Abstract:
A coaxial dielectric resonator for use at high frequency. The outer or inner peripheral surface of a tubular dielectric member is stepped so as to provide a greater suppression of spurious resonance. The dielectric member has a prism-shaped outer configuration so as to provide a high Q value, as well as improved space factor.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a coaxial dielectric resonator (referred to as dielectric S.I.R. (Stepped Impedance Resonator) hereinafter) which is used at high frequency. 
     FIG. 9 shows a typical conventional dielectric S.I.R. This dielectric S.I.R. is composed of a cylindrical outer conductor 1, an inner conductor 2 and a dielectric member 3. The outside diameter of the outer conductor 1 is reduced at the open end of the outer conductor 1 over a predetermined axial length as compared with the closed end of the outer conductor 1 so as to reduce impedance. Another known dielectric S.I.R. is shown in FIG. 10. Both these known dielectric S.I.R.s have cylindrical configurations. 
     In these known dielectric S.I.R.s, a large reduction of the Q value in the resonance circuit is inevitable, which makes it difficult to reduce the size of the filter in the resonator. 
     Another problem encountered with the known dielectric S.I.R.s is that the space factor is reduced due to the fact that the filter has to be placed in a case which has a cylindrical configuration. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a dielectric S.I.R. which has a higher value of Q than known dielectric S.I.R.s. 
     Another object of the present invention is to provide a dielectric S.I.R. which enables the spurious resonance frequency to be set freely and which provides an improved space factor. 
     To this end, according to one aspect of the invention, there is provided a dielectric stepped impedance resonator comprising: a hollow dielectric member the inside diameter of which is varied at an intermediate portion thereof so as to provide a step, the dielectric member having a prism-form outer configuration; inner and outer conductors covering the inner and outer surfaces of the dielectric member; and a short-circuiting member provided on one end of the dielectric member so as to electrically connect the inner and outer conductors to each other. 
     According to another aspect of the invention, there is provided a dielectric stepped impedance resonator comprising: a prism-shaped outer conductor; a cylindrical inner conductor disposed in the outer conductor coaxially therewith; a dielectric member disposed between the outer and inner conductors; and a short-circuiting member which electrically connects one end of the outer conductor to the adjacent end of the inner conductor; the end portion of the outer conductor opposite to the short-circuiting member and extending over a predetermined length from the end extremity has a cylindrical or prism form the outer dimension of which is smaller than the outer dimension of the end portion of the outer conductor adjacent to the short-circuiting member. 
     According to still another aspect of the present invention, there is provided a dielectric stepped impedance resonator comprising: a prism-shaped outer conductor; a cylindrical inner conductor disposed in the outer conductor coaxially therewith; a dielectric member disposed between the outer and inner conductors; and a short-circuiting member through which one end of the outer conductor is electrically connected to the adjacent end of the inner conductor; the end portion of the inner conductor opposite to the short-circuiting member and extending over a predetermined length from the end extremity has a cylindrical or prism form the inner dimension of which is greater than the inner dimension of the end portion of the inner conductor adjacent to the short-circuiting member. 
     The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments when the same is read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an embodiment of a dielectric S.I.R. in accordance with the present invention; 
     FIG. 2 is a perspective view of another embodiment of the dielectric S.I.R. in accordance with the present invention; 
     FIGS. 3a and 3b are a perspective view and a sectional view of still another embodiment of the dielectric S.I.R. of the present invention; 
     FIG. 4a and 4b are a sectional view and a right end view of a different embodiment of the dielectric S.I.R. of the present invention; 
     FIGS. 5a and 5b are a perspective view and a sectional view of a different embodiment of the dielectric S.I.R. of the present invention; 
     FIG. 6 is a characteristic curve showing the relationship between a ratio Q and a length L 1  with parameters of resonance frequency and band width; 
     FIGS. 7a and 7b are a cross-sectional view and a longitudinal sectional view of a band-pass filter making use of a known dielectric S.I.R.; 
     FIGS. 8a and 8b are a cross-sectional view and a longitudinal sectional view of a band-pass filter making use of the dielectric S.I.R. of FIG. 3; 
     FIG. 9 is a perspective view of a known dielectric S.I.R.; and 
     FIGS. 10a and 10b are a sectional view and a side elevational view of another known dielectric S.I.R. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of the dielectric S.I.R. of the present invention will be described with reference to the accompanying drawings. 
     Referring to FIG. 1 showing the first embodiment, an outer conductor 11 has a closed or short-circuited end portion 11a which has a prism form and an open end portion 11b which has a cylindrical form. The cylindrical open end portion 11b has a predetermined axial length and has an outside diameter which is smaller than the length of the side of cross-section of the closed end portion 11a. An inner conductor 12 is disposed in the outer conductor 11 coaxially therewith. A dielectric member 13 is disposed in the annular space between the outer and inner conductors 11 and 12. The dielectric member 13 is formed by dry-forming and firing a BaTi 4  O 9  type ceramic material and shaping the open end portion into a cylindrical form. The outer conductor 11 and the inner conductor 12 are formed by metallizing the outer and inner surfaces of the dielectric member 13 with silver. Thus, the conductors 11 and 12 actually have a film-like form. The outer conductor 11 and the inner conductor 12 are connected to each other at one end of the dielectric member 13. 
     The outer dimension, length and the impedance of the closed end portion of the thus formed dielectric S.I.R. are represented by b 1 , L 1  and Z 1 , respectively. The outer diameter, length and the impedance of the open end portion are represented by b 2 , L 2  and Z 2 , respectively. The overall length of the dielectric S.I.R. is represented by LT. 
     Using these expressions, the condition of resonance is given as follows: 
     
         tanβL.sub.1.tanβL.sub.2 =Z.sub.2 /Z.sub.1 =K 
    
     where, ##EQU1## while K represents the impedance ratio, ε r  represents the dielectric constant of the dielectric member. 
     The Q value is given by: 
     
         1/Q=1/Q.sub.c +tanδ 
    
     where tanδ represents the dielectric loss. The value Q c  is given as follows: ##EQU2## where, 
     
         A.sub.1 =2βL.sub.1 +sin2βL.sub.1, B.sub.1 =cos.sup.2 βL.sub.1 
    
     
         A.sub.2 =βL.sub.2 -sin2βL.sub.1, B.sub.2 =cos.sup.2 βL.sub.1 /sin.sup.2 βL.sub.2 
    
     where, δ represents the thickness of the surface layer. 
     The Q values were calculated on the conventional dielectric S.I.R. of FIG. 9 and the dielectric S.I.R. of this embodiment, while varying L 1  and b 2 , the results of which are shown in FIG. 6. 
     Table 1 shows the dimensions and numerical data of the conventional dielectric S.I.R. employed in the calculations of FIG. 6, while Table 2 shows those of the dielectric S.I.R. of the first embodiment, employed in the calculations. 
     From FIG. 6 and Tables 1 and 2, it will be understood that the deterioration of the Q value can remarkably be suppressed by the present invention as compared with the conventional dielectric S.I.R. 
     
                       TABLE 1______________________________________                                    Spurious                                    ResonanceNO   a.sub.1 b.sub.2              b.sub.1                    L.sub.1                        K      Q    Frequency______________________________________1    2       7     8     6   0.90   680  3.06f.sub.02    2       6     8     6   0.79   630  3.31f.sub.03    2       5     8     7   0.66   665  3.50f.sub.04    2       4     8     8   0.50   630  3.38f.sub.0______________________________________ 
    
     
                       TABLE 2______________________________________                                    Spurious                                    ResonanceNO   a.sub.1 b.sub.2              b.sub.1                    l.sub.1                        K      Q    Frequency______________________________________1    2       7     8     6   0.80   730  3.28f.sub.02    2       6     8     6   0.70   685  3.50f.sub.03    2       5     8     7   0.58   715  3.66f.sub.04    2       4     8     8   0.44   690  3.51f.sub.0______________________________________ 
    
     Referring to FIG. 6, the impedance ratio K of the conventional dielectric S.I.R. is calculated to be K=0.66, when the dimensions and the dielectric constant are selected to be b 1  =8 mm, b 2  5 mm, a 1  =2 mm and ε r  =38. 
     On the other hand, the impedance ratio K of the dielectric S.I.R. of this embodiment is calculated as K=0.58 on condition of b 1  =8 mm, b 2  =5 mm, a 1  =2 mm and ε r  =38. 
     It will be understood that the dielectric S.I.R. of the invention provides a smaller impedance ratio K as compared with the conventional dielectric S.I.R. 
     As a matter of fact, a smaller impedance ratio K provides a greater suppression of the spurious resonance frequency. Thus, the described embodiment provides a greater suppression of the spurious resonance frequency (f s ) than the conventional dielectric S.I.R. 
     Although in FIG. 1 the open end portion 116 of the outer conductor 11 has a cylindrical form, this is not exclusive and the outer end portion 11b of the outer conductor 11 can have a prism form as shown in FIG. 2. 
     FIGS. 3a and 3b show another embodiment of the invention. In these Figures, the same reference numerals are used to denote the same parts or members as those used in the first embodiment, and detailed description of such parts or members is omitted. This embodiment is characterized in that the inside diameter of the open end portion 12a of the inner conductor, having a predetermined axial length from the end extremity, is determined to be greater than that of the closed end of the inner conductor 12. On the other hand, the outer conductor 11 has a constant outer diameter over its entire length. 
     FIG. 4 shows a different embodiment in which the relationship between the inside diameters of both ends of the inner conductor 12 are reversed from that of the embodiment shown in FIGS. 3a and 3b. 
     FIGS. 5a and 5b show a different embodiment in which the open end portion 12a of the inner conductor 12 has a prism form, unlike the embodiment shown in FIGS. 3a and 3b. 
     The embodiments shown in FIGS. 3a, 3b to FIGS. 5a and 5b produce effects substantially equivalent to those produced by the first embodiment. 
     As has been described, according to the present invention, the closed end portion of the outer conductor has a prism form, while the open end portion of the outer conductor has a cylindrical or a prism form with outer dimension smaller than that of the closed end. Alternatively, the open end portion of the inner conductor has a cylindrical or prism form with a greater inner diameter. With this arrangement, it is possible to reduce deterioration of the Q value so as to provide a greater effect of suppression of the spurious characteristics of the dielectric S.I.R. 
     By using the dielectric S.I.R. of the present invention, therefore, it is possible to obtain a filter which does not suffer from a reduced insertion loss and which exhibits a significantly improved spurious characteristics, thus offering a great advantage from an industrial point of view. 
     A description will be given of a 2-staged band-pass filter incorporating a dielectric S.I.R. of the present invention. 
     FIGS. 7a and 7b show a filter which incorporates a conventional dielectric S.I.R. of the type shown in FIG. 10, having a diameter of 10 mm. 
     FIGS. 8a and 8b show a filter incorporating the dielectric S.I.R. of the invention shown in FIG. 3, having outside dimensions of 10 mm and 8 mm. In FIGS. 7a to FIG. 8b, numerals 50 and 60 denote, respectively, coupling capacitors, while 51 and 61 denote dielectric members. The outer and inner conductors are denoted by 52, 62 and 58, 68, respectively. Numerals 54, 64 denote steps, while 55 and 65 denote connectors. Numerals 55 and 56 denote cases, while 57 and 67 denote solder terminals. The resonator element used in the filters of FIGS. 7a to 8b have the same value of Q so that they exhibit the same characteristics at the fundamental frequencies. The primary spurious characteristics, outer dimensions and the volume of these filters are shown in Table 3. From this Table, it will be understood that the volume of the filter shown in FIGS. 8a and 8b is reduced in amount of about 18% as compared with the conventional filter of FIGS. 7a and 7b, though the spurious characteristics of both filters are materially the same. 
     
         ______________________________________FIGS. 7a, 7b        FIGS. 8a, 8b______________________________________Primary  3.9 × f.sub.0                   3.9 × f.sub.0SpuriousFre-quencyOuter  Width × Depth × Height                   Width × Depth × Heightdimen- 26mm × 16mm × 11.5mm                   22mm × 16mm × 11.5mmsionsVolume 4.9cc            4.0cc______________________________________ f.sub.0 fundamental frequency 
    
     As will be understood from the foregoing description, according to the present invention, it is possible to freely set the spurious resonance frequency by virtue of the provision of a step in the hollow of the dielectric S.I.R. incorporated in the filter. 
     The invention also offers the following advantages: 
     (1) Space factor us significantly improved so as to enable the size of the product to be reduced. 
     Namely, wasteful or dead space is minimized by virtue of the prism form of the outer conductor. 
     In addition, resonators having variety of operation characteristics can be obtained by changing the length of one side of the prism form, so that the degree of freedom of design can be enhanced with a constant width. 
     (2) The no-load Q value is increased as compared with conventional cylindrical resonator having an equal volume. 
     Thus, the present invention enables dielectric S.I.R.s to be designed to have variety of operation characteristics adaptable to wide variety of use, thus offering a significantly increased degree of freedom in design as compared with conventional resonators which have cylindrical configurations. In addition, the dielectric S.I.R.s of the present invention can easily be mass-produced, thus offering a great advantage from an industrial point of view.