1. Field of the Invention
The present invention relates to a dielectric filter which is stable mechanically and electrically.
2. Description of the Prior Art
In mobile radio communication systems such as land mobile radio communication systems and the like, the frequency used has been raised from the VHF band to the UHF band and then to the microwave band so as to meet the demand for a greater number of channels. Mobile radio communication systems call for small-sized, lightweight and economical radio equipment. To satisfy the requirements, it has been proposed to employ a dielectric filter as a filter or antenna duplexer for separating transmitting and receiving waves of vehicular radio equipment, for example, for a 800 MHz band land mobile radio communication system, so that an antenna may be used in common.
The aforesaid dielectric filter is a multi-section filter which is produced by metalizing required surfaces of cylindrical dielectric rods of different lengths to form coaxial and/or re-entrant resonators and arranging them in a metal case in predetermined positions. Assuming that the dielectric constant of the dielectric material used is 40, the size of the dielectric filter can be made about 1/.sqroot.40 that of an ordinary waveguide type filter (a coaxial and/or re-entrant filter which does not use the dielectric material).
Since vehicular radio equipment is exposed to severe environmental conditions as of vibration, shock, temperature, humidity and so forth, however, the dielectric filter is also required to stably operate under such conditions; accordingly, in the multi-section filter, the resonators must be housed in a rigid metal case. This is an obstacle to mass production of the dielectric filter and reduction of its size and weight.