Dielectric filter

Abstract: Stronger electromagnetic coupling than in conventional devices can be provided between adjacent resonator holes (2a, 2b) in a dielectric filter or a dielectric duplexer without changing the external shape and dimensions of a dielectric block (1). Resonator holes (2a, 2b) pass through opposing surfaces of a dielectric block (1), each including a large-diameter hole section (22a, 22b) and a small-diameter hole section (23a, 23b). The small-diameter hole sections (23a, 23b) may be formed near a short-circuit end face (1b) of the dielectric block (1). The large-diameter hole sections (22a, 22b) and the small-diameter hole sections (23a, 23b) are connected to each other with their axes shifted from each other. The radius R of the large-diameter hole sections (22a, 22b), the radius r of the small-diameter hole sections (23a, 23b), and the shift distance P between the axes of the large-diameter hole sections and those of the small-diameter hole sections satisfy the expression R - r < P < R + r .

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to dielectric filters and dielectric duplexers, and more particularly, to dielectric filters and dielectric duplexers in which a plurality of dielectric resonators are provided in a single dielectric block. 2. Related Art A known dielectric filter in which a plurality of dielectric resonators are provided in a single dielectric block is shown in Fig. 18. In this dielectric filter, two resonator holes 32a and 32b pass through opposing surfaces 31a and 31b of a dielectric block 31. The resonator holes 32a and 32b have large-diameter hole sections 42a and 42b, and small-diameter hole sections 43a and 43b connecting to the large-diameter hole sections 42a and 42b. The axes of the small-diameter hole sections 43a and 43b are eccentrically shifted from those of the large-diameter hole sections 42a and 42b. In other words, as shown in Fig. 19, the axes of the small-diameter hole sections 43a and 43b are shifted a distance P from those of the large-diameter hole sections 42a and 42b wherein P is within a range defined by 0 < P ≤ R - r, where R indicates the radius of the large-diameter hole section 42a and 42b, r indicates the radius of the small-diameter hole section 43a and 43b, and P indicates the shift distance between the respective axes of the large-diameter hole sections 42a and 42b and those of the small-diameter hole sections 43a and 43b (see Fig. 19). An outer conductor 34 is formed on almost all the outer surface of the dielectric block 31. One pair of input and output electrodes 35 is formed on the outer surface of the dielectric block 31. The pair of electrodes 35 are not electrically connected to the outer conductor 34 because of a gap maintained between them. Inner conductors 33 are formed on almost all the surface inside the resonator holes 32a and 32b. Gaps 38 are provided between the inner conductors 33 and the portions of the outer conductor 34 extending into the openings of the large-diameter hole sections 42a and 42b. In the known dielectric filter having the structure described above, as shown in Fig. 19, when the distance d1 between the axes of the small-diameter hole sections 43a and 43b is set longer than the distance d2 between the axes of the large-diameter hole sections 42a and 42b, the electromagnetic coupling between the resonator holes 32a and 32b becomes capacitive coupling. Conversely, when the distance d1 between the axes of the small-diameter hole sections 43a and 43b is set shorter than the distance d2 between the axes of the large-diameter hole sections 42a and 42b, the electromagnetic coupling between the resonator holes 32a and 32b becomes inductive coupling. The level of the electromagnetic coupling between the resonator holes 32a and 32b is set to the desired strength by changing the distance d1 between the axes of the small-diameter hole sections 43a and 43b. However, since the axes of the small-diameter hole sections 43a and 43b are shifted eccentrically to those of the large-diameter hole sections 42a and 42b only in a range of 0 < P ≤ R - r in the conventional dielectric filter, the range over which the distance d1 between the axes of the small-diameter hole sections 43a and 43b can be varied is narrow. Therefore, the strength of the level of the electromagnetic coupling between the adjacent resonator holes 32a and 32b cannot be varied over a wide range. Consequently, when a stronger electromagnetic coupling is required between the adjacent resonator holes 32a and 32b, the external shape and dimensions of the dielectric block 31 need to be changed. EP-A-731 522 discloses a dielectric filter according to the preamble of claim 1. SUMMARY OF THE INVENTION Accordingly, there is a need to provide a dielectric filter and a dielectric duplexer in which a strong electromagnetic coupling can be set between adjacent resonator holes without changing the external shape and dimensions of a dielectric block. These objectives have been achieved by the dielectric filter and the dielectric duplexer defined in respective claims 1 and 12. To achieve the foregoing, the present invention provides a dielectric filter and a dielectric duplexer, comprising a dielectric block, a plurality of resonator holes provided inside the dielectric block, inner conductors provided on the inner surfaces of the plurality of resonator holes, and an outer conductor formed on the outer surface of the dielectric block. At least one of the plurality of resonator holes has a large-diameter hole section, and a small-diameter hole section connected to said large-diameter hole section. The axis of said large-diameter hole section is shifted from the axis of said small-diameter hole section to form a bent-shaped hole, and the radius R of said large-diameter hole section, the radius r of said small-diameter hole section, and the shift distance P between the axis of said large-diameter hole section and the axis of said small-diameter hole section satisfy the expression R - r < P < R + r. In the above dielectric filter and a dielectric duplexer, a plurality of the bent-shaped resonator holes may be formed adjacently, and the distance between the axes of the small-diameter hole sections in adjacent resonator holes may set to be longer than, shorter than, or equal to the distance between the axes of the large-diameter hole sections. According to the dielectric filter and a dielectric duplexer of the present invention, the variable range of the distance between the axes of the small-diameter hole sections or of the distance between the axes of the large-diameter hole sections becomes wider than in the conventional dielectric filter and the dielectric duplexer. Therefore, when a strong electromagnetic coupling is required between adjacent resonator holes, the external shape or dimensions of the dielectric block do not need to be changed. Since the axes of the small-diameter hole sections are shifted from those of the large-diameter hole sections in a range of R - r < P < R + r, where R indicates the radius of the large-diameter hole sections, r indicates the radius of the small-diameter hole sections, and P indicates the shift distance between the axes of the large-diameter hole sections and those of the small-diameter hole sections, the electromagnetic coupling between the resonator holes is made stronger than in the conventional dielectric filter and a dielectric duplexer can be made without changing the external shape and dimensions of the dielectric block. In addition, the attenuation pole formed at the lower frequency side (or the higher frequency side) of the passband can be moved in the lower-frequency direction (or the higher-frequency direction). A compact dielectric filter and dielectric duplexer having high performance and a steep attenuation characteristic can be readily made with a wider passband. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an external perspective view of a dielectric filter according to a first embodiment of the present invention. Fig. 2 is a view of an open end face of the dielectric filter shown in Fig. 1. Fig. 3 is a graph showing the attenuation characteristic of the dielectric filter shown in Fig 1. Fig. 4 is a view of a dielectric filter according to a second embodiment of the present invention. Fig. 5 is a graph showing the attenuation characteristic of the dielectric filter shown in Fig 4. Fig. 6 is a view of a dielectric filter according to a third embodiment of the present invention. Fig. 7 is an external perspective view of a dielectric filter according to a fourth embodiment of the present invention. Fig. 8 is a view of an open end face of the dielectric filter shown in Fig. 7. Fig. 9 is a graph showing the attenuation characteristic of the dielectric filter shown in Fig 7. Fig. 10 is an external perspective view of a dielectric duplexer according to a fifth embodiment of the present invention. Fig. 11 is a view of a short-circuited end face of the dielectric duplexer shown in Fig. 10. Fig. 12 is a plan view of the dielectric duplexer shown in Fig. 11. Fig. 13 is an end view of a dielectric filter according to a sixth embodiment of the present invention. Fig. 14 is a horizontal cross-section of a dielectric filter according to a seventh embodiment of the present invention. Fig. 15 is an end view of a dielectric filter according to an eighth embodiment of the present invention. Fig. 16 is a horizontal cross-section of a dielectric filter according to a ninth embodiment of the present invention. Fig. 17 is an external perspective view of a dielectric filter according to a tenth embodiment of the present invention. Fig. 18 is an external perspective view of a conventional dielectric filter. Fig. 19 is an end view of the dielectric filter shown in Fig. 18 viewed from an open end face. DETAILED DESCRIPTION OF EMBODIMENTS THE PRESENT INVENTION Embodiments of dielectric filters and dielectric duplexers of the present invention will be described below by referring to the accompanying drawings. First embodiment (Fig. 1 to Fig. 3) In a dielectric filter according to a first embodiment, as shown in Fig. 1, two resonator holes 2a and 2b passing through opposing surfaces 1a and 1b of a dielectric block 1 are formed. The resonator holes 2a and 2b include large-diameter hole sections 22a and 22b having a circular transverse cross section and small-diameter hole sections 23a and 23b having a circular transverse cross section and connecting to the large-diameter hole sections 22a and 22b. The small-diameter hole sections 23a and 23b are spaced away from each other with the axes of the small-diameter hole sections 23a and 23b being eccentrically shifted from those of the large-diameter hole sections 22a and 22b in the direction away from each other. The axes of the small-diameter hole sections 23a and 23b are shifted away from those of the large-diameter hole sections 22a and 22b by a distance in a range defined by R - r < P < R + r, where R indicates the radius of the large-diameter hole sections 22a and 22b, r indicates the radius of the small-diameter hole sections 23a and 23b, and P indicates the shift distance between the axes of the large-diameter hole sections 22a and 22b and those of the small-diameter hole sections 23a and 23b (see Fig. 2). Therefore, the resonator holes 2a and 2b have bent shapes. The distance d1 between the axes of the small-diameter hole sections 23a and 23b is wider than the distance d2 between the axes of the large-diameter hole sections 22a and 22b, and further, the distance d1 is set wider than the distance between the axes of the small-diameter hole sections in the conventional dielectric filter shown in Fig. 15. An outer conductor 4 and a pair of input and output electrodes 5 are formed on the outer surface of the dielectric block 1. The pair of input and output electrodes 5 are not electrically connected to the outer conductor 4 since a distance is maintained between them. The outer conductor 4 is formed on almost all the outer surface except for the area where the input and output electrodes 5 are formed and except for a surface 1a (hereinafter called an open end face la) on which the openings of the large-diameter hole sections 22a and 22b are disposed. Inner conductors 3 are formed on the entire surface inside the resonator holes 2a and 2b. The inner conductors 3 are electrically open (separated) from the outer conductor 4 at the open end face 1a, and are electrically short-circuited (connected) to the outer conductor 4 at a surface 1b (hereinafter called an short-circuit end face 1b) where the openings of the small-diameter hole sections 23a and 23b are disposed. The axial length of the resonator holes 2a and 2b is set to λ/4 (where λ indicates the central wavelength of a resonator formed in each of the resonators 2a and 2b). Between the inner conductors 3 in the resonator holes 2a and 2b and the input and output electrodes 5, external coupling capacitors are generated. In the dielectric filter having this structure, since the distance d2 between the axes of the large-diameter hole sections 22a and 22b of the resonator holes 2a and 2b is fixed at the open end face 1a, the amount of electric-field coupling energy coupling between the resonator holes 2a and 2b is hardly changed from that in the conventional dielectric filter. However, since the distance d1 between the axes of the small-diameter hole sections 23a and 23b is set longer than the distance d2 between the axes of the large-diameter hole sections 22a and 22b at the short-circuit end face 1b, the amount of magnetic-field coupling energy is reduced and the level of capacitive coupling is increased. In addition, since the distance d1 between the axes of the small-diameter hole sections 23a and 23b is set longer than in the conventional dielectric filter shown in Fig. 15, stronger capacitive coupling is obtained, so that two resonators formed in each of the resonator holes 2a and 2b are capacitively coupled strongly. Therefore, a dielectric filter having a stronger capacitive coupling is obtained without changing the external shape and dimensions of the dielectric block 1. In general, in a dielectric filter in which a plurality of dielectric resonators is coupled, when the coupling between adjacent resonators is of a capacitive type, one attenuation pole GL is obtained at the lower-frequency side of the passband. This attenuation pole GL moves in the lower-frequency direction as the capacitive coupling becomes stronger. Therefore, as shown in Fig. 3, an attenuation pole GL (see a solid line 11) at the lower-frequency side of the passband of the dielectric filter according to the first embodiment is formed at a position lower in frequency than an attenuation pole GL (see a dotted line 12) of the conventional dielectric filter shown in Fig. 15. Thus, the passband of the dielectric filter according to the first embodiment is wider than that in the conventional dielectric filter. Second embodiment (Fig. 4 and Fig. 5) As shown in Fig. 4, a dielectric filter according to a second embodiment has the same structure as the dielectric filter according to the first embodiment except that the distance d1 between the axes of the small-diameter hole sections 23a and 23b is shorter than the distance d2 between the axes of the large-diameter hole sections 22a and 22b, and is set shorter than the distance between the axes of the small-diameter hole sections in the conventional dielectric filter. In the dielectric filter having this structure, since the distance d2 between the axes of the large-diameter hole sections 22a and 22b of the resonator holes 2a and 2b is fixed at the open end face 1a, the amount of electric-field energy related to the coupling between the resonator holes 2a and 2b is hardly changed from that in the conventional dielectric filter. However, since the distance d1 between the axes of the small-diameter hole sections 23a and 23b is set shorter than the distance d2 between the axes of the large-diameter hole sections 22a and 22b at the short-circuit end face 1b, the amount of magnetic-field energy related to the coupling is increased so that the level of inductive coupling is increased. In addition, since the distance d1 between the axes of the small-diameter hole sections 23a and 23b is set shorter than in the conventional dielectric filter, stronger inductive coupling is obtained, so that two resonators formed in each of the resonator holes 2a and 2b are inductively coupled strongly. Therefore, a dielectric filter having a stronger inductive coupling is obtained without changing the external shape and dimensions of the dielectric block 1. In general, in a dielectric filter in which a plurality of dielectric resonators is coupled, when the coupling between adjacent resonators is of an inductive type, one attenuation pole GH is obtained at the higher-frequency side of the passband. This attenuation pole GH moves in the higher-frequency direction as the inductive coupling becomes stronger. Therefore, as shown in Fig. 5, an attenuation pole GH (see a solid line 13) at the higher-frequency side of the dielectric filter according to the second embodiment is formed at a position higher in frequency than an attenuation pole GH (see a dotted line 14) at the higher-frequency side of the conventional dielectric filter. The passband of the dielectric filter according to the second embodiment is made wider than in the conventional dielectric filter. Third embodiment (Fig. 6) As shown in Fig. 6, a dielectric filter according to a third embodiment has the same structure as the dielectric filter according to the first embodiment except that the distance d1 between the axes of the small-diameter hole sections 23a and 23b is set equal to the distance d2 between the axes of the large-diameter hole sections 22a and 22b. This dielectric filter provides a higher degree of freedom in designing the level of electromagnetic coupling. Fourth embodiment (Fig. 7 and Fig. 8) In a dielectric filter according to a fourth embodiment, as shown in Fig. 7, three resonator holes 2a, 2b, and 2c passing through an open end face 1a and a short-circuit end face 1b of a dielectric block 1 are formed in line. The resonator holes 2a, 2b, and 2c include large-diameter hole sections 22a, 22b, and 22c having a circular transverse cross section and small-diameter hole sections 23a, 23b, and 23c having a circular transverse cross section and connecting to the large-diameter hole sections 22a, 22b, and 22c. The axes of the small-diameter hole sections 23a, 23b, and 23c are eccentrically shifted from those of the large-diameter hole sections 22a, 22b, and 22c. In other words, the axes of the small-diameter hole sections 23a, 23b, and 23c are eccentric to those of the large-diameter hole sections 22a, 22b, and 22c in a range of R - r < P < R + r, where R indicates the radius of the large-diameter hole sections 22a, 22b, and 22c, r indicates the radius of the small-diameter hole sections 23a, 23b, and 23c, and P indicates the shift distance between the axes of the large-diameter hole sections 22a, 22b, and 22c and those of the small-diameter hole sections 23a, 23b, and 23c (see Fig. 8). Therefore, the resonator holes 2a, 2b, and 2c have bent shapes. As shown in Fig. 8, the distance d3 between the axes of the small-diameter hole sections 23a and 23c is set shorter than the distance d4 between the axes of the large-diameter hole sections 22a and 22c, and is set shorter than the distance between the axes of the small-diameter hole sections in the conventional dielectric filter. The distance d5 between the axes of the small-diameter hole sections 23b and 23c is set longer than the distance d6 between the axes of the large-diameter hole sections 22b and 22c, and is set longer than in the conventional dielectric filter. In the dielectric filter having this structure, the coupling between two resonators formed of the resonator holes 2a and 2c is of a strong inductive type, and the coupling between resonators formed of the resonator holes 2b and 2c is of a strong capacitive type. Therefore, as shown in Fig. 9, the attenuation characteristic of the filter has one attenuation pole GL at the lower-frequency side of the passband and one attenuation pole GH at the higher-frequency side of the passband. Thus, the width of the passband is made larger by making the distance d3 between the axes of the small-diameter hole sections 23a and 23c shorter, and by making the distance d5 between the axes of the small-diameter hole sections 23b and 23c longer. Fifth embodiment (Fig. 10 to Fig. 12) The fifth embodiment is a dielectric duplexer which can be used for a mobile communication apparatus such as a car phone or a mobile phone. Fig. 10 is an external perspective view of a dielectric duplexer viewed from the side of an end face 51a, indicating a mounting surface (bottom surface) 51c as seen from above. Fig. 11 is a view from the side of an end face 51b, indicating the mounting surface 51c at the bottom of the figure. Fig. 12 is a plan view of the dielectric duplexer shown in Fig. 11. In this dielectric duplexer, seven resonator holes 52a - 52g passing through a pair of opposite end surfaces 51a, 51b of a dielectric block 51 having substantially parallelepiped shape are formed in line. External coupling holes 55a, 55b and 55c and grounding holes 56a, 56b and 56c are formed between resonator holes 52a and 52b, 52c and 52d, and 52f and 52g, respectively. The resonator holes 52a - 52g respectively include large-diameter hole sections 62a - 62g having a circular transverse cross-section and small-diameter hole sections 63a ~ 63g having a circular transverse cross-section and connecting to the large-diameter hole sections 62a - 62g. The axes of the small-diameter hole sections 63c - 63f are eccentrically shifted from those of the large-diameter hole sections 62c - 62f. The axes of the small-diameter hole sections 63c - 63f are shifted away from those of the large-diameter hole sections 62c - 62f by a distance in a range defined by R - r < p < R + r, where R indicates the radius of the large-diameter hole sections 62c - 62f, r indicates the radius of the small-diameter hole sections 63c - 63f, and P indicates the shift distance between the axes of the large-diameter hole sections 62c - 62f and those of the small-diameter hole sections 63c - 63f (see Fig. 12). Therefore, the resonator holes 52c - 52f have bent shapes. As shown in Fig.12, the distance d11 between the axes of the small-diameter hole sections 63b and 63c is narrower than the distance d14 between the axes of the large-diameter hole sections 62b and 62c, and is set narrower than the distance between the axes of the small-diameter hole sections in the conventional dielectric filter. The distance d12 between the axes of the small-diameter hole sections 63d and 63e is wider than the distance d15 between the axes of the large-diameter hole sections 62d and 62e, and is set wider than the distance between the axes of the small-diameter hole sections in the conventional dielectric filter. The distance d13 between the axes of the small-diameter hole sections 63e and 63f is equal to the distance d16 between the axes of the large-diameter hole sections 62e and 62f, and is set equal to the distance between the axes of the small-diameter hole sections in the conventional dielectric filter. Almost all of the outer surface of the dielectric block 51 is covered with an outer conductor 54. A transmission electrode Tx and a reception electrode Rx serving as input/output electrodes and an antenna electrode ANT are formed on the dielectric block 51 with a distance maintained around them such that the electrodes are not electrically connected to the outer conductor 54. The input/output electrodes extend from the mounting surface 51c to the end surface 51b. Inner conductors 53 (shown in Fig. 10) are formed on almost the entire inner peripheral surfaces of the respective resonator holes 52a - 52g. Nonconducting portions 58 are formed between the inner conductors 53 and the outer conductor 54 and extend into openings of the respective large-diameter hole sections 62a - 62g. The end surface 51a having the openings of the large-diameter sections 62a - 62g with non-conducting portions 58 is the open-circuited end, and the end surface 51b having the openings of the small-diameter sections 63a - 63g is the short-circuited end. The inner conductors 53 are electrically open (separated) from the outer conductor 54 at the open-circuited end 51a, and is electrically short-circuited (connected) to the outer conductor 54 at the short-circuited end 51b. The axial length of the resonator holes 52a - 52g is set to λ/4 (where λ indicates the central wavelength of a resonator formed in each of the resonator holes 52a - 52g). Inner conductors 53 are formed on the entire inner peripheral surfaces of the external coupling holes 55a, 55b and 55c and the grounding holes 56a, 56b and 56c. The external coupling holes 55a, 55b and 55c are respectively connected to the transmission electrode Tx, the antenna electrode ANT, and the reception electrode Rx at the short-circuited end 51b and electrically separated from the outer conductor 54. On the other hand, the inner conductors 53 of the respective external coupling holes 55a - 55c are electrically connected to the outer conductor 54 at the open-circuited end 51a. On the other hand, the grounding holes 56a - 56c are respectively provided in the vicinity of the external coupling holes 55a - 55c in parallel fashion. The inner conductors of the respective grounding holes 56a - 56c are electrically connected to the outer conductor 54 at both the open-circuited end 51a and the short-circuited end 51b. The self capacitance of each of the external coupling holes 55a - 55c can be increased or decreased by changing the location, shape and dimensions of each of the grounding holes 56a - 56c. Therefore, the external coupling can be changed to thereby obtain the appropriate external coupling. Note that the self capacitance of each of the external coupling holes 55a - 55c is the capacitance generated between the inner conductors 53 of the respective external coupling holes 55a - 55c and the grounding conductors (the outer conductor 54 and the inner conductor 53 of each of the grounding holes 56a - 56c). This dielectric duplexer comprises a transmission filter (band pass filter) having two resonators formed by the resonator holes 52b and 52c, a reception filter (band pass filter) having three resonators formed by the resonator holes 52d, 52e and 52f, and two traps (band elimination filters) having resonators formed by the resonator holes 52a, 52g located at both sides. The external coupling hole 55a and the adjacent resonator holes 52a and 52b, the external coupling hole 55b and the adjacent resonator holes 52c and 52d, and the external coupling hole 55c and the adjacent resonator holes 52f and 52g are electromagnetically coupled to each other respectively. External coupling is obtained by this electromagnetic coupling. In the above described dielectric duplexer, a transmission signal from a transmission circuit (not shown in the drawings) to the transmission electrode Tx is output from the antenna electrode ANT via the transmission filter having the resonator holes 52b and 52c, and a reception signal from the antenna electrode ANT is output to a reception circuit (not shown in the drawings) via the reception filter having the resonator holes 52d, 52e, 52f and the reception electrode Rx. And, the coupling between the two resonators formed of the resonator holes 52b, 52c is of a strong inductive type, and the coupling between the two resonators formed of the resonator holes 52d, 52e is of a strong capacitive type. Therefore, a dielectric duplexer having a strong inductive coupling or capacitive coupling is obtained without changing the external shape and dimensions of the dielectric block 51. When the distance d1 between the axes of the small-diameter hole sections 63e and 63f of the resonator holes 52e and 52f is set equal to the distance d16 between the axes of the large-diameter hole sections 62e and 62f, electromagnetic coupling between the two resonators formed of the resonator holes 52e, 52f is kept constant to thereby obtain a higher degree of freedom in designing the dielectric duplexer. In addition, the attenuation pole formed at the lower frequency side (or the higher frequency side) of the passband can be moved in the lower-frequency direction (or the higher-frequency direction). A compact dielectric filter having high performance and a steep attenuation characteristic can readily be made with a wider passband. Other embodiments According to a sixth embodiment, shown in Fig. 13, for example, four resonator holes 2a, 2b, 2c, and 2d may be formed in the dielectric block 1. In this case, the resonator holes 2a and 2c are formed such that the axes of the small-diameter hole sections 23a and 23c are shifted with respect to those of the large-diameter hole sections 22a and 22c by a distance in a range defined by 0 < P ≤ R - r, where R indicates the radius of the large-diameter hole sections 22a to 22d, r indicates the radius of the small-diameter hole sections 23a to 23d, and P indicates the shift distance between the axes of the large-diameter hole sections 22a to 22d and those of the small-diameter hole sections 23a to 23d. The resonator holes 2b and 2d are formed such that the axes of the small-diameter hole sections 23b and 23d are shifted with respect to those of the large-diameter hole sections 22b and 22d by a distance in a range defined by R - r < P < R + r. The coupling between two resonators formed in each of the resonator holes 2a and 2c is of a strong inductive type, and the coupling between two resonators formed in each of the resonator holes 2c and 2d is of a strong capacitive type. Two resonators formed at each of the resonator holes 2b and 2d are inductively coupled stronger than the inductive coupling between the resonator holes 2a and 2c. Therefore, the degree of freedom in designing the electromagnetic coupling in a dielectric filter can be further increased, and a band-pass filter and a duplexer can also be readily designed. Five resonator holes may be provided. The axial length of a resonator hole is not limited to λ/4. It may be λ/2, for example. In this case, both opening surfaces of a resonator hole need to be short-circuit end faces or open end faces. As shown in Fig. 14, in a seventh embodiment; the boundary step sections 24a and 24b between the large-diameter hole sections 22a and 22b and the small-diameter hole sections 23a and 23b in resonator holes 2a and 2b are not necessarily disposed at the same position in the axial direction, and may be disposed at different positions in the axial direction of the resonator holes 2a and 2b. As shown in an eighth embodiment in Fig. 15, the shapes of the large-diameter hole sections 22e and 22f and the small-diameter hole sections 23e and 23f of resonator holes 2e and 2f may be rectangular in transverse cross-section as well as circular. As shown in a ninth embodiment in Fig. 16, the large-diameter hole sections 22g and 22h and the small-diameter hole sections 23g and 23h of resonator holes 2g and 2h may be formed such that the large-diameter hole section 22g is disposed near an open end face 1a and the small-diameter hole section 23g is disposed near a short-circuit end face 1b, while the small-diameter hole section 23h is disposed near the open end face 1a and the large-diameter hole section 22h is disposed near the short-circuit end face 1b. A dielectric filter according to a tenth embodiment may be formed as shown in Fig. 17. In this dielectric filter, an outer conductor 4 is formed on almost all of the outer surface of a dielectric block 1. One pair of input and output electrodes 5 is formed on the outer surface of the dielectric block 1. The electrodes 5 are not electrically connected to the outer conductor 4 because of a gap maintained between them. Inner conductors 3 are formed on almost the entire surface inside the resonator holes 2a and 2b. Gaps 8 are provided between the inner conductors 3 and the portion of the outer conductor 4 extending into the openings of the large-diameter hole sections 22a and 22b. An open end face 1a is adjacent to the large-diameter hole sections 22a and 22b on which the gaps 8 are provided, and a short-circuit end face 1b is adjacent to the small-diameter hole sections 23a and 23b. The axial length of the inner conductors 3 of the resonator holes 2a and 2b is set to λ/4. A dielectric filter and a dielectric duplexer may include a resonator hole having a constant inner diameter. A dielectric filter may be configured with another electromagnetic coupling structure such as a coupling groove provided in a dielectric block in order to greatly change the level of coupling. In the above embodiments, the large-diameter hole sections are usually formed near the open end face and the small-diameter hole sections are formed near the short-circuit end face in the resonator holes. The structure of the resonator holes is not limited to this structure. Resonator holes may be configured such that large-diameter hole sections are formed near the short-circuit end face, and wherein the distance between the axes of small-diameter hole sections formed near the open end face is changed. In this case, the coupling relationship between adjacent resonator holes is opposite that described in the above embodiments. In other words, as the distance between the axes of the small-diameter hole sections is reduced, the level of capacitive coupling gradually becomes strong. As the distance between the axes of the small-diameter hole sections is extended, the level of inductive coupling becomes strong. In the above embodiments, input/output coupling is provided by the pair of input and output electrodes on the outer surface of the dielectric block in the dielectric filter. However, instead of the input and output electrodes, a resin pin may be used to connect the dielectric filter to an external circuit. In the above embodiments, the axes of the small-diameter hole sections are shifted from the axes of the large-diameter hole sections, which are disposed at a specified pitch. However, instead, the axes of the large-diameter hole sections may be shifted from the axes of the small-diameter hole sections, which are disposed at the specified pitch. In the above embodiments, the axes of the large-diameter hole sections and the small-diameter hole sections are arranged in line in the resonator holes. Instead, however, the axes of the large-diameter hole sections and those of the small-diameter hole sections may be disposed, for example, in a zigzag pattern at different positions in the height direction of a dielectric filter.

Claims: A dielectric filter, comprising a dielectric block (1), a plurality of resonator holes (2a-2f) provided inside the dielectric block (1), inner conductors (3) provided on the inner surfaces of the plurality of resonator holes (2a-2f), and an outer conductor (4) formed on the outer surface of the dielectric block (1), wherein at least one of the plurality of resonator holes (2a-2f) has a large-diameter hole section (22a-22f) having an axis and a small-diameter hole section having (23a-23f) an axis, said small-diameter hole section (23a-23f) being connected to said large-diameter hole section (22a-22f), characterized in that the axis of said large-diameter hole section (22a-22f) is shifted by a shift distance P from the axis of said small-diameter hole section (23a-23f) to form a bent-shaped hole (2a-2f), and a radius R of said large-diameter hole section (22a-22f), a radius r of said small-diameter hole section (23a-23f), and the shift distance P between the axis of said large-diameter hole section (22a-22f) and the axis of said small-diameter hole section (23a-23f) satisfy the expression R - r < P < R + r. The dielectric filter according to Claim 1, comprising a pair of adjacent bent-shaped resonator holes (2a, 2b), wherein the distance (d1) between the axes of the small-diameter hole sections (23a, 23b) in the pair of adjacent resonator holes (2a, 2b) is longer than the distance (d2) between the axes of the large-diameter hole sections (22a, 22b) in said pair of adjacent resonator holes (2a, 2b). The dielectric filter according to Claim 1, comprising a pair of adjacent bent-shaped resonator holes (2a, 2b), wherein the distance (d1) between the axes of the small-diameter hole sections (23a, 23b) in the pair of adjacent resonator holes (2a, 2b) is shorter than the distance (d2) between the axes of the large-diameter hole sections (22a, 22b) in said pair of adjacent resonator holes (2a, 2b). The dielectric filter according to Claim 1, comprising a pair of adjacent bent-shaped resonator holes (2a, 2b), wherein the distance (d1) between the axes of the small-diameter hole sections (23a, 23b) in the pair of adjacent resonator holes (2a, 2b) is equal to the distance (d2) between the axes of the large-diameter hole sections (22a, 22b) in said pair of adjacent resonator holes (2a, 2b). The dielectric filter according to Claim 2, comprising another pair of adjacent bent-shaped resonator holes, wherein the distance between the axes of the small-diameter hole sections in the other pair of adjacent resonator holes is shorter than the distance between the axes of the large-diameter hole sections in said first-mentioned pair of adjacent resonator holes. The dielectric filter according to Claim 1, wherein said dielectric block (1) has an open end face (1a) which is substantially free of said outer conductor (4) adjacent to said resonator hole inner conductors (3), said large-diameter hole section being disposed adjacent to said open end face (la). The dielectric filter according to Claim 1, wherein at least one of said plurality of resonator holes (2a-2f) has an open end defined by a gap (8) between the inner conductor (3) thereof and said outer conductor (4), and said gap (8) is formed in said large-diameter hole section (22a-22f) of said at least one resonator hole (2a-2f). The dielectric filter according to Claim 1, comprising a pair of bent-shaped resonator holes, wherein a respective boundary (24a, 24b) is defined between the large-diameter and small-diameter hole sections in each of said pair of resonator holes, and said boundaries (24a, 24b) are disposed at different positions along the corresponding axes of said pair of resonator holes. The dielectric filter according to claim 1, comprising a pair of bent-shaped resonator holes (2e, 2f), wherein said large-diameter hole sections (22e, 22f) thereof have different shapes. The dielectric filter according to claim 9, wherein said small-diameter hole sections (23e, 23f) of said pair of resonator holes (2e, 2f) have different shapes. The dielectric filter according to claim 1, comprising a pair of bent-shaped resonator holes, wherein said small-diameter hole sections of said pair of resonator holes have different shapes. A dielectric duplexer, comprising a dielectric block (51), first (52b, 52c) and second (52d, 52e, 52f) pluralities of resonator holes (52a-52g) provided inside the dielectric block (51) for providing said duplexer with at least first and second filters, inner conductors (53) provided on the inner surfaces of the first (52b, 52c) and second (52d, 52e, 52f) pluralities of resonator holes (52a-52g), and an outer conductor (54) formed on the outer surface of the dielectric block (51), wherein at least one resonator hole among said first and second pluralities of resonator holes (52a-52g) has a large-diameter hole section (62a-62g) having an axis and a small-diameter hole section (63a-63g) having an axis, said small-diameter hole section (63a-63g) being connected to said large-diameter hole section (62a-62g), characterized in that the axis of said large-diameter hole section (62c-62f) is shifted from the axis of said small-diameter hole section (63c-63f) by a shift distance P to form a bent-shaped hole (52c-52f) , and a radius R of said large-diameter hole section (62c-62f), a radius r of said small-diameter hole section (63c-63f), and the shift distance P between the axis of said large-diameter hole section (62c-62f) and the axis of said small-diameter hole section (63c-63f) satisfy the expression R - r < P < R + r. The dielectric duplexer according to Claim 12, wherein a pair (52d, 52e) of the bent-shaped resonator holes (52c-52f) are formed adjacently, and the distance (d12) between the axes of the small-diameter hole sections (63d, 63e) in the pair (52d, 52e) of adjacent resonator holes (52c-52f) is longer than the distance (d15) between the axes of the large-diameter hole sections (62d, 62e) in said pair (52d, 52e) of adjacent resonator holes (52c-52f). The dielectric duplexer according to Claim 12, wherein a pair of the bent-shaped resonator holes are formed adjacently, and the distance between the axes of the small-diameter hole sections in the pair of adjacent resonator holes is shorter than the distance between the axes of the large-diameter hole sections in said pair of adjacent resonator holes. The dielectric duplexer according to Claim 12, wherein a pair (52e, 52f) of the bent-shaped resonator holes (52c-52f) are formed adjacently, and the distance (d13) between the axes of the small-diameter hole sections (63e, 63f) in the pair (52e, 52f) of adjacent resonator holes (52e, 52f) is equal to the distance (d16) between the axes of the large-diameter hole sections (62e, 62f) in said pair (52e, 52f) of adjacent resonator holes (52c-52f). The dielectric duplexer according to Claim 13, comprising another pair of adjacent bent-shaped resonator holes, wherein the distance between the axes of the small-diameter hole sections in the other pair of adjacent resonator holes is shorter than the distance between the axes of the large-diameter hole sections in said first-mentioned pair of adjacent resonator holes. The dielectric duplexer according to Claim 12, wherein at least one of said plurality of resonator holes (52a-52g) has an open end defined by a gap (58) between the inner conductor (53) thereof and said outer conductor (54), and said gap (58) is formed in said large-diameter hole section (62a-62g) of said at least one resonator hole (52a-52g).

Inventors: MIYAMOTO HIROFUMI, MIYAMOTO, HIROFUMI
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1998
Main classification: H01P1
IPC codes: H01P1, H01P7