TM mode dielectric resonator and TM mode dielectric filter and duplexer using the resonator

Abstract: A dielectric resonator designed so that there is substantially no loss in a conductor on the surface of a casing forming a shielded cavity, and so that the unloaded Q and the resonant frequency can be changed independently of each other. A cylindrical dielectric block (2) having a pair of electrodes (3, 4) formed on its two opposite surfaces is disposed in a metallic shielded-cavity casing (5) so that one (4) of the electrodes (3, 4) is in contact with an inner bottom surface of the shielded-cavity casing (5). This electrode (4) is electrically connected to the shielded-cavity casing (5) by soldering or the like.

Description: The present invention relates to a transverse magnetic (TM) mode dielectric resonator and to a TM mode dielectric and a TM mode dielectric duplexer using the resonator. As a dielectric filter using a TM mode dielectric resonator, a dielectric filter having a structure such as that shown in Fig. 13 is known. Each of dielectric resonators shown in Fig. 13 is arranged as a dual mode type in such a manner that dielectric blocks of short-circuit type TM110 mode dielectric resonators are integrally combined in a crisscross fashion. This structure enables one TM mode dielectric resonator to have the function of two TM mode dielectric resonators while being formed so as to be equal in size to one ordinary dielectric resonator of this kind. Referring to Fig. 13, a dielectric filter 101 has four TM dual mode dielectric resonators 102, 103, 104, and 105, which are arranged in a row with their openings facing in the same direction. Metallic panels 106 and 107 are attached to these dielectric resonators so as to cover the openings. The TM dual mode dielectric resonator 102 has a cavity casing 102a having openings on the front and rear sides as viewed in Fig. 13, and a dielectric crisscross block 102XY. The cavity casing 102a and the dielectric crisscross block 102XY are integrally formed of the same dielectric material. A conductor 102b is formed on the outer surface of the cavity casing 102a except on the front and rear opening edges. The cavity casing 102a with the conductor 102b forms a shielded cavity. The dielectric block 102XY is formed of a horizontal portion 102X and a vertical portion 102Y as viewed in Fig. 13. Thus, one TM dual mode dielectric resonator 102 is formed as a two-stage resonator. Each of the TM dual mode dielectric resonator 103, 104, and 105 has the same structure as the TM dual mode dielectric resonator 102. An input loop 108 and an output loop 109 are mounted on the panel 106. The input loop 108 and the output loop 109 are connected to external circuits via coaxial connectors (not shown). Coupling loops 107a, 107b, 107c, and 107d for coupling each adjacent pair of the TM dual mode dielectric resonators are mounted on the panel 107. In dielectric resonators for use in such a dielectric filter, the resonant frequency of each dielectric resonator is determined by the size of the cavity and the size of the dielectric block. For example, in the case of an ordinary TM110 mode dielectric resonator having a single vertical dielectric block structure, the resonant frequency becomes lower if the width of the cavity is increased while the width, thickness and height of the dielectric block and the height of the cavity are fixed. The resonant frequency becomes lower if the width or thickness of the dielectric block is increased while the size of the cavity is fixed. Also, when the frequency is fixed, an increase in the unloaded Q of the dielectric resonator is attained by increasing the height of the dielectric block. In such a case, if the height of the dielectric block is increased, the height of the cavity is necessarily increased. Since a real current flows through the conductor on the cavity casing surface in the TM110 mode dielectric resonator, the loss in the conductor on the cavity casing surface becomes larger if the size of the cavity casing is increased. However, an increase in unloaded Q achieved by enlarging the cavity is sufficiently large in comparison with the loss in the conductor on the cavity casing surface. Consequently, the unloaded Q becomes higher if the height of the dielectric block is increased. If the loss in the conductor on the cavity casing surface can be reduced, the unloaded Q can be increased while the increase in the height of the dielectric block is limited. Therefore, there has been a need for a dielectric resonator designed to reduce the loss in the conductor on the cavity casing surface. In the TM dual mode dielectric resonator shown in Fig. 13, when the sizes of the vertical and horizontal portions of the dielectric block are adjusted according to a predetermined frequency, the size of the cavity is also determined. To increase the unloaded Q, therefore, it is necessary to increase both the width and height of the cavity, resulting in an increase in the overall size of the dielectric filter. Also, the resonant frequency becomes lower if the cavity size is increased while the size of the dielectric block is fixed. Therefore, if the size of the cavity is increased, the width or thickness of the dielectric block is necessarily reduced. Thus, in the conventional TM dual mode dielectric resonator, it is difficult to independently change each of the unloaded Q and the frequency. The article: "Low-profile filter using open disk dual-mode dielectric resonators" from Y. Ishikawa et al., Electronics and Communications in Japan, Part 2, Vol. 79, No. 3, pages 16 to 24) discloses a TM dual-mode dielectric resonator comprising two circular dielectric disks with thick film electrodes on both sides stacked in their axial direction and disposed in a shielded cavity, wherein one electrode of each disk is placed on an inner surface of the shielding cavity. EP 0786822 A2 which has been published after the priority date of the present application discloses a high frequency resonator with a dielectric substrate sandwiched between thin film multi-layered electrodes each having a structure wherein thin film conductors and thin film dielectrics are layered alternately with one another. The substrate with the electrodes is disposed in a cylindrically shaped case wherein one electrode is in contact with the case and the other electrode is spaced apart from the case by a void. WO-A-9642118, which was published after the priority date of the present application, describes a resonator comprising a dielectric substrate covered on two sides by a super-conducting film and situated in a cavity housing. Connectors are provided for the input and output, respectively of microwave signals. Probes are provided for coupling the microwave signals in and out of the resonator. In an alternative embodiment, loops are provided for coupling microwave signals in and out of the resonator. In a similar embodiment, the connectors comprise pins for capacitive coupling of the microwave signals in and out of the resonator. In view of the above-described problems, an object of the present invention is to provide a dielectric resonator which has substantially no loss in the conductor on the cavity casing surface, and in which the unloaded Q and the resonant frequency can be changed independently of each other. This object is solved by a dielectric resonator according to claim 1. Another object of the present invention is to provide a dielectric filter and a dielectric duplexer having an improved unloaded Q and having a reduced thickness. Particular aspects of the invention are set out in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is a partially fragmentary perspective view of a dielectric filter useful for understanding the invention; Fig. 1B is a cross-sectional view taken along the line A-A of Fig. 1A; Fig. 2A is a partially fragmentary perspective view of a dielectric filter useful for understanding the invention; Fig. 2B is a cross-sectional view taken along the line B-B of Fig. 2A; Fig. 3A is a partially fragmentary perspective view of a modification of the dielectric filter shown in Figs. 2A and 2B; Fig. 3B is a cross-sectional view taken along the line C-C of Fig. 3A; Fig. 4A is a partially fragmentary perspective view of a dielectric filter which represents an embodiment of the present invention; Fig. 4B is a cross-sectional view taken along the line D-D of Fig. 4A; Fig. 5A is a partially fragmentary perspective view of a dielectric filter which represents a second embodiment of the present invention; Fig. 5B is a cross-sectional view taken along the line E-E of Fig. 5A; Fig. 6 comprises plan views of inner portions of upper and lower sections of the dielectric filter shown in Figs. 5A and 5B; Fig. 7 is a cross-sectional view of a modification of the dielectric filter shown in Figs. 5A, 5B, and 6; Fig. 8 is a partially fragmentary perspective view of a dielectric duplexer which represents a third embodiment of the present invention; Fig. 9 is an exploded perspective view of the dielectric duplexer shown in Fig. 8; Fig. 10 is a cross-sectional view of a modification of the dielectric duplexer shown in Fig. 8 and 9; Fig. 11 is a cross-sectional view of another modification of the dielectric duplexer shown in Fig. 8 and 9; Fig. 12 is a cross-sectional view of a dielectric filter useful for understanding the invention; and Fig. 13 is an exploded perspective view of a conventional TM mode dielectric filter. DESCRIPTION OF THE PREFERRED EMBODIMENTS A dielectric filter will be described with reference to Figs. 1A and 1B. Fig. 1A is a partially fragmentary perspective view of a dielectric filter 1, and Fig. 1B is a cross-sectional view taken along the line A-A of Fig. 1A. As shown in Figs. 1A and 1B, the dielectric filter 1 has a dielectric block 2 provided in a casing 5 made of a metal and forming a shielded cavity. The dielectric block 2 is a cylindrical member formed of a dielectric material. Electrodes 3 and 4 are formed on two opposite surfaces of the dielectric block 2. The dielectric block 2 is placed so that the electrode 4 is in contact with an inner bottom surface of the shielded-cavity casing 5. The electrode 4 is fixed and electrically connected to the shielded-cavity casing 5 by soldering or the like. The electrode 3 of the dielectric block 2 faces an inner ceiling surface of the shielded-cavity casing 5 and is uniformly spaced apart from this surface. When a high-frequency signal is input to the thus-constructed dielectric filter 1, an electric field is generated between the electrodes 3 and 4 in the dielectric block 2 and a magnetic field is generated along the circumference of the dielectric block 2. As a result, an electromagnetic field is concentrated at and confined in the dielectric block 2 in an electromagnetic field distribution approximate to a TM010 mode. At this time, the dielectric block 2 functions as a one-stage dielectric resonator. A pair of coaxial connectors 6 for external input and output are attached to side wall potions of the shielded-cavity casing 5. Center electrodes of the coaxial connectors 6 are electrically connected to electrodes sheets 7 by, for example, wires. Each of the electrode sheets 7 is formed of a sheet of an insulating material such as a resin and an electrode film formed on the upper surface of the insulating material sheet. No electrode film is formed on the lower surface of the insulating material sheet. The electrode sheets 7 are disposed on and attached to the electrode 3 formed on the upper surface of the dielectric block 2. The lower surfaces of the electrode sheets 7, on which no electrode film are formed, are brought into contact with the electrode 3. The thus-constructed dielectric filter 1 functions as described below. A high-frequency signal is input to one of the coaxial connectors 6. The capacitance across the insulating material between the electrode 3 of the dielectric block 2 and the electrode film on the upper surface of one of the electrode sheets 7 connected to the center electrode of the coaxial connector 6 acts for coupling between the center electrode of the coaxial connector 6 and the dielectric block 2. The dielectric block 2 resonates with the input signal by this coupling. A signal is thereby output through the capacitance of the other electrode sheet 7 and through the other coaxial connector 6 connected to the electrode film on this electrode sheet 7. The thus-arranged dielectric filter can be much smaller in thickness than the conventional dielectric filter using short-circuit type TM110 mode dielectric resonators. The resonant frequency and the unloaded Q of the dielectric filter of this example are determined by the same factors as the conventional dielectric filter using short-circuit type TM110 mode dielectric resonators. That is, the resonant frequency is determined by the sectional area along a plane perpendicular to the direction of height while the unloaded Q is determined by the height of the dielectric block. In this example, however, substantially no real current flows through the side surface of the shielded-cavity casing corresponding to the conventional cavity casing. Accordingly, substantially no deterioration in unloaded Q results with respect to this portion. Consequently, the increase in the height of the dielectric block necessary for obtaining the desired unloaded Q can be limited, thereby limiting the increase in the height of the entire dielectric filter. This example has been described with respect to use of a cylindrical dielectric block. However, such a cylindrical dielectric block is not exclusively used and dielectric blocks having any other shapes may also be used as long as they have electrodes corresponding to the two electrodes 3 and 4 shown in Fig. 1. Among such dielectric blocks, however, a cylindrical dielectric block, such as the dielectric block 2 of the embodiment described above, is used particularly advantageously for a reason described below. In the surface of such a cylindrical dielectric block on which an electrode is formed, the distance from the center of the circle to the edge of the circuit, i.e., the circumferential, is constant. In other dielectric blocks in the form of polygonal prisms, the distance from the center to the vertices of the polygonal shape is different from the distance from the center to other edge portions. In such dielectric blocks, therefore, a potential difference occurs to cause a current at the edge of the electrode along the polygonal shape, resulting in occurrence of a loss in the electrode. In contrast, in a cylindrical dielectric block, substantially no current flows due to such a potential difference since the distance between the center of the circle and the circumferential end of the surface on which the electrode is formed is constant. The resulting loss in this case is advantageously small. Because of the above-described effect of using a cylindrical shape, a superconductor, with which a serious problem of loss at the electrode edge may arise, can be used as electrodes 3 and 4. If a superconductor is used as electrodes 3 and 4, a dielectric resonator or filter having a higher unloaded Q can be obtained. A second example will next be described with reference to Figs. 2A and 2B. Fig. 2A is a partially fragmentary perspective view and Fig. 2B is a cross-sectional view taken along the line B-B of Fig. 2A. Components of this embodiment identical to those of the first example are indicated by the same reference numerals and will not be described in detail. Referring to Figs. 2A and 2B, a dielectric filter 11 has dielectric blocks 12a and 12b disposed in a metallic shielded-cavity casing 5. Electrodes 13a and 14a are formed on two opposite surfaces of the dielectric block 12a. Electrodes 13b and 14b are formed on two opposite surfaces of the dielectric block 12b. The electrode 13a of the dielectric block 12a is fixedly connected to an inner ceiling surface of the shielded-cavity casing 5 by soldering or the like while the electrode 14b of the dielectric block 12b is fixedly connected to an inner bottom surface of the shielded-cavity casing 5 by soldering or the like. The electrode 14a of the dielectric block 12a and the electrode 13b of the dielectric block 13b are electrically connected to each other. Electrode sheets 7 are formed in the same manner as those in the first embodiment. Each of electrode sheets 7 is attached to the joint between the dielectric blocks 12a and 12b, the surface of the electrode sheet 7 on which no electrode film is formed being in contact with the dielectric blocks 12a and 12b. If the balance of an electromagnetic field distribution through the upper and lower dielectric blocks is considered, it is preferable to attach the electrode sheets 7 to the joint between the dielectric blocks 12a and 12b. However, the electrode sheets 7 may be attached to other portions. The center electrodes of the coaxial connectors 6 attached to side surfaces of the shielded-cavity casing 5 are electrically connected to the electrode films on the electrode sheets 7 by, for example, wires. The center electrodes of the coaxial connectors 6 may be directly connected to the electrodes 13b and 14a without using electrode sheets 7. In such a case, a wide-band dielectric filter can be formed because the degree of external coupling is maximized. The thus-constructed dielectric filter 11 functions as a one-stage dielectric filter and has an improved unloaded Q in comparison with the dielectric filter of the first embodiment if these dielectric filters are equal in height. A modification of this example such as that shown in Figs. 3A and 3B may be made. Fig. 3A is a partially fragmentary perspective view and Fig. 3B is a cross-sectional view taken along the line C-C of Fig. 3A. Components of this example identical to those of the first or second embodiment are indicated by the same reference numerals and will not be described in detail. Referring to Fig. 3A and 3B, dielectric blocks 22a and 22b constructed in the same manner as the dielectric block 2 shown in Figs. 1A and 1B and the dielectric blocks 12a and 12b shown in Figs. 2A and 2B are placed in a shielded-cavity casing 5. A dielectric block 22c, newly provided, is interposed between the dielectric blocks 22a and 22b, thus constructing a dielectric filter 21. In this arrangement, the dielectric blocks 22a and 22c form one-stage resonator and the dielectric blocks 22b and 22c also form one-stage resonator. Accordingly, the dielectric blocks 22a to 22c superposed one on another in the dielectric filter 21 shown in Figs. 3A and 3B function as a dual mode dielectric resonator, so that the dielectric filter 21 can be used as a filter having a two-stage resonator. On the basis of this structure, a dielectric filter having n-1 dielectric resonator stages may be constructed by further superposing dielectric blocks so as to form a stack of n dielectric blocks. The above-described TM dual mode dielectric resonator of this example having the structure shown in Figs. 3A and 3B uses dielectric blocks thin enough to reduce the overall thickness relative to that of the conventional short-circuit type TM dual mode dielectric resonator having the same resonant frequency. In this example, as well as in the first example, the shape of the dielectric blocks is not limited to a cylindrical shape and may have the shape of any polygonal prism. However, it is preferred that each of the dielectric blocks be formed into a cylindrical shape for the reason described above with respect to the first example. Also, the shapes of the plurality of dielectric blocks of the dielectric filter shown in Figs. 2A and 2B or 3A and 3B may be varied. An embodiment of the present invention will next be described with reference to Figs. 4A and 4B. Fig. 4A is a partially fragmentary perspective view and Fig. 4B is a cross-sectional view taken along the line D-D of Fig. 4A. Components of this embodiment identical to those of the first or second example are indicated by the same reference numerals and will not be described in detail. Referring to Fig. 4A and 4B, a dielectric filter 31 has such a structure that an electrode 34a of a dielectric block 32a and an electrode 33b of a dielectric block 32b are electrically insulated from each other by spacing therebetween. The dielectric blocks 32a and 32b function as resonators independent of each other, such that the dielectric filter 31 is formed of a two-stage resonator. A coupling control plate 39 having a coupling control hole 39a formed generally at its center is disposed between the electrode 34a of the dielectric block 32a and the electrode 33b of the dielectric block 32b. The degree of coupling between the resonator formed by the dielectric block 32a and the resonator formed by the dielectric block 32b is controlled by selecting the size of the coupling control hole 39a. If the coupling control hole 39a is larger, the degree of coupling between the resonator formed by the dielectric block 32a and the resonator formed by the dielectric block 32b is higher. If the coupling control hole 39a is smaller, the degree of coupling between the resonator formed by the dielectric block 32a and the resonator formed by the dielectric block 32b is lower. In this embodiment, as well as in the first and second examples, the shape of the dielectric blocks is not limited to a cylindrical shape. However, it is preferred that each of the dielectric blocks be formed into a cylindrical shape for the reason described above with respect to the first embodiment. Also, the shapes of the two dielectric blocks used may be different from each other. A second embodiment of the present invention will next be described with reference to Figs. 5A, 5B, and 6. Fig. 5A is a partially fragmentary perspective view and Fig. 5B is a cross-sectional view taken along the line E-E of Fig. 5A. Fig. 6 comprises plan views of upper and lower sections of the dielectric filter shown in Figs. 5A and 5B. Supporting members 48 shown in Figs. 5B are omitted in Fig. 6. In this embodiment, a dielectric filter 41 formed of a four-stage resonator is constructed by disposing, in a side-by-side fashion, two dielectric filters 31 described above as the first embodiment. Components of this embodiment identical to those above described are indicated by the same reference numerals and will not be described in detail. Referring to Fig. 5A and 5B, the dielectric filter 41 has four cylindrical dielectric blocks 42a to 42d, and pairs of electrodes 43a and 44a, 43b and 44b, 43c and 44c, and 43d and 44d are respectively formed on two major opposite surfaces of the dielectric blocks 42a to 42d. The structure of each of the dielectric blocks 42a to 42d is the same as that of the above-described dielectric blocks of the first embodiment, and will not be described in detail. The shielded-cavity casing 45 is formed of a dielectric material having the same thermal expansion coefficient as the dielectric blocks 42a to 42d, and an electrode 45a formed on its outer surface and, therefore, has the same shielding function as a metallic shielded-cavity casing. Since the shielded-cavity casing 45 has the same thermal expansion coefficient as the dielectric blocks, it is free from the problem of the difference between the thermal expansion coefficients of a metal and a dielectric. The shielded-cavity casing 45 is formed by combining separate upper and lower sections. Recesses for accommodating the dielectric blocks 42a to 42d are formed in each of the upper and lower sections. Further, input/output electrodes 46 are formed on one of the side surfaces of the shielded-cavity casing 45 while being electrically separated from the electrode 45a formed on the outer surface of the shielded-cavity casing 45. The input/output electrodes 46 extend vertically from the bottom surface of the shielded-cavity casing 45 used as a mounting surface. One of the input/output electrodes 46 is coupled to the dielectric block 42b through an electrode sheet 7. The dielectric block 42b is coupled to the dielectric block 42a uniformly spaced apart from the dielectric block 42b. The dielectric block 42a is in turn coupled to the dielectric block 42c adjacent to the dielectric block 42a through an electrode sheet 7. Further, the dielectric block 42c is coupled to the dielectric block 42d uniformly spaced apart from the dielectric block 42c. The dielectric block 42d is coupled to the other input/output electrode 46 through an electrode sheet 7. Supporting member 48 made of a dielectric material having a smaller dielectric constant is disposed between the dielectric blocks 42a and 42b uniformly space these dielectric blocks from each other. Another supporting member 48 is disposed between the dielectric blocks 42c and 42d for the same purpose. A coupling control plate 49 made of a metal is integrally combined with each supporting member 48 by being partially embedded in the supporting member 48. Each coupling control plate 49 has a coupling control holed 49a for controlling the coupling between the dielectric blocks 42a and 42b or the dielectric blocks 42c and 42d The thus-constructed dielectric filter can be obtained as a filter smaller in thickness and capable of being mounted in a surface mount manner. The dielectric blocks 42a to 42d may have different characteristic resonant frequencies. That is, in the dielectric blocks 42b and 42d coupled to the input/output electrodes 46 and respectively forming the initial-stage and final-stage dielectric resonators, the circumferential side surface on which no electrode is formed is partially cut off to adjust the resonance frequency of the corresponding dielectric resonator to a frequency higher than that of the resonators formed by the other dielectric blocks 42a and 42c. This is because, when input and output means are respectively coupled to the initial-stage and final-stage dielectric resonators by capacitive coupling, the capacitance due to each coupling reduces the apparent resonant frequency of each of the initial-stage and final-stage dielectric resonators by such an amount that the desired filtering characteristic of the dielectric filter formed by the dielectric resonators cannot be obtained. That is, to present this phenomenon, the resonant frequency of each of the initial-stage and final-stage dielectric resonators in the state of operating alone is increased so that the apparent resonant frequencies of all the dielectric resonators become approximately equal to each other when the dielectric resonator is formed. A structure such as shown in Fig. 7 may alternatively be used as means for increasing the resonant frequency of each of the initial-stage and final-stage dielectric resonators. Fig. 7 is a cross-sectional view of a dielectric filter 41a corresponding to the cross section of the dielectric filter shown in Fig. 5B. As shown in Fig. 7, dielectric blocks 42e and 42f smaller in diameter than the dielectric blocks 42b and 42d forming the initial-stage and final-stage dielectric resonators are provided in place of the dielectric blocks 42b and 42d. That is, the dielectric block 42e is provided in the initial stage while the dielectric block 42f having the same diameter as the dielectric block 42e is provided in the final stage, thereby increasing the resonant frequency of each of the initial-stage and final-stage dielectric resonators in the state of operating alone. In this embodiment, the shape of the dielectric blocks is not limited to a cylindrical shape. However, it is preferred that each of the dielectric blocks be formed into a cylindrical shape for the reason described above with respect to the first embodiment. Also, the shape of one of the plurality of dielectric blocks may be changed. In this embodiment, the input and output means are not coaxial connectors such as those used in the first, second or third embodiment but surface mount type input/output electrodes. In this embodiment, however, coaxial connectors arranged in the same manner as those in the first, second or third embodiment may alternatively be used. Needless to say, the input/output electrode structure of this embodiment suitable for surface mounting may be used in place of the coaxial connectors in the dielectric filters described above as the first embodiment. A third embodiment of the present invention will next be described with reference to Figs. 8 and 9. Fig. 8 is a partially fragmentary perspective view and Fig. 9 is an exploded perspective view. Components of this embodiment identical to those of the first or second, embodiment are indicated by the same reference numerals and will not be described in detail. Referring to Fig. 8, a dielectric duplexer 51 is formed of a first dielectric filter 51a having a first frequency band and a second dielectric filter 51b having a second frequency band. The first dielectric filter 51a is formed of dielectric blocks 52a to 52d shown in Fig. 9. In the dielectric filter 51a, a coaxial connector 56a is coupled to the dielectric block 52b through an electrode sheet 7, and the dielectric block 52b is coupled to the dielectric block 52a. The dielectric block 52a is coupled to the dielectric block 52c through an electrode sheet 7. The dielectric block 52c is coupled to the dielectric block 52d, which is coupled to a coaxial connector 56b through an electrode sheet 7 and a coil L1 and a capacitor C1 provided as matching means. Thus, the dielectric filter 51a having a four-stage dielectric resonator is formed, as shown in Fig. 8. The second dielectric filter 51b is formed of dielectric blocks 52e to 52h shown in Fig. 9. In the dielectric filter 51b, a coaxial connector 56b is coupled to the dielectric block 52f through a capacitor C1 and a coil L1 provided as matching means and through an electrode sheet 7. The dielectric block 52f is coupled to the dielectric block 52e. The dielectric block 52e is coupled to the dielectric block 52g through an electrode sheet 7. The dielectric block 52g is coupled to the dielectric block 52h, which is coupled to a coaxial connector 56c through an electrode sheet 7. Thus, the dielectric filter 51b having a four-stage dielectric resonator is formed, as shown in Fig. 8. As shown in Fig. 9, a shielded-cavity casing 55 is formed by combining separate upper and lower sections. Recesses for accommodating the dielectric blocks 52a to 52h are formed in each of the upper and lower sections. The dielectric blocks 52a to 52h are electrically connected to recessed surfaces of the shielded-cavity casing 55 by annular grounding plates 60. As shown in Fig. 9, sets of supporting members 58 for supporting the dielectric blocks 52a to 52h and a coupling control plate 59 supported by being interposed between upper and lower supporting members 58 are provided between the groups of dielectric blocks 52a, 52c, 52e, and 52g and the group of dielectric blocks 52b, 52d, 52f, and 52h. Supporting members 58 are made of a material having a small dielectric constant. Three supporting members 58 form one set for supporting one dielectric block in a three-point supporting manner. Cuts 58a are formed in the supporting members 58 to enable the electrode sheets 7 to be fixed by being pinched between the dielectric blocks and the supporting members 58a. Coupling control holes 59a are formed in the coupling control plate 59. The diameter and the shape of the coupling control holes 59a are selected to control coupling between the dielectric blocks 52a and 52b, between the dielectric blocks 52c and 52d, between the dielectric blocks 52e and 52f and between the dielectric blocks 52g and 52h. The thus-constructed dielectric duplexer 51 can be obtained as a small-loss thin duplexer formed of an eight-stage dielectric resonator. The initial-stage and final-stage dielectric blocks of the dielectric filters 51a and 52b of the dielectric duplexer 51 may be reduced in diameter, as are those in the above-described modification of the fourth embodiment. Fig. 10 is a cross-sectional view of a dielectric duplexer 61 in which the diameters of the initial-stage and final-stage dielectric blocks of each of dielectric filters are reduced. A structure about coaxial connectors of this dielectric duplexer is the same as that in the dielectric duplexer 51 shown in Figs. 8 and 9, and the description for it will not be repeated. As shown in Fig. 10, the diameters of the dielectric blocks 62b, 62d, 62f, and 62h corresponding to the initial and final stages of the dielectric filters are reduced relative to those of the other dielectric blocks 62a, 62c, 62e, and 62g. The shapes of supporting members 68a and grounding plates 60a for supporting the dielectric blocks 62b, 62d, 62f, and 62h are also changed according to the sizes of these dielectric blocks. In this manner, the resonant frequencies of the initial-stage and final-stage dielectric resonators in the state of operating alone are increased to ensure that, in each of the first and second dielectric filters, the apparent resonant frequencies of the dielectric resonators are approximately equal to each other. Needless to say, the apparent resonant frequency of the dielectric resonators forming the first dielectric filter and the apparent resonant frequency of the dielectric resonators forming the second dielectric filter are set so as to be different from each other. A structure such as that as shown in Fig. 11 can also be used as a structure for enabling the first and second dielectric filters to have different frequency bands. A structure about coaxial connectors of the dielectric duplexer shown in Fig. 11 is the same as that in the dielectric duplexer 51 shown in Figs. 8 and 9, and the description for it will not be repeated. As shown in Fig. 11, dielectric blocks 72a to 72d forming a first dielectric filter and dielectric blocks 72e to 72h forming second dielectric filter are made different in shape from each other; the dielectric blocks 72a to 72d are smaller in diameter than the dielectric blocks 72e to 72h, thereby enabling the first and second dielectric filters to have different frequency bands. While in this modification the diameters of dielectric blocks are made different from each other, any other various means for setting different frequency bands, e.g., making rectangular and cylindrical dielectric blocks, are also possible. The frequency bands of the first and second dielectric filters may be made different from each other by adding reactance elements such as capacitors and inductors without changing the shape of the dielectric blocks or by cutting the dielectric blocks. Each of the dielectric duplexers shown in Figs. 8 to 11 can be used as a common antenna device for a transmitter-receiver in such a manner that the first frequency band of the first dielectric filter is used as a receiving frequency band of a receiving filter while the second frequency band is used as a transmitting frequency band of a transmitting filter. Also, the first and second dielectric filters may be used as two transmitting filters or two receiving filters. Another example useful for understanding the invention will next be described with reference to Fig. 12. This example uses the same construction as that of the dielectric filter 1 shown in Fig. 1. Components or portions identical or corresponding to those shown in Fig. 1 are indicated by the same reference numerals and will not be described in detail. A dielectric filter 81 shown in Fig. 12 differs from the dielectric filter 1 shown in Fig. 1 in the structure of electrodes formed on the dielectric block. That is, while each of the electrodes 3 and 4 of the dielectric block 2 in the dielectric filter 1 shown in Fig. 1 is formed of a single-layer conductor, each of electrodes 83 and 84 of a dielectric block 82 in the dielectric filter 81 shown in Fig. 12 is formed of a thin-film multilayer electrode formed by alternately laminating a thin-film conductor and a thin-film dielectric. Such a thin-film multilayer electrode, e.g., one described in Japanese Patent Application No. 310900/1994, can be used with a reduced insertion loss in comparison with a single-layer conductor. Therefore, if such an thin-film multilayer electrode is used in a resonator, the resonator can have a higher unloaded Q. An arrangement using a thin-film multilayer electrode in the dielectric filter shown in Fig. 1 has been described by way of example. Needless to say, such a thin-film multilayer electrode can also be applied to each of the dielectric filters of the first to second embodiments and the dielectric duplexer of the third embodiment to obtain a dielectric filter or dielectric duplexer having a higher unloaded Q. According to the present invention, substantially no real current flows in the shielded-cavity casing for accommodating the dielectric block, so that there is substantially no loss in the shielded cavity casing. As a result, a dielectric resonator, a dielectric filter and a dielectric duplexer each having a high unloaded Q can be obtained. According to the second aspect of the present invention, a plurality of dielectric blocks are disposed in a space where an electromagnetic field distribution is generated, thereby making it possible to obtain a dielectric resonator, a dielectric filter and a dielectric duplexer each having a higher unloaded Q. According to the third aspect of the present invention, a plurality of dielectric blocks are arranged in the direction of height while being spaced apart from each other to form a multi-stage resonator, thereby achieving a reduction in bottom surface area. According to the fourth aspect of the present invention, a thin-film multilayer electrode is used to obtain a dielectric resonator, a dielectric filter and a dielectric duplexer each having a much higher unloaded Q. According to the fifth aspect of the present invention, the dielectric block is formed into a cylindrical shape such that the edge of the electrode surface is at a constant distance from the center of the surface, thereby preventing occurrence of a potential difference and, hence, a current at the edge. The loss in the electrode can be further reduced thereby. As a result, a dielectric resonator having a higher unloaded Q can be obtained. According to the ninth aspect of the present invention, an electrode sheet formed of a dielectric sheet and an electrode formed on one surface of the dielectric sheet is used as a coupling means, and the desired degree of coupling can easily be achieved by suitably selecting the dielectric constant of the dielectric and the size of the electrode sheet. According to the tenth aspect of the present invention, the resonant frequency of the initial-stage and final-stage TM mode dielectric resonators in the state of operating alone is increased, thereby equalizing the resonant frequencies of the TM mode dielectric resonators when the resonators form a dielectric filter. According to the eleventh aspect of the present invention, a plurality of TM mode dielectric filters described above are combined to form a first TM mode dielectric filter having a first frequency band, and a second TM mode dielectric filter having a second frequency band, and the first frequency band and the second frequency band are made different from each other, thereby obtaining a dielectric duplexer having a higher unloaded Q. According to the twelfth aspect of the present invention, the shape of TM mode dielectric resonator forming the first TM mode dielectric filter and the shape of the TM mode dielectric resonator forming the second TM mode dielectric filter are made different from each other to make the first frequency band and the second frequency band different from each other. A need for adding a circuit for relatively shifting the frequency bands is thereby eliminated while such a circuit is required in the case of using TM mode dielectric resonators equal in shape.

Claims: A transverse magnetic mode dielectric resonator comprising: a shielded-cavity casing (5; 45) having electrical conductivity; and a first and a second dielectric block (22a, 22b; 32a, 32b; 42a - 42f;52a - 52h; 62a - 62h; 72a - 72h) disposed in said shielded-cavity casing (5; 45), wherein electrodes ( 23a, 24a; 33a, 34a) are formed on two surfaces of said first dielectric block (22a; 32a; 42a, 42c; 42a - 42d; 52a, 52c, 52e, 52g) opposite from each other and electrodes (24b, 24c; 33b, 34b) are formed on two surfaces of said second dielectric block (22b; 32b; 42b, 42d; 42e, 42f; 52b, 52d, 52f, 52h) opposite from each other, and one (23a; 33a) of the electrodes on said first dielectric block is placed on an inner surface of said shielded-cavity casing (5; 45), and one (24b; 34b) of the electrodes on said second dielectric block is placed on another inner surface of said shielded-cavity casing (5; 45) characterised in that another (24a; 34a) of the electrodes of said first dielectric block and another (24c; 33b) of the electrodes of said second dielectric block are opposed to each other and spaced apart from each other. A transverse magnetic mode dielectric resonator according to claim 1, wherein at least one of the electrodes formed on the two surfaces of each of said dielectric blocks is formed of a thin-film multilayer electrode formed by alternately superposing a thin-film conductor and a thin-film dielectric. A transverse magnetic mode dielectric resonator according to claim 1 or 2, wherein at least one of said dielectric blocks (22a, 22b; 32a, 32b; 42a - 42d; 42a - 42f) is cylindrical. A transverse magnetic mode dielectric filter (21; 31; 41; 41a; 51; 61; 71) comprising: at least one transverse magnetic mode dielectric resonator according to any one of claims 1 to 3; and input and output means (6; 46; 56a - 56c) coupled to said transverse magnetic mode dielectric resonator. A transverse magnetic mode dielectric filter (21; 31; 41; 41a) according to Claim 4, wherein coupling means (7) are disposed between said transverse magnetic mode dielectric resonator and said input and output means (6; 46; 56a - 56c). A transverse magnetic mode dielectric filter (41; 41a; 51; 61; 71) according to Claim 4 or 5, wherein a plurality of said transverse magnetic mode dielectric resonators are disposed and coupling means (7) are disposed between the plurality of transverse magnetic mode dielectric resonators. A transverse magnetic mode dielectric filter (21; 31; 41; 41a; 81) according to Claim 5 or 6, wherein said coupling means (7) comprises an electrode sheet formed of a dielectric sheet and an electrode formed on one surface of the dielectric sheet. A dielectric filter (21; 31; 41; 41a; 81) according to any one of Claims 5 to 7, wherein a plurality of said transverse magnetic mode dielectric resonators are disposed, and wherein, in said plurality of transverse magnetic mode dielectric resonators, the resonant frequency of the initial-stage and final-stage transverse magnetic mode dielectric resonators is increased relative to the resonant frequency of the other transverse magnetic mode dielectric resonators. A transverse magnetic mode dielectric duplexer (51; 61; 71) in which a plurality of transverse magnetic mode dielectric filters (51a, 51b; 62a - d, 62e - h; 72a - d, 72e - h) according to any one of claims 4 to 8 are combined, said duplexer comprising: a first transverse magnetic mode dielectric filter (51a; 62a - d; 72a - d) having a first frequency band; and a second transverse magnetic mode dielectric filter (51b; 62e - h; 72e - h) having a second frequency band, wherein the first frequency band and the second frequency band are different from each other. A transverse magnetic mode dielectric duplexer (61; 71) according to Claim 9, wherein the shape of the transverse magnetic mode dielectric resonator forming the first transverse magnetic mode dielectric filter and the shape of the transverse magnetic mode dielectric resonator forming the second transverse magnetic mode dielectric filter are made different from each other to make the first frequency band and the second frequency band different from each other. A transverse magnetic mode dielectric duplexer (51; 61; 71) according to Claim 9 or 10, wherein the first transverse magnetic mode dielectric filter is used as a transmitting filter while the second transverse magnetic mode dielectric filter is used as a receiving filter.

Inventors: HIDAKA SEIJI, ISE TOMOYUKI, ISHIKAWA YOHEI, KUBOTA KAZUHIKO, MATSUI NORIFUMI, HIDAKA, SEIJI, ISE, TOMOYUKI, ISHIKAWA, YOHEI, KUBOTA, KAZUHIKO, MATSUI, NORIFUMI
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1997
Main classification: H01P1
IPC codes: H01P7, H01P1