Patent Publication Number: US-4320368-A

Title: Filter for microwaves

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a filter for microwaves, and more particularly to a microwave filter of the capacitive coupling type having a plurality of resonator cavities. 
     Conventional microwave filters have a construction wherein resonator rods are fixed to the respective resonator cavities by the use of screws or the like. This has led to the disadvantage that, when they are subjected to vibrations or impact, the resonance characteristics may vary. The applicant of the present application filed on July 27, 1979 a U.S. patent application Ser. No. 61,415, now U.S. Pat. No. 4,275,369, which described a microwave filter which has solved the aforecited disadvantage. The present invention concerns further improvements in such microwave filters. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a microwave filter wherein the resonant frequency thereof can be trimmed readily. 
     Another object of the present invention is to provide a microwave filter which is immune against vibrations and shocks. 
     Still another object of the present invention is to provide a capacitive coupling type microwave filter which is structurally simple and is accordingly inexpensive. 
     Further objects and advantages of the present invention will become apparent from the following description taken with reference to the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of an embodiment of a microwave filter according to the present invention. 
     FIG. 2 is a side view of the embodiment. 
     FIG. 3 is a sectional view taken along line III--III in FIG. 1. 
     FIG. 4 is an exploded perspective view of parts of the microwave filter according to the present invention, for explaining the relationship among a trimming cap, a cover and a resonator rod. 
     FIG. 5 is a sectional view taken along line V--V in FIG. 3. 
    
    
     PREFERRED EMBODIMENT OF THE INVENTION 
     Hereunder, an embodiment illustrated in FIGS. 1 to 5 will be described. A housing 1 made of metal is formed with a plurality of resonator cavities 10, 11 and 12 which are separated by a plurality of partition walls 3 and 3. The resonator cavities 10, 11 and 12 are provided with respective resonator rods 2 of λ/4 wavelength in a manner to extend from the bottom of the housing 1. Over the partition walls 3 and 3, there are formed coupling windows 3a and 3a for capacitively coupling the respectively adjacent resonator cavities. The upside of the housing 1 has an open end, to which a cover 4 made of metal is attached. The cover 4 is formed with inserting holes 4a each opposing a respective resonator rod 2 and these holes are each adapted to receive a respective trimming cap 6 made of a dielectric substance. A respective mounting slot 4b is provided in proximity to each inserting hole 4a, and deformable bridge pieces 5 are thus formed between each hole 4a and its associated slot 4b. 
     As best seen from FIG. 4, notches 4c are formed in suitable portions of the border area of each inserting hole 4a. These notches 4c serve to receive a respective trimming cap 6 made of a dielectric substance such as polypropylene. The trimming caps each include a longitudinal recess 6a into which a respective resonator rod 2 may be snugly fitted; a plurality of projections 6b extending within the recess 6a and which are brought into resilient contact with the resonator rod 2; and serrations 6c which are formed in the outer periphery of the body of the cap. In mounting the trimming cap 6 onto the mounting plate or cover 4, the trimming cap 6 is inserted through the cap inserting hole 4a of the mounting plate 4 assembled on the housing 1, and the recess 6a of the trimming cap 6 is subsequently brought into tight engagement with the resonator rod 2 so as to embrace the resonator rod 2 by means of the projections 6b. 
     Therefore, the trimming cap 6 can be moved up and down (in the directions of arrows A and B of FIG. 3) with the resonator rod 2 as a guide. The upward or downward movement of the trimming cap 6 results in varying the dielectric constants between the resonator rod 2 and the surrounding members (housing 1, partition wall 3), whereby the trimmings of the resonant frequency and the characteristic impedance can be carried out. 
     After the frequency has been trimmed through the upward and downward movement of a trimming cap 6, a jig is inserted into the adjacent mounting hole 4b of the mounting plate 4, and the bridge piece 5 is deformed so as to push it against the side of the trimming cap 6. Then, the serrations 6c of the trimming cap 6 are pressed against the peripheral wall of the inserting hole 4a, and tips 4d (refer to FIG. 4) of the notches 4c provided in the inserting hole 4a bite into the serrations 6c. Thus, the installation of the trimming cap 6 on the mounting plate 4 is complete. 
     Numeral 7 designates a signal coupling loop which is disposed within the housing 1. A printed circuit board 8 is attached to the housing 1, and external lead terminals 9 are mounted on the latter. Shown at 4e are adjusting holes which are provided in the mounting plate 4 and which are used for adjusting the signal coupling loop 7. 
     As described above, according to the present invention, a trimming cap 6 can be passed through the inserting hole 4a of the cover or mounting plate 4, and the recess 6a of the trimming cap 6 is brought into engagement with the resonator rod 2. Therefore, the job of assembling the trimming cap 6 is easy, so productivity is high, and the trimming cap 6 can be moved up and down with the resonator rod 2 as the guide, so the trimming of the resonant frequency is facilitated. In addition, the trimming cap 6 is fixed in such a manner that one end part thereof embraces the resonator rod 2, while the other end part thereof is held against the inserting hole 4a of the mounting plate 4 by means of the bridge piece 5. Therefore, the trimming cap 6 is firmly supported, and a stable electric performance is ensured even against external vibrations and the like. 
     With the construction as in the foregoing embodiment wherein the serrations 6c are provided in the outer periphery of the trimming cap 6 and the notches 4c  are provided in the appropriate portions of the inserting hole 4a of the mounting plate 4, the tips 4d of the notches 4c bite into the serrations 6c when the bridge piece 5 is deformed, so that the support of the trimming cap 6 can be secured still more. Further, with the construction wherein the resonator rod 2 is embraced by means of the projections 6b of the trimming cap 6, the external vibrations and the like can be absorbed by these projections 6b, so that an enhancement in the electric performance can be achieved. 
     Although, in the above embodiment, the trimming cap 6 has been explained as embracing the resonator rod 2 by means of the projections 6b, it is also possible to remove the projections 6b and embrace the resonator rod 2 directly by means of the inner wall of the recess 6a of the trimming cap 6. Needless to say, the shape of the trimming cap 6 is not restricted to the column as illustrated, but it can be put into various shapes including a cylinder, a hexagonal prism, an octagonal prism or the like and still be within the scope of the present invention set forth appended claims.