Dielectric Filter Having Variable Rectangular Cross Section Inner Conductors

This invention relates to a dielectric filter for use in various kinds of radiocommunication equipment.

FIG. 3(a) is a perspective view of the construction of a conventional dielectric filter. FIG. 3(b) is a cross-sectional view taken along the line A--A' of FIG. 3(a). FIG. 3(c) is a cross-sectional view taken along the line B--B' of FIG. 3(a). Referring to FIGS. 3(a) to 3(c), a generally box-like dielectric body 1 has an open end 1A formed as its one end face, and through holes 2a, 2b, and 2c formed so as to extend from the open end 1A to a short-circuit end 1B provided at the other end face. The length of the through holes 2a, 2b, and 2c is set to approximately 1/4 the wavelength .lambda. corresponding to the desired resonance frequency. Each of the through holes 2a, 2b, and 2c has a circular cross-sectional shape and the diameter of each hole is changed at an intermediate position from a smaller diameter on the open end side to a larger diameter on the short-circuit end side. Inner conductors 3a, 3b, and 3c are formed on inner surfaces of the through holes 2a, 2b, and 2c, respectively. An outer conductor 4 is formed over outer surfaces of the dielectric block 1 except for the open end 1A. The inner conductors 3a, 3b, and 3c and the outer conductor 4 are connected at the short-circuit end 1B of the dielectric body 1. The inner conductors 3a and 3c are connected to input/output terminals Ta and Tc through input/output capacitors Ca and Cc, respectively.

In this type of coupling distribution constant line circuit open at its one end and connected at the other end, coupling in an odd mode is strong at the open end 1A while coupling in an even mode is strong at the short-circuit end 1B. The odd mode coupling is capacitive coupling, and the even mode coupling is inductive coupling. The odd mode coupling and the even mode coupling cancel out each other. Accordingly, if the through holes 2a, 2b, and 2c have a cylindrical shape uniform in diameter, no coupling occurs between the inner conductors 3a, 3b, and 3c. Ordinarily, to obtain desired band-pass characteristics, the shape of the through holes 2a, 2b, and 2c and other factors are determined so that suitable coupling occurs between the inner conductors 3a, 3b, and 3c. In the case of the construction shown in FIGS. 3(a) to 3(c), the diameter of the through holes 2a, 2b, and 2c is reduced on the open end 1A side to weaken odd mode coupling while the diameter is increased on the short-circuit end 1B side to strengthen even mode coupling. Inductive couplings thereby occur between the inner conductors 3a, 3b, and 3c. The inductive couplings thereby provided attenuate signals having frequencies higher than the desired pass band. Also, the small-diameter portions of the inner conductors 3a, 3b, and 3c function as a small-capacity distribution constant line circuit.

In the conventional dielectric filter described above, the diameter of the inner conductors on the open end side is reduced in order to weaken capacitive coupling between the adjacent inner conductors. For this reason, the distance between the inner and outer conductor is large, and the areas of portions of these conductors facing each other are small, and the capacitances between the inner and outer conductors are small. For desired sharpness of the filter, skirt characteristics, however, a certain capacitance between inner and outer conductors is required. If a large capacitance between inner and outer conductors is required, it is necessary to increase the axial size of the inner conductors in order to achieve that capacitance. It is therefore difficult to reduce the overall size of the dielectric filter.

The present invention has been achieved in view of the above-described circumstances, and an object of the present invention is to provide a dielectric filter having desired pass band characteristics and capable of being reduced in size.

According to the present invention, there is provided a dielectric filter comprising a dielectric block generally in the form of a rectangular parallelepiped, an outer conductor covering side surfaces of the dielectric block, a plurality of inner conductors arranged in a row in the longitudinal direction of the dielectric block so that their axes extend parallel to each other from an upper face of the dielectric block to a lower face of same, and a short-circuit conductor provided in the lower face of the dielectric block to connect the outer and inner conductors, the dielectric filter being characterized in that at least one of the plurality of inner conductors has a rectangular cross-sectional shape, and the size of this cross-sectional shape in the longitudinal direction of the dielectric block is smaller at the upper face and larger at the lower face.

In this arrangement, the distance between one inner conductor having a rectangular sectional shape and another adjacent inner conductor is increased on the upper face side of the dielectric block so that odd mode coupling between the conductors is weakened. It is therefore possible to freely set the distance between the outer conductor and two side surfaces of the inner conductor having the rectangular sectional shape. A large fringing (edge) capacitance exists between the outer conductor and angular portions of the inner conductors corresponding to the ends of the two side surfaces. Consequently, the capacitance between the inner and outer conductors per unit length in the axial direction can be increased in comparison of the arrangement in which the sectional shape of the inner conductors is circular, and the overall size of the dielectric filter can therefore be reduced even if a large capacitance value is required.

FIGS. 1(a) to 1(c) show the construction of a dielectric filter in accordance with an embodiment of the present invention;

FIG. 1(a) is a perspective view of the embodiment;

FIG. 1(b) is a cross-sectional view taken along the line A--A' of FIG. 1(a);

FIG. 1(c) is a cross-sectional view taken along the line B--B' of FIG. 1(a);

FIGS. 2(a) to 2(c) show the construction of another dielectric filter provided by applying a tapped line type input/output structure to the embodiment shown in FIGS. 1(a) to 1(c);

FIG. 2(a) is a perspective view;

FIG. 2(b) is a cross-sectional view taken along the line A--A' of FIG. 2(a);

FIG. 2(c) is a cross-sectional view taken along the line B--B' of FIG. 2(a);

FIGS. 3(a) to 3(c) show the construction of a conventional dielectric filter;

FIG. 3(a) is a perspective view;

FIG. 3(b) is a cross-sectional view taken along the line A--A' of FIG. 3(a); and

FIG. 3(c) is a cross-sectional view taken along the line B--B' of FIG. 3(a) .

An embodiment of the present invention will be described below with reference to the accompanying drawings.

Referring to FIGS. 1(a) to 1(c), a dielectric filter in accordance with the present invention is illustrated. In FIGS. 1(a) to 1(c), components corresponding to those shown in FIGS. 3(a) to 3(c) are indicated by the same reference symbols, and the description for them will not be repeated. The dielectric filter of this embodiment differs from the arrangement shown in FIGS. 3(a) to 3(c) in that through holes 12a, 12b, and 12c having a rectangular cross-sectional shape are formed in a dielectric block 1, with inner conductors 13a, 13b, and 13c formed on inner surfaces of the through holes 12a, 12b, and 12c. The size of the through holes 12a, 12b, and 12c in the longitudinal direction of the dielectric block 1, i.e., the direction of arrangement of the through holes is comparatively reduced on the open end 1A side and is comparatively increased on the short-circuit end 1B side. The size of each hole is constant in the direction perpendicular to the longitudinal direction.

In this arrangement, the distance between adjacent two of the inner conductors 13a, 13b, and 13c is larger on the open end side 1A, so that the odd mode coupling is weakened, and that the inner conductors 13a, 13b, and 13c are in an inductive coupling state. Two side surfaces of the inner conductors 13a, 13b, and 13c face the outer conductor 4, and large fringing capacitances exist between the outer conductor 4 and angular portions of the inner conductors 13a, 13b, and 13c. For this reason, even though the width of the side surfaces of the inner conductors 13a, 13b, and 13c is reduced on the open end 1A side, the capacitances between these conductors and the outer conductor 4 are larger in comparison with the arrangement in which the cross-sectional shape of the inner conductors is circular. Consequently, the axial size of the inner conductors 13a, 13b, and 13c can be reduced so that the dielectric filter has a smaller overall size.

It is not always necessary to make the cross-sectional shape of all the inner conductors 12a, 12b, and 12c rectangular. Inner conductors having a circular cross-sectional shape may be mixed. In such a case also, the obtained effect is similar to that of the above-described embodiment. With respect to inner conductors having a rectangular cross-sectional shape, it is not always necessary to change the sectional shape in the longitudinal direction, i.e., the direction of arrangement of the through holes in a stepping manner as shown in FIG. 1(b). The width of the sectional shape in this direction may be gradually reduced from the short-circuit end toward the open end in a tapering manner. Also, the sectional shape may be changed in a stepping or tapering manner in a direction perpendicular to the direction of arrangement of the through holes.

FIGS. 2(a) to 2(c) show the construction of another dielectric filter provided by applying a tapped line type input/output structure to the above-described embodiment. In this dielectric filter, regions 5a and 5c of two side surfaces of the dielectric block 1 are not covered with the outer conductor 4, connection holes 6a and 6c are respectively formed through the dielectric body 1 for communication between the regions 5a and 5c and the through holes 12a and 12c, and connection conductors 6a, and 6c connected to the inner conductors 13a and 13c are formed on inner surfaces of the connection holes 6a and 6c. A signal which is to be input into this dielectric filter is applied to the connection conductor 6a, and an output signal is extracted through the connection conductor 6c. This construction enables input and output parts to be mounted easily.

According to the present invention, as described above, the capacitance between the inner and outer conductors per unit length in the axial direction can be increased and the overall size of the dielectric filter can therefore be reduced.