CA1292785C - Dual mode dielectric resonator filters without iris - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A microwave band pass filter including dual-mode dielectric resonators mounted in a tubular enclosure to achieve coupling among the resonators without an iris. The filter is implemented in a canonical symmetric form, a longitudinal dual-mode realization or a canonical asymmetric form. The microwave band pass filter has input and output coaxial probes located along the enclosure with tuning and coupling screws provided to enable adjustment control of the frequency of resonance of the dielectric resonators and to control the coupling of energy from one resonant mode to an orthogonal mode in the same resonator. Lastly, the coupling of energy from one resonator to an adjacent resonator is accomplished by properly placed coupling screws.

Description

- lZ9Z785 TITLE OF THE INVENTION
DUAL MODE DIELECTRIC RESONATOR ~ILTERS WITHOUT IRIS
BACKGROUND OF THE INVENTION

Field of the Invention:
The.present.invention is related to dual hybrid mode dielectric resonator band pass filters. More particularly, the present invention concerns filter realizations in tubular enclosures which are suitable for use in broadcast receivers, phased array radar applications and other applications requiring large quantity microwave narrow band pass filters.

Discussion of Background:
The use of low pass, high pass and band pass filters in microwave systems is well known and is used to achleve results simllar to the use of such filters at low frequencies to separate frequency components of a complex wave.
Early attempts at providing waveguide type of filters involve the utilization of the lumped-circuit method of cascading several filter sections together which was copied in the sense that microwave filter sections were cascaded with the spacing between the sections being any odd number of quarter wavelengths.

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lZ9Z785 The theory being that the greater the number ofcavities used, the flatter the pass band and the skirts of the pagg batld.~become gteeper. A9 a practlcal matter, however, the insertion loss in the pass band increases with the number of resonators.
Recent developments with respect to dual-mode band pass filters as in the article entitled "Narrow Band Pass Waveguide Filters" by Atia and Williams, IEEE
Transactions on Microwave Theory and Techniques, Vol.
MTT-20, pages 258-265, April 1974 and "Dual-Mode Canonical Waveguide Filters" by Williams and Atia, IEEE
Transactions on Microwave Theory and Techniques, Vol.
MTT-25, pages 1021-1025, December 1977, and U.S. Patent No. 3,969,692, July 13, 1976, "Generalized Waveguide ~and Pass Filters", and U.S. Patent No. 4,060,779, November 29, 1977, "Canonical Dual Mode Filters"
possess slgnificant performance advantages over the above discussed conventional waveguide realizations which are detailed for example in "Microwave Filters, Impedance Matching Networks and Coupling Structures" by Matthaei, Young and Jones, New York: McGraw-Hill, 1965. These advantages of the dual mode band pass filters are especially signiEicant in applications where the mass and the volume are critical. Other dramatic reductions in the filter size and the mass are achieved by using dielectric loading of the cavities - - - - - - - - -r ~ _3_ lZ9Z785 with high-dielectric~ low-1osg temperature stable materials as reflected in the article by Fiedziusko entitled "Dual-Mode Dielectric Resonator Loaded Cavity Filters", IEEE Transactiong on Microwave Theory and Techniques, Vol. MTT-30, pages 1311-1316, September 1982.
Whether the filters are air-filled or dielectric-loaded dual-mode filters each of these type of structures in the prior art required that physically adjacent resonators be coupled to each other through iris slots or holes. These iris slots or holes required an extremely high degree of precision to provide the required accuracy for achievement of an exact filter response. Therefore, between each resonator, there was required a iris which had to be machined and silver plated which naturally led to major cost in producing to such extreme tolerances.
Thereore, in view of the high cost the utilization of these filters is restricted to applications where the performance, mass and size are extremely critlcal factors, as for example communication satellites. Normally their use was precluded in areas where cost i9 the major factor as where there are an extremely large number of filters to be used in, for example, phased arrays.

lZ~Z7t~5 A filter such as hybrid dual-mode dielectric resonators may comprise dielectric resonators separated by circular irises. The realization of cascade couplings produced by the iris separation of resonators presents the above-discussed difficulties concerning the manufacture of these iris elements and the extreme accuracy with which they must be manufactured. Thus, although dual mode dielectric resonator filters, which are extremely light and extremely space conservative, are available from a practical standpoint, the cost of manufacture prohibits the use in most large quantity microwave narrow band pass filter applications.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a realization of the most general form of multiple coupled cavity transfer functions in dual mode dielectric resonators without using an iris.
It is a further object of the present invention to provide a microwave band pas~ filter consisting of high dielectric constant ceramic cylindrical disks having input and output provided by coaxial probes and wherein adjustment and control of the frequency of the resonators is accomplish-ed by tuning and coupling screws.
It is a further object of the present invention to provide a microwave band pass filter whereby the need for expensive machined parts requiring tight tolerances is elimi-nated in a configuration for dual-mode dielectric resonators in simple tubular enclosures.

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It is a further object of the present invention to provide a microwave band pass filter which achieves lower mid-band insertion losses than comparable filters having irises by eliminating conduction currents on the metallic cavity ends.
It is a further object of the present invention to provide a canonical form microwave band pass filter having a general band pass transfer function which is realized by multiple coupled cavity structure and which contains a minimum number of coupling elements without the use of an iris wherein proper cascade coupling values between two adjacent resonators excited in the hybrid modes are obtained by adjusting the spacing between the resonators.
It is also an object of the present invention to provide a canonical dual-mode filter without coupling holes or irises in which the coupling between the input and output cavities is achieved by the orientation of a coupling screw which creates, by its orientation, two additional trans-mission zeros in the stop band of the filter which increases the filter's selectivity.
It is a further object of the present invention to maximize the out-of-band isolation achievable with dual-mode canonical band pass filter by utilizing an asymmetric coupling structure or by maximizing the number of realizable finite transmission zeros as is possible with longitudinal dual-mode filters.

-"` lZ~Z~785 It is a further object to realize the utilization of longitudinal dual-mode filters by providing a structure whereby unequal couplings between any two corresponding modes of adjacent dual-mode resonators is provided.
In accordance with the invention there is provided a dual hybrid mode dielectric resonator band pass filter, comprising a tubular enclosure including a plurality of cascade coupled ceramic dielectric-loaded disk resonators, wherein each of said resonators is spaced from each other and coaxially supported in said tubular enclosurei means for exciting said resonators in said tubular enclosure so as to provide dual hybrid modes, said means for exciting including first and second probes fixed to and penetrating into said enclosure for providing input and output ports, coupling means for providing cross coupling of two orthogo-nal modes of each resonator, wherein said probes couple the radial electric fields of said two orthogonal modes of said resonators and wherein the depth of penetration into said enclosure and the thickness of said probe is proportional to the amount of coupling of said dual modes; and wherein the cascade coupling of said cascade coupled ceramic resona-tors between any two resonators is determined by the spacing between said any two disk resonators.
In accordance with a further broad embodiment of the invention there is provided a microwave band pass filter comprising a plurality of dielectric ceramic disks resona-tors coaxially placed in a cylindrical metallic tube; first lZ92'785 and second coaxial connec~ors having center conductors extending inside of said metallic tube wherein said coaxial connectors serve as input and output ports of said filter;
a first set of tuning screws provided to adjust the resonant frequency of one set of resonant modesi a second set of tuning screws provided to adjust the resonant frequencies of an orthogonal set of resonant modes of said dielectric resonators; a third set of screws placed midway between adjacent resonators in order to couple the energy between the resonant modes in one di.rection; a fourth set of screws placed midway between adjacent resonators for controlling the coupling of energy between the resonant modes in the orthogonal set of resonant modes; and wherein said input and output ports are associated with different ones of said resonators.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
FIGURE 1 is a schematic illustration of a canonical form of an equivalent filter circuit of n=2m coupled cavities;
FIGURE 2 is a schematic perspective view of a realiza-tion of the canonical form of the filter of Figure 1 using dielectric-loaded resonators excited in hybrid (HEHll) mode5 with coupling holes/iris;

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FIGURE 3(a) is a perspective view of a canonical dual-mode dielectric resonator filter without iris according to the present invention, and FIGURE 3(b) is an end view of the filter of Figure 3(a) additionally showing alternative orientations of coupling screw Mln with respect to the connectors (input/output ports);
FIGURE 4(a) is a schematic perspective view of a longi-tudinal dual-mode dielectric resonator filter without iris according to another embodiment of the present invention, and FIGURE 4(b) and 4(c) respectively show a side view and an end view of the coupling adjustment between two hybrid mode dielectric resonators;
FIGURE 5 is a graph showing the measured insertion loss response of two separate 4-pole filters in accordance with the Figure 3 embodiment;
FIGURE 6 is a graph showing the measured and computed insertion loss response of the longitudinal filter of Figure 4(a)-(c);
FIGURE 7 is a perspective view showing an alternate embodiment of the filter of the present invention in the form of a canonical nonsymmetric dual-mode dielectric resonator filter without iris; and FIGURE 8 is a graph showing the computed and measured coupling between two resonators for the hybrid HEHll mode.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
When filters such as hybrid dual mode dielectric resonators are configured, the most general band pass lZ9Z~5 g transfer function which is realizable utilizes a multiple coupled cavity structure which can be reduced to a canonical form containing the minimum number of coupling elements.
Figure 1 shows an equivalent circuit of a canonical form which consists of a number of identical resonant circuits 10 coupled in cascade by frequency invariant coupling i, i+l, i=l, 2, . m having the same sign. Each resonant circuit 10 in one half is coupled to the corres-ponding circuit in the other half by means of a specified sign cross coupling element Mi n 1 i 1 2 m-When the dielectric loaded resonators excited in hybridmode (HEHll) are used in the canonical form the result is shown in Figure 2. The hybrid mode characteristics are discussed in Applicant's article entitled "New Results in Dielectric Loaded Resonators", IEEE Transactions on Micro-wave Theory and Techniques, Vol. MTT-34, No. 7, July 1986, pages 815-824. The realization of Figure 2 is similar to the realization of a circular waveguide form excited in TElll modes described in the above-referred to article, "Dual-Mode Canonical Waveguide Filters", 1977 and U.S. Patent 4,060,779 (Nov. 1977). The cascade couplings of Figure 1 are provided in Figure 2 by the circular ir~s 20 separating each dielectric resonator 25. The coupling screws 27 are located at a 45 angle to the direction of ~he degenerate dual modes and provide cross couplings. The relative signs of any two cross couplings are determined by the relative directions of the corresponding coupling screws with the same sign being dictated by parallel screws and opposite . ~ ' 1~ 785 - 9a -signs being dictated by perpendicular screws. Although not shown, it is a feature of the dual-mode structure, whether air-filled or dielectric, that there are two tuning screws associated with each resonator in order to adjust the resonant frequency of each set of orthogonal modes. This is discussed in the above-discussed April 1974 and December 1977 articles by Atia and Williams.
As above-mentioned, the realization of cascade couplings produced by the iris separation of the resonators in Figure 7 presents the above-discussed difficulties concerning the manufacture of these iris elements and the extreme accuracy with which they must be manufactured.
Referring now to Figure 3(a), there is shown a dual-mode dielectric resonator in tubular enclosure 6 which is a reduction of the most general band pass transfer function realizable by a multiple coupled cavity structure. The reduced system shown in Figure 3(a) is a canonical form containing the minimum number of coupling elements which has no iris. The proper cascade coupling values between any two adjacent dual ~2~;~7~5 mode resonators 25 excited in the hybrid modes is obtained by adjusting the spacing S (Sl, S2, S3) between the dielectric ceramic disc resonators 25. The input and output ports shown by the connectors 28 and 29 are located in the same physical cavity and can be realized by having the coaxial probes couple the radial electric fields of each of the two orthogonal dual modes of the resonators as shown in FIGURE 3a and in FIGURE 3b. The amount of coupling (or external QJ of the two orthogonal modes is controlled by the depth of penetration and by the thickness (diameter) of the probe as shown in FIGVRE 3b with the probes being shown attached to the connectors 28 and 29 in an orthogonal position. The maximum isolation achievable between the input and output ports is in this case only limited by the ability to maintain the probes mechanically at right angles with respect to each other and partially by spurious mode couplings. The maximum isolation has been shown to have a top limlt of approximately 30 dB, which can be achieved by the two orthogonal probe coupling mechanism.
As discussed previously, the center frequency of a filter of the prior art which use an iris as in FIGURE
2, is to a first order determined by the resonant frequency of the dielectric resonators and to a lesser extent by the metallic boundary of the cylindrical tube -11- lZ927~5 .

and the end planes containing the iri~ 20 of FIGURE
2. If each iris is completely removed, as is accomplished in the FIGURE 3a embodiment of the present invention, the resonant frequencies of the dielectric resonators will be slightly changed, and the couplings between the two corresponding pairs of hybrid modes existing in two adjacent resonators will be equal because of the circular symmetry. The value of these couplings are determined by the aforementioned spacing Sl, S2, S3 between the resonators. The configuration whereby the input and output ports are derived from two orthogonal modes in the same resonator provides the realization of the symmetrical canonical form of FIGURE
3a.
The screws 30 and 32 shown in the FIGURE 3a and more particularly in the FIGURE 3b, provide two embodiments for regulating the coupling between the input and output cavities 28 and 29. The orientation "A" shown by screw 30 provides two additional transmission zeros in the stop band of the filter.
These additional zeros ag well as of course any other zeros help to improve the selectivity of the filter.
On the other hand, the orientation "B" does not introduce these real frequency transmission zeros, and the filter formed by the screw 32 becomes less selective than the orientation shown by screw 30. The . ` -12- l~Z785 orientation of the screw 30, called the orientation "A", involves a location which is symmetrical at a 45 orientation with respect to the projections of the input and output coaxial probes 28 and 29 through the tubular enclosure. Although not shown in Figure 3(a), there are two tuning screws associated with each resonator, as is known in the art of dual mode resonators, with each screw adjusting the resonant frequency of one mode. Figure 3(b) illustrates a set of tuning screws 16, 17 for the first resonator.
FIGURE 5 illustrates the differences between the embodiment with a filter "B" formed by a coupling screw 32 when compared with a filter "A" formed by an orientation of a coupling screw 30. It can be seen quite clearly by way of examination of the insertion loss response as indicated in FIGURE 5 that the filter "A" uslng the screw 3~ has two additional zeros to provide an improved selectivity or the filter.
The position of the coupling screw 3~ or 32 for the first resonator determines that the second resonator have a coupling screw which ls 9~ from the location of the first resonator coupling screw as is shown in the Figure 2 coupling screw relationship between the consecutive resonators 25.
It should also be noted that instead of the configuration for the connectors 28 and 29 shown in -13- 1~,92'785 FIGURE 3a and 4a, a single coaxial probe port can be used with the other probe port being a dipole, a loop or waveguide slot, which couples to the magnetic field of the mode near the end wall of the resonator. Such a utilization of a dipole or a loop in conjunction with a coaxial probe port or a slot in conjunction with a waveguide port, can provide better isolation between the input and output ports than the two orthogonal probes of FIGURES 3a and 3b because it is less suscept~ble to spurious couplings. However, the use of a dipole or a loop is more difficult to realize than using two simple coaxial probes and is also more sensitive to dimensional tolerances.
The parameters which are required for the design of a filter configured of the dual-hybrid-mode dielectric resonators in the cylindrical tube as shown in EIGURES 3a and 3b, include the resonant frequency of the resonators in the tube, the coupling between two adjacent resonator, and the external Q of the probe.
The theoretical calculation o the resonant frequency can be performed using previous methods, as for example those described in "New Results in Dielectric Loaded Resonators" IEEE Transactions Microwave Theory Techniques Vol. MTT-34, pages 815-824, July 1986 by Zaki and Chen. The selection of the optimum resonator dimensions which result in the widest spurious-free -14- 1Z9~85 stop band can be made based on mode charts of the resonators as de~ailed in the above reerenced Zaki and Chen article. The variations of the ratio between the closest spurious modes to the desired mode of a resonator in an infinitely long waveguide with a diameter to length ratio shows that when both the desired mode frequency and the ratio of the diameter of the tube to the diameter of the dielectric were held constant, then the optimum ratio of the diameter of the dielectric to the length of the dielectric for the mode would be approximately 3.5, which results in a spurious-free region approximately 30% of the resonant frequency, From these techniques, the optimum diameter of the dielectric resonator is approximated by the ormula 2a=(c/fO)(2.24/~r)2 where c is the speed of light and fO equals the desired mode frequ.ency.
Coupling calculations between hybrid modes are described in the article entitled "Coupling of Non-Axially-Symmetric Hybrld Modes in Dielectric Resonators" IEEE Transactions Microwave Theory Techniques pages 1136-1142 December 1987. The computed and experimentally measured data show the variation of the coupllng coefficient between two resonators as a function of separation as graphically illustrated in FIGVRE 8.

-15- 12~Z~85 As indicated previously, the maximum out-of-band isolation achievable with the dual-mode canonical filter realizations described in conjunction with the FIGURES 3a and 3b is limited due to the incidental couplinq between the input and output ports 28 and 29 which always exist in the same cavity. Although it may be possible to improve this isolation by using a dipole, loop, or waveguide slot in one of the ports as previously discussed, such improvement involves complicating the structure. In order to achieve anywhere close to the theoretically possible isolation in the out-of-band insertion loss of the filters, the input and output ports must be located in two different physical resonators. Although this is not possible with the symmetrical canonical form described in conjunction with the embodiment of FIGURE 3, there are realizations which achieve the same response with asymmetric coupling structure as shown in FIGURE 7 or achieve the required isolation without the maximum number of realizable finite transmission zeros (e.g.
longitudinal dual-mode filters).
In order to realize s~ch type of filters, a way of provlding unequal coupling between any two corresponding modes of adjacent dual-mode resonators is required. Because of the structure described with respect to the canonical configuration of FIGURE 3, the lZ9Z785 . -16-couplings are always equal due to circular symmetry, a modification to the type of structure of FIGURE 3 is required in order to achieve this unequal coupling and therefore provide desirable realizations either having asymmetric coupling or required isolation without the maximum number of realizable finite transmission zeros.
The coupling configuration shown in FIGURE 4b consist of two dielectrlc resonators separated by a dlstance S. Screws for coupllng adjustments are placed midway between the resonators parallel to the maximum of the radial electric fields of the two hybrid -modes. By changing the penetration of these screws, the coupling between the two pairs of hybrid modes can be changed independently of each other. Thus, the coupling Mk k+3 between the two modes (k,k+3) can be changed by adjusting the penetration of screws A-A as shown in FIGURE 4c. This change of the screws A-A is made without effecting coupling between the modes (k+l, k+2). In a similar manner the coupling Mk+l k+2 between the two modes (k+l,k+2) can be adjusted by changing the penetration of the screws B-B without effecting the coupling Mk k+3. Thus unequal coupling between each of the two pairs of hybrid modes can be reallzed without the need for an iris. It is further noted that these couplings can be simply and -17- ~ 9Z~85 independently controlled by means of the coupling screws and it is important to note that the increase in the depth of penetration of the screws increases the corresponding coupling between the mode pair. Thus, in the design of filters, the spacing S from FIGURE 4b between the two resonators is chosen to correspond to a coupling value which is slightly less than the minimum required of the two couplings Mk,k~3 and Mk+l,k+2- The screws A-A and B-B can then be used to adjust the coupllng to achleve preclge deslred values for these unequal couplings.
A filter which employs the principles of FIGURES 4 and 4c is shown in FIGURE 4a which illustrates an eight-pole dual-mode longitudinal filter which can achieve two pairs of finlte transmission zeros.
The embodiment of FIGURE 4a which illustrates the longitudinal dual-mode filter consists of high dielectric constant ceramic cylindrical disks 11-14 placed inside a metalllc tubular enclosure 19 with the disk resonators 11-14 being coaxial and supported by foam ~upports, inside the tube. Coaxial connectors 31 and 32 with their center conductors extend inside the enclosure 19 and these connectors 31 and 32 serve as input and output ports of the filter. The tuning screws 41-44 are provided to adjust the resonant frequencies of one set of resonant modes, while the other set of tuning screws 45-48 are provided to adjust the resonant frequencies of the orthogonal set o~
resonant modes of the dielectric resonators 11-14. The screws 51-53 which are placed midway between adjacent resonators serve to control the coupling of energy between the resonant modes in one direction, while the set of screws 54-56 serve the same function for the orthogonal set of resonant modes.
In order to design the filter of FIGURE 4a, the dimensiong of the dlelectrlc resonators are determlned so that the resonant frequency is the HEHll mode which corresponds to the desired center frequency of the filter with the other spurious modes separated as far as possible, as discussed previou~ly. The distances between each of the dlsk resonators 11-14 are computed so as to yield couplings which are slightly less than a imum ~M14, M23), the minimum (M36, M45) and the minimum (M58, M67) respectively. The rest of the coupllng matrix elements, i-e- M12~ M34~ Ms6 and M78 are realized by means of 45 coupling screws approximately located in the planes bisecting the lengths of the corresponding resonators. These coupling ~crews are not shown ln Figure 4(a) for the sake of simplicity but are similar to those coupling screws in the canonical embodiment of Figure 3. The subscripts for the couplings are determined in -19- 1;Z9Z'78S

accordance with the Eormula for cross coupling of PIGURE 4b and the respectlve labeling in FIGURE 4a concerning each of the eight pole conEigurations.
The eight pole filter which is shown in PIGURE 7 can realize the optimum transfer function available by the symmetric canonical form of FIGURE 3. However, the advantage of the form of PIGURE 7 iS that this realization allows the input and output ports to be located in two different resonators thereby eliminating the limitation imposed on the maximum out-of-band isolation which exist in the canonical form. The synthesis procedure for developing this type of filter is similar to the procedure with regard to the dual-mode longitudinal filter. The figure has the resonant modes labeled with their coupling and the connectors 78 and 79 associated with two different resonators 25.
As a validation of the above embodiments, three experimental four-pole elliptical function filters were designed, constructed and tested in accordance with the parameters of Table I.

-- -20- 1~9Z7~5 TA~LE I
Filter Parameters Parameter Canonical Fllter Longltudinal Filter Center Frequency (CHz) 3.9145 3-9Z~
~andwidth (MHz) 21.0 47.0 Normalized input impedance R ~ 1 . 3 1 150 Normallzed output impedance R2 13 1 150 0 .9~ 0 -.21 0. .86 0 -0.26 .98 0 .84 0 .86 0. .80 0 Coupling Matrlx M 0 .84 0 .98 0 .80 .0 .86 .-.21 0 .98 0 -0.26 0 .86 0 Two of the three experimental filters were of the canonical dual-mode type having a M14 for coupling screw located in accordance with the orientation "A" or orientation "B" as prevlougly discussed with respect to the EIGURES 3a and 3b. The distance S between adjacent resonators determines the coupllngs M12 and M34 (which are equal, as indicated by the coupling matrix M of Table I). The couplings M23 are realized by screws located at 90 angles from the respective M14 screws.
The measured insertion loss responses of the two filters either A or B over a wide frequency band are illustrated at FIGURE S, as discussed previously.

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The Coupllng Matrlx associated with the Canonical filter uses the convention established in Figure 3(a) wherein the Matrix for the Longitudinal filter use the convention shown in ~igure 4(a).
The third Eilter which was designed has input and output ports in separate resonators (i.e. a longitudinal type). The computed and the measured insertion loss response of the longitudinal filter are shown in FIGURE 6 wherein the improvement in the out-of-band isolatlon is due to location of the input and output ports at two different resonators.
Thus, whether implemented in the form of the longitudinal dual mode of FIGURE 4a or the canonical symmetric form of FIGURE 3a or the asymmetric canonical form of FIGURE ?, there ls disclosed a microwave band pass filter consisting of high dielectric constant ceramic cylindrical disks placed inside a metallic tubular enclosure which provides a realizable form of the most general form of multiple coupled cavity transfer functions without using an iris in order to drastically reduce the cost of the production of the filters and to open up these type of dual mode dielectric resonators for use in microwave filter applications for direct broadcast receivers, phased array radar applications and any of a number of other large quantity microwave narrow band pass filter applications which require high quality, miniaturi--22- 1~2'~85 zat~on and low cost.
Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practlced otherwise than as specifically described herein.

Claims (9)

1. A dual hybrid mode dielectric resonator band pass filter, comprising:
a tubular enclosure including a plurality of cascade coupled ceramic dielectric-loaded disk resonators, wherein each of said resonators is spaced from each other and coaxially supported in said tubular enclosure:
means for exciting said resonators in said tubular enclosure so as to provide dual hybrid modes, said means for exciting including first and second probes fixed to and penetrating into said enclosure for providing input and output ports, coupling means for providing cross coupling of two orthogonal modes of each resonator, wherein said probes couple the radial electric fields of said two orthogonal modes of said resonators and wherein the depth of penetration into said enclosure and the thickness of said probe is proportional to the amount of coupling of said dual modes: and wherein the cascade coupling of said cascade coupled ceramic resonators between any two resonators is determined by the spacing between said any two disk resonators.

2. The filter according to Claim 1, wherein said first and second probes are positioned 90° from each other on said enclosure and wherein both said probes are located on the radial projection of one of said disk resonators.

3. The filter according to Claim 2, wherein said coupling means comprises:
a coupling screw positioned on the radial extension of said one of said disks at a symmetric 45°
angle with respect to the projections of the penetration through said enclosure of each of said first and second probes.

4. The filter according to Claim 1, wherein said coupling means comprises:
coupling screw means including a coupling screw associated with each of said resonators and positioned on said tubular enclosure in such a manner that a coupling screw associated with any one of said resonators is positioned on said tubular enclosure 90°
away from the coupling screw associated with an adjacent resonator.

5. The filter according to Claim 4, further comprising:
a first and second set of tuning screws wherein one of said first set and one of said second set of tuning screws is associated with each of said resonators in order to adjust both resonant modes of each of said resonators.

6. The filter according to Claim 1, wherein:
said first probe and said second probe are each associated with different ones of said plurality of disk resonators; and said coupling means includes means for controlling the coupling of energy between each of the resonant modes in both orthogonal directions, wherein said means for coupling includes a first series of screw means each placed midway between adjacent resonators in order to control the coupling of energy between the resonant modes in one direction and a second series of screw means wherein each of said screw means of said second series of screw means is placed midway between the adjacent resonators in order to control the coupling of energy between the orthogonal set of the resonant modes.

7. The filter according to Claim 6, wherein each of said first series and said second series of screw means includes a set of two screws positioned opposite each other on said tubular enclosure.

8. The filter according to Claim 1 further comprising:
means for providing unequal couplings between any two corresponding modes of adjacent dual-mode resonators.

9. A microwave band pass filter comprising:
a plurality of dielectric ceramic disks resonators coaxially placed in a cylindrical metallic tube;
first and second coaxial connectors having center conductors extending inside of said metallic tube wherein said coaxial connectors serve as input and output ports of said filter;
a first set of tuning screws provided to adjust the resonant frequency of one set of resonant modes;
a second set of tuning screws provided to adjust the resonant frequencies of an orthogonal set of resonant modes of said dielectric resonators;
a third set of screws placed midway between adjacent resonators in order to couple the energy between the resonant modes in one direction;
a fourth set of screws placed midway between adjacent resonators for controlling the coupling of energy between the resonant modes in the orthogonal set of resonant modes; and wherein said input and output ports are associated with different ones of said resonators.