the dielectric filter in accordance with the first embodiment of the present invention depicted in FIGS. 1, 2. In FIG. 3, a reference numeral 13 represents an equivalent circuit of the dielectric resonator 11 and a reference numeral 14 represents an equivalent circuit of the other dielectric resonator 12 depicted in FIGS. 1, 2. C1 represents a capacitance between the electrode 11g and the inner conductor 11d, and C3 represents a capacitance between the electrode 12g and the inner conductor 12d. C2 represents a composite capacitance of two capacitances--one is a capacitance between the electrodes 11f and 11d, and the other is a capacitance between the electrodes 12f and 12d. As understood from this equivalent circuit, the dielectric filter of the present embodiment has substantially the same circuit configuration as the conventional dielectric filter. Nevertheless, the structure of this embodiment is very simplified and compact when compared with that of the conventional one. In more detail, this embodiment no longer requires the central conductor and the dielectric substrate of the conventional dielectric filter. This results in 50% reduction of overall size. 
     FIG. 4 is a perspective view showing the dielectric filter mounted on a substrate in accordance with the first embodiment of the present invention. In FIG. 4, a reference numeral 15 represents a printed circuit board constituted by insulating material, such as glass . epoxy resin. Reference numerals 16 and 17 represent input/output pathways formed on the printed circuit board 15. Similarly, a reference numeral 18 represents a grounded pathway formed on the printed circuit board 15. The electrode 11g of the dielectric resonator 11 is connected onto the input/output pathway 16 by a solder 19, and the electrode 12g of the dielectric resonator 12 is connected onto the input/output pathway 17 by a solder 20. Furthermore, the outer conductors 11c and 12c of the dielectric resonators 11 and 12 are connected onto the grounded pathway 18 by solder 21. The input/output pathways 16, 17 and the grounded pathway 18 are formed by coating conductive paste, such as Ag paste, on the printed circuit board 15 in a predetermined pattern and then fixing it by printing. 
     FIG. 5 is a graph showing comparison of damping characteristics between the first embodiment and the prior art. In FIG. 5, a line A represents a characteristic curve of the first embodiment and B represents a characteristic curve of the prior art filter. As apparent from FIG. 5, the characteristic curve A of the first embodiment has two extreme values A1 and A2 at low and high frequencies. This is because, as shown in the equivalent circuit of FIG. 3, the interstage coupling causes a small amount of electromagnetic coupling M besides the coupling capacitance (C1, C2, C3). On the contrary, the characteristic curve B of the prior art does not generate the similar extreme values. 
     A manufacturing method of the above-described dielectric filter will be explained hereinafter. 
     First of all, starting materials (for example, BaO, TiO 2 , Nd 2  O 3  or the like) are blended at a predetermined ratio. Then, the blended material is mixed by using a mill or else. Next, the mixed material is granulated by using a spray dryer or the like, so as to adjust the particle size and add binder. Subsequently, the granulated material is pelletized by a dry press so as to be formed into a predetermined shape. In turn, the pelletized material is sintered in a kiln at the temperature of 1300° C. to 1400° C. Thus, the dielectric base body 11a of cylindrical shape shown in FIG. 6 is obtained. Then, the conductor film is formed on the dielectric base body 11a. There are various method for forming the conductor film, several of which will be explained below. 
     A first method is applied in a case where copper is used as a material constituting the conductor film. The surface of the dielectric base body 11a is roughened by a barrelling machine or a blast device. Thereafter, the dielectric base body 11a is processed by etching until the roughness of the surface of the dielectric base body 11a becomes 5 μm to 9 μm. Etchant to be used in this etching will be, for example, HF--HNO 2  series. Subsequently, all the surface of the dielectric base body 11a is processed by stannous chloride or the like to give sensitivity. Then, palladium qualifying as catalytic metal is attached on all the surface of the dielectric base body 11a. Next, as shown in FIG. 7, a resist film 23 is formed partially on the dielectric base body 11a. Namely, this resist film 23 defines a region on which no conductor film of the dielectric base body 11a is provided--a region becoming the cutout 11k or the open end 11j as depicted in FIG. 2. In the formation of this resist film 23, resist ink is coated on the dielectric base body 11a by the use of printing technology or else and then thus printed resist ink is dried until it hardens. Next, on thus manufactured dielectric base body 11a, there is formed a thin, first copper film by the electroless copper plating method. In this case, the first copper film is selectively formed only within a region where the resist film 23 is not provided. Subsequently, a second copper film is laminated on the first copper film by the electrolytic copper plating to form the conductor film whose thickness is approximately 5 μm. After the resist film 23 is removed by solvent or else, the dielectric resonator 11 (or 12) shown in FIGS. 1 and 2 is manufactured. Although the above manufacturing method coats the resist ink on the predetermined portion of the dielectric base body 11a using the printing technology and then dries and hardens the resist ink, another manufacturing method will allow the use of a photosensitive resist as a resist. That is, after the catalytic metal, such as palladium, is attached on the dielectric base body 11a, photosensitive resist is coated on all the surface of the dielectric base body 11a. Then, a predetermined portion of the photosensitive resist is exposed and hardened. Thereafter, the portion not being hardened by exposure is washed away by developing solution. Then, the resist film 23 shown in FIG. 7 will be obtained. 
     Next, still another manufacturing method of the conductor film will be explained. First of all, a conductor film 24 is formed on all the surface of the dielectric base body 11a of FIG. 2 as shown in FIG. 8. In this case, the conductor film 24 can be formed in the double-layer structure of copper as previously described. Furthermore, the conductor film 24 will be formed on the entire surface of the dielectric base body 11a by printing Ag paste on the entire surface of the dielectric base body 11a, drying this Ag paste, and applying a thermal treatment at the temperature of 800° C. to 900° C. In turn, a resist film 25 is formed on the conductor film 24 in a predetermined pattern as shown in FIG. 9. The method of forming this resist film 25 is the same as in the previous method. The resist film 25 is formed to define the region where the conductors and electrodes are formed. Thereafter, the unnecessary portion of the conductor film 24 is removed by using the etching technology, such as chemical etching or dry etching. Thus, the dielectric resonator 11 (or 12) shown in FIGS. 1 and 2 is manufactured. 
     Yet another manufacturing method (not shown) of the conductor film will be explained. After forming the conductor film 24 on the entire surface of the dielectric base body 11a as shown in FIG. 8, cutting or laser machining is applied on the surface of the dielectric base body 11a to physically remove the predetermined portion of the surface. Thus, the dielectric resonator 11 (or 12) shown in FIGS. 1 and 2 is manufactured. 
     The dielectric resonators 11 and 12 thus constructed are disposed in such a manner that the electrodes 11f and 12f confront with each other. Then, the outer conductors 11i and 12i are connected by means of cream solder or the like. Similarly, the electrodes 11f and 12f are connected by means of cream solder or the like as depicted in FIG. 2. 
     Next, a modified embodiment of the first embodiment will be explained. 
     FIG. 10 is a perspective view showing the modified dielectric filter of the first embodiment of the present invention, and FIG. 11 is an exploded perspective view showing the modified dielectric filter of the first embodiment of the present invention. 
     In FIGS. 10 and 11, reference numerals 26 and 27 represent dielectric resonators whose constructions are almost identical with those of FIGS. 1 and 2 except the configuration of the through hole. First of all, the dielectric resonator 26 will be explained. A reference numeral 26a represents a dielectric base body made of dielectric material. The outer configuration of the dielectric base body 26a is rectangular parallelepiped having a square cross section. Furthermore, a centrally axially extending through hole is formed in the dielectric base body 26a. The through hole consists of a large hole 26b and a small hole 26c (see FIG. 10) extending centrally and axially and communicated with each other. The large hole 26b is positioned near the open end and has a square cross section, while the small hole 26c has a circular cross section. 
     Although the dielectric resonator of FIGS. 1 and 2 has the through hole of constant diameter extending from the open end to the connecting end--from one base to the other base of the rectangular parallelepiped--of the dielectric base body 11a, this modified embodiment is different from the embodiment of FIGS. 1 and 2 and characterized in that the through hole has a stepped portion. With this arrangement, an inner conductor 26d formed inside the large hole 26b can be made large. This means that it becomes possible to increase not only the input/output coupling capacitance between the electrode 26e and the inner conductor 26d but also the interstage coupling capacitance between the electrodes 26f and the inner conductor 26d; therefore, it becomes possible to manufacture a wide-band dielectric filter. In the same manner, the other dielectric resonator 27 includes a dielectric base body 27a with a centrally axially extending through hole consisting of a large square hole 27b and a small circular hole 27c (see FIG. 10). By providing an inner conductor 27d inside this through hole, the input/output coupling capacitance between the electrodes 27e and 27d can be increased but the interstage coupling capacitance between the electrode 27f and the inner conductor 27d can be increased. 
     The remainder of the construction is substantially the same as that of FIGS. 1 and 2. 
     Second Embodiment 
     FIGS. 12 and 13 are perspective and exploded views showing a dielectric filter of the second embodiment of the present invention. In FIGS. 12 and 13, reference numerals 28 and 29 represent dielectric resonators. Hereinafter, taking the dielectric resonator 28 of these two dielectric resonators, a construction of the dielectric resonator will be explained. A dielectric base body 28a made of dielectric material, such as BaO--TiO 2  --Nd 2  O 3 , BaO--TiO 2 , ZrO 2  --SnO 2  --TiO 2 , BaO--Sm 2  O 3  --TiO 2 , is formed with a centrally axially extending through hole 28b. The dielectric base body 28a has an outer configuration of rectangular parallelepiped having a square cross section, while the through hole 28b has a circular cross section. 
     An outer conductors 28c is provided on the outer side surface of the dielectric base body 28a so as to surround it. An inner conductor 28d is provided along the inner side surface of the through hole 28b. The outer conductor 28c and the inner conductor 28d are connected with each other via a connecting conductor 28e. This connecting conductor 28e is provided on a closed end surface, i.e. a base of the rectangular parallelepiped, of the dielectric base body 28a, in the same manner as the connecting conductor 11e of the first embodiment explained with reference to FIGS. 35 and 36. Of four outer side surfaces of the dielectric base body 28a, one side surface 28h is partly cut off so that the dielectric base body 28a is exposed near the open end 28j. Namely, a cutout 28k of the outer conductor 28c is formed near the open end 28j of the dielectric base body 28a. Furthermore, an interstage coupling electrode 28f is provided within the region of the cutout 28k (see FIG. 13), so that this interstage coupling electrode 28f does not contact with other conductors. In the same manner, an input/output coupling electrode 28g is provided within the region of the cutout 28k, so that this input/output coupling electrode 28g does not contact with other conductors. 
     The arrangement of the interstage coupling electrode 28f and the input/output coupling electrode 28g is different from that of the first embodiment. Namely, the interstage coupling electrode and the input/output coupling electrode are separately provided on different side surfaces of the dielectric resonator in the first embodiment. On the contrary, the second embodiment provides the cutout 28k on only one outer side surface 28h and disposes the electrodes 28f and 28g within the region of this cutout 28k. 
     The other dielectric resonator 29 comprises: a dielectric base body 29a, a through hole 29b, an outer conductor 29c, an inner conductor 29d, a connecting conductor 29e, an interstage coupling electrode 29f, an input/output coupling electrode 29g, an outer side surface 29h, an open end 29j, and a cutout 29k (see FIG. 13). The like parts between the dielectric resonators 28 and 29 are denoted by the same reference alphabet throughout views. (For example, the dielectric base body 28a is substantially identical with the dielectric base body 29a) However, as apparent from FIG. 13, the relationship between the dielectric resonator 28 and the dielectric resonator 29 are mirror symmetry. Therefore, in production of each dielectric resonator, it is necessary to pay attention to the positional relationship between the electrodes 28f, 28g and 29f, 29g. 
     More specifically, the dielectric resonator 28 and the dielectric resonator 29 are connected by means of, for example, cream solder in such a manner that the outer side surfaces 28h and 29h do not confront with each other and are placed on the same plane. Furthermore, there is provided a terminal 30 electrically connecting both the electrodes 28f and 29f. This terminal 30 is made of conductive material, such as silver, copper, and aluminum. Conductive bonding material, such as solder, is used to connect the terminal 30 with the electrodes 28f, 29f. Furthermore, the electrode 28g is connected with a terminal 31 and the electrode 29g is connected with a terminal 32 by means of conductive bonding material, such as solder. The terminals 31 and 32 are made of conductive material, such as copper and aluminum. 
     Furthermore, the outer conductors 28c, 29c, inner conductors 28d, 29d, connecting conductors 28e, 29e, electrodes 28f, 29f, and electrodes 28g, 29g (hereinafter, these components respectively are referred to as conductor film) are basically thin film made of conductive material such as copper and silver. The thickness of the film is approximately 5 μm. Although the conductor film is a single layer in this embodiment, it is needless to say that two or more layer structure can be allowed. Regarding the film thickness of approximately 5 μm, this value should be adequately changed depending on the condition of the dielectric filter in service. 
     Although the electrodes 28g, 29g and 28f, 29f are rectangular in this second embodiment, the configuration of these electrodes 28g, 29g and 28f, 29f can be any other shape, such as circle, ellipse, and polygon. 
     As apparent from the foregoing description, the dielectric filter of this embodiment is very simplified and compact in structure. In particular, the present embodiment no longer requires the central conductor and the dielectric substrate of the conventional dielectric filter. This results in 50% reduction of overall size. 
     FIG. 14 is a perspective view showing the dielectric filter mounted on a substrate in accordance with the second embodiment of the present invention. In FIG. 14, a reference numeral 33 represents a printed circuit board constituted by insulating material, such as glass . epoxy resin. Reference numerals 34 and 35 represent input/output pathways formed on the printed circuit board 33. Similarly, a reference numeral 36 represents a grounded pathway formed on the printed circuit board 33. The terminal 31 of the dielectric resonator 28 is connected onto the input/output pathway 34 by conductive bonding material, such as solder, and the terminal 32 of the dielectric resonator 29 is connected onto the input/output pathway 35 by conductive bonding material, such as solder. Furthermore, the outer conductors 28c and 29c of the dielectric resonators 28 and 29 are respectively connected onto the grounded pathway 36 by conductive bonding material, such as solder. The input/output pathways 34, 35 and the grounded pathway 36 are formed by coating conductive paste, such as Ag paste, on the printed circuit board 33 in a predetermined pattern and then fixing it by printing. Although this embodiment uses the terminal 30 to connect the electrodes 28f and 29f and uses the terminals 31 and 32 to connect the input/output pathways and the dielectric filter, it is also possible to provide a connecting electrode on the printed circuit board 33 instead of the terminal 30 to connect the electrodes 28f and 29f (see FIGS. 12, 13). The terminals 31 and 32 are also omitted in this case. 
     More specifically, there is provided a connecting electrode (not shown) between the input/output pathways 34, 35 made of conductive material. The dielectric resonators 28 and 29 (being not equipped with the terminals 30, 31 and 32) are placed on the connecting electrode so that both the electrodes 28f, 29f (see FIGS. 12, 13) contact with the connecting electrode, the input/output terminals 28g, 29g contact with the input/output pathways 34, 35, and the outer conductors 28c, 29c contact with the grounded conductor 36. Thereafter, the dielectric resonators 28 and 29 are tightly fixed on the printed circuit board by means of conductive bonding material, such as cream solder. In other words, if the terminal 30 is omitted, no gap is generated between the electrodes 28g, 29g (see FIGS. 12, 13) and the printed circuit board 33. Therefore, the terminals 31 and 32 are no longer necessary. 
     The prospective manufacturing methods of the above-described dielectric filter are substantially the same as those of the previously described first embodiment and, therefore, will be no more explained. 
     Next, a modified embodiment of the second embodiment will be explained. 
     FIG. 15 is a perspective view showing the modified dielectric filter of the second embodiment of the present invention, and FIG. 16 is an exploded perspective view showing the modified dielectric filter of the second embodiment of the present invention. 
     The modified embodiment of FIGS. 15 and 16 is different from the dielectric filter of FIGS. 12 and 13 in the configuration of a through hole. The dielectric filter of FIGS. 12, 13 has the through hole of constant diameter extending from the open end to the opposite end. On the other hand, this modified dielectric filter is different from the embodiment of FIGS. 12, 13 and characterized in that the through hole has a stepped portion. That is, the dielectric filter 28 has a dielectric base body 28a formed with a centrally axially extending through hole. The through hole consists of a large hole 28m and a small hole 28n (see FIG. 15) extending centrally and axially and communicated with each other. The large hole 28m is positioned near the open end and has a square cross section, while the small hole 28n has a circular cross section. An inner conductor 28q is provided on the inside surface of this through hole. Likewise, the other dielectric filter 29 has a dielectric base body 29a and a centrally axially extending through hole consisting of a large hole 29m and a small hole 29n (see FIG. 15) extending centrally and axially and communicated with each other. The large hole 29m is positioned near the open end and has a square cross section, while the small hole 29n has a circular cross section. An inner conductor 29q is provided on the inside surface of this through hole. Providing the stepped portion in the through hole and forming the square holes 28m, 29m near the open end so as to have a larger cross section than the corresponding holes 29n, 29m is advantageous in increasing not only the input/output coupling capacitance between the electrode 28g and the inner conductor 28q and between the electrode 29g and the inner conductor 29q but also the interstage coupling capacitance between the electrode 28f (see FIG. 16) and the inner conductor 28q and between the electrode 29f (see FIG. 16) and the inner conductor 29q. Thus, it becomes possible to manufacture a wide-band dielectric filter. The remainder of the construction is substantially the same as that of FIGS. 12 and 13. 
     Third Embodiment 
     FIGS. 17 and 18 are perspective and exploded views showing a dielectric filter of the third embodiment of the present invention. In FIGS. 17 and 18, reference numerals 37 and 38 represent dielectric resonators. Hereinafter, taking the dielectric resonator 37 of these two dielectric resonators, a construction of the dielectric resonator will be explained. A dielectric base body 37a made of dielectric material, such as BaO--TiO 2  --Nd 2  O 3 , BaO--TiO 2 , ZrO 2  --SnO 2  --TiO 2 , BaO--Sm 2  O 3  --TiO 2 , is formed with a centrally axially extending through hole 37b. 
     The dielectric base body 37a has an outer configuration of rectangular parallelepiped having a square cross section, while the through hole 37b has a circular cross section. An outer conductors 37c is provided on the outer side surface of the dielectric base body 37a so as to surround it. An inner conductor 37d is provided along the inner side surface of the through hole 37b. The outer conductor 37c and the inner conductor 37d are connected with each other via a connecting conductor 37e. This connecting conductor 37e is provided on a closed end surface, i.e. a base of the rectangular parallelopiped, of the dielectric base body 37a as explained in the first embodiment with reference to FIGS. 35 and 36. Of four outer side surfaces of the dielectric base body 37a, two side surfaces 37i and 37h are partly cut off so that the dielectric base body 37a is bared near the open end 37j. Namely, cutouts 37p (see FIG. 18) and 37k of the outer conductor 37c are formed on the two side surfaces 37i and 37h, respectively, near the open end 37j of the dielectric base body 37a. 
     Furthermore, an interstage coupling electrode 37f is provided within the region of the cutout 37k on the outer side surface 37h, so that this interstage coupling electrode 37f does not contact with other conductors. In the same manner, an input/output coupling electrode 37g (not shown) is provided within the region of the cutout 37p on the outer side surface 37i, so that this input/output coupling electrode does not contact with other conductors. The cutout 37p extends beyond the corner and along an adjacent outer side surface--a surface to be confronted with the printed circuit board. It will be preferable to connect an L-shaped terminal 39 (see FIG. 17) with the input/output coupling electrode. Namely, depending on the condition of the printed circuit board on which the dielectric filter is mounted, the terminal 39 and others are connected with the input/output coupling electrode. Then, the terminal 39 is firmly connected with a conductive film (e.g. input/output pathway) on the printed circuit board by means of solder or the like. Instead of providing the terminal 39, it will be also possible to directly connect the input/output coupling electrode with the conductive film on the printed circuit board by means of solder or the like. For the latter case, the cutout 37p extending within the region of the surface to be confronted with the printed circuit board serves to prevent the solder electrically connecting the conductive film and the input/output coupling electrode from contacting with the outer conductor 37c. 
     Likewise, the other dielectric resonator 38 comprises: a dielectric base body 38a, a through hole 38b, an outer conductor 38c, an inner conductor 38d, a connecting conductor 38e, an interstage coupling electrode 38f, an input/output coupling electrode 38g (which is similar to the input/output coupling electrode 37g of the dielectric resonator 37), outer side surfaces 38h and 38i, an open end 38j, and cutouts 38k, 38p. The like parts between the dielectric resonators 37 and 38 are denoted by the same reference alphabet throughout views. (For example, the dielectric base body 37a is substantially identical with the dielectric base body 38a) 
     However, as apparent from FIG. 18, the relationship between the dielectric resonator 37 and the dielectric resonator 38 are mirror symmetry. Therefore, in production of each dielectric resonator, it is necessary to pay attention to the positional relationship between the electrodes 37f, 37g and 38f, 38g. 
     Furthermore, the electrode 38g may be connected with a terminal 39 (see FIG. 17) depending on the condition of the printed circuit board on which the dielectric resonator 38 is mounted, in the same manner as the dielectric resonator 37. 
     More specifically, the dielectric resonator 37 and the dielectric resonator 38 are connected by means of, for example, cream solder in such a manner that the electrodes 37f and 38f confront and contact with each other and the electrodes 37g and 38g are placed in parallel and remotely opposed relationship so as to face both sides of the resonators. This is because these dielectric resonators are disposed in mirror symmetry relationship as described before. 
     Furthermore, the outer conductors 37c, 38c, inner conductors 37d, 38d, connecting conductors 37e, 38e, electrodes 37f, 38f, and electrodes 37g, 38g (hereinafter, these components respectively are referred to as conductor film) are basically thin film made of conductive material such as copper and silver. The thickness of the film is approximately 5 μm. Although the conductor film is a single layer in this embodiment, it is needless to say that two or more layer structure can be allowed. Regarding the film thickness of approximately 5 μm, this value should be adequately changed depending on the condition of the dielectric filter in service. 
     Although the electrodes 37g, 38g and 37f, 38f are rectangular in this third embodiment, the configuration of these electrodes 37g, 38g and 37f, 38f can be any other shape, such as circle, ellipse, and polygon. 
     As apparent from the foregoing description, the dielectric filter of this embodiment is very simplified and compact in structure. In particular, the present embodiment no longer requires the central conductor and the dielectric substrate of the conventional dielectric filter. This results in 50% reduction of overall size. 
     FIGS. 19 and 20 are perspective views showing the dielectric filter mounted on a substrate in accordance with the third embodiment of the present invention. In FIG. 19, a reference numeral 40 represents a printed circuit board constituted by insulating material, such as glass . epoxy resin. Reference numerals 41 and 42 represent input/output pathways formed on the printed circuit board 40. Similarly, reference numerals 43, 44 represent grounded pathways formed on the printed circuit board 40 so as to sandwich the input/output pathways 41, 42, respectively. The terminal 39, connected to the input/output coupling electrode 37g (not shown) of the dielectric resonator 37, is connected onto the input/output pathway 41 by conductive bonding material, such as solder. And the terminal 39, connected to the electrode 38g of the dielectric resonator 38 is connected onto the input/output pathway 42 by conductive bonding material, such as solder. Furthermore, the outer conductors 37c and 38c of the dielectric resonators 37 and 38 are connected onto the grounded pathways 43, 44, respectively, by conductive bonding material, such as solder. The input/output pathways 41, 42 and the grounded pathways 43, 44 are formed by coating conductive paste, such as Ag paste, on the printed circuit board 40 in a predetermined pattern and then fixing it by printing. 
     Furthermore, as shown in FIG. 20, the terminal 39 can be omitted. Namely, in mounting the dielectric filter onto the printed circuit board 40, the input/output coupling electrode 37g of the dielectric resonator 37 and the electrode 38g of the dielectric resonator 38 can be directly connected with the input/output pathways 41 and 42 by means of a conductive bonding material 45, such as solder. In this case, the conductive bonding material 45 should be spaced from the outer conductor 38g (as the outer conductor 37g of the dielectric resonator 37 is spaced from the conductive material). 
     The prospective manufacturing methods of the above-described dielectric filter are substantially the same as those of the previously described first embodiment and, therefore, will be no more explained. 
     FIG. 21 shows a graph showing the comparison of damping characteristics outside the band with respect to frequencies between the first and third embodiments. In FIG. 21, a curve A represents the damping characteristics of the third embodiment and a curve B represents the damping characteristics of the first embodiment. As understood from FIG. 21, the third embodiment can improve the damping amount outside the band. 
     In the same manner as the first and second embodiments, the third embodiment can form the through holes provided in the dielectric base bodies 37a, 38a to have a stepped portion as shown in FIG. 15. With this arrangement, the input/output coupling capacitance can be increased and also the interstage coupling capacitance can be enlarged. Thus, it becomes possible to obtain a wide-band dielectric filter. 
     The remainder of the construction is substantially the same as that of FIGS. 17 and 18. 
     Fourth Embodiment 
     FIGS. 22 and 23 are perspective and exploded views showing a dielectric filter of the fourth embodiment of the present invention. In FIGS. 22 and 23, reference numerals 46 and 47 represent dielectric resonators. Hereinafter, taking the dielectric resonator 46 of these two dielectric resonators, a construction of the dielectric resonator will be explained. A dielectric base body 46a made of dielectric material, such as BaO--TiO 2  --Nd 2  O 3 , BaO--TiO 2 , ZrO 2  --SnO 2  --TiO 2 , BaO--Sm 2  O 3  --TiO 2 , is formed with a centrally axially extending through hole 46b. The dielectric base body 46a has an outer configuration of rectangular parallelepiped having a square cross section, while the through hole 46b has a circular cross section. An outer conductors 46c is provided on the outer side surface of the dielectric base body 46a so as to surround it. An inner conductor 46d is provided along the inner side surface of the through hole 46b. The outer conductor 46c and the inner conductor 46d are connected with each other via a connecting conductor 46e. This connecting conductor 46e is provided on a closed end surface, i.e. a base of the rectangular parallelepiped, of the dielectric base body 46a as explained in the first embodiment with reference to FIGS. 35 and 36. Of four outer side surfaces of the dielectric base body 46a, two side surfaces 46i and 46h (see FIG. 22) are partly cut off so that the dielectric base body 46a is exposed near the open end 46j. Namely, cutouts 46p and 46k (see FIG. 23) of the outer conductor 46c are formed on the two side surfaces 46i and 46h, respectively, near the open end 46j (see FIG. 23) of the dielectric base body 46a. 
     Furthermore, an interstage coupling electrode 46f is provided within the region of the cutout 46k on the outer side surface 46h, so that this interstage coupling electrode 46f does not contact with other conductors as depicted in FIG. 23. In the same manner, an input/output coupling electrode (not shown) is provided within the region of the cutout 46p on the outer side surface 46i, so that this input/output coupling electrode does not contact with other conductors. The cutout 46p extends beyond the corner and along an adjacent outer side surface--a surface to be confronted with the printed circuit board. It will be preferable to connect an L-shaped terminal (not shown) with the input/output coupling electrode. Namely, depending on the condition of the printed circuit board on which the dielectric filter is mounted, the terminal and others are connected with the input/output coupling electrode. Then, the terminal is firmly connected with a conductive film (e.g. input/output pathway) on the printed circuit board by means of solder or the like. Instead of providing the terminal, it will be also possible to directly connect the input/output coupling electrode with the conductive film on the printed circuit board by means of solder or the like. For the latter case, the cutout 46p extending within the region of the surface to be confronted with the printed circuit board serves to prevent the solder electrically connecting the conductive film and the input/output coupling electrode from contacting with the outer conductor 46c. 
     Likewise, the other dielectric resonator 47 comprises: a dielectric base body 47a, a through hole 47b, an outer conductor 47c, an inner conductor 47d, a connecting conductor 47e, an interstage coupling electrode 47f (which is similar to the interstage coupling electrode 46f of the dielectric resonator 46), an input/output coupling electrode 47g (which is similar to the input/output coupling electrode 47f of the dielectric resonator 46 as depicted in FIG. 23), outer side surfaces 47h and 47i, an open end 47j, and cutouts 47k (not shown in FIGS. 22 and 23), 47p. The like parts between the dielectric resonators 46 and 47 are denoted by the same reference alphabet throughout views. (For example, the dielectric base body 46a is substantially identical with the dielectric base body 47a) Furthermore, the electrode 47g may be connected with a terminal depending on the condition of the printed circuit board on which the dielectric resonator 47 is mounted, in the same manner as the dielectric resonator 46. 
     More specifically, the dielectric resonator 46 and the dielectric resonator 47 are connected by means of, for example, cream solder in such a manner that the electrodes 46f and 47f confront and contact with each other and the input/output coupling electrodes 47g are placed in parallel and remotely opposed relationship so as to face both sides of the resonators. Furthermore, the dielectric resonators 46 and 47 are placed in opposed relationship so that respective open ends thereof face opposite directions. 
     Furthermore, the outer conductors 46c, 47c, inner conductors 46d, 47d, connecting conductors 46e, 47e, electrode 46f as well as the interstage coupling electrode of the dielectric resonator 47, and electrode 47g as well as the input/output coupling electrode of the dielectric resonator 46 (hereinafter, these components are referred to as conductor film) are basically thin film made of conductive material such as copper and silver. The thickness of the film is approximately 5 μm. Although the conductor film is a single layer in this embodiment, it is needless to say that two or more layer structure can be allowed. Regarding the film thickness of approximately 5 μm, this value should be adequately changed depending on the condition of the dielectric filter in service. 
     Although the electrode 47g (as well as the input/output coupling electrode of the dielectric resonator 46) and the electrode 46f (as well as the interstage coupling electrode of the dielectric resonator 47) are rectangular in this embodiment, the configuration of these electrodes can be any other shape, such as circle, ellipse, and polygon. 
     As apparent from the foregoing description, the dielectric filter of this embodiment is very simplified and compact in structure. In particular, the present embodiment no longer requires the central conductor and the dielectric substrate of the conventional dielectric filter. This results in 50% reduction of overall size. 
     In the same manner as the first and second embodiments, this embodiment can form the through hole to have a stepped portion as shown in FIG. 15. With this arrangement, the input/output coupling capacitance can be increased and also the interstage coupling capacitance can be enlarged. Thus, it becomes possible to obtain a wide-band dielectric filter. 
     The remainder of the construction is substantially the same as that of FIGS. 17 and 18. 
     Variations of First to Fourth Embodiments 
     Although the above first to fourth embodiments are explained based on a dielectric filter consisting of two dielectric resonators, the same effect will be obtained even if more than two dielectric resonators are assembled as shown in FIGS. 24, 25 and 26. 
     FIG. 24 is a perspective view showing a modified dielectric filter of the first embodiment, wherein three dielectric resonators are assembled. In FIG. 24, reference numerals 11 and 12 represent dielectric resonators having the same construction as the above-described first embodiment. A reference numeral 48 represents a dielectric resonator interposed between the dielectric resonators 11 and 12. The dielectric resonator 48 has interstage coupling electrodes 48a, 48b on the opposite outer side surfaces and has no input/output coupling electrode. The electrode 48a is connected with the interstage coupling electrode 11f of the dielectric resonator 11, and the electrode 48b is connected with the interstage coupling electrode 12f of the dielectric resonator 12. In this manner, when the dielectric filter includes not less than three resonators, the dielectric resonators equipped with both interstage and input/output coupling electrodes (hereinafter referred to as end resonators) are placed at both ends. Then, one or more dielectric resonators equipped with only interstage coupling electrodes (hereinafter referred to as interstage resonators) are interposed in series between the two end resonators, so that mutually confronting interstage coupling electrodes are connected one another. With this arrangement, the dielectric filter of the first embodiment can include not less than three dielectric resonators. 
     FIG. 25 is a perspective view showing a modified dielectric filter of the second embodiment, wherein three dielectric resonators are assembled. In FIG. 25, reference numerals 28 and 29 represent dielectric resonators having the same construction as the above-described second embodiment. A reference numeral 49 represents a dielectric resonator interposed between the dielectric resonators 28 and 29. The dielectric resonator 49 has interstage coupling electrodes 49a, 49b on the same outer side surface and has no input/output coupling electrode. The electrode 49a is connected with the interstage coupling electrode 28f of the dielectric resonator 28 through a terminal 50, and the electrode 49b is connected with the interstage coupling electrode 29f of the dielectric resonator 29 through another terminal 50. In this manner, when the dielectric filter includes not less than three resonators, the dielectric resonators equipped with both interstage and input/output coupling electrodes (hereinafter referred to as end resonators) are placed at both ends. Then, one or more dielectric resonators equipped with only interstage coupling electrodes (hereinafter referred to as interstage resonators) are interposed in series between the two end resonators, so that mutually confronting interstage coupling electrodes are connected one another. With this arrangement, the dielectric filter of the second embodiment can include not less than three dielectric resonators. 
     FIG. 26 is a perspective view showing a modified dielectric filter of the third embodiment, wherein three dielectric resonators are assembled. In FIG. 26, reference numerals 37 and 38 represent dielectric resonators having the same construction as the above-described third embodiment. A reference numeral 51 represents a dielectric resonator interposed between the dielectric resonators 37 and 38. The dielectric resonator 51 has interstage coupling electrodes 51a, 51b on the opposite outer side surfaces and has no input/output coupling electrode. The electrode 51a is connected with the interstage coupling electrode 37f of the dielectric resonator 37, and the electrode 51b is connected with the interstage coupling electrode 38f of the dielectric resonator 38. In this manner, when the dielectric filter includes not less than three resonators, the dielectric resonators equipped with both interstage and input/output coupling electrodes (hereinafter referred to as end resonators) are placed at both ends. Then, one or more dielectric resonators equipped with only interstage coupling electrodes (hereinafter referred to as interstage resonators) are interposed in series between the two end resonators, so that mutually confronting interstage coupling electrodes are connected one another. With this arrangement, the dielectric filter of the third embodiment can include not less than three dielectric resonators. 
     In the same manner, the dielectric filter of the fourth embodiment can include not less than four resonators by interposing one or more interstage resonators between two end resonators. 
     Fifth Embodiment 
     FIGS. 29 and 30 are perspective and exploded views showing a dielectric filter of the fifth embodiment of the present invention. In FIGS. 29 and 30, reference numerals 111 and 112 represent dielectric resonators. Hereinafter, taking the dielectric resonator 111 of these two dielectric resonators, a construction of the dielectric resonator will be explained. A dielectric base body 111a made of dielectric material, such as BaO--TiO 2  --Nd 2  O 3 , BaO--TiO 2 , ZrO 2  --SnO 2  --TiO 2 , BaO--Sm 2  O 3  --TiO 2 , Ln 2  O 3  --BaO--TiO 2  series, is formed with a centrally axially extending through hole 111b. 
     The dielectric base body 111a has an outer configuration of rectangular parallelepiped having a square cross section, while the through hole 111b has a circular cross section. An outer conductors 111c is provided on the outer side surface of the dielectric base body 111a so as to surround it. An inner conductor 111d is provided along the inner side surface of the through hole 111b. The outer conductor 111c and the inner conductor 111d are connected with each other via a connecting conductor 111e. This connecting conductor 111e is provided on a closed end surface, i.e. a base of the rectangular parallelepiped, of the dielectric base body 111a as explained in the first embodiment with reference to FIGS. 35 and 36. Of four outer side surfaces of the dielectric base body 111a, one side surface 111i (see FIG. 29) is partly cut off so that the dielectric base body 111a is bared near the open end 111j. Namely, a cutout 111k (see FIG. 29) of the outer conductor 111c is formed near the open end of the dielectric base body 111a. 
     Furthermore, an interstage electrode 111f is provided on an outer side surface 111l (see FIG. 30), which is adjacent and perpendicular to the outer side surface 111i, near the open end 111j. The electrode 111f is provided within the region of a cutout 111o (see FIG. 30) formed on the outer side surface 111l (see FIG. 30), so that the electrode 111f does not contact with the outer conductor 111c. 
     Moreover as depicted in FIG. 30, there is provided input/output electrodes 111g and 111h along outer side surfaces 111m, 111n near the open end 111j. These electrodes 111g and 111h are provided within the region of a cutout 111p formed on the outer side surfaces 111m, 111n. 
     Likewise, the other dielectric resonator 112 comprises substantially the same components: namely, a dielectric base body 112a, a through hole 112b, an outer conductor 112c, an inner conductor 112d, a connecting conductor 112e, an interstage coupling electrode 112f, input/output coupling electrodes 112g, 112h, outer side surfaces 112i (see FIG. 29), 112l, 112m and 112n (see FIG. 30), an open end 112j, and cutouts 112k (see FIG. 29), 112o and 112p. The like parts between the dielectric resonators 111 and 112 are denoted by the same reference alphabet throughout views. (For example, the dielectric base body 111a is substantially identical with the dielectric base body 112a) However, as apparent from FIG. 30, the relationship between the dielectric resonator 111 and the dielectric resonator 112 are mirror symmetry. Therefore, in production of each dielectric resonator, it is necessary to pay attention to the positional relationship between the electrodes 111f, 111g, 111h and 112f, 112g, 112h. 
     More specifically, the dielectric resonator 111 and the dielectric resonator 112 are connected by means of, for example, cream solder in such a manner that the electrodes 111f and 112f confront and contact with each other and the electrodes 111g and 112g are placed in opposed and parallel relationship and the electrodes 111h and 112h are placed on the same plane. 
     This is because the dielectric resonators 111 and 112 are disposed in mirror symmetry relationship as explained previously. 
     Furthermore, the outer conductors 111c, 112c, inner conductors 111d, 112d, connecting conductors 111e, 112e, electrodes 111f, 112f, 111g, 112g, 111h, 112h (hereinafter, these components respectively are referred to as conductor film) are basically thin film made of conductive material such as copper and silver. The thickness of the film is approximately 5 μm. Although the conductor film is a single layer in this embodiment, it is needless to say that two or more layer structure can be allowed. Regarding the film thickness of approximately 5 μm, this value should be adequately changed depending on the condition of the dielectric filter in service. 
     FIG. 31 is a graph showing a relationship between an area S of the cutouts 111k and 112k--an area defined by Z1×Z2 in FIG. 29--and an interstage coupling degree K in accordance with the fifth embodiment of the present invention. 
     As understood from FIG. 31, the interstage coupling degree K increases with increasing area S of the cutouts 111k and 112k. Furthermore, the interstage coupling degree K can be enlarged when compared with the conventional one (as illustrated by the &#34;Prior Art&#34; point located between the values two and three on the vertical axis). Thus, it becomes possible to provide a wide-band dielectric filter. 
     A manufacturing method of the above-described dielectric filter will be explained hereinafter. 
     First of all, starting materials (for example, BaO, TiO 2 , Nd 2  O 3  or the like) are blended at a predetermined ratio. Then, the blended material is mixed by using a mill or else. Next, the mixed material is granulated by using a spray dryer or the like, so as to adjust the particle size and add binder. Subsequently, the granulated material is pelletized by a dry press so as to be formed into a predetermined shape. In turn, the pelletized material is sintered in a kiln at the temperature of 1300° C. to 1400° C. Thus, the dielectric base body 111a of cylindrical shape is obtained. Then, the conductor film is formed on the dielectric base body 111a. There are various method for forming the conductor film, several of which will be explained below. 
     A first method is applied in a case where copper is used as a material constituting the conductor film. The surface of the dielectric base body 111a is roughened by a barrelling machine or a blast device. Thereafter, the dielectric base body 111a is processed by etching until the roughness of the surface of the dielectric base body 111a becomes 5 μm to 9 μm. Etchant to be used in this etching will be, for example, HF--HNO 2  series. Subsequently, all the surface of the dielectric base body 111a is processed by stannous chloride or the like to give sensitivity. Then, palladium qualifying as catalytic metal is attached on all the surface of the dielectric base body 111a. And, a resist film is partially formed on the dielectric base body 111a. 
     Namely, this resist film 23 defines a region on which no conductor film of the dielectric base body 111a is provided--a region becoming the cutout 111k, 111o and 111p. In the formation of this resist film, resist ink is coated on the dielectric base body 111a by the use of printing technology or transfer technology or else and then thus printed resist ink is dried until it hardens. Next, on thus manufactured dielectric base body 111a, there is formed a thin, first copper film by the electroless copper plating method. In this case, the first copper film is selectively formed only within a region where the resist film is not provided. 
     Subsequently, a second copper film is laminated on the first copper film by the electrolytic copper plating to form the conductor film whose thickness is approximately 5 μm. After the resist film is removed by solvent or else, the electrodes are formed. Although the above manufacturing method coats the resist ink on the predetermined portion of the dielectric base body 111a using the printing technology and then dries and hardens the resist ink, another manufacturing method will allow the use of a photosensitive resist as a resist. That is, after the catalytic metal, such as palladium, is attached on the dielectric base body 111a, photosensitive resist is coated on all the surface of the dielectric base body 111a. Then, a predetermined portion of the photosensitive resist is exposed and hardened. Thereafter, the portion not being hardened by exposure is washed away by developing solution. Then, the electrodes will be obtained. 
     Another manufacturing method will form the conductor film on the dielectric base body 111a by coating Ag paste on the entire surface of the dielectric base body 111a by printing or the like method, drying this Ag paste, and applying a thermal treatment at the temperature of 800° C. to 900° C. Thereafter, the unnecessary portion of the conductor film may be removed by using the etching technology, such as chemical etching or dry etching. Thus, the electrodes are provided at the predetermined places. 
     Still another manufacturing method will form the electrodes, after forming the conductor film on the entire surface of the dielectric base body 111a, by cutting or laser machining the surface of the dielectric base body 111a to physically remove the predetermined portion of the surface. 
     The dielectric resonators 111 and 112 thus constructed are disposed in such a manner that the electrodes 111f and 112f confront with each other. Then, the outer conductors 111c and 112c are connected by means of cream solder or the like. Similarly, the electrodes 111f and 112f are connected by means of cream solder or the like. 
     Although this embodiment is explained based on the cutouts 111k and 112k shown in FIG. 29, the cutouts 11k and 112k can be made variously as long as they reach the open end 29m. Furthermore, it will be allowed to provide the cutout on either of these dielectric resonators 111 and 112. 
     Sixth Embodiment 
     FIGS. 32 and 33 are perspective and exploded views showing a dielectric filter of the sixth embodiment of the present invention. In FIGS. 32 and 33, reference numerals 301 and 302 represent dielectric resonators. Hereinafter, taking the dielectric resonator 301 of these two dielectric resonators, a construction of the dielectric resonator will be explained. A dielectric base body 301a made of dielectric material, such as BaO--TiO 2  --Nd 2  O 3 , BaO--TiO 2 , ZrO 2  --SnO 2  --TiO 2 , BaO 2  --Sm 2  O 3  --TiO 2 , is formed with a centrally axially extending through hole 301b. The dielectric base body 301a has an outer configuration of rectangular parallelopiped having a square cross section, while the through hole 301b has a circular cross section. An outer conductors 301c is provided on the outer side surface of the dielectric base body 301a so as to surround it. An inner conductor 301d is provided along the inner side surface of the through hole 301b. The outer conductor 301c and the inner conductor 301d are connected with each other via a connecting conductor 301e. This connecting conductor 301e is provided on a closed end surface, i.e. a base of the rectangular parallelepiped, of the dielectric base body 301a as explained in the first embodiment with reference to FIGS. 35 and 36. Of four outer side surfaces of the dielectric base body 301a, outer side surfaces 301i, 301h and 301l (see FIG. 33) are partly cut off so that the dielectric base body 301a is exposed near the open end 301j. Namely, as depicted in FIG. 33 three cutouts 301p, 301k and 301m of the outer conductor 301c are formed near the open end of the dielectric base body 301a. Furthermore, an interstage electrode 301f is provided within the cutout 301k formed on the outer side surface 301h, so that the electrode 301f does not contact with other conductors. Likewise, input/output coupling electrode 301g (not shown), 301n are provided within the respective cutouts 301p, 301m formed on the corresponding outer side surfaces 301i, 301l, so that the electrodes 301g, 301n do not contact with other conductors. In mounting the dielectric filter on a printed circuit board, the input/output coupling electrodes 301 and 301n are directly connected with the conductive film (e.g. input/output pathways) by means of solder or the like. 
     The other dielectric resonator 302 (see FIGS. 32, 33) comprises substantially the same components: namely, a dielectric base body 302a, a through hole 302b, an outer conductor 302c, an inner conductor 302d, a connecting conductor 302e, an interstage coupling electrode 302f, input/output coupling electrodes 302g (which is similar to the input/output coupling electrode of the dielectric resonator 301), 302n (see FIG. 33), outer side surfaces 302i, 302h (see FIG. 33), 302l, an open end 302j, and cutouts 302p, 302m (see FIG. 33) and 302k. The like parts between the dielectric resonators 301 and 302 are denoted by the same reference alphabet throughout views. (For example, the dielectric base body 301a is substantially identical with the dielectric base body 302a) However, as apparent from FIG. 33, the relationship between the dielectric resonator 301 and the dielectric resonator 302 are mirror symmetry. Therefore, in production of each dielectric resonator, it is necessary to pay attention to the positional relationship between the electrodes 301f, 301g, 301n and 302f, 302g, 302n. 
     More specifically, the dielectric resonator 301 and the dielectric resonator 302 are connected by means of, for example, cream solder in such a manner that the electrodes 301f and 302f confront and contact with each other and the electrodes 301g and 302g are placed in opposed and parallel relationship and the electrodes 301n and 302 n are placed on the same plane. This is because the dielectric resonators 301 and 302 are disposed in mirror symmetry relationship as explained previously. 
     Furthermore, the outer conductors 301c, 302c, inner conductors 301d, 302d, connecting conductors 301e, 302e, electrodes 301f, 302f, 301g, 302g, 301n, 302n (hereinafter, these components are referred to as conductor film) are basically thin film made of conductive material such as copper and silver. The thickness of the film is approximately 5 μm. Although the conductor film is a single layer in this embodiment, it is needless to say that two or more layer structure can be allowed. Regarding the film thickness of approximately 5 μm, this value should be adequately changed depending on the condition of the dielectric filter in service. 
     Moreover, as depicted in FIG. 33 the sixth embodiment allows to vary a cutout width H of the cutouts 301k, 302k, thereby finely adjusting the interstage coupling degree of the filter. Thus, it becomes possible to set the interstage coupling degree at a desired value. 
     In the same manner as the first and second embodiments, this embodiment can form the through hole to have a stepped portion as shown in FIG. 15. With this arrangement, the input/output coupling capacitance can be enlarged; thus it becomes possible to obtain a wide-band dielectric filter. 
     Furthermore, by increasing the inner diameters of the inner conductors 301d, 302d of the dielectric resonators 301 and 302, the input/output coupling capacitance and the interstage coupling capacitance can be enlarged. Thus, it becomes possible to provide a wide-band dielectric filter, 
     The sixth embodiment can include three dielectric resonators as shown in FIG. 34. In this case, dielectric resonators 301, 302 placed at both ends (hereinafter, referred to as end resonators) are identical with those shown in FIGS. 32 and 34. A central dielectric resonator 401 has two interstage coupling electrodes formed at opposite outer side surfaces which are to be connected to the confronting interstage coupling electrodes of the end resonators. 
     As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appending claims rather than by the description preceding them, and all changes that fall within meets and bounds of the claims, or equivalence of such meets and bounds are therefore intended to embraced by the claims.