Patent Publication Number: US-6989722-B2

Title: Dielectric filter, dielectric duplexer, and communication apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a divisional of application Ser. No. 10/076,705, entitled DIELECTRIC FILTER, DIELECTRIC DUPLEXER, AND COMMUNICATION APPARATUS, filed Feb. 13, 2002, now U.S. Pat. No. 6,566,987. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to dielectric filters, dielectric duplexers, and communications apparatuses used mainly in the microwave band. 
   2. Description of the Related Art 
   In a known type of dielectric filter including a substantially rectangular dielectric block, the dielectric block, inner conductors, and an outer conductor constitute resonators in TEM modes, and the resonators are comb-line coupled with each other via stray capacitance generated at portions of the resonators where no conductors are formed, whereby the dielectric filter is formed. 
   However, in a dielectric duplexer in which an outer conductor is formed on the outer surface of such a substantially rectangular dielectric block, the dielectric block and the outer conductor cause a resonance in a mode, for example, the TE 101  mode, other than the TEM mode which is the fundamental resonance mode. 
     FIG. 22A  is a diagram showing the distribution of a magnetic field in the TE 101  mode generated in the dielectric filter according to the related art, and  FIG. 22B  is a graph showing the attenuation characteristics of the dielectric filter. 
   As shown in  FIG. 22B , when resonance occurs in a mode other than the fundamental mode, for example, in the TE mode, a plurality of resonance frequencies in the TE mode, in addition to the resonance frequency in the desired TEM mode, appear outside the band necessary for obtaining the desired characteristics of the filter, whereby the spurious-response characteristics of the dielectric filter are degraded. 
   Proposals have been made in order to avoid the effects of the TE mode. In a first proposed dielectric filter, because the frequency in the TE mode is affected by the outer dimensions of the dielectric filter, the outer dimensions are altered so as to shift the resonance frequency in the TE mode, whereby degradation of the spurious-response characteristics is avoided. In a second proposed dielectric filter, a portion of an outer conductor is cut, so that a perturbation is caused in the TE-mode resonance of the dielectric block and the outer conductor, shifting the frequency in the TE mode, whereby degradation of the spurious-response characteristics is avoided. 
   However, the dielectric filters according to the related art have suffered the following problems to be solved. 
   According to the first proposed dielectric filter, the filter must be designed for the TEM mode while also taking the effects of TE mode into consideration. In addition, because size reduction of dielectric filters is constantly desired, larger outer dimensions are inhibited. Thus, flexibility in designing filters is diminished. 
   In the second proposed dielectric filter, because a separate process of cutting the outer conductor is required, lead time and workload are increased, incurring additional manufacturing cost. 
   SUMMARY OF THE INVENTION 
   To address these problems, the present invention provides a dielectric filter, a dielectric duplexer, and a communications apparatus in which the resonance frequency in the TE mode is shifted so as to improve the spurious-response characteristics without incurring additional manufacturing cost or altering the overall outer dimensions. 
   To this end, the present invention, in one aspect thereof, provides a dielectric filter including a substantially rectangular dielectric block; a plurality of inner-conductor holes having respective apertures in a first end surface of the dielectric block and in a second end surface which is opposite to said first end surface of the dielectric block; a plurality of inner conductors formed respectively on the inner surfaces of the plurality of inner-conductor holes; at least one concavity formed either in one of the end surfaces in which the apertures of the plurality of inner-conductor holes are formed, or in one of the third and fourth end surfaces of the dielectric block which are arranged with the inner-conductor holes therebetween in the direction of array of the plurality of inner-conductor holes; and an outer conductor formed on the outer surface of the dielectric block including the inner surface of the at least one concavity; wherein the resonance frequency in a TE mode in which the electric field is aligned in the direction perpendicular to both the axial direction and the direction of array of the plurality of inner-conductor holes is shifted towards higher frequencies. Thus, the effects of TE modes can be readily diminished without altering the outer dimensions, so that the spurious-response characteristics are improved. 
   The at least one concavity may be formed substantially in the central portion of at least one of the first and second end surfaces in which the apertures of the plurality of inner-conductor holes are formed. Thus, mainly the effects of the TE 101  mode can be readily reduced without altering the outer dimensions, so that the spurious-response characteristics are improved. 
   The at least one concavity may be formed in at least one of the first and second end surfaces in which the apertures of the plurality of inner-conductor holes are formed, at a position spaced away from a corresponding nearest end surface in the direction of array of the plurality of inner-conductor holes, by a distance of approximately a quarter of the dimension of the dielectric block in said direction of array of the inner-conductor holes. Thus, mainly the effects of the TE 201  mode can be readily reduced without altering the outer dimensions, so that the spurious-response characteristics are improved. 
   The at least one concavity may be formed in a localized region not including spaces between the plurality of inner-conductor holes. Thus, the at least one concavity can be readily formed without altering the coupling capacitance between the inner-conductor holes. In addition, the effects of TE modes can be readily diminished without altering the outer dimensions, so that the spurious-response characteristics are improved. 
   The at least one concavity may be formed substantially in the central portion of at least one of the third and fourth end surfaces which are arranged at the ends in the direction of array of the plurality of inner-conductor holes. Thus, the effects of TE modes in general can be readily diminished without altering the outer dimensions, so that the spurious-response characteristics are improved. 
   The present invention, in another aspect thereof, provides a dielectric duplexer including a dielectric filter described above, so that the spurious-response characteristics can be readily improved to achieve good attenuation characteristics. 
   The present invention, in still another aspect thereof, provides a communications apparatus including the dielectric filter or the dielectric duplexer described above, so that the communications characteristics are improved. 
   Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A ,  1 B, and  1 C are, respectively, an external perspective view, a side view, and a bottom view of a dielectric filter according to a first embodiment; 
       FIGS. 2A and 2B  are diagrams showing the distributions of magnetic fields in the TE 101  mode generated in the dielectric filter according to the first embodiment; 
       FIG. 3  is a graph showing the attenuation characteristics of the dielectric filter according to the first embodiment; 
       FIG. 4  is a graph showing the relationship between the position of a concavity and the amount of shift in the resonance frequency in the TE 101  mode; 
       FIGS. 5A ,  5 B, and  5 C are graphs showing variations in the resonance frequency in each TE mode in relation to the depth and width of a concavity; 
       FIGS. 6A and 6B  are, respectively, an external perspective view and a side view of a dielectric filter according to a second embodiment; 
       FIGS. 7A ,  7 B, and  7 C are diagrams showing the distributions of magnetic fields in each TE mode generated in the dielectric filter according to the second embodiment; 
       FIG. 8  is a graph showing the attenuation characteristics of the dielectric filter according to the second embodiment; 
       FIGS. 9A and 9B  are, respectively, an external perspective view and a side view of a dielectric filter according to a third embodiment; 
       FIGS. 10A ,  10 B, and  10 C are diagrams showing the distributions of magnetic fields in each TE mode generated in the dielectric filter according to the third embodiment; 
       FIG. 11  is a graph showing the attenuation characteristics of the dielectric filter according to the third embodiment; 
       FIGS. 12A and 12B  are, respectively, an external perspective view and a side view of a dielectric filter according to a fourth embodiment; 
       FIGS. 13A and 13B  are, respectively, an external perspective view and a side view of a dielectric filter according to a fifth embodiment; 
       FIGS. 14A and 14B  are, respectively, an external perspective view and a side view of another dielectric filter according to the fifth embodiment; 
       FIGS. 15A and 15B  are, respectively, an external perspective view and a side view of a dielectric filter according to a sixth embodiment; 
       FIGS. 16A ,  16 B, and  16 C are diagrams showing the distributions of magnetic fields in each TE mode generated in the dielectric filter according to the sixth embodiment; 
       FIG. 17  is a graph showing the attenuation characteristics of the dielectric filter according to the sixth embodiment; 
       FIGS. 18A and 18B  are, respectively, an external perspective view and a side view of a dielectric filter according to a seventh embodiment; 
       FIG. 19  is a diagram showing the distribution of a magnetic field in the TE 101  mode generated in the dielectric filter according to the seventh embodiment; 
       FIGS. 20A and 20B  are, respectively, an external perspective view and a side view of a dielectric duplexer according to an eighth embodiment; 
       FIG. 21  is a block diagram of a communications apparatus according to a ninth embodiment; and 
       FIG. 22A  is a diagram showing the distribution of a magnetic field in a TE mode generated in a known dielectric filter, and  FIG. 22B  is a graph showing the attenuation characteristics of the known dielectric filter. 
   

   DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
   The construction of a dielectric filter according to a first embodiment will be described with reference to  FIGS. 1A to 1C ,  FIGS. 2A and 2B ,  FIG. 3 ,  FIG. 4 , and  FIGS. 5A to 5C . 
     FIGS. 1A ,  1 B, and  1 C are, respectively, an external perspective view, a side view, and a bottom view of the dielectric filter. 
     FIGS. 2A and 2B  are, respectively, a perspective view and a side view showing the distribution of a magnetic field in the TE 101  mode generated in the dielectric filter. 
     FIG. 3  is a graph showing the attenuation characteristics of the dielectric filter. 
     FIG. 4  is a graph showing the relationship between the position of a concavity and the amount of shift in the resonance frequency in the TE 101  mode. 
     FIGS. 5A ,  5 B, and  5 C are graphs showing variations in the resonance frequency in relation to the depth and width of the concavity, respectively in the TE 101  mode, the TE 201  mode, and the TE 301  mode. 
   In  FIGS. 1A to 1C ,  1  indicates a dielectric block,  2   a  to  2   c  indicate inner-conductor holes,  3   a  to  3   c  indicate inner conductors,  4   a  to  4   c  indicate non-conductor portions,  5  indicates an outer conductor,  6  indicate input and output electrodes, and  7  indicates a concavity. 
   Referring to  FIGS. 1A to 1C , from the top surface to the bottom surface of the substantially rectangular dielectric block  1 , the inner-conductor holes  2   a  to  2   c  are formed, and the inner conductors  3   a  to  3   c  are formed respectively on the inner surfaces of the inner-conductor holes  2   a  to  2   c . The outer conductor  5  is formed substantially over the entire outer surface of the dielectric block  1 . 
   In the inner-conductor holes  2   a  to  2   c , the non-conductor portions  4  are formed respectively in the proximity of one of the first and second end surfaces in which the apertures of the inner-conductor holes  2   a  to  2   c  are formed. These portions define the open ends of the inner conductors  3   a  to  3   c , and the other surface defines the shorted ends. On the outer surface of the dielectric block  1 , the input and the output electrodes  6 , isolated from the outer conductor  5 , are formed so as to be capacitively coupled with the open ends. 
   Furthermore, in the proximity of the central portion of the shorted-end surface, the convexity  7  is cut into the dielectric block  1  in the axial direction of the inner-conductor holes  2   a  to  2   c , the inner surface thereof being covered with the outer conductor  5 , whereby the entire dielectric filter is formed. 
   In the dielectric filter of the above construction, a magnetic field in the TE 101  mode is distributed as shown in  FIGS. 2A and 2B . 
   Referring to  FIGS. 2A and 2B ,  2   a  to  2   c  are the inner-conductor holes, and  7  is the concavity.  11  and  12  each show the distribution of a magnetic field in the TE 101  mode, respectively in a case where the concavity  7  is not provided and in a case where the concavity  7  is provided. 
   A indicates the length of the longer sides of the surfaces in which the apertures of the inner-conductor holes  2   a  to  2   c  are formed, B indicates the length of the shorter sides thereof, C indicates the length of the dielectric block in the axial direction of the inner-conductor holes  2   a  to  2   c , C′ is the distance from the inner surface of the concavity to the open-end surface, D is the depth of the concavity (length in the direction parallel to the axial direction of the inner-conductor holes  2   a  to  2   c ), and w is the width of the concavity (length in the direction parallel to the direction of array of the inner-conductor holes  2   a  to  2   c ). 
   The resonance frequency f in TE mns  mode generated in the dielectric filter including the dielectric block can be expressed as: 
             f   =         v   c         ɛ   r         ⁢             (     m   A     )     2     +       (     n   B     )     2     +       (     s   C     )     2         2               (   1   )             
 
where vc is the speed of light, ∈r is the relative dielectric constant of the dielectric material, and A, B, and C are the dimensions shown in  FIG. 2A .
 
   As shown in  FIGS. 2A and 2B , due to the concavity  7  provided at the central portion of the shorted-end surface, a magnetic field in the TE 101  mode is distributed as indicated by  12 , not as indicated by  11 , so that the wavelength of the magnetic field component is equivalently shortened. That is, the length of the dielectric block  1  in the axial direction of the inner-conductor holes  2   a  to  2   c  is equivalently shortened from the length C to the length C′, so that the resonance frequency becomes higher according to Eq. 1. 
   In terms of attenuation characteristics, as shown in  FIG. 3 , because the resonance frequency in the TE 101  mode is shifted, unwanted signals in the proximity of the resonance frequency in the TE 101  mode are suppressed, so that spurious-response characteristics in the proximity of the resonance frequency in the TE 101  mode are improved. 
   The concavity  7  may be provided at positions other than the central portion of the shorted-end surface. However, as shown in  FIG. 4 , the amount of shift in the resonance frequency in the TE 101  mode increases in accordance with the distance of the position of the concavity  7  from the nearer end surface of the dielectric block  1  in the direction of array of the inner conductor holes  2   a  to  2   c , reaching the maximum at the central portion (A/2 distant from the end surface), when maximal improvement in spurious-response characteristics is obtained. 
     FIGS. 5A to 5C  are graphs showing variations in the resonance frequency in TE modes when the depth D and the width w of the concavity are changed, when the dimensions in  FIG. 2A  are such that A is 10.4 mm, B is 2.0 mm, and C is 6.0 mm, and when the relative dielectric constant of the dielectric block  1  is 47. 
   As shown in  FIGS. 5A to 5C , in each of the TE 101  mode, the TE 201  mode, and the TE 301  mode, the amount of shift in the resonance frequency can be increased by increasing the depth and the width of the concavity. 
   Next, the construction of a dielectric filter according to a second embodiment will be described with reference to  FIGS. 6A and 6B ,  FIGS. 7A to 7C , and  FIG. 8 . 
     FIGS. 6A and 6B  are, respectively, an external perspective view and a side view of the dielectric filter. 
     FIGS. 7A to 7C  show the distributions of magnetic fields generated in the dielectric filter, respectively in the TE 101  mode, the TE 201  mode, and the TE 301  mode. 
     FIG. 8  is a graph showing the attenuation characteristics of the dielectric filter. 
   In  FIGS. 6A and 6B  and  FIGS. 7A to 7C ,  1  indicates a dielectric block,  2   a  to  2   c  indicate inner-conductor holes,  3   a  to  3   c  indicate inner conductors,  4   a  to  4   c  indicate non-conductor portions,  5  indicates an outer conductor,  6  indicate input and output electrodes, and  7  indicate concavities.  11  and  12  show the distributions of magnetic fields in each of the TE modes, respectively for a case where the concavities  7  are not provided and for a case where the concavities  7  are provided. 
   In the dielectric filter shown in  FIGS. 6A and 6B , the concavities  7  are formed respectively in the central portions of both of the surfaces on which the apertures of the inner-conductor holes  2   a  to  2   c  are formed. The construction of the dielectric filter is otherwise the same as that of the dielectric filter according to the first embodiment. 
   According to the above construction, the concavities  7  are formed in the regions where the magnetic field in the TE 101  mode is most intense, as shown in FIG.  7 A. Thus, the distribution of magnetic field is significantly altered, so that the wavelength of the magnetic field component in the TE 101  mode is equivalently shortened, whereby the resonance frequency is shifted towards higher frequencies. 
   Furthermore, with respect to the TE 201  mode, the concavities  7  are formed in the regions where the magnetic field is weak, as shown in  FIG. 7B , exerting almost no effect. Thus, the distribution of magnetic field is not altered, and the resonance frequency remains substantially unchanged. 
   Furthermore, with respect to TE 301  mode, only the portion of the magnetic field in the center is affected and the other portions of the magnetic field are not affected, as shown in  FIG. 7C . Thus, the overall distribution of magnetic field is not significantly altered, causing only a small shift in the resonance frequency. 
     FIG. 8  is a graph representing the content described above in terms of attenuation characteristics. As shown in  FIG. 8 , only the attenuation in the TE 101  mode is significantly affected. 
   As described above, the resonance frequency in the TE 101  mode is shifted, so that unwanted signals in the proximity of the resonance frequency in the TE 101  mode are blocked, whereby the spurious-response characteristics in the proximity of the resonance frequency in the TE 101  mode are improved. 
   Next, the construction of a dielectric filter according to a third embodiment will be described with reference to  FIGS. 9A and 9B ,  FIGS. 10A to 10C , and  FIG. 11 . 
     FIGS. 9A and 9B  are, respectively, an external perspective view and a side view of the dielectric filter. 
     FIGS. 10A ,  10 B, and  10 C show the distributions of magnetic fields generated in the dielectric filter, respectively in the TE 101  mode, the TE 201  mode, and the TE 301  mode. 
     FIG. 11  is a graph showing the attenuation characteristics of the dielectric filter. 
   In  FIGS. 9A and 9B  and  FIGS. 10A to 10C ,  1  indicates a dielectric block,  2   a  to  2   d  indicate inner-conductor holes,  3   a  to  3   d  indicate inner conductors,  4   a  to  4   d  indicate non-conductor portions,  5  indicates an outer conductor,  6  indicate input and output electrodes, and  7  indicate concavities.  11  and  12  show the distributions of magnetic fields in each of the TE modes, respectively for a case where the concavities  7  are not provided and for a case where the concavities  7  are provided. 
   Referring to  FIGS. 9A and 9B , from the top surface to the bottom surface of the substantially rectangular dielectric block  1 , the inner-conductor holes  2   a  to  2   d  are formed, and the inner conductors  3   a  to  3   d  are formed respectively on the inner surfaces of the inner-conductor holes  2   a  to  2   d . The outer conductor  5  is formed substantially over the entire outer surface of the dielectric block  1 . 
   In the inner-conductor holes  2   a  to  2   d , the non-conductor portions  4   a  to  4   d  are formed respectively in the proximity of one of the surfaces in which the apertures of the inner conductor holes  2   a  to  2   d  are formed. These portions define the open ends of the inner conductors  3   a  to  3   d , and the other surface defines the shorted ends. On the outer surface of the dielectric block  1 , the input and output electrodes  6 , isolated from the outer conductor  5 , are formed so as to be capacitively coupled with the open ends. 
   On both of the surfaces in which the apertures of the inner-conductor holes  2   a  to  2   d  are formed, the concavities  7  are extended in the axial direction of the inner-conductor holes  2   a  to  2   d , each being disposed at a respective position distant from a corresponding nearest end surface in the direction of array of the inner-conductor holes  2   a  to  2   d  by a quarter of the width of the dielectric block  1  in said direction. The inner surfaces of the concavities  7  are covered with the outer conductor  5 , whereby the entire dielectric filter is formed. 
   According to the above construction, the concavities  7  are formed in the regions where the magnetic field in the TE 201  mode is most intense. Thus, the distribution of magnetic field is significantly altered, so that the wavelength of the magnetic field component in the TE 201  mode is equivalently shortened, whereby the resonance frequency is shifted towards higher frequencies. 
   Furthermore, with respect to the TE 101  mode and the TE 301  mode, the concavities  7  are formed in the regions where the magnetic fields are weak, as shown in  FIGS. 10A and 10C . Thus, the distributions of magnetic fields are not altered, and the resonance frequency remains substantially unchanged. 
     FIG. 11  is a graph representing the content described above in terms of attenuation characteristics. As shown in  FIG. 11 , only the attenuation in the TE 201  mode is significantly affected. 
   As described above, the resonance frequency in the TE 201  mode is shifted, so that unwanted signals in the proximity of the resonance frequency in the TE 201  mode are blocked, whereby the spurious-response characteristics in the proximity of the resonance frequency in the TE 201  mode are improved. 
   Next, the construction of a dielectric filter according to a fourth embodiment will be described with reference to  FIGS. 12A and 12B . 
     FIGS. 12A and 12B  are, respectively, an external perspective view and a side view of the dielectric filter. 
   In  FIGS. 12A and 12B ,  1  indicates a dielectric block,  2   a  to  2   d  indicate inner-conductor holes,  3   a  to  3   d  indicate inner conductors,  4   a  to  4   d  indicate non-conductor portions,  5  indicates an outer conductor,  6  indicate input and output electrodes, and  7  indicate concavities. 
   In the dielectric filter shown in  FIGS. 12A and 12B , the plurality of concavities  7  is formed in localized regions not including spaces between the inner-conductor holes  2   a  to  2   d  in one of the surfaces in which the apertures of the inner-conductor holes  2   a  to  2   d  are formed, and not in the two edges of that surface parallel to the direction of array of the inner-conductor holes  2   a  to  2   d . The construction is otherwise the same as that of the dielectric filter shown in  FIGS. 9A and 9B . 
   According to the above construction, in a case in which the adjacent inner-conductor holes are disposed very close to each other, the concavities  7  can be formed without altering the capacitive coupling between the inner conductors. In addition, due to the concavities  7 , the wavelength of the magnetic field component in each of the TE modes is equivalently shortened, so that the resonance frequency is shifted toward higher frequencies, whereby the spurious-response characteristics are improved. 
   Also in the above embodiment, it is seen that the concavities can have various cross-sectional shapes while still obtaining the advantages of the invention. 
   Next, the constructions of dielectric filters according to a fifth embodiment will be described with reference to  FIGS. 13A and 13B  and  FIGS. 14A and 14B . 
     FIGS. 13A and 13B  are, respectively, an external perspective view and a side view of a dielectric filter. 
     FIGS. 14A and 14B  are, respectively, an external perspective view and a side view of another dielectric filter. 
   In  FIGS. 13A and 13B ,  1  indicates a dielectric block,  2   a  to  2   c  indicate inner-conductor holes,  3   a  to  3   c  indicate inner conductors,  4   a  to  4   c  indicate non-conductor portions,  5  indicates an outer conductor,  6  indicate input and output electrodes, and  7  indicate concavities. Similarly, in  FIGS. 14A and 14B ,  1  indicates a dielectric block,  2   a  to  2   d  indicate inner-conductor holes,  3   a  to  3   d  indicate inner conductors,  4   a  to  4   d  indicate non-conductor portions,  5  indicates an outer conductor,  6  indicate input and output electrodes, and  7  indicate concavities. 
   In the dielectric filters shown in  FIGS. 13A and 13B  and  FIGS. 14A and 14B , each of the plurality of concavities  7  is cut perpendicularly into one of the surfaces in which the apertures of the inner-conductor holes are formed, and also perpendicularly into one of the surfaces parallel to both the axial direction and the direction of array of the inner-conductor holes, so that a portion of the edge adjacent to those two surfaces is cut out. The construction of the dielectric filter shown in  FIGS. 13A and 13B  is basically the same as that of the dielectric filter shown in  FIGS. 1A to 1C , and the construction of the dielectric filter shown in  FIGS. 14A and 14B  is basically the same as that of the dielectric filter shown in  FIGS. 9A and 9B . Other concavities can optionally be provided, in addition to those shown. 
   According to the constructions, because the concavities are formed at the edges of one of the surfaces in which the apertures of the inner-conductor holes  2   a  to  2   c  are formed, the concavities can be readily formed by a simple process and without cutting the inner-conductor holes  2   a  to  2   c . In addition, due to the concavities, the wavelength of the magnetic field component in each of the TE modes is equivalently shortened, so that the resonance frequency is shifted toward higher frequencies, whereby the spurious-response characteristics are improved. 
   Next, the construction of a dielectric filter according to a sixth embodiment will be described with reference to  FIGS. 15A and 15B ,  FIGS. 16A to 16C , and  FIG. 17 . 
     FIGS. 15A and 15B  are, respectively, an external perspective view and a side view of the dielectric filter. 
     FIGS. 16A ,  16 B, and  16 C are diagrams showing the distributions of magnetic fields generated in the dielectric filter, respectively in the TE 101  mode, the TE 201  mode, and in the TE 301  mode.  FIG. 17  is a graph showing the attenuation characteristics of the dielectric filter. 
   Referring to  FIGS. 15A and 15B  and  FIGS. 16A to 16C ,  1  indicates a dielectric block,  2   a  to  2   d  indicate inner-conductor holes,  3   a  to  3   d  indicate inner conductors,  4   a  to  4   d  indicate non-conductor portions,  5  indicates an outer conductor,  6  indicate input and output electrodes, and  7  indicate concavities.  11  and  12  show the distributions of magnetic fields in each of the TE modes, respectively for a case where the concavities  7  are not provided and for a case where the concavities  7  are provided. 
   Referring to  FIGS. 15A and 15B , from the top surface to the bottom surface of the substantially rectangular dielectric block  1 , the inner-conductor holes  2   a  to  2   d  are formed, and the inner conductors  3   a  to  3   d  are formed respectively on the inner surfaces of the inner-conductor holes  2   a  to  2   d . The outer conductor  5  is formed substantially over the entire outer surface of the dielectric block  1 . 
   In the inner-conductor holes  2   a  to  2   d , the non-conductor portions  4   a  to  4   d  are formed respectively in the proximity of one of the surfaces in which the apertures of the inner-conductor holes  2   a  to  2   d  are formed. These portions provide open ends of the inner conductors  3   a  to  3   d , and the other surface provides shorted ends. On the outer surface of the dielectric block  1 , the input and output electrodes  6 , isolated from the outer conductor  5 , are formed so as to be capacitively coupled with the open ends. 
   Furthermore, in the central portions of the end surfaces in the direction of array of the inner-conductor holes  2   a  to  2   d , the concavities  7  are cut in the direction of array of the inner-conductor holes  2   a  to  2   d , the inner surfaces thereof being covered with the outer conductor  5 , whereby the entire dielectric filter is formed. 
   According to the construction, as shown in  FIGS. 16A to 16C , the concavities  7  are provided on the regions where the magnetic fields are most intense. Thus, the distributions of magnetic fields in the TE 101 , TE 201 , and TE 301  modes are significantly altered, so that the wavelength of each of the magnetic fields components in the TE modes is equivalently shortened, whereby the resonance frequency is shifted towards higher frequencies. As for higher TE modes such as TE 401  mode, the resonance frequency is also shifted towards higher frequencies. 
     FIG. 17  is a graph representing the content described above in terms of attenuation characteristics. As shown in  FIG. 17 , the attenuation in each of the TE modes is significantly affected. 
   As described above, the resonance frequency in each of the TE modes is shifted, so that unwanted signals in the proximity of the resonance frequency in each of the TE modes are blocked, whereby the spurious-response characteristics are improved. 
   Next, the construction of a dielectric filter according to a seventh embodiment will be described with reference to  FIGS. 18A and 18B  and  FIG. 19 . 
     FIGS. 18A and 18B  are, respectively, an external perspective view and a side view of the dielectric filter. 
     FIG. 19  is a diagram showing the distribution of a magnetic field in the TE 101  mode generated in the dielectric filter. 
   In  FIGS. 18A and 18B  and  FIG. 19 ,  1  indicates a dielectric block,  2   a  to  2   c  indicate inner-conductor holes,  3   a  to  3   c  indicate inner conductors,  5  indicates an outer conductor,  6  indicates input and output electrodes, and  7  indicates a concavity.  11  and  12  show the distribution of a magnetic field in the TE 101  mode, respectively for a case where the concavity  7  is not provided and for a case where the concavity  7  is provided. 
   Referring to  FIGS. 18A and 18B , from the top surface to the bottom surface of the substantially rectangular dielectric block  1 , the inner-conductor holes  2   a  to  2   c  are formed, and the inner conductors  3   a  to  3   c  are formed respectively on the inner surfaces of the inner-conductor holes  2   a  to  2   c . The outer electrode  5  is formed over five of the outer surfaces of the dielectric block  1 , the remaining surface being the top surface, i.e., one of the surfaces in which the apertures of the inner-conductor holes  2   a  to  2   c  are formed. 
   With the uncovered surface as the open-end surface and the opposite surface as the shorted-end surface, the input and output electrodes  6 , isolated from the outer conductor  5 , are formed so as to be coupled with the open-end surface. 
   Furthermore, the concavity  7  is cut in the axial direction of the inner-conductor holes  2   a  to  2   c  substantially at the central portion of the shorted-end surface, the inner surface thereof being covered with the outer conductor  5 , whereby the entire dielectric filter is formed. 
   In the dielectric filter of the above construction, a magnetic field in the TE 101  mode is distributed as shown in  FIG. 19 . 
   As shown in  FIG. 19 , the concavity  7  is provided in a region where the magnetic field in the TE 101  mode is most intense in the dielectric filter. Thus, the magnetic field is significantly altered, so that the wavelength of the magnetic field component in the TE 101  mode is equivalently shortened, whereby the resonance frequency is shifted toward higher frequencies. 
   As described above, the resonance frequency in the TE 101  mode is shifted, so that unwanted signals in the proximity of the resonance frequency in TE 101  mode are blocked, whereby the spurious-response characteristics in the proximity of the resonance frequency in the TE 101  mode are improved. 
   Next, the construction of a dielectric duplexer according to an eighth embodiment will be described with reference to  FIGS. 20A and 20B . 
     FIGS. 20A and 20B  are, respectively, an external perspective view and a side view of the dielectric duplexer. 
   In  FIGS. 20A and 20B ,  1  indicates a dielectric block,  2   a  to  2   f  indicate inner-conductor holes,  3   a  to  3   f  indicate inner conductors,  4   a  to  4   f  indicate non-conductor portions,  5  indicates an outer conductor,  6  indicates input and output electrodes, and  7  indicates concavities. 
   Referring to  FIGS. 20A and 20B , from the top surface to the bottom surface of the substantially rectangular dielectric block  1 , the inner-conductor holes  2   a  to  2   f  are formed, and the inner conductors  3   a  to  3   f  are formed respectively on the inner surfaces of the inner-conductor holes  2   a  to  2   f . The outer conductor  5  is formed substantially over the entire outer surface of the dielectric block  1 . 
   In the inner-conductor holes  2   a  to  2   f , the non-conductor portions  4   a  to  4   f  are formed respectively in the proximity of one of the surfaces in which the apertures of the inner-conductor holes  2   a  to  2   f  are formed. These portions provide the open ends of the inner conductors  3   a  to  3   f , and the other surface provides the shorted ends. The input and output electrodes  6 , isolated from the outer conductor  5 , are formed so as to be capacitively coupled with the open ends. 
   Furthermore, in the proximity of the central portions of the end surfaces in the direction of array of the inner-conductor holes  2   a  to  2   f , the concavities  7  are formed in the direction of array of the inner-conductor holes  2   a  to  2   f , the inner surfaces thereof being covered with the outer conductor  5 . 
   The inner-conductor holes  2   a  to  2   c  constitute a transmitting filter, and the inner-conductor holes  2   d  to  2   f  constitute a receiving filter, whereby the entire dielectric duplexer is formed. 
   According to this construction, as described previously in relation to the sixth embodiment, the magnetic fields in each of the TE modes are altered, so that the wavelengths of the magnetic field components are equivalently shortened. Thus, the resonance frequency in each of the TE modes is shifted, so that unwanted signals in the proximity of the resonance frequency in each of the TE modes are blocked, whereby the spurious-response characteristics are improved. 
   Similarly to the previous embodiments which relate to a discrete dielectric filter, in a dielectric duplexer as well, concavities may be formed in the surfaces in which the apertures of inner-conductor holes are formed and are not limited to being in the end surfaces in the direction of array of the inner-conductor holes. 
   Furthermore, similarly to the dielectric filter according to the seventh embodiment, concavities may be formed in a dielectric duplexer in which the open ends are provided by not forming the outer conductor on one of the surfaces in which the apertures of the inner-conductor holes are formed. 
   In the dielectric filters and the dielectric duplexer described above, the sectional shape of the inner-conductor holes is not limited to being a circular shape, and may be an elliptical shape, an oval shape, a polygon shape, etc. Likewise, the cross-sectional shape of the concavity is not limited to the disclosed shapes. 
   Furthermore, in a dielectric filter or a dielectric duplexer in which concavities are formed in one of the surfaces in which the apertures of the inner-conductor holes are formed, similar advantages can be obtained whether the concavities are formed in the open-end surface or in the shorted-end surface. 
   Next, the construction of a communications apparatus according to a ninth embodiment will be described with reference to  FIG. 21 . 
   Referring to  FIG. 21 , ANT indicates a transmitting and receiving antenna, DPX indicates a duplexer, BPFa and BPFb respectively indicate band-pass filters, AMPa and AMPb respectively indicate amplification circuits, MIXa and MIXb respectively indicate mixers, OSC indicates an oscillator, SYN indicates a synthesizer, and IF indicates an intermediate-frequency signal. 
   The band-pass filters BPFa and BPFb shown in  FIG. 21  can each be implemented by one of the dielectric filters shown in  FIGS. 1A and 1B ,  FIGS. 6A and 6B ,  FIGS. 9A and 9B ,  FIGS. 12A and 12B ,  FIGS. 13A and 13B ,  FIGS. 14A and 14B , FIGS.  15 A and  15 B, and  FIGS. 18A and 18B . The duplexer DPX can be implemented by the dielectric duplexer shown in  FIGS. 20A and 20B . As described above, by using a dielectric filter and a dielectric duplexer having good attenuation characteristics, a communications apparatus having good communications characteristics can be implemented. 
   Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. Therefore, the present invention is not limited by the specific disclosure herein.