1. Field of the Invention
The present invention relates to a dielectric filter for use in a high frequency circuit, a dielectric duplexer, and communication device having the same.
2. Description of the Related Art
In conventional dielectric filters, a plurality of through holes are provided in a dielectric body. The outer surfaces of the dielectric body and the inner surfaces of the through holes are plated to form a series of resonator cavities. Each resonator cavity will have an open end and a short-circuit end. In many filters of this type the inner diameters of each through hole vary so that the line impedances of the resonator cavities are different between the open end and the short-circuit end of the resonator cavity.
FIG. 6 shows an example of one such filter 1. In this example, a plurality of through holes 2a, 2b, and 2c are provided in a rectangular parallelepiped dielectric block 3 whose inner and outer surfaces are substantially coated with a conductive material to form a plurality of resonator cavities. To this end, the inner surfaces of the through holes 2a, 2b and 2c are coated with respective inner conductors 4 and an outer conductor 5 is formed so as to cover substantially the entire outer surface of the dielectric block. The outer conductor 5 is directly coupled to the inner conductor 4 at the end 6 of dielectric block 3 to form short-circuit ends of the resonator cavities. Unplated areas or gaps g are formed in the through holes 2a, 2b and 2c at locations slightly removed from the end 7 of the dielectric block to form open ends of the resonator cavities. Input-output electrodes block 3 and are separated from the outer conductor 5 by unplated portions of the outer surface of the dielectric block. The input-output electrodes are used to couple input and output signals into the adjacent resonators 2a, 2b. 
To adjust the coupling between adjacent resonators and the resonant frequencies of the respective resonators, a step is formed in the resonator cavities so that each resonator cavity is divided into an open end portion and the short-circuit end portion having different diameters.
When dielectric blocks used for these filters are molded, different molds are required for different filters due to the fact that the diameters of the through holes vary from filter to filter.
Moreover, the gaps g are formed by first fully plating the inner walls of the through holes with a conductive material and then using a cutting tool to remove a ring-shaped area of the conductive plating. As a result of variations in the size of the inner diameters on the open end portions of the through holes, control programs for controlling the operation of the cutting tools are required. Additionally, when the positions and widths of the gaps are varied from resonator cavity to resonator cavity, adjustable tools and further control programs are needed.
For these reasons, the production cost of the filter is increased.
One way to reduce production costs is to keep the time which it takes for the tip of the cutting tool to make one revolution when it cuts each of the unplated areas constant. However, because the open end of the through holes have different inner diameters, the cutting rates per unit arc length differ as a function of the inner diameters. Therefore, variations in the amount of the inner conductors and the dielectric block which are cut during this process become large. These variations exert an undesired influence over the characteristics of the filter.
Accordingly, it is an object of the present invention to provide a dielectric filter and a dielectric duplexer in each of which the above-described problems can be solved, the manufacturing cost can be reduced, and a predetermined characteristic can be obtained, and a communication device having the same.
To achieve the above object, according to the present invention, there is provided a dielectric filter which comprises:
a dielectric block having a plurality of through holes, each of said through holes having an open end section and short-circuit end section, each of said open end sections having the same circular cross-section, at least one of said short-circuit end sections having an angular cross-section;
inner conductors formed on inner walls of said respective through holes, gaps being formed in said inner conductors to define said open end sections of said through holes;
an outer conductor covering most of the outer surface of the dielectric block, said outer conductor being electrically coupled to said inner conductors at one respective end of each of the inner conductor.
In the preferred embodiment, the open end sections of the through holes are cylindrical in shape and are of the same diameter. Therefore, the same type of molds for forming the open end sections of the through holes can be used irrespective of the shapes and sizes of the short-circuit end sections of the through holes. Moreover, the same cutting tool and the same control program for the tool can be used to cut the gaps in the plating located on the inner walls of the open end sections of the through holes. This is true even if adjustments in the size and depth of the gaps are formed to adjust the electrical characteristics of the filter. The foregoing filter can be used in a dielectric duplexer as a transmission filter, a reception filter and/or a transmission-reception filter.
The present invention is also directed towards a duplexer including at least a transmission filter section, a reception filter section and/or a transmission-reception filter section, at least one of said filter sections comprising:
a dielectric block having a plurality of through holes, each of said through holes having an open end section and short-circuit end section, each of said open end sections having a circular cross section, at least one of said short-circuit end sections having an angular cross section;
inner conductors formed on inner walls of the respective through holes, gaps being formed in said inner conductors to define said open end sections of said through holes;
an outer conductor covering most of the outer surface of the dielectric block, said outer conductor being electrically coupled to said inner conductors at one respective end of each of inner conductor.
Moreover, according to the present invention, a communication device provided with the above-described filter or duplexer, e.g., in the filter circuit unit for a transmission-reception signal of a high frequency circuit is formed.