Patent Publication Number: US-6661314-B2

Title: Dielectric band pass filter having an evanescent waveguide

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a band pass filter, and particularly, to a highly compact band pass filter that has excellent mechanical strength. 
     DESCRIPTION OF THE PRIOR ART 
     In recent years, marked advances in miniaturization of communication terminals, typically mobile phones, has been achieved thanks to miniaturization of the various components incorporated therein. One of the most important components incorporated in a communication terminal is a filter component. 
     As one type of filter component, Japanese Patent Laid Open No. 2000-68711 and Japanese Patent Laid Open No. 2000-183616, for example, teach band pass filters comprising a dielectric block formed with a plurality of holes whose inner walls are coated with metal plates. As another type of a filter component, band pass filters constituted by forming metal plates on irregular surfaces of a dielectric block are described in “Novel Dielectric Waveguide Components—Microwave Applications of New Ceramic Materials (PROCEEDINGS OF THE IEEE, VOL.79, NO.6, JUNE 1991), p734, FIG. 31.” 
     As a need continues to be felt for still further miniaturization of communication terminals such as mobile phones, further miniaturization of filter components, e.g., band pass filters, incorporated therein is also required. 
     The mechanical strength of the above-mentioned types of filter components is, however, low because holes are formed in, or irregularities are formed on, the dielectric block constituting the main body. Miniaturization of the filter component is therefore impossible. Specifically, in the former type of filter component having holes formed in a dielectric block, mechanical strength of the dielectric block is low around the holes and in the latter type of filter component having irregularities formed on the surface of a dielectric block, mechanical strength is low around the recesses. Therefore, miniaturization of the filter component must be limited to ensure the mechanical strength at such portions. 
     Thus, in the prior art it is difficult to miniaturize filter components while ensuring sufficient mechanical strength. Therefore, a compact band pass filter that has excellent mechanical strength is desired. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a compact band pass filter having excellent mechanical strength. 
     The above and other objects of the present invention can be accomplished by a band pass filter comprising a dielectric block of substantially rectangular prismatic shape constituted of a first portion lying between a first cross-section of the dielectric block and a second cross-section of the dielectric block substantially parallel to the first cross-section and second and third portions divided by the first portion and metal plates formed on the surfaces of the dielectric block, thereby enabling the first portion of the dielectric block and the metal plates formed thereon to act as an evanescent waveguide, the second portion of the dielectric block and the metal plates formed thereon to act as a first resonator, and the third portion of the dielectric block and the metal plates formed thereon to act as a second resonator, the metal plates including a capacitive stub formed on a first surface of the dielectric block which is substantially perpendicular to the cross-sections. 
     According to this aspect of the present invention, a filter function can be obtained even using a rectangular prismatic dielectric block because a desired coupling constant can be produced between the first and second resonators by the capacitive stub formed on the first surface of the dielectric block. Since the band pass filter according to the present invention is constituted of a rectangular prismatic dielectric block, the mechanical strength is extremely high. Thus, even if the overall size of the band pass filter is reduced, sufficient mechanical strength can be ensured. 
     In a preferred aspect of the present invention, the capacitive stub is formed on at least surfaces of the second and third portions of the dielectric block. 
     In a further preferred aspect of the present invention, the capacitive stub is further formed on a surface of the first portion of the dielectric block to form a continuous and integral capacitive stub on the surfaces of the first to third portions of the dielectric block. 
     In a further preferred aspect of the present invention, a portion of the capacitive stub formed on the surface of the second portion of the dielectric block and another portion of the capacitive stub formed on the surface of the third portion of the dielectric block have the same dimensions. 
     In a further preferred aspect of the present invention, the metal plates further include a first exciting electrode formed on a second surface of the dielectric block which is substantially parallel to the cross-sections and a second exciting electrode formed on a third surface of the dielectric block which is substantially parallel to the cross-sections. 
     In a further preferred aspect of the present invention, the second and third portions of the dielectric block have the same dimensions. 
     The above and other objects of the present invention can be also accomplished by a band pass filter comprising: 
     a first flat resonator and a second flat resonator each having top and bottom surfaces on which metal plates are formed, a shorting surface electrically short-circuiting the metal plates formed on the top and bottom surfaces, a first open surface opposite the shorting surface, a second open surface perpendicular to the shorting surface, and a third open surface opposite the second open surface; 
     an evanescent waveguide provided between the first and second flat resonators such that the evanescent waveguide is in contact with the entire second open surfaces of the first and second flat resonators; 
     a first capacitive stub formed on the first open surface of the first flat resonator; 
     a second capacitive stub formed on the first open surface of the second flat resonator; 
     a first exciting electrode formed on the third open surface of the first flat resonator; and 
     a second exciting electrode formed on the third open surface of the second flat resonator. 
     According to this aspect of the present invention, a band pass filter having no surface irregularities can be obtained because a desired coupling constant can be produced between the first and second flat resonators by the first and second capacitive stubs. Since the band pass filter according to the present invention has no surface irregularities, its mechanical strength is extremely high. Thus, even if the overall size of the band pass filter is reduced, sufficient mechanical strength can be ensured. 
     In a preferred aspect of the present invention, the band pass filter is substantially a rectangular prism in overall shape. 
     In a further preferred aspect of the present invention, the first and second flat resonators have the same dimensions. 
     In a further preferred aspect of the present invention, the first open surfaces of the first and second flat resonators are coplanar. 
     In a further preferred aspect of the present invention, the metal plates formed on the bottom surfaces of the first and second flat resonators are short-circuited by a metal plate formed on a bottom surface of the evanescent waveguide. 
     In a further preferred aspect of the present invention, the first and second capacitive stubs are short-circuited by a metal plate formed on a side surface of the evanescent waveguide. 
     In a further preferred aspect of the present invention, the first capacitive stub is connected to the metal plate formed on the bottom surface of the first flat resonator and the second capacitive stub is connected to the metal plate formed on the bottom surface of the second flat resonator. 
     In a further preferred aspect of the present invention, the first exciting electrode is formed on the third open surface of the first flat resonator at a portion adjacent to the first open surface of the first flat resonator, the second exciting electrode is formed on the third open surface of the second flat resonator at a portion adjacent to the first open surface of the second flat resonator, the first exciting electrode is prevented from being in contact with the metal plates formed on the top and bottom surfaces of the first flat resonator, and the second exciting electrode is prevented from being in contact with the metal plates formed on the top and bottom surfaces of the second flat resonator. 
     In a further preferred aspect of the present invention, the first exciting electrode is formed on the third open surface of the first flat resonator at a portion adjacent to the shorting surface of the first flat resonator, the second exciting electrode is formed on the third open surface of the second flat resonator at a portion adjacent to the shorting surface of the second flat resonator, the first exciting electrode is prevented from being in contact with the metal plate formed on the bottom surface of the first flat resonator and is connected to the metal plate formed on the top surface of the first flat resonator, and the second exciting electrode is prevented from being in contact with the metal plate formed on the bottom surface of the second flat resonator and is connected to the metal plate formed on the top surface of the second flat resonator. 
     The above and other objects of the present invention can be also accomplished by a band pass filter comprising: 
     a first flat resonator and a second flat resonator each having top and bottom surfaces on which metal plates are formed, a shorting surface electrically short-circuiting the metal plates formed on the top and bottom surfaces, a first open surface opposite the shorting surface, a second open surface perpendicular to the shorting surface, and a third open surface opposite the second open surface; 
     an evanescent waveguide provided between the first and second flat resonators such that the evanescent waveguide is in contact with the entire second open surfaces of the first and second flat resonators; 
     a first exciting electrode formed on the third open surface of the first flat resonator; and 
     a second exciting electrode formed on the third open surface of the second flat resonator, 
     whereby a first resonation circuit is established between the first exciting electrode and the metal plates, a second resonation circuit is established between the second exciting electrode and the metal plates, and a coupling circuit is established between the first and second resonation circuits, 
     the band pass filter further comprising means for providing an additional capacitance in parallel with the first resonation circuit and another additional capacitance in parallel with the second resonation circuit. 
     In a preferred aspect of the present invention, the band pass filter is substantially a rectangular prism in overall shape. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic perspective view from one side showing a band pass filter  1  that is a preferred embodiment of the present invention. 
     FIG. 2 is a schematic perspective view from the opposite side showing the band pass filter of FIG.  1 . 
     FIG. 3 is a schematic perspective view showing an ordinary TEM-mode half-wave (λ/2) dielectric resonator. 
     FIG. 4 is a schematic perspective view showing an ordinary quarter-wave (λ/4) dielectric resonator. 
     FIG. 5 is a schematic diagram for explaining an electric field and a magnetic field generated by a quarter-wave (λ/4) dielectric resonator. 
     FIG. 6 is an equivalent circuit diagram of the band pass filter  1  shown in FIGS. 1 and 2. 
     FIG. 7 is graph showing the frequency characteristic curve of the band pass filter  1  shown in FIGS. 1 and 2. 
     FIG. 8 is a schematic perspective view from one side showing a model in which first and second capacitive stubs  12  and  13  are eliminated from the band pass filter  1  shown in FIGS. 1 and 2. 
     FIG. 9 is a schematic perspective view from the opposite side showing the model of FIG.  8 . 
     FIG. 10 is an equivalent circuit diagram of the model shown in FIGS. 8 and 9. 
     FIG. 11 is a graph showing the relationship between the heights h of the first and second capacitive stubs  12  and  13  and an even mode resonant frequency f even  and an odd mode resonant frequency f odd . 
     FIG. 12 is a graph showing the relationship between the heights h of the first and second capacitive stubs  12  and  13  and a coupling constant k total . 
     FIG. 13 is a schematic perspective view for explaining the relationship between an electric field generated by the band pass filter  1  shown in FIGS. 1 and 2 and the first and second capacitive stubs  12  and  13 . 
     FIG. 14 is a schematic perspective view from one side showing a band pass filter  47  that is another preferred embodiment of the present invention. 
     FIG. 15 is a schematic perspective view from the opposite side showing the band pass filter  47  of FIG.  14 . 
     FIG. 16 is a schematic perspective view from one side showing a band pass filter  62  that is a further preferred embodiment of the present invention. 
     FIG. 17 is a schematic perspective view from the opposite side showing the band pass filter  62  of FIG.  16 . 
     FIG. 18 is a schematic perspective view from one side showing a band pass filter  78  that is a further preferred embodiment of the present invention. 
     FIG. 19 is a schematic perspective view from the opposite side showing the band pass filter  78  of FIG.  18 . 
     FIG. 20 is a schematic perspective view of the band pass filter  1  showing an example in which the capacitive stubs  12  and  13  and metal plate  9  formed on the bottom surfaces of a dielectric block  2  are separated. 
     FIG. 21 is a schematic perspective view of the band pass filter  78  showing an example in which a capacitive stub  89  and metal plate  86  formed on the bottom surfaces of a dielectric block  79  are separated. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be explained with reference to the drawings. 
     As shown in FIGS. 1 and 2, a band pass filter  1  that is a preferred embodiment of the present invention is constituted of a dielectric block  2  and various metal plates formed on the surface thereof. The dielectric block  2  is made of dielectric material whose dielectric constant εΓ is relatively high, i.e., εΓ=93, and has the shape of a rectangular prism whose length, width, and thickness are 5.0 mm, 3.4 mm, and 1.0 mm. That is, the dielectric block  2  has no holes or surface irregularities. 
     Further, the dielectric block  2  is composed of a first portion lying between a first cross-section and a second cross-section parallel to the first cross-section and second and third portions divided by the first portion. It is worth noting that this does not mean that the dielectric block  2  is a combination of the first to third portions of physically different components. The dielectric block  2  constitutes a single dielectric unit, i.e., the first to third portions are names used solely for convenience of description. 
     The first portion whose length, width, and thickness are 0.2 mm, 3.4 mm, and 1.0 mm, is located at the center of the rectangular prismatic dielectric block  2 . The second and third portions of the dielectric block  2  are symmetrically located relative to the first portion. Each measures 2.4 mm, 3.4 mm, and 1.0 mm in length, width and thickness. Directions defining the “length,” “width,” and “thickness” of the first to third portions are the same as the directions defining the “length,” “width,” and “thickness” of the dielectric block  2 . 
     The dielectric block  2  has a top surface, a bottom surface, and four side surfaces. Among the four side surfaces of the dielectric block  2 , the end surface of the second portion is defined as a “first side surface,” end surface of the third portion is defined as a “second side surface,” and the remaining surfaces are defined as a “third side surface” and a “fourth side surface.” Therefore, both the top and bottom surfaces measure 5.0 mm (length)×3.4 mm (width), both the first and second side surfaces measure 1.0 mm (thickness)×3.4 mm (width), and both the third and fourth side surfaces measure 5.0 mm (length)×1.0 mm (thickness). 
     As shown in FIGS. 1 and 2, metal plates  3  and  4  are formed on, among the top surface of the dielectric block  2 , entire surfaces corresponding to the second and third portions, respectively. Metal plates  5  and  6  are formed on parts of the third surface of the dielectric block  2  corresponding to the entire surfaces of the second and third portions, respectively. A metal plate  9  is formed on the bottom surface of the dielectric block  2  except at clearance portions  7  and  8 . These metal plates  3 ,  4 ,  5 ,  6 , and  9  are short-circuited with one another and grounded. 
     As shown in FIG. 1, an exciting electrode  10  whose height and width are 0.9 mm and 1.2 mm is formed on the first side surface of the dielectric block  2  where the clearance portion  7  prevents the exciting electrode  10  from being in contact with the metal plate  9  formed on the bottom surface. Similarly, as shown in FIG. 2, an exciting electrode  11  whose height and width are 0.9 mm and 1.2 mm is formed on the second side surface of the dielectric block  2  where the clearance portion  8  prevents the exciting electrode  11  from being in contact with the metal plate  9  formed on the bottom surface. One of the exciting electrodes  10  and  11  is used as an input electrode, and the other is used as an output electrode. 
     As shown in FIG. 1, a first capacitive stub  12  whose height and width are 0.35 mm and 1.6 mm is formed on the fourth side surface of the dielectric block  2  corresponding to the second portion and a second capacitive stub  13  whose height and width are 0.35 mm and 1.6 mm is formed on the fourth side surface of the dielectric block  2  corresponding to the third portion. The first and second capacitive stubs  12  and  13  are connected to the metal plate  9  formed on the bottom surface of the dielectric block. The direction defining the “width” of the first and second capacitive stubs  12  and  13  is coincident with the direction defining the “length” of the dielectric block  2 . 
     The metal plates  3 ,  4 ,  5 ,  6 , and  9 , the exciting electrodes  10  and  11 , and the first and second capacitive stubs  12  and  13  are made of silver. However, the present invention is not limited to using silver and other kinds of metal can be used instead. It is preferable to use a screen printing method to form them on the surfaces of the dielectric block  2 . 
     No metal plate or electrode is formed on the remaining surfaces of the dielectric block  2 , which therefore constitute open ends. 
     According to the above described structure, the first portion of the dielectric block  2  and the metal plate formed thereon act as an evanescent waveguide  14 , the second portion of the dielectric block  2  and the metal plate formed thereon act as a first resonator  15 , and the third portion of the dielectric block  2  and the metal plate formed thereon act as a second resonator  16 . The evanescent waveguide  14  is an E-mode waveguide, and each of the first and second resonators  15  and  16  is a quarter-wave (λ/4) dielectric resonator. 
     The principle of the quarter-wave (λ/4) dielectric resonators constituted by the first resonator  15  and the second resonator  16  will now be explained. 
     FIG. 3 is a schematic perspective view showing an ordinary TEM-mode half-wave (λ/2) dielectric resonator. 
     As shown in FIG. 3, the ordinary half-wave (λ/2) dielectric resonator is constituted of a dielectric block  20 , a metal plate  21  formed on the upper surface of the dielectric block  20 , and a metal plate  22  formed on the lower surface of the dielectric block  20 . The metal plate  21  formed on the upper surface of the dielectric block  20  is electrically floated whereas the metal plate  22  formed on the lower surface of the dielectric block  20  is grounded. All of the four side surfaces of the dielectric block  20  are open to the air. In FIG. 3, the length and width of the dielectric block  20  are indicated by a and t. 
     For propagation of the dominant TEM-mode along the z direction of this half-wave (λ/2) dielectric resonator, if electric field is negative maximum at z=0 plan, then it should be positive maximum at z=a plan as indicated by the arrow  23  in this Figure. Definitely there should be minimum (zero) electric field at z=a/2 plan, which is the symmetry plan  24  of the resonator. 
     Cutting such a half-wave (λ/2) dielectric resonator along the symmetry plan  24 , two quarter-wave (λ/4) dielectric resonators can be obtained. In this quarter-wave (λ/4) dielectric resonator, a plan z=a/2 acts as a perfect electric conductor (PEC). 
     FIG. 4 is a schematic perspective view showing the quarter-wave (λ/4) dielectric resonator obtained by above described method. 
     As shown in FIG. 4, the quarter-wave (λ/4) dielectric resonator is constituted of a dielectric block  30 , a metal plate  31  formed on the upper surface of the dielectric block  30 , a metal plate  32  formed on the lower surface of the dielectric block  30 , and a metal plate  34  formed on one of the side surfaces of the dielectric block  30 . The remaining three side surfaces of the dielectric block  30  are open to the air. The metal plate  32  formed on the lower surface of the dielectric block  30  is grounded. The metal plate  34  formed on one of the side surfaces of the dielectric block  30  corresponds to the perfect electric conductor (PEC) of the half-wave (λ/2) dielectric resonator to short-circuit the metal plate  31  and the metal plate  32 . In FIG. 4, arrows  33  indicate electric field, and arrows  35  indicate current flow. 
     Ideally, the quarter-wave (λ/4) dielectric resonator shown in FIG.  4  and the half-wave (λ/2) dielectric resonator shown in FIG. 3 should have the same resonant frequency. If a material having a relatively high dielectric constant is used for the dielectric block  30 , electromagnetic field confinement inside the resonator is adequately strong. Moreover, the distribution of the electromagnetic field of the quarter-wave (λ/4) dielectric resonator becomes substantially the same as that of the half-wave (λ/2) dielectric resonator. As shown in FIGS. 3 and 4, the volume of the quarter-wave (λ/4) dielectric resonator is half the volume of the half-wave (λ/2) dielectric resonator. As a result, the total energy of the quarter-wave (λ/4) dielectric resonator is also half the total energy of the half-wave (λ/2) dielectric resonator. However, the unloaded quality factor (Q 0 ) of the quarter-wave (λ/4) dielectric resonator remain almost same that of the half-wave (λ/2) dielectric resonator because the energy loss of the quarter-wave (λ/4) dielectric resonator decreases to around 50% that of the half-wave (λ/2) dielectric resonator. The quarter-wave (λ/4) dielectric resonator therefore enables miniaturization without substantially changing the resonant frequency and the unloaded quality factor (Q 0 ). 
     FIG. 5 is a schematic diagram for explaining the electric field and the magnetic field generated by the quarter-wave (λ/4) dielectric resonator. 
     As shown in FIG. 5, the magnetic field  36  of the quarter-wave (λ/4) dielectric resonator is maximum throughout the metal plate  34  formed on one of the side surfaces of the dielectric block  30 . By linking the metal plate  34 , the magnetic field  36  causes the additional series inductance to resonator equivalent circuit. Thus, the resonant frequency of the quarter-wave (λ/4) dielectric resonator therefore becomes slightly lower than that of the half-wave (λ/2) dielectric resonator. 
     In this type of quarter-wave (λ/4) dielectric resonator, the unloaded quality factor (Q 0 ) depends on the thickness and the length of the dielectric block. Specifically, the unloaded quality factor (Q 0 ) of the quarter-wave (λ/4) dielectric resonator increases in proportion to the thickness of the dielectric block in a first thickness region of the dielectric block smaller than a predetermined thickness and decreases in proportion to the thickness of the dielectric block in a second thickness region of the dielectric block greater than the predetermined thickness. Further, the unloaded quality factor (Q 0 ) of the quarter-wave (λ/4) dielectric resonator increases in proportion to the length of the dielectric block in a first length region of the dielectric block smaller than a predetermined length and becomes substantially constant in a second length region of the dielectric block greater than the predetermined length. A quarter-wave (λ/4) dielectric resonator having the desired unloaded quality factor (Q 0 ) can therefore be obtained by optimizing the thickness and the length of the dielectric block constituting the quarter-wave (λ/4) dielectric resonator. 
     Further, in this type of quarter-wave (λ/4) dielectric resonator, the resonant frequency mainly depends on the width of the dielectric block but has very little dependence upon thickness and length of the resonator. Specifically, the resonant frequency increases with shorter width of the dielectric block. A quarter-wave (λ/4) dielectric resonator having the desired resonant frequency can therefore be obtained by optimizing the width of the dielectric block constituting the quarter-wave (λ/4) dielectric resonator. 
     The band pass filter  1  of this embodiment is constituted of two quarter-wave (λ/4) dielectric resonators, whose operating principle was explained in the foregoing, and an evanescent waveguide  14  which acts as an E-mode waveguide disposed therebetween. 
     FIG. 6 is an equivalent circuit diagram of the band pass filter  1  shown in FIGS. 1 and 2. 
     In this Figure, the evanescent waveguide  14  is represented by a L-C parallel circuit  40 , and the first resonator  15  and the second resonator  16  are represented by two L-C parallel circuits  41  and  42 , respectively. Capacitances Cp of the L-C parallel circuits  41  and  42  are produced by the first and second capacitive stubs  12  and  13 . The exciting electrodes  10  and  11  are represented by two capacitances Ce. 
     FIG. 7 is a graph showing the frequency characteristic curve of the band pass filter  1  shown in FIGS. 1 and 2. 
     In this Figure, S 11  represents a reflection coefficient, and S 21  represents a transmission coefficient. As shown in FIG. 7, the resonant frequency of the band pass filter  1  is approximately 2.45 GHz and its 3-dB band width is approximately 120 MHz. 
     The function of the first and second capacitive stubs  12  and  13  of the band pass filter  1  will be explained. 
     To explain the function of the first and second capacitive stubs  12  and  13 , a model in which first and second capacitive stubs  12  and  13  are eliminated from the band pass filter  1  will be explained first. 
     FIG. 8 is a schematic perspective view from one side showing the model in which the first and second capacitive stubs  12  and  13  are eliminated from the band pass filter  1  shown in FIGS. 1 and 2. FIG. 9 is a schematic perspective view from the opposite side showing the model of FIG.  8 . 
     This model is constituted of the evanescent waveguide  14 , a first resonator  43 , and a second resonator  44 . Each of the first and second resonators  43  and  44  is a quarter-wave (λ/4) dielectric resonator. 
     FIG. 10 is an equivalent circuit diagram of the model shown in FIGS. 8 and 9. 
     In this Figure, the evanescent waveguide  14  is represented by the L-C parallel circuit  40 , and the first resonator  43  and the second resonator  44  are represented by two L-C parallel circuits  45  and  46 , respectively. Unlike the L-C parallel circuits  41  and  42 , the L-C parallel circuits  45  and  46  do not include capacitances Cp because the model does not employ the first and second capacitive stubs  12  and  13 . 
     The coupling constant k total  ascribed to the evanescent waveguide  14  can be represented by the following formula. 
     
       
           k   total   =k   c + k   i   
       
     
     where k c  represents the capacitive coupling constant and k i  represents the inductive coupling constant. These constants can be represented by the following formulas. 
     k c =(coupling capacitance between the resonators)/(capacitance of each resonator) 
     k i =(coupling inductance between the resonators)/(inductance of each resonator) 
     In the model shown in FIGS. 8 and 9, therefore, the capacitive coupling constant k c  is represented by Cm/C and the inductive coupling constant k i  is represented by Lm/L. In this model, the coupling constant k total  becomes zero because 
     
       
         
           Cm/C=−Lm/L. 
         
       
     
     Therefore, the model in FIGS. 8 and 9 does not have a filter function. 
     As apparent from the foregoing, the filter function disappears when the first and second capacitive stubs  12  and  13  are eliminated from the band pass filter  1 . 
     In contrast, in the band pass filter  1  of the preferred embodiment, since the capacitances Cp are added to the L-C parallel circuits  41  and  42  in parallel by the first and second capacitive stubs  12  and  13 , the capacitive coupling constant k c  is represented by Cm/(C+Cp) and the inductive coupling constant k i  is represented by Lm/L. In this case, the coupling constant k total  becomes other than zero because 
     
       
           Cm /( C+Cp )≠− Lm/L.   
       
     
     Thus, the band pass filter  1  of the preferred embodiment has desired filter function. 
     As apparent from the foregoing, the first and second capacitive stubs  12  and  13  function to provide a predetermined coupling constant k total  between the first and second resonators  15  and  16 . 
     FIG. 11 is a graph showing the relationship between the heights h of the first and second capacitive stubs  12  and  13  and an even mode resonant frequency f even  and an odd mode resonant frequency f odd . The widths of both the first and second capacitive stubs  12  and  13  are fixed at 1.6 mm. 
     As shown in FIG. 11, both the even mode resonant frequency f even  and the odd mode resonant frequency f odd  decrease with increasing height h of the first and second capacitive stubs  12  and  13 , whereas the even mode resonant frequency f even  and the odd mode resonant frequency f odd  are the same when the height h is 0 mm, i.e., without capacitive stubs. As is apparent from FIG. 11, because the odd mode resonant frequency f odd  decreases more rapidly than the even mode resonant frequency f even , the frequency difference between them increases with increasing height h of the first and second capacitive stubs  12  and  13 . This means that the coupling constant k total  increases with increasing height h of the first and second capacitive stubs  12  and  13 . 
     FIG. 12 is a graph showing the relationship between the heights h of the first and second capacitive stubs  12  and  13  and a coupling constant k total . The widths of both the first and second capacitive stubs  12  and  13  are fixed at 1.6 mm. 
     As is apparent from FIG. 12, the coupling constant k total  exponentially increases with increasing height h of the first and second capacitive stubs  12  and  13 . In the case where the height h of the first and second capacitive stubs  12  and  13  are set at 0.35 mm as in the band pass filter  1  of this embodiment, a coupling constant k total  of approximately 0.034 can be obtained as shown in FIG.  12 . 
     As described above, the first and second capacitive stubs  12  and  13  give the band pass filter  1  a filter function, and a desired coupling constant k total  can be obtained by controlling their height h. Because the coupling constant k total  also increases with increasing width of the first and second capacitive stubs  12  and  13 , a desired coupling constant k total  can be also obtained by controlling their width. 
     FIG. 13 is a schematic perspective view for explaining the relationship between an electric field generated by the band pass filter  1  shown in FIGS. 1 and 2 and the first and second capacitive stubs  12  and  13 . 
     As is apparent from FIG. 13, the first and second capacitive stubs  12  and  13  are formed at the fourth side surface of the dielectric block  2  where the electric field  18  is the strongest. A radiation loss arising at the fourth side surface of the dielectric block  2  is therefore reduced. Further, the exciting electrode  10  ( 11 ) is formed at a region of the first (second) side surface of the dielectric block  2  where the electric field  18  is relatively strong. The widths and heights of the exciting electrodes  10  and  11  are 1.2 mm and 0.9 mm, as mentioned above. An external quality factor Q of 29 can therefore be obtained. 
     Because, as described above, the band pass filter  1  according to this embodiment is constituted of the rectangular prismatic dielectric block  2  having no holes or surface irregularities and the metal plates  3 ,  4 ,  5 ,  6 , and  9 , the exciting electrodes  10  and  11 , and the first and second capacitive stubs  12  and  13  formed on the surfaces thereof, the mechanical strength is extremely high compared with conventional filters. Thus, even if the overall size of the band pass filter  1  is reduced, sufficient mechanical strength can be ensured. 
     Moreover, because the band pass filter  1  according to this embodiment can be fabricated by only coating various metal plates on the dielectric block  2 , i.e., forming holes or inequalities is not necessary as in conventional filters, the fabrication cost thereof can be substantially reduced. 
     Another preferred embodiment of the present invention will now be explained. 
     FIG. 14 is a schematic perspective view from one side showing a band pass filter  47  that is another preferred embodiment of the present invention. FIG. 15 is a schematic perspective view from the opposite side showing the band pass filter  47  of FIG.  14 . 
     As shown in FIGS. 14 and 15, the structure of the band pass filter  47  that is another preferred embodiment is similar to that of the band pass filter  1  of the embodiment explained earlier, but the band pass filter  47  differs from the band pass filter  1  in that a capacitive stub is formed as a single unit. 
     Specifically, the band pass filter  47  that is another preferred embodiment is constituted of a regular prismatic dielectric block  48  whose dielectric constant εΓ is 93, metal plates  49  and  50  formed on, of the top surface of the dielectric block  48 , the entire surfaces corresponding to the second and third portions, respectively, metal plates  51  and  52  formed on, of the third surface of the dielectric block  48 , the entire surfaces corresponding to the second and third portions, respectively, a metal plate  55  formed on the bottom surface of the dielectric block  48  except at clearance portions  53  and  54 , an exciting electrode  56  formed on the first side surface of the dielectric block  48 , an exciting electrode  57  formed on the second side surface of the dielectric block  48 , and a capacitive stub  58  formed on the fourth side surface of the dielectric block  48  continuously at the first to third portions. 
     As shown in FIGS. 14 and 15, the exciting electrodes  56  and  57  are prevented from being in contact with the metal plate  55  formed on the bottom surface of the dielectric block  48  by the clearance portions  53  and  54 , respectively, whereas the capacitive stub  58  is connected to the ground plane  55 . One of the exciting electrodes  56  and  57  is used as an input electrode, and the other is used as an output electrode. The capacitive stub  58  is symmetrical with respect to the center of the dielectric block  48  so that a part of the capacitive stub  58  which is formed on the second portion and another part of the capacitive stub  58  which is formed on the third portion have the same dimensions. 
     The first to third portions of the dielectric block  48  are defined the same as the corresponding portions of the dielectric block  2  of the embodiment explained earlier. The top surface, bottom surface, and first to fourth side surfaces of the dielectric block  48  are defined the same as the corresponding surfaces of the dielectric block  2  of the embodiment explained earlier. Further, the length, width, and thickness are defined the same as the embodiment explained earlier. 
     According to the above-described structure, the first portion of the dielectric block  48  and the metal plate formed thereon act as an evanescent waveguide  59 , the second portion of the dielectric block  48  and the metal plate formed thereon act as a first resonator  60 , and the third portion of the dielectric block  48  and the metal plate formed thereon act as a second resonator  61 . The evanescent waveguide  59  is an E-mode waveguide, and each of the first and second resonators  60  and  61  is a quarter-wave (λ/4) dielectric resonator. 
     The band pass filter  47  having the above-described configuration has the same advantages as the band pass filter  1  of the embodiment described earlier. Specifically, because the mechanical strength of the band pass filter  47  is extremely high compared with conventional filters, even if its overall size is reduced, sufficient mechanical strength can be ensured. In addition, according to this embodiment, because the capacitive stub  58  is formed on the fourth side surface of the dielectric block  48  continuously at the first to third portions, the area of the capacitive stub  58  is large. Thus, the advantages produced by the capacitive stub  58  can be obtained more effectively. 
     A further preferred embodiment of the present invention will now be explained. 
     FIG. 16 is a schematic perspective view from one side showing a band pass filter  62  that is a further preferred embodiment of the present invention. FIG. 17 is a schematic perspective view from the opposite side showing the band pass filter  62  of FIG.  16 . 
     As shown in FIGS. 16 and 17, the structure of the band pass filter  62  that is a further preferred embodiment is similar to that of the band pass filter  1  of the embodiment explained earlier. but the band pass filter  62  differs from the band pass filter  1  in that the exciting electrodes are of inductive type. 
     Specifically, the band pass filter  62  that is a further preferred embodiment is constituted of a rectangular prismatic dielectric block  63  whose dielectric constant εΓ is 93, metal plates  64  and  65  formed on, of the top surface of the dielectric block  63 , the entire surfaces corresponding to the second and third portions, respectively, metal plates  66  and  67  formed on, of the third surface of the dielectric block  63 , the entire surfaces corresponding to the second and third portions, respectively, a metal plate  70  formed on the bottom surface of the dielectric block  63  except at clearance portions  68  and  69 , an exciting electrode  71  formed on the first side surface of the dielectric block  63 , an exciting electrode  72  formed on the second side surface of the dielectric block  63 , a first capacitive stub  73  formed on the fourth side surface of the dielectric block  63  corresponding to the second portion, and a second capacitive stub  74  formed on the fourth side surface of the dielectric block  63  corresponding to the third portion. 
     As shown in FIGS. 16 and 17, the exciting electrodes  71  and  72  are connected to the metal plates  64  and  65  formed on the top surface of the dielectric block  63 , respectively, while they are prevented from being in contact with the metal plate  70  formed on the bottom surface of the dielectric block  63  by the clearance portions  68  and  69 , respectively. The first and second capacitive stubs  73  and  74  are connected to the metal plate  70 . As shown in FIGS. 16 and 17, the exciting electrodes  71  and  72  are formed at the first and second side surfaces of the dielectric block  63  where the electric field is relatively weak (the magnetic field is relatively strong). One of the exciting electrodes  71  and  72  is used as an input electrode, and the other is used as an output electrode. 
     The first to third portions of the dielectric block  63  are defined the same as the corresponding portions of the dielectric block  2  of the embodiment explained earlier. The top surfaces, bottom surfaces, and first to fourth side surfaces of the dielectric block  63  are defined the same as the corresponding surfaces of the dielectric block  2  of the embodiment explained earlier. Further, the length, width, and thickness are defined the same as the embodiment explained earlier. 
     According to the above described structure, the first portion of the dielectric block  63  and the metal plate formed thereon act as an evanescent waveguide  75 , the second portion of the dielectric block  63  and the metal plate formed thereon act as a first resonator  76 , and the third portion of the dielectric block  63  and the metal plate formed thereon act as a second resonator  77 . The evanescent waveguide  75  is an E-mode waveguide, and each of the first and second resonators  76  and  77  is a quarter-wave (λ/4) dielectric resonator. 
     The band pass filter  62  having the above-described configuration has the same advantages as the band pass filter  1  of the embodiment described earlier. Specifically, because the mechanical strength of the band pass filter  62  is extremely high compared with conventional filters, even if its overall size is reduced, sufficient mechanical strength can be ensured. 
     A further preferred embodiment of the present invention will now be explained. 
     FIG. 18 is a schematic perspective view from one side showing a band pass filter  78  that is a further preferred embodiment of the present invention. FIG. 19 is a schematic perspective view from the opposite side showing the band pass filter  78  of FIG.  18 . 
     As shown in FIGS. 18 and 19, the structure of the band pass filter  78  that is a further preferred embodiment is similar to that of the band pass filter  47  of the embodiment explained earlier, but the band pass filter  78  differs from the band pass filter  47  in that the exciting electrodes are of inductive type. 
     Specifically, the band pass filter  78  that is a further preferred embodiment is constituted of a rectangular prismatic dielectric block  79  whose dielectric constant εΓ is 93, metal plates  80  and  81  formed on, of the top surface of the dielectric block  79 , the entire surfaces corresponding to the second and third portions, respectively, metal plates  82  and  83  formed on, of the third surface of the dielectric block  79 , the entire surfaces corresponding to the second and third portions, respectively, a metal plate  86  formed on the bottom surface of the dielectric block  79  except at clearance portions  84  and  85 , an exciting electrode  87  formed on the first side surface of the dielectric block  79 , an exciting electrode  88  formed on the second side surface of the dielectric block  79 , and a capacitive stub  89  formed on the fourth side surface of the dielectric block  79  continuously at the first to third portions. 
     As shown in FIGS. 18 and 19, the exciting electrodes  87  and  88  are connected to the metal plates  80  and  81  formed on the top surface of the dielectric block  79 , respectively, while they are prevented from being in contact with the metal plate  86  formed on the bottom surface of the dielectric block  79  by the clearance portions  84  and  85 , respectively. As shown in FIGS. 18 and 19, the exciting electrodes  87  and  88  are formed at the first and second side surfaces of the dielectric block  79  where the electric field is relatively weak (the magnetic field is relatively strong). One of the exciting electrodes  87  and  88  is used as an input electrode, and the other is used as an output electrode. The capacitive stub  89  is symmetrical with respect to the center of the dielectric block  79  so that a part of the capacitive stub  89  which is formed on the second portion and another part of the capacitive stub  89  which is formed on the third portion have the same dimensions. 
     The first to third portions of the dielectric block  79  are defined the same as the corresponding portions of the dielectric block  2  of the embodiment explained earlier. The top surface, bottom surface, and first to fourth side surfaces of the dielectric block  79  are defined the same as the corresponding surfaces of the dielectric block  2  of the embodiment explained earlier. Further, the length, width, and thickness are defined the same as the embodiment explained earlier. 
     According to the above described structure, the first portion of the dielectric block  79  and the metal plate formed thereon act as an evanescent waveguide  90 , the second portion of the dielectric block  79  and the metal plate formed thereon act as a first resonator  91 , and the third portion of the dielectric block  79  and the metal plate formed thereon act as a second resonator  92 . The evanescent waveguide  90  is an E-mode waveguide, and each of the first and second resonators  91  and  92  is a quarter-wave (λ/4) dielectric resonator. 
     The band pass filter  78  having the above-described configuration has the same advantages as the band pass filter  47  of the embodiment described earlier. Specifically, because the mechanical strength of the band pass filter  78  is extremely high compared with the conventional filters, even if its overall size is reduced, sufficient mechanical strength can be ensured. Moreover, because the capacitive stub  89  is formed on the fourth side surface of the dielectric block  79  continuously at the first to third portions, the area of the capacitive stub  89  is large. Thus, the advantages produced by the capacitive stub  89  can be obtained more effectively. 
     The present invention has thus been shown and described with reference to specific embodiments. However, it should be noted that the present invention is in no way limited to the details of the described arrangements but changes and modifications may be made without departing from the scope of the appended claims. 
     For example, in the above described embodiments, the dielectric block portions for the resonators and the evanescent waveguide are made of dielectric material whose dielectric constant εΓ is 93. However, a material having a different dielectric constant can be used according to purpose. 
     Further, the dimensions of the resonators and the evanescent waveguide specified in the above-described embodiments are only examples. Resonators and an evanescent waveguide having different dimensions can be used according to purpose. 
     Further, in the above-described embodiments, the capacitive stubs are formed such that they are in contact with the metal plates formed on the dielectric block. However, the present invention is not limited to the capacitive stubs being in contact with the metal plates and they can be formed separately from the metal plates. An example in which the first and second capacitive stubs  12  and  13  and metal plate  9  are formed separately in the band pass filter  1  is shown in FIG.  20 . Another example in which the capacitive stub  89  and metal plate  86  are formed separately in the band pass filter  78  is shown in FIG.  21 . This configuration of the capacitive stubs also enables a filter function to be obtained by producing a desired coupling constant k total . It is worth noting that to obtain the effects efficiently it is preferable that the capacitive stubs and the metal plates be connected. 
     As described above, according to the present invention, a highly compact band pass filter of excellent mechanical strength can be provided. 
     Therefore, the present invention provides a band pass filter that can be preferably utilized in communication terminals such as mobile phones and the like, Wireless LANs (Local Area Networks), and ITS (Intelligent Transport Systems) and the like.