1. Field of the Invention
The present invention generally relates to a dielectric filter applicable to an antenna duplexer of a car telephone and, more particularly, to a dielectric filter having an attenuation pole which is tunable to a predetermined frequency.
2. Description of the Prior Art
A dielectric filter customarily has a plurality of dielectric resonators implemented by center electrodes which may be arranged substantially parallel to each other in a homogeneous monolithic block of dielectric material. The dielectric block is provided with an input electrode pattern and an output electrode pattern thereon. The dielectric resonators in combination constitute a series resonance circuit and define a pass band frequency of the filter.
A conductive electrode pattern for frequency adjustment is provided on one surface of the dielectric block and connected to one end of each center conductor. Another conductive electrode pattern is provided on the above-mentioned surface of the dielectric block in such a manner as to intervene between nearby dielectric resonators for the purpose of adjusting coupling capacitance or coupling inductance. A metalized pattern is formed on opposite sides and bottom of the dielectric block and connected to ground.
An insulated wire having an insulative coating is laid above the dielectric resonators and connected at one end to the metalized pattern and at the other end to the output electrode pattern. The insulated wire may be implemented as an ICXL-PVC wire having a diameter of 0.32 millimeter, for example. An ICXL-PVC wire is a wire having a single conductor and a coating of vinyl chloride, as is well known in the art. Such an insulated wire has the following effect in the electrical aspect.
The ICXL-PVC wire is connected to the output electrode pattern and spaced apart from the dielectric resonators of the dielectric filter by a predetermined distance. Since the dielectric resonators serve as .lambda./4 semicoaxial resonators, the electric field is most intensive at their open end. A certain capacitance exists between the dielectric resonators and the ICXL-PVC wire which is spaced apart from the open end of the dielectric resonators, setting up a capacitive coupling. In this kind of dielectric filter, therefore, a parallel resonance circuit is completed by the coupling capacitance between the ICXL-PVC wire and the dielectric resonators, self-inductances of the ICXL-PVC wire, coupling capacitance between the input electrode pattern and the dielectric resonator, coupling capacitance between the dielectric resonators themselves, and coupling capacitance between the dielectric resonators and the output electrode pattern. The resonance frequency of the parallel resonance circuit is the zero transmission point, i.e., infinite attenuation point or attenuation pole. The parallel resonance circuit made up of the .lambda./4 semicoaxial resonators defines a pass band.
The prior art dielectric filter having the above construction has some problems left unsolved. Specifically, the use of an ICXL-PVC wire for achieving an attenuation pole makes it difficult to tune the attenuation pole to a predetermined frequency range. While the ICXL-PVC wire has to be surely fixed to the dielectric block in order to set up an accurate attenuation pole, the fixation is not easy and, therefore, the reliability of operation is not satisfactory. This, coupled with the poor tunability of the pole, adds to the cost involved in the fabrication of a high performance polar dielectric filter.