1. Field of the Invention
The present invention relates to a filter in a millimeter wave band, and more particularly to a millimeter wave band filter to which the technology of a non-radiative dielectric waveguide (xe2x80x9cNRD guidexe2x80x9d) is applied.
2. Description of Prior Art
An NRD guide circuit has attracted attention as a transmission line for a micro wave band, particularly a millimeter wave band above 30 GHz, due to its small transmission loss in comparison with a microstrip line and due to its easiness in manufacturing the transmission line in comparison with prior waveguides.
The structure of a general prior NRD guide circuit is illustrated in FIG. 1. The NRD guide circuit has a structure that a dielectric line 10 through which an electromagnetic wave is transmitted is sandwiched between two parallel conductive plates 12a, 12b made from conductive metal. A space h of the two parallel plates 12a, 12b is less than half a free space wavelength of a using frequency. Accordingly, the electromagnetic wave is blocked in places other than the dielectric line 10 and its radiation is restricted, so that the NRD guide circuit can transmit the electromagnetic wave along the dielectric line 10 at a small loss. Paying attention to such transmission superiority of the NRD guide circuit, there have been proposed NRD guide filters of the 35 GHz and 50 GHz bands.
FIGS. 2 and 3 illustrate the structure of a prior air gap coupled filter using an NRD guide. The prior air gap coupled filter has a structure that multi-staged dielectric blocks are sandwiched between the parallel conductive plates 12a, 12b. One dielectric line is cut into multiple dielectric blocks with proper lengths. The dielectric blocks 14axcx9c14e are straight aligned, with maintaining certain gaps therebetween, in the direction to which a signal proceeds and is air gap coupled with dielectric lines 10a, 10b on input and output sides, respectively. Each of the dielectric blocks operates as a dielectric resonator at each stage of the filter. The number of the dielectric resonator blocks is proportional to an order number of the filter. The air gap coupled filter shown in FIG. 2 is the fifth order filter because it has five dielectric resonator blocks 14axcx9c14e. 
The typical raw material for the dielectric line of the NRD guide which is applicable to millimeter waves is teflon. Teflon has an advantage that transmission loss is small whereas it has such disadvantages arising from its material characteristic that its processing is difficult due to its weakness and that its assembly is difficult because it does not easily adhere to other materials like metal. These disadvantages are the reason why the NRD guide has not been commercially used since the first introduction by Professor Yoneyama in the early 1980""s.
Since the using frequency is as high as the millimeter wave band, a wavelength of the electromagnetic wave transmitted along the dielectric resonator blocks in the waveguide, i.e., within the parallel conductive plates, is very short. The characteristic of the filter, in this case, is sensitively changed in accordance with the physical dimensions of structural bodies and fixtures for setting the resonator. Thus, it is necessary not only that a length of each of the dielectric resonator blocks 14axcx9c14e should be so accurately calculated as to be resonated at a certain frequency within a passing band, but also that each of the dielectric resonator blocks should be made as precisely as a predetermined length to obtain a wanted characteristic of the filter.
Further, each of the multi-staged dielectric resonator blocks 14axcx9c14e should be spaced to maintain a proper gap apart from its adjacent dielectric resonator blocks. This gap should be determined to obtain an optimal impedance matching between the two adjacent resonator blocks. That is, in order to obtain a good characteristic of a designed filter, there should be a precision of several microns not only in the length of each of the dielectric resonator blocks 14axcx9c14e but also in the distance between the resonators.
However, in manufacturing the prior air gap coupled filter using the NRD guide, it is difficult to make the dielectric resonator blocks 14axcx9c14e have such a precision. And also, with maintaining the precision of several microns, it is difficult to align the dielectric resonator blocks 14axcx9c14e which have different lengths in a straight line in the direction that a wave proceeds. In doing so, a lot of time and labor are required. Due to these reasons, the prior air gap coupled filter is a disadvantageous structure in terms of making, assembly and production, and is not suitable for a commercial model which is applicable to a high frequency in the millimeter wave band.
That is, in the case of the prior NRD guide air gap coupled filter, the resonator of each stage exists as a single independent block having a different length from one another, and has a structure that an impedance of each stage is controlled by adjusting a distance between each resonator. In this kind of structure, it is difficult to precisely make the dielectric resonators and to align the independent block of each stage in the right position within the filter housing with maintaining a predetermined distance.
In order to improve the above problems, an object of the present invention is to provide a metal post filter assembly, using an NRD guide, which is designed for an easy making and a good productivity resulting from a convenient and accurate assembly and is capable of stably having filter characteristics to a wanted degree.
To accomplish the object of the present invention, there is provided a metal post filter assembly using a non-radiative dielectric waveguide, comprising: a filter housing which includes parallel conductive plates facing each other; and a filter, disposed between said parallel conductive plates, for filtering a certain frequency band of a traveling electromagnetic wave, said filter including, a single body type dielectric line, made from a non-radiative dielectric, whose side surface is formed with a plurality of inserting holes running parallel to said parallel conductive plates, said dielectric line being segmented into multi-stages by one or more sets of said inserting holes which are spaced apart by a predetermined distance along a length direction of said dielectric line, and the number of the sets corresponding to a filtering order of said filter assembly, and a plurality of metal posts, each of which having a diameter to be fittingly inserted in each of said inserting holes.
The filter is formed with multi-staged dielectric resonators cascaded as a single body and segmented by said metal posts, said multi-staged dielectric resonators providing a filtering function which selectively allows only the certain frequency band of the traveling electromagnetic wave to pass therethrough by a predetermined impedance coupling relationship. It is particularly preferable that an impedance of said multi-staged dielectric resonators is largest in a middle stage and becomes gradually and symmetrically smaller to both end stages. Further, it is preferable that a length of said multi-staged dielectric resonators is longest in a middle stage and becomes gradually and symmetrically shorter to both end stages.
According to one preferred embodiment of the filter, one inserting hole is formed per each stage on the side surface of said dielectric line, said inserting hole in each stage having a diameter which is largest in the middle stage and becomes gradually and symmetrically smaller to both end stages and being disposed along an approximately half-height of said side surface in line.
According to another preferred embodiment of the filter, two inserting holes whose diameters are identical are disposed, per each stage, above and below an approximately half-height point of said side surface of said dielectric line and a vertical distance of said two inserting holes in each stage is narrowest in the middle stage and becomes gradually and symmetrically wider to both end stages.
According to a third preferred embodiment of the filter, one or more inserting holes are formed per each stage on said dielectric line, the number of, and diameters of, the inserting holes in each stage being determined in the manner that a reflection rate in each stage against the electromagnetic wave traveling through said dielectric line is highest in the middle stage and becomes gradually and symmetrically lower to both end stages.
The metal window filter assembly further comprises a plurality of tuning screws inserted, parallel to said metal posts toward said dielectric line, through both side walls of said filter housing, for tuning a resonance frequency of the filter by adjusting insertion lengths of said tuning screws.
According to the metal post filter of the present invention, the filter can be made by forming the inserting holes in the dielectric line made from a material which is difficult for being processed, and the dielectric resonators having the desired number of stages can be made as a single body by inserting the metal posts in the inserting holes. And also, the filter assembly is easily assembled by inserting the filter between the upper and lower conductive plates of the filter housing.
Therefore, the filter assembly according to the present invention can remarkably maximize production efficiency and reduce manufacturing costs due to its simple structure and superiority in the processing and assembling thereof. Further, since the filter assembly of the present invention is designed to minimize the factors of error occurrence during its assembly, it has advantages of maintaining the precision of the processing machine for the filter structure of the millimeter wave band which requires the precision of several microns and of accurately maintaining the characteristic of the filter to the designed level as it can be assembled without an extra auxiliary zig.
Other characteristics and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the accompanying drawings.