EP1962371A1

EP1962371A1 - Dielectric multimode resonator

Info

Publication number: EP1962371A1
Application number: EP07003579A
Authority: EP
Inventors: Michael Höft
Original assignee: Panasonic Corp; Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Intellectual Property Management Co Ltd
Priority date: 2007-02-21
Filing date: 2007-02-21
Publication date: 2008-08-27

Abstract

The present invention relates to a dielectric multimode resonator that comprises walls (2a, 2b, 2c) enclosing a resonator cavity (3), and a resonator element (4) made of dielectric material and disposed in the resonator cavity (3). The resonator element (4) comprises a plurality of interconnected elongate portions (5), each extending longitudinally from a connection to a wall (2a, 2b, 2c) to an adjacent branching region (6) from which the elongate portion (5) and at least two further of the elongate portions (5) extend longitudinally in different directions or between two adjacent such branching regions (6). At least a section of each elongate portion (5) between its two longitudinal ends (5a, 5b) is spaced from the walls (2a, 2b, 2c) and from connections to the walls (2a, 2b, 2c). The elongate portions (5) are only joined at longitudinal ends (5b) thereof at the branching regions (6). Each two adjacent branching regions (6) are separated by an elongate portion (5) extending longitudinally between them. The plurality of elongate portions (5) includes a group of at least three interconnected elongate portions (5), wherein each of the two longitudinal ends (5a, 5b) of each elongate portion (5) included in this group is either connected to a wall (2a, 2b, 2c) or is directly joined to exactly one of the two longitudinal ends (5b) of each of exactly two of the other elongate portions (5) included in this group, so that there is at least one branching region (6) from which exactly three elongate portions (5) extend in three different directions.

Description

The present invention relates to a dielectric multimode resonator comprising walls enclosing a resonator cavity, and a resonator element made of dielectric material and disposed in the resonator cavity, wherein the resonator element comprises a plurality of at least three interconnected elongate portions. The invention further relates to a microwave filter comprising at least one of such dielectric multimode resonators.
Dielectric resonators are commonly used as basic components of microwave filters which are e.g. utilized in various devices, such as base stations and mobile units, of wireless communications systems. Generally, a dielectric resonator comprises a piece of material having a large dielectric constant and disposed within an electrically conductive housing or enclosure acting as a shield against coupling of radiation between the inside and the outside of the enclosure. Electromagnetic energy coupled into the piece of dielectric material is internally reflected at the interfaces between the dielectric material and air. In this way, at certain frequencies resonances are supported by the piece of dielectric material, so that the piece of dielectric material functions as a miniature microwave resonator or resonator element. This results in the electric field being guided by the resonator element and, thus, in confinement of electromagnetic energy within and in the vicinity of the resonator element. Such resonance modes may therefore be referred to as "guided modes". Depending on their shape and construction, such resonator elements may support one or more TE (transverse electric) modes and/or one or more TM (transverse magnetic) modes.
At the resonance frequency of a dielectric resonator, the magnetic field energy equals the electric field energy and electromagnetic fields can be transmitted with minimal loss. The resonance frequencies of a dielectric resonator are controlled by the shape, the cross sectional area and the permittivity constant of its resonator element. Important characteristics of a dielectric resonator are the field patterns, the Q factor, the resonance frequencies and the spurious free bandwidth. It is known that these factors depend on the dielectric material used, the shape of the resonator element, and the resonance mode(s) used. The quality factor Q, which is determined by losses in a structure, is an important design parameter in the design of dielectric resonator filters. The resonator bandwidth is inversely proportional to Q. A high Q is a desirable property of a dielectric resonator as it infers low insertion losses.
Another factor that is important in the design of dielectric resonator filters is the tuning of the individual resonance frequencies of the dielectric resonator(s) to achieve a desired filter response. Such adjusting means are usually realized by a screw extending in a direction orthogonal to the reflection surface effective to change the resonance frequency of a particular resonator element or resonance mode. Further tuning of the filter response may be effected by a screw between two dielectric resonators to adjust the coupling between these dielectric resonators.
The first dielectric resonator arrangements included cylindrical resonator elements commonly known as pucks. As a fundamental mode such pucks support the TE01δ mode in which the electric field is concentrated within the dielectric material and rotates inside the puck forming closed circular rings. To avoid ohmic losses, any contact between the dielectric puck and the walls of the enclosure has to be avoided and sufficient distance between the puck and the walls has to be provided to minimize the surface currents which are induced by the magnetic field circularly surrounding the electric field and not confined by the dielectric material. For these purposes, the pucks were usually supported within the enclosure by a supporting structure made of low dielectric constant material.
Other common dielectric resonator elements are formed by a straight dielectric rod disposed centrally inside a cylindrical cavity extending between and in electrical contact with the bottom wall and the top wall. As a fundamental mode such resonator elements support the TM010 mode, wherein for mode designation purposes the direction of extension of the rod is chosen as z axis. In this mode, the electric field is again concentrated within and guided by the dielectric material, i.e. the electric field lines extend along the direction of extension of the dielectric rod and are perpendicular to the bottom wall and the top wall. The magnetic field lines are circularly closed and surround the rod in planes perpendicular to the rod. Surface currents are induced, which are flowing between the two contact locations of the rod with the enclosure and together with the electric field lines form closed loops.
For dielectric rods supporting the TM010 mode, good electric contact between the dielectric material and the top wall and the bottom wall has to be maintained because an air gap between the dielectric rod and the walls leads to an undesired frequency shift. Mechanical stress due to different coefficients of thermal expansion for the walls and the dielectric rod poses a problem which has to be taken into account upon construction of the dielectric resonator. For example, it is known to avoid mechanical stress and increase temperature stability by letting the dielectric rod extend into bores in the top wall and the bottom wall (see e.g. Y. Kobayashi, S. Yoshida, "Bandpass filters using TM010 dielectric rod resonators", Proc. IEEE MTT-Symposium, 1978, pages 233-235). However, this construction has been found to be insufficient in solving the problem of frequency stability. Another approach utilizes a dielectric shielding enclosure made of the same material as the dielectric rod. This technique was improved by constructing the dielectric shielding enclosure and the rod integrally in one piece (see e.g. Y. Ishikawa, J. Hattori, M. Andoh, T. Nishikawa, "800 MHz high power duplexer using TM dual mode dielectric resonators", Proc. IEEE MTT-Symposium, 1992, pages 1617-1620).
As compared to such single mode dielectric resonators, in general multimode dielectric resonators, realized by using two or more distinct dielectric resonator elements and/or a dielectric resonator element structure, parts of which form different dielectric resonator components, are superior with regard to filter production. This is because the filter characteristics are commonly enhanced when the number of resonance modes excited in the filter is increased. Thus, a single dielectric resonator having a resonator element supporting more than one mode enables a reduction in the size of the filter, because a plurality of coupled single mode dielectric resonators is avoided.
Therefore, a variety of different dielectric resonators with resonator elements simultaneously supporting two or more resonance modes are known in the prior art. A common resonator element used for achieving TM dual mode operation is cross- or X-shaped, i.e. the resonator element comprises two dielectric rods arranged at angles and intersecting each other. With such a resonator element, each of the two straight rod components supports a fundamental TM resonance mode having a field configuration similar to that described above for the case of a single straight rod.
US 4,642,591 discloses an example of a TM dual mode dielectric resonator including such a resonator element consisting of two separate, interconnected dielectric rods that are arranged in a cross-shaped configuration, i.e. the two dielectric rods are perpendicular to each other.
It is also known to form such a cross- or X-shaped resonator element integrally in one piece with the two intersecting rods extending in the same plane. In this case, the resonator element may also be described as consisting of four straight elongate portions that are directly joined with only one of their two longitudinal ends such that the four joined ends form a node or branching region from which the four elongate portions extend in four different directions with an angular distance between adjacent elongate portions of 90°. The branching region is formed by the region of intersection of the two rods. An example of a TM dual mode dielectric resonator including such a resonator element is disclosed in US 2006/0176129 A1 and US 5,638,037 .
US 5,325,077 discloses a dielectric resonator including a resonator element consisting of six cross-shaped sections that are interconnected such that adjacent cross-shaped sections share a common elongate portion and such that the resonator element has a spherical shape. The resonator element is integrally formed in one piece.
With regard to the terminology used to designate the resonance modes it has to be noted that different designations may exist for a particular resonator or mode. For example, instead of using the name TM mode resonator or filter the name dielectric-loaded waveguide filter is used in the textbook I.C. Hunter, "Theory and Design of microwave filters", IEE electromagnetic waves series No. 48, London: IEE, 2001, chapter 7.5.1 pages 314 et sqq., since the field patterns of this type of resonator are comparable to similar waveguide filters which are using air cavity resonators, i.e. resonators not comprising dielectric resonator elements. As another example, the TM010 mode in a cylindrical cavity is comparable to the TMI10 mode in a cuboidal cavity. Furthermore, the mode names may depend on the axis chosen to be the direction of propagation for the corresponding waveguide modes leading to the resonances. This is explained in the textbook S. Ramo, J.R. Whinnery, T. van Duzer, "Fields and waves in communication electronics", 3rd ed. New York: John Wiley & Sons, 1993, chapter 10.4 pages 494 et sqq.
Therefore, to avoid ambiguities, it is more convenient to include the direction of propagation into the mode designation. For example, in a cuboidal resonator the TMy110 mode is identical to the TEz101 mode. Using this terminology, the above mentioned TM dual mode resonances of two intersecting, perpendicular dielectric rods are designated as TMy110 and TMx110 in US 6,278,344 (Figures 12a and 12b). However, they could also be designated as TEz101 and TEz011. In the summary of US 6,278,344 , the modes are designated as "pseudo TM110".
The above mentioned prior art dielectric TM dual mode resonators including a cross- or X-shaped resonator element or a resonator element consisting of a plurality of interconnected cross- or X-shaped sections have the disadvantage that, taking the number of resonance modes supported as a measure, the resonator elements are relatively heavy and bulky, in particular for mobile communications systems. Further, their construction is relatively complex, so that manufacturing of these dielectric resonators is relatively expensive. In addition, there is the problem that the frequencies of the spurious modes tend to be relatively close to the frequencies of the desired modes.
It is an object of the present invention to provide a dielectric multimode resonator, and in particular a dielectric TM multimode resonator, which is adapted to overcome the above mentioned disadvantages.
This object is achieved by a dielectric multimode resonator with the features of claim 1. Further preferred embodiments of the dielectric multimode resonator are the subject-matter of the dependent claims.
The dielectric multimode resonator comprises walls defining a resonator cavity enclosed by the walls. For example, the resonator cavity can have a (circularly or elliptically) cylindrical, cuboidal or cubical shape or the shape of a right prism with a polygonal base having three or more sides. In general, the walls consist of or comprise, e.g. in the form of a coating, a conductive material, such as a metal material.
The dielectric multimode resonator further comprises a resonator element made of dielectric material and disposed within the resonator cavity. The dielectric material may e.g. be a ceramic, preferably a mixture of mainly zirconate and titanate, in particular about 48% ZrO₂ and about 48% TiO₂, such as e.g. a Zr-Ti-Mg-Nb-O based dielectric ceramic. Preferably, the dielectric material has a dielectric constant ε_r of between 20 and 80, preferably of between 35 and 45, and most preferred of about 42.
The resonator element comprises a plurality of interconnected elongate portions, i.e. portions having a length dimension that is greater than the two width dimensions. Due to the fact that the resonance frequencies of undesired modes, in which the electric field lines are guided annularly closed within an elongate portion, are lower for plate-shaped portions than for rod-shaped portions, it is preferred that the elongate portions have the shape of a straight, curved and/or angled rod, i.e. that the two width dimensions are comparable.
The elongate portions are interconnected such that each elongate portion extends longitudinally, i.e. between its two longitudinal ends, either from a connection to a wall to an adjacent branching region from which the respective elongate portion and at least two further of the elongate portions extend longitudinally in different directions, or from one branching region, from which the respective elongate portion and at least two further of the elongate portions extend longitudinally in different directions, to an adjacent branching region, from which the respective elongate portion and at least two further of the elongate portions extend longitudinally in different directions. Thus, each elongate portions has only one of its two longitudinal ends joined at a particular branching region. Two adjacent branching regions may share more than one common elongate portion extending from the two branching regions. For example, for two adjacent branching regions from each of which exactly three elongate portions extend and which are interconnected by an elongate portion, the two pairs of two further elongate portions extending from these two adjacent branching regions may include one or two or no common member.
At least a section of each elongate portion between the two longitudinal ends of the respective elongate portion is spaced from the walls and from connections to the walls, such as interconnecting elements disposed between the elongate portion and a wall. With other words, each of the elongate portions has a longitudinal section between its two longitudinal ends that is freely extending, i.e. is not in contact with or connected to a wall and is not in contact with or connected to an interconnecting element disposed between a wall and the respective elongate portion. For example, in one such arrangement each of the elongate portions, that are connected to a wall, is only connected to a wall at one of the two longitudinal ends of the respective elongate portion, so that the remainder of the elongate portion is not connected to a wall.
The connection between an elongate portion and the wall is preferably such that an electrical connection between the wall and the elongate portion is established. The connection of a longitudinal end of an elongate portion to a wall can be effected by directly joining the end to the wall, or by connecting the end to the wall via one or more spacer or interconnecting elements or a dielectric shielding cavity provided between the electrically conductive portions of the walls and the elongate portions. The resonator element may also be integrally formed in one piece with such a shielding cavity. Preferably, such spacer elements are made of a ceramic or other dielectric material having a much lower dielectric constant than the resonator element, e.g. a dielectric constant ε_r of between 8 and 12, and preferably of about 10. Advantageous materials are alumina, forsterite or quartz.
The elongate portions are only joined to each other at longitudinal ends thereof at the branching regions. Further, each two adjacent branching regions are separated by an elongate portion extending longitudinally from one branching region to the other.
As defined above, a branching region is a region or portion of the resonator element at which or from which at least three different elongate portions extend longitudinally in different directions. In this context, it is expressly pointed out that, in the usual manner, two elongate portions extending away from each other at an angle of 180° are regarded as extending in different directions. Thus, for e.g. a cross-shaped or X-shaped resonator element integrally formed in one piece, the branching region is the central portion of the resonator element constituted by the region of intersection of the two rod shaped components from which the resonator element can be thought of as being comprised, i.e. the node portion or mid portion of the cross or X. With other words, since the longitudinal ends of the elongate portions are directly joined at the branching regions, it will be appreciated that each branching region is formed by the longitudinal ends of the respective elongate portions themselves. For example, in case of a resonator element formed integrally in one piece, i.e. in case the elongate portions do not constitute physically distinct and separate components that are actually attached to each other at a branching region, the branching region may be considered as being formed by overlaying longitudinal end sections of the elongate portions and fusing them together.
In contrast to the prior art resonator elements discussed above with four elongate portions extending from a common branching region to the resonator walls, the present resonator element is constructed and arranged such that the plurality of elongate portions includes a group of at least three interconnected elongate portions, wherein each of the two longitudinal ends of each elongate portion included in this group is either connected to a wall or is directly joined to exactly one of the two longitudinal ends of each of exactly two of the other elongate portions included in this group. With other words, there is at least one branching region from which exactly three elongate portions extend in three different directions, i.e. exactly three elongate portions extend from one or more or all of the branching regions in three different directions. In a preferred version, the present resonator element is constructed and arranged such that each of the two longitudinal ends of each elongate portion is either connected to a wall or is directly joined to exactly one of the two longitudinal ends of each of exactly two of the other elongate portions at one of the branching regions. Thus, exactly three elongate portions extend from each branching region in three different directions.
With other words, the resonator element can also be regarded as comprising or preferably being comprised of one or more bifurcated portions consisting of an elongate portion that bifurcates at one of its two longitudinal ends into two elongate portions extending in different directions. Such bifurcated portions are portions topologically identical to a Y- or a T-shaped portion. As will be described in more detail below, the resonator elements of the present invention may also be regarded as comprising or preferably being comprised of one such bifurcated portion or a plurality of bifurcated portions which are interconnected such that two adjacent bifurcated portions share one, two or three elongate portions, wherein the number of bifurcated portions is identical to the number of branching regions.
This dielectric multimode resonator has the advantage of being capable to provide the same number of independently usable, guided resonance modes as the known dielectric multimode resonators with resonator elements having four elongate portions extending from the branching region(s), but with one fewer elongate portion per branching region. Thus, the amount of dielectric material required to build the resonator element can be decreased without decreasing the number of orthogonal resonance modes guided by the resonator element generally along the direction of extension of the elongate portions (i.e. guided resonance modes with the electric field extending along the direction of extension of the elongate portions). Therefore, the resonator element is simple in construction and requires a relatively small amount of dielectric material for its production. In this way, manufacturing of the dielectric elements is facilitated and their volume and weight is reduced as compared to prior art resonator elements supporting a comparable number of resonance modes. Furthermore, it has been found that advantageously the frequencies of spurious modes are shifted to higher values as compared to the known dielectric multimode resonators with resonator elements having four elongate portions extending from the branching region(s).
The elongate portions may be straight, curved or angled. For example, it is possible that one or more of the elongate portions are straight, whereas others are curved or angled. For example, the three elongate portions may have an arcuate shape, and each elongate portion is extending along a plane. It is also possible that one or more elongate portions are angled or bent at one or more locations, wherein one or more or all of the sections to the both sides of the bend(s) are straight and/or one or more or all of the sections to the both sides of the bend(s) are curved. As will be explained in more detail below, the shape of the elongate portions will depend at least in part on the resonance frequencies and filter characteristics to be achieved with the resonator element, because the dimensions, shape, orientation and relative arrangement of the elongate portions influence these properties.
In a preferred embodiment, for one or more or all of the branching regions from which exactly three elongate portions extend in three different directions the three elongate portions longitudinally extending from the respective branching regions in three different directions extend from the branching regions with an angular distance of 120° between each two of the three elongate portions. In such an arrangement, for all possible pairs of elongate portions extending from a particular branching region, the electric field experiences minimum turning when passing from one elongate portion to the other. This is advantageous, because it is difficult for the electric field to follow bends that are too sharp.
In a preferred embodiment, the resonator element comprises exactly three elongate portions, and preferably consists of three elongate portions. Such a resonator element is directly comparable to the cross- or X-shaped prior art resonator elements discussed above and supporting TM dual mode operation. As will be illustrated in more detail below, the number of orthogonal resonance modes for dual mode operation is not decreased by the omission of one elongate portion per branching region.
In this embodiment, the three elongate portions may be symmetrically arranged about the branching region from which they extend in different directions.
Further, in this embodiment each of the three elongate portions is preferably connected to a wall with the longitudinal end that is disposed opposite the branching region from which the three elongate portions extend in different directions.
In a preferred version of this embodiment the dielectric element is planar with the three elongate portions extending along or in a common plane. Such a resonator element is particularly easy to manufacture and to secure within the resonator cavity. In this case, the three elongate portions may advantageously be straight, and the angle between each two adjacent elongate portions may advantageously be 120°, thereby resulting in a Y-shaped arrangement. Due to the 120° symmetry of this structure, the resonance frequencies of the two fundamental orthogonal resonance frequencies are immediately identical. In this connections, it should be noted that 120° rotational symmetry may also advantageously be achieved with elongate portions that are not straight, such as e.g. with curved elongate portions. Alternatively, the three elongate portions may be straight, and the angle between one of the three elongate portions and each of the two adjacent elongate portions may be 90°, thereby resulting in a T-shaped arrangement. Such resonator elements are particularly easy to manufacture.
The resonator element of this embodiment may advantageously be utilized in a dielectric multimode resonator having walls that include a, preferably planar, base wall, a, preferably planar, upper cover wall and either a (circularly or elliptically) cylindrical sidewall to define a cylindrical resonator cavity, or three or more, preferably planar, sidewalls to define a resonator cavity having the shape of a right prism and preferably four, preferably planar, sidewalls to define a cuboidal or cubical resonator cavity. The resonator element may then be arranged within the resonator cavity such that the three elongate portions are connected with their longitudinal ends, that are located opposite the branching region, to the same wall, and in particular to the cylindrical sidewall of a cylindrical resonator cavity. In this case, it is advantageous if the resonator element is planar and extends parallel to the base wall and perpendicular to the cylindrical sidewall. In another possible arrangement, two of the three elongate portions are connected with their longitudinal ends, that are located opposite the branching region, to the same wall and the third elongate portion is connected with its longitudinal end, that is located opposite the branching region, to a different wall. In still a further possible arrangement, each of the three elongate portions is connected with its longitudinal end, that is located opposite the branching region, to a different wall.
It is also possible that the three longitudinal ends of the three elongate members opposite the branching region are likewise directly joined such that the three elongate portions are directly joined at both of their two ends at two branching regions separated by each of the three elongate portions. Such a resonator element is a closed structure without any free longitudinal ends.
In the embodiment comprising or consisting of three elongate portions, it is preferred that the three elongate portions are constructed and arranged such that the resonator element supports two particular orthogonal resonance modes.
In the first of these two resonance modes the electric field is concentrated in all three elongate portions and is guided longitudinally in the three elongate portions in such a manner that, with respect to the branching region, the direction of extension of the electric field in one of the three elongate portions is opposite to the direction of extension of the electric field in the other two elongate portions. With other words, the electric field is guided in one elongate portion towards the branching region where it splits up and is guided away from the branching region in the other two elongate portions, or the electric field is guided in two elongate portions towards the branching region where it is merged and is guided away from the branching region in the remaining elongate portion.
In the second of these two resonance modes the electric field is concentrated in only two of the three elongate portions, i.e. only dominant and preferably only present in two of the three elongate portions. With other words, while a small amount of electric field may be present in the third leg portion, the electric field lines are essentially confined or concentrated in the other two leg portions. In the present application, wordings such as "the electric field is only dominant in a particular part of the resonator element" mean that the electric field strength in the remainder of the resonator element is negligible as compared to the electric field strength in the particular part. Preferably, the maximum electric field strength in the remainder of the resonator element is less than 5% of the maximum electric field strength in the particular part, more preferably less than 1%, and most preferably less than 0.5%. In this context it has to be noted, however, that some electric field "leaks out of" parts of the resonator element in which the electric field is guided, so that e.g. even in case of a leg portion in which essentially no electric field is concentrated and guided, some electric field may be present immediately adjacent the end connected to a part of the annularly closed portion in which electric field is concentrated and guided. Such electric field components, that exponentially decrease with the distance from the guiding part, are disregarded in the above definition. In the second resonance mode, the electric field is guided longitudinally in the two elongate portions in which the electric field is concentrated in such a manner that, with respect to the branching region, the direction of extension of the electric field in one of the two elongate portions in which the electric field is concentrated is opposite to the direction of extension of the electric field in the other of the two elongate portions in which the electric field is concentrated. Accordingly, the electric field is guided towards the branching region in one of the two elongate portions, and away from the branching region in the other of the two elongate portions.
The three elongate portions are further constructed and arranged, e.g. by choosing suitable material, dimensions such as width and/or length, shape and relative positions of the individual elongate portions, such that the central frequencies of the first and the second resonance mode are within the same pass band of the dielectric multimode resonator, so that they contribute to this pass band, thereby achieving dual mode operation. Preferably, these central frequencies are equal, or they are substantially equal to deviate not more than 25% from their mean value, preferably not more than 20%, more preferably not more than 15%, even more preferably not more than 10%, even more preferably not more than 5%, even more preferably not more than 2% and most preferably not more than 1%, i.e. the two resonance modes are degenerated.
In a further preferred embodiment, the plurality of elongate portions includes at least four elongate portions. Preferably, the resonator element consists of the plurality of interconnected elongate portions.
In this embodiment, each of the two longitudinal ends of each elongate portion may be directly joined to exactly one of the two longitudinal ends of each of exactly two of the other elongate portions at one of the branching regions, so that the elongate portions form a closed structure without free longitudinal ends.
In one version of the embodiment with at least four elongate portions, the resonator element consists of six elongate portions. A first, a second and a third of the six elongate portions are directly joined with one of their two longitudinal ends at a branching region from which they extend longitudinally in different directions. The longitudinal ends of the first, the second and the third elongate portion opposite the branching region from which they extend in different directions are interconnected by the remaining three elongate portions which together form a closed loop. Such a resonator element may be planar, or may be three dimensional defining an interior space. In the latter case, the elongate portions may for example advantageously be arranged such that they form a tetrahedral structure, the four corner portions of which constitute the four branching regions of the resulting structure.
In this embodiment consisting of six elongate portions, it is preferred that the six elongate portions are constructed and arranged such that the resonator element supports three particular orthogonal resonance modes.
In the first of these three resonance modes the electric field is concentrated in the first, the second and the third elongate portion and in only two of the three remaining elongate portions, i.e. only dominant and preferably only present the first, the second and the third elongate portion and in only two of the three remaining elongate portions. The electric field is guided longitudinally in these five elongate portions in such a manner that, with respect to the branching region at which the first, the second and the third elongate portion are directly joined, the direction of extension of the electric field in the second elongate portion is opposite to the direction of extension of the electric field in the first and the third elongate portion, and such that, following the closed loop, i.e. in a particular circumferential direction of the closed loop, the circumferential direction of extension of the electric field in the elongate portion connecting the first and the second elongate portion is opposite to the circumferential direction of extension of the electric field in the elongate portion connecting the second and the third elongate portion.
In the second of these three resonance modes the electric field is concentrated only in the first and the third elongate portion and in the three remaining elongate portions connecting the longitudinal ends of the first, the second and the third elongate portion opposite the branching region from which the first, the second and the third elongate portion extend in different directions. The electric field is guided longitudinally in these five elongate portions in such a manner that, with respect to the branching region at which the first, the second and the third elongate portion are directly joined, the direction of extension of the electric field in the first elongate portion is opposite to the direction of extension of the electric field in the third elongate portion, and such that, following the closed loop, the circumferential direction of extension of the electric field in the elongate portion connecting the first and the third elongate portion is opposite to the circumferential direction of extension of the electric field in the two remaining elongate portions connecting the first and the second elongate portion and the second and the third elongate portion, respectively.
In the third of these three resonance modes the electric field is only concentrated in and guided annularly closed in the closed loop.
The six elongate portions are further constructed and arranged, e.g. by choosing suitable material, dimensions such as width and/or length, shape and relative positions of the individual elongate portions, such that the central frequencies of the first, the second and the third resonance mode are within the same pass band of the dielectric multimode resonator, so that they contribute to this pass band, thereby achieving triple mode operation. Preferably, these central frequencies are equal, or they are substantially equal to deviate not more than 25% from their mean value, preferably not more than 20%, more preferably not more than 15%, even more preferably not more than 10%, even more preferably not more than 5%, even more preferably not more than 2% and most preferably not more than 1%, i.e. the three resonance modes are degenerated.
In the embodiment with at least four elongate portions, it may be advantageous that the elongate portions extend on the outer surface of an imaginary three dimensional body, so that they define an interior space occupied by the imaginary three dimensional body. This interior space my e.g. have a spherical shape or an approximately spherical shape with indentations in the outer surface.
In the embodiment with at least four elongate portions, it may be advantageous if the elongate portions are arranged such that the resonance modes in a plurality of orthogonal resonance modes supported by the resonator element are degenerated, i.e. have the same or substantially the same resonance frequency, such that the resonator element supports a set of non-orthogonal resonance modes, in each of which the electric field is dominant only in a different one of each possible group of three elongate portions serially interconnected end to end in an annularly closed fashion. Thus, there is a non-orthogonal resonance mode with the above characteristics for each possible group of three elongate portions interconnected in the above manner. This set of non-orthogonal resonance modes is obtainable by superposition of the set of degenerated orthogonal resonance modes. For example, in the case of exactly six elongate portions described above, the three orthogonal resonance modes would be degenerated so that three non-orthogonal resonance modes could be formed by superposition, wherein in each of the three non-orthogonal resonance modes the electric field is dominant only in a different group of three elongate portions interconnected in the above-referenced manner.
In such a resonator element, the set of non-orthogonal resonance modes are essentially decoupled with respect to one of the elongate portions of each group of three elongate portions and adjacently coupled by means of the remaining elongate portions. It is then advantageously possible to provide an input coupling means for coupling electromagnetic energy into the resonator element, wherein the input coupling means is arranged to couple electromagnetic energy selectively and predominantly to a decoupled elongate portion, and to provide an output coupling means for coupling electromagnetic energy out of the resonator element, wherein the output coupling means is arranged to couple electromagnetic energy selectively and predominantly out of another decoupled elongate portion, wherein the latter decoupled elongate portion is adjacent the first decoupled elongate portion. In this way, the non-orthogonal resonance modes are coupled in series. Thus, the dielectric resonator can be regarded as comprising a number of individual resonators connected in series between input and output. It is further advantageous if the common elongate portion between the two corresponding adjacent groups of three elongate portions is shorter than the remaining elongate portions common to other pairs of groups of three elongate portions in order to decrease the coupling strength between the input mode and the output mode. However, it may, of course, also be desirable to arrange the input coupling means to simultaneously excite all of the decoupled non-orthogonal resonance modes and to arrange the output coupling means to simultaneously receive electromagnetic energy from all of the decoupled non-orthogonal resonance modes. The latter arrangement results in the non-orthogonal modes being connected in parallel.
It is also preferred that the entire resonator element is made of the same dielectric material, and in particular that the resonator element is integrally formed in one piece. This construction avoids internal surfaces at which electromagnetic energy may be reflected.
The resonator element may be constructed such that all elongate portions have the same length and diameter or at least one of the elongate portions has a different length and/or diameter as compared to another elongate portion. The dimensions of the individual elongate portions may be adapted to adapt the resonance frequencies of the individual resonance modes supported by the resonator element.
For coupling electromagnetic energy into or out of the resonator cavity, an input coupling means and an output coupling means are provided, which are preferably inductive coupling means. Such inductive input coupling means and inductive output coupling means may e.g. comprise an electrically conductive rod, wire-shaped element or plate, and at least one of the input coupling means and the output coupling means is preferably arranged such that the distance between its rod, wire-shaped element or plate or portions thereof and the resonator element and/or its width or the width of portions thereof is adjustable in order to adjust the coupling strength.
It is also preferred that the dielectric multimode resonator comprises at least one frequency adjustment screw extending through a wall portion into the resonator cavity towards or within the resonator element and/or at least one coupling adjustment screw extending through a wall portion into the resonator cavity towards or within the resonator element. In case of an adjustment screw extending towards the resonator element, such screw is arranged such that the distance between their terminal ends and resonator element can be adjusted. For a frequency adjustment screw of this type, it is preferred that it extends into the resonator cavity towards a region of an elongate portion remote from a branching region, and for a coupling adjustment screw of this type, it is preferred that it extends into the resonator cavity towards a branching region. In case of an adjustment screw extending within the resonator element, it is preferred to provide a bore in a portion of a wall to which a longitudinal end of one of the elongate leg portions is connected, which bore extends longitudinally into the respective elongate leg portion. The screw extends through the bore in the wall into the bore provided longitudinally in the elongate leg portion. The extension of the tuning screw into the bore in the elongate leg portion can be modified to thereby adjust the resonator element.
The dielectric multimode resonators described above can be advantageously used in a microwave filter comprising a plurality of coupled resonators. The coupling to and/or from the at least one dielectric resonator to the adjacent resonators may preferably be effected by means of coupling loops or coupling apertures. Such microwave filter may only comprise dielectric resonators of the present invention, or at least one dielectric resonator may be mixed with other types of microwave resonators, such as e.g. other dielectric resonators or coaxial resonators.
In the following, the invention is explained in more detail for preferred embodiments with reference to the figures.

Figs. 1a and 1b: schematically show a cross sectional side view and top view, respectively, of a prior art dielectric multimode resonator including a cross-shaped resonator element disposed in a cylindrical resonator cavity.
Figs. 2a and 2b: schematically show a cross sectional side view and top view, respectively, of a dielectric multimode resonator according to the present invention including Y-shaped resonator element disposed in a cylindrical resonator cavity.
Figs. 3a and 3b: show the distribution of the electric field in the resonator element shown in Figures 2a and 2b for a set of two fundamental orthogonal resonance modes supported by the resonator element.
Figs. 4a and 4b: show the distribution of the electric field in the resonator element shown in Figures 2a and 2b for the resonance mode depicted in Figure 3a after being rotated by +120° and -120°, respectively.
Figs. 5 and 6: show the distribution of the electric field in the resonator element shown in Figures 2a and 2b for a set of two fundamental orthogonal resonance modes supported by the resonator element and derivable from the two resonance modes depicted in Figures 4a and 4b by superposition.
Figs. 7a and 7b: schematically show a cross sectional side view and top view, respectively, of a dielectric multimode resonator according to the present invention including a T-shaped resonator element disposed in a cuboidal resonator cavity.
Figs. 8a and 8b: schematically show a cross sectional side view and top view, respectively, of a dielectric multimode resonator according to the present invention including a generally Y-shaped resonator element with two angled elongate portions disposed in a cuboidal resonator cavity.
Figs. 9a and 9b: schematically show a cross sectional side view and top view, respectively, of a dielectric multimode resonator according to the present invention including a resonator element in a cuboidal resonator cavity with all three elongate portions being connected to the same wall.
Figs. 10a and 10b: schematically show a cross sectional side view and top view, respectively, of a dielectric multimode resonator according to the present invention including a resonator element in a cylindrical resonator cavity with all three elongate portions being connected to the same planar wall.
Fig. 11: shows a schematic elevational view of the dielectric multimode resonator depicted in Figures 10a and 10b.
Figs. 12a and 12b: schematically show two cross sectional side views of a dielectric multimode resonator according to the present invention including a resonator element in a cuboidal resonator cavity with three perpendicular elongate portions connected to different walls.
Fig. 13: shows a schematic elevational view of the dielectric multimode resonator depicted in Figures 12a and 12b.
Fig. 14: shows schematically a top view of a further dielectric multimode resonator according to the present invention.
Fig. 15: shows schematically a top view of a further dielectric multimode resonator according to the present invention.
Fig. 16: shows schematically a top view of a further dielectric multimode resonator according to the present invention.
Figs. 17a and 17b: schematically show a cross sectional side view and top view, respectively, of a further dielectric multimode resonator according to the present invention.
Fig. 18: shows a schematic elevational view of the dielectric multimode resonator depicted in Figures 17a and 17b.
Fig. 19: shows a schematic elevational view of a further dielectric multimode resonator according to the present invention with a resonator element topologically identical to the resonator elements shown in Figures 16 to 18.
Fig. 20: schematically shows an elevational view of a resonator element having a tetrahedral structure.
Fig. 21: schematically shows an elevational view of a resonator element having a modified but topologically identical structure as compared to the resonator element depicted in Figure 20.
Fig. 22: schematically shows a top view of a planar resonator element that is topologically identical to the resonator elements depicted in Figures 20 and 21.
Figs. 23a to 23c: show the distribution of the electric field in the resonator element shown in Figure 22 for a set of three fundamental orthogonal resonance modes supported by the resonator element.
Figs. 24a to 24c: show the distribution of the electric field in the resonator element shown in Figure 22 for a set of three non-orthogonal resonance modes supported by the resonator element which can be derived from the set shown in Figures 23a to 23c.
Fig. 25: schematically shows an elevational view of a dielectric multimode resonator according to the present invention having a resonator element consisting of two joined resonator elements of Figure 13.
Fig. 26: schematically shows an elevational view of a dielectric multimode resonator according to the present invention having a resonator element consisting of two joined resonator elements of Figure 24.
Fig. 27: schematically shows an elevational view of a dielectric multimode resonator according to the present invention having a resonator element consisting of two joined resonator elements of Figure 25.
Fig. 28: schematically shows an elevational view of a resonator element having a modified but topologically identical structure as compared to the resonator element depicted in Figure 27.

In Figures 1a and 1b a cross sectional side view and top view, respectively, of a prior art dielectric multimode resonator 101 is schematically shown. The prior art dielectric resonator 101 comprises a planar, circular bottom wall 102a, a planar, circular top wall 102b, and a cylindrical sidewall 102c, which together form an enclosure defining a cylindrical resonator cavity 103. The walls 102a, 102b, 102c are, at least in part, electrically conductive. Inside the resonator cavity 103 a planar resonator element 104 made of dielectric material is disposed extending in the x-y-plane parallel to the bottom wall 102a and the top wall 102b and electrically connected to the cylindrical sidewall 102c.
The resonator element 104, that is integrally formed in one piece, can be regarded as consisting of four interconnected elongate portions 105. One longitudinal end 105a of each elongate portion 105 is directly connected to the sidewall 102c, and the other longitudinal end 105b of each elongate portion 105 is directly joined to the longitudinal ends 105b of each of the remaining three elongate portions 105. The longitudinal ends 105b are joined at a branching region 106 from which the four elongate portions 105 extend longitudinally in four different directions. The resonator element 104 can also be regarded as consisting of two perpendicular, intersecting straight rods, each formed by opposite elongate portions 105. The branching region 106 is constituted by the region of intersection of these two rods, i.e. the central portion 106a of the cross. When described in terms of the four elongate portions 105, the central portion may be regarded as being a common part of all four elongate portions 105, so that the resonator element may be thought of as being formed by overlaying the four longitudinal end sections 105b of the four elongate portions 105 and fusing them together.
Such a resonator element 104 supports dual mode operation by supporting two TM modes in which the electric field is guided longitudinally within each rod formed by opposite elongate portions 105.
In Figures 2a and 2b a cross sectional side view and top view, respectively, of a dielectric multimode resonator 1 according to one exemplary embodiment of the present invention is schematically shown. The dielectric resonator 1 comprises a planar, circular bottom wall 2a, a planar, circular top wall 2b, and a cylindrical sidewall 2c, which together form an enclosure defining a cylindrical resonator cavity 3. The walls 2a, 2b, 2c are, at least in part, electrically conductive. Inside the resonator cavity 3 a planar resonator element 4 made of dielectric material is disposed extending in the x-y-plane parallel to the bottom wall 2a and the top wall 2b and electrically connected to the cylindrical sidewall 2c. This construction is identical to the construction of the prior art dielectric resonator 101 shown in Figures 1a and 1b.
In contrast to the resonator element 104 shown in Figures 1a and 1b, the resonator element 4, that is integrally formed in one piece, can be regarded as consisting of only three interconnected elongate portions 5. One longitudinal end 5a of each elongate portion 5 is directly connected to the sidewall 2c, and the other longitudinal end 5b of each elongate portion 5 is directly joined to the longitudinal ends 5b of each of the remaining two elongate portions 5. The longitudinal ends 5b are joined at a branching region 6 from which the three elongate portions 5 extend longitudinally in three different directions, wherein each two adjacent elongate portions are spaced by 120°. Thus, the resonator element 4 has the shape of a "Y". It constitutes a bifurcated portion as defined above. The branching region 6 is constituted by the central portion that has a triangular cross section in Figure 2a and that may be regarded as being a common part of all three elongate portions 5, so that the resonator element may be thought of as being formed by overlaying the three longitudinal end sections 5b of the three elongate portions 5 and fusing them together. Thus, each such longitudinal end section 5b has a triangular cross sectional shape identical to the cross sectional shape of the branching region 6.
Although such a resonator element 4 has one elongate portion 5 less than the prior art resonator element 104 shown in Figures 1a and 1b, it likewise supports dual mode operation by supporting two orthogonal fundamental TM modes.
In Figures 3a and 3b the distribution of the electric field (open arrows) for these two fundamental orthogonal resonance modes supported by the resonator element 4 and guided along the general direction of extension of the elongate portions 5 is illustrated. In the first of the two resonance modes illustrated in Figure 3a, the electric field is concentrated in all three elongate portions 5. The electric field extends longitudinally within the upper elongate portion 5 from the sidewall 2c towards the branching region 6. In the branching region 6, the electric field splits up into two halves, each of which extends longitudinally within the lower left elongate portion 5 and the lower right elongate portion 5, respectively, from the branching region 6 to the sidewall 2c. In the second of the two resonance modes illustrated in Figure 3b, the electric field is concentrated and present in only the two lower elongate portions 5. The electric field extends longitudinally within the lower right elongate portion 5 from the sidewall 2c towards the branching region 6, and then within the lower left elongate portion 5 from the branching region 6 to the sidewall 2c. For both modes, it will be appreciated that within one period of oscillation the direction of the electric field will be reversed as compared to the shown distributions.
In this regard, it should be noted that throughout the description, terms like "upper", "lower", "inner" or "outer" are to be understood to describe the particular orientation shown in the Figures, but beyond this are not to be understood in a limiting sense.
Thus, the dielectric multimode resonator 1 has at least the same performance characteristics as the prior art dielectric multimode resonator 101 shown in Figures 1a and 1b, but with less material, so that the manufacturing costs are lower. For example, in case of a cylindrical resonator cavity with a diameter of 40 mm and a height (in the z-direction) of 25 mm, and a resonator element 4, 104 having a height of 7 mm and a relative dielectric constant of 42, the width of the four elongate portions 105 in the prior art cross shape must be 7 mm to achieve a resonance frequency of 2.05 GHz, while the width of the three branches 5 in the Y-shape of the invention must be less than 8 mm. Therefore, in the resonator element 4 shown in Figures 2a and 2b, approximately only 88% of the dielectric material used in the resonator element 104 is required. Furthermore, the spurious frequency is around 3.6 GHz instead of 2.4 GHz which is a further advantage. The quality factors of both resonators 1, 101 is similar, i.e. the resonator 1 exhibits no other performance degradation.
Due to the 120° symmetry of the resonator element 4, the resonance frequencies of the two fundamental resonance modes are identical to achieve dual mode operation, i.e. the two modes are degenerated. This frequency identity may be derived from the following considerations:
In the coordinate system indicated in Figures 2a and 2b, the resonator element 4 shows mirror symmetry with regard to the central y-z-plane. The resonance mode shown in Figure 3a has magnetic symmetry with respect to the central y-z-plane, i.e. the magnetic fields are perpendicular to this plane and, as a result, all electric (E) and magnetic (H) field components exhibit symmetric behavior: $H_{x} (x y z) = + H_{x} (- x, y, z),$
$H_{y} (x y z) = - H_{y} (- x, y, z),$
$H_{z} (x y z) = - H_{z} (- x, y, z),$
$E_{x} (x y z) = - E_{x} (- x, y, z),$
$E_{y} (x y z) = + E_{y} (- x, y, z),$
$E_{z} (x y z) = + E_{z} (- x, y, z) .$
The behavior is similar to the case in which a magnetic conductor is disposed in the y-z-plane (image theory). Similarly, the resonance mode shown in Figure 3b has electric symmetry with respect to the central y-z-plane, i.e. the electric fields are perpendicular to the plane: $H_{x} (x y z) = - H_{x} (- x, y, z),$
$H_{y} (x y z) = + H_{y} (- x, y, z),$
$H_{z} (x y z) = + H_{z} (- x, y, z),$
$E_{x} (x y z) = + E_{x} (- x, y, z),$
$E_{y} (x y z) = - E_{y} (- x, y, z),$
$E_{z} (x y z) = - E_{z} (- x, y, z) .$
The behavior is similar to the case in which an electric conductor is disposed in the y-z-plane.
From this symmetry, it can be concluded for the Y-shaped resonator element 4 that the two shown electric field distributions represent two possible eigenmodes, because there are no other simple guided modes possible (only higher order modes). In Figure 4a the result of a clockwise rotation of the field distribution of the resonance mode of Figure 3a by 120° is shown, whereas in Figure 4b the result of a counter-clockwise rotation of the field distribution of the resonance mode of Figure 3a by 120° is shown. Due to the rotational symmetry of the resonator element 4, these configurations should yield the same behavior as the eigenmode shown in Figure 3a, i.e. should have the same resonance frequency. The two field distributions shown in Figures 4a and 4b can be split up into their even and odd part (i.e. magnetic or electric symmetry with respect to the y-z-plane) by adding or subtracting the two field distributions. This is illustrated in Figure 5 and 6, respectively.
If the two field distributions of Figures 4a and 4b are added as shown in Figure 5, this results in a field distribution identical to the distribution shown in Figure 3a, but with opposite sign. This similarity or identity to multiples of the zero degree solution must occur, because otherwise the field distribution would not be an eigenmode of the system. Therefore, this could be used as a condition to determine the field strengths of the eigenmodes in the resonator by using unknown variables for their values and establishing a set of equations based on the superposition of the field distributions rotated by 120° in the clockwise and the counter-clockwise direction which must be equal to multiples of the zero degree solution. If the two field distributions of Figures 4a and 4b are subtracted as shown in Figure 6, this results in a field distribution identical to the distribution shown in Figure 3b. This proves that the resonance mode of Figure 3b has the same resonance frequency as the resonance mode of Figure 3a.
It should be noted that although in Figures 2 to 6 the Y-shaped resonator element 4 is shown to be disposed in a cylindrical resonator cavity with circular bottom and top walls, it could also be disposed in a resonator cavity having another shape. For example, it may be advantageous to provide a resonator cavity with bottom and top walls that have e.g. the shape of an isosceles triangle or a hexagonal shape. The corners of such walls may be sharp or they may be rounded or cut off. As a further example, a barrel shaped resonator cavity is also possible.
In Figures 7a and 7b a cross sectional side view and top view, respectively, of a modified dielectric multimode resonator 1 according to another exemplary embodiment of the present invention is schematically shown. Like the case shown in Figures 2a and 2b, this dielectric resonator 1 comprises a planar dielectric resonator element 4 that is disposed within a resonator cavity 3 and that consists of three elongate portions 5 extending from a branching region 6, at which the three elongate portions are interconnected with one longitudinal end 5b thereof. However, in this embodiment the resonator cavity 3 is cuboidal and is defined by a planar, rectangular bottom wall 2a, a planar, rectangular top wall 2b, and four distinct planar, rectangular sidewalls 2c. Again, the six walls 2a, 2b, 2c are, at least in part, electrically conductive, and the resonator element 4 is extending in the x-y-plane parallel to the bottom wall 2a and the top wall 2b and perpendicularly to the sidewalls 2c.
The resonator element 4 shown in Figures 7a and 7b differs from the resonator element 4 shown in Figures 2a and 2b only in that there is an angle of 90° between the upper elongate portion 5 and the two adjacent elongate portions 5, i.e. the element has a T-shape instead of a Y-shape. Further, while all elongate portions 5 of the resonator element 4 shown in Figures 2a and 2b are connected to the same (cylindrical) sidewall, the elongate portions 5 of the resonator element 4 shown in Figures 7a and 7b are connected to three different walls, namely to three different (planar) sidewalls 2c.
It should be noted that the branching region 6, at which the longitudinal ends 5b of the three elongate portions 5 are directly joined and from which the three elongate portions 5 extend longitudinally in three different directions, is constituted by the central cubical portion that may be regarded as being a common part of all three elongate portions 5, so that the resonator element may be thought of as being formed by overlaying the three longitudinal end sections 5b of the three elongate portions 5 and fusing them together. Thus, each such longitudinal end section 5b has a rectangular cross sectional shape identical to the cross sectional shape of the branching region 6.
For this multimode resonator 1, there is still mirror symmetry with respect to the y-z-plane, but since the rotational symmetry is lost, the two fundamental resonance modes generally split up into two different frequencies. In order to maintain them at the same frequency, the elongate portions have to be properly dimensioned, taking for example their width and height and/or their longitudinal shape, e.g. straight, curved and/or angled, as design parameters.
Figures 8a and 8b and Figures 9a and 9b show two modified versions of the multimode resonator 1 of Figures 7a and 7b. In Figures 8a and 8b the lower two elongate portions 5 are connected to the sidewall 2c opposite the sidewall 2c to which the upper elongate portion 5 is connected. Thus, the three elongate portions 5 are connected to only two walls. For this purpose, the two lower elongate portions 5 are angled at bends 7. In Figures 9a and 9b, all elongate portions 5 are connected to the same sidewall 2c. For this purpose, the two outer elongate portions 5 are continuously curved.
In Figures 10a, 10b and 11 a modified version of the multimode resonator shown in Figures 9a and 9b is shown. As in Figures 2a and 2b, this multimode resonator 1 again comprises a cylindrical resonator cavity 3 defined by a planar, circular bottom wall 2a, a planar, circular top wall 2b, and a cylindrical sidewall 2c. The resonator element 4 differs from the resonator element 4 shown in Figures 9a and 9b in that it exhibits, like the Y-shaped resonator element 4 of Figures 2a and 2b, 120° rotational symmetry with regard to the z-axis. The elongate portions 5 are not continuously curved, but are angled at bends 7, and are connected to the bottom wall 2a.
Another resonator element 4 exhibiting rotational symmetry is shown disposed in a cubical resonator cavity 3 in Figures 12a, 12b and 13. The three straight elongate portions 5 extend from the branching region 6 in three perpendicular directions and are connected to three perpendicular walls 2a, 2c of the multimode resonator 1. The axis of rotational symmetry extends from one corner of the cubical cavity 3 to the opposite corner diagonally through the resonator cavity.
Other exemplary embodiments of the present invention can be derived by applying image theory. This means, that such embodiments may be thought of as being constructed by removing one of the walls 2a, 2b, 2c of the multimode resonators shown in Figures 1 to 13, to which wall 2a, 2b, 2c the longitudinal end 5a of at least one of the elongate portions 5 is connected, and by mirroring the entire resonator 1 at the plane of the removed wall. Such embodiments, in which the resonator structure is "doubled", has to advantage of an increased quality factor. Further, besides the electric symmetry with regard to this plane - which results in the dual mode operation - additional modes might be taken into account, since also magnetic symmetry might lead to additional modes, which could contribute to the resonator to achieve e.g. triple mode operation of the resonator.
In Figure 14 a multimode resonator 1 is depicted which can be thought of as being constructed by mirroring the resonator of Figures 8a and 8b at the plane of its sidewall 2c to which only one of the elongate portions 5 is connected. In this resonator 1, three orthogonal TM modes are supported, which are similar to those designated as "TMx110", "TMy110" and "TM111" in e.g. US 6,278,344 mentioned above. The resonator element 4 shown in Figure 14 consists of five elongate portions 5 that are interconnected at two space branching regions 6 by directly joining one of their longitudinal ends 5b. From each branching region 6, exactly three of the elongate portions 5 extend in different directions. One straight elongate portion 5 extends between the two branching regions 6, and the remaining four elongate portions 5 are angled at bends 7, so that the two uppermost elongate portions are connected to one wall and the two lowermost elongate portions are connected to an opposite wall.
While each of the resonator elements 4 shown in Figures 1 to 13 constitute a single bifurcated portion as defined above, the resonator element 4 shown in Figure 14 may be regarded as being comprised of two bifurcated portions that share one common elongate portion 5, namely the straight inner elongate portion 5 connecting the two branching regions 6.
In Figure 15 a multimode resonator 1 is depicted which can be thought of as being constructed by mirroring the resonator of Figures 8a and 8b at the plane of its sidewall 2c to which two of the elongate portions 5 are connected. The resonator element consists of four elongate portions that are interconnected at two spaced branching regions 6 by directly joining one of their longitudinal ends 5b. From each of the branching regions 6, exactly three elongate portions 5 extend in different directions. The uppermost and the lowermost elongate portions 5 are straight, wherein each of the two elongate portions 5 connecting the two branching regions 6 is angled at two spaced bends 7. Since the magnetic symmetry expects electric fields which are parallel to the mirror plane (i.e. the perpendicular component should be zero), there are no additional fundamental modes (with continuously guided electric fields along the elongate portions crossing that plane), so that only two fundamental resonance modes are present. In one fundamental mode, the electric field enters the resonator element 4 via one of the outer straight elongate portions 5, extends longitudinally within this elongate portion 5 towards the respective branching region 6, splits up at the branching region 6 into two halves, which extend longitudinally within the two angled elongate portions 5 towards the second branching region 6, where it is combined to extend from the second branching region 6 longitudinally within the other straight elongate portion 5 towards the respective wall. This resonance mode is a TM mode. In the second mode, the electric field is concentrated and circumferentially guided in the closed loop formed by the two angled elongate portions 5. This latter mode is a TE mode.
The resonator element 4 shown in Figure 15 may also be regarded as being comprised of two bifurcated portions that share two common elongate portions 5, namely the two angled inner elongate portions 5 connecting the two branching regions 6.
In Figure 16 a multimode resonator 1 is depicted which can be thought of as being constructed by mirroring the resonator of Figures 9a and 9b at the plane of its sidewall 2c to which all three elongate portions 5 are connected. Similar to the case of Figure 15, this resonator supports only two fundamental modes. As with the resonator elements 4 of Figures 14 and 15, the resonator element 4 shown in Figure 16 has exactly two spaced branching regions 6. However, this resonator element 4 consists of only three interconnected elongate portions 5. One central, straight elongate portion 5 and two outer, curved elongate portions 5 connect the two branching regions 6. It should be noted that this resonator element 4 has no free longitudinal end 5a to be connected to a wall. Rather, the resonator element 4 has to be supported within the resonator cavity be suitable means. Due to the resonator element 4 being spaced from the walls, the quality factor of both resonance modes is increased.
The resonator element 4 shown in Figure 16 may also be regarded as being comprised of two bifurcated portions that share all three elongate portions 5.
In Figures 17a, 17b and 18 a multimode resonator 1 is depicted which can be thought of as being constructed by mirroring the resonator of Figures 10a and 10b at the plane of its bottom wall 2a to which all three elongate portions 5 are connected.
The resulting resonator element 4 is topologically identical to the resonator element 4 shown in Figure 16, but exhibits 120° rotational symmetry with respect to the z-axis. Like the resonator element 4 shown in Figure 16, it does not have any free longitudinal ends 5a to be connected to a wall. A further topologically identical resonator element 4 is shown in Figure 19. All these resonator elements 4 yield dual mode operation. As compared to the resonator elements of other dual mode resonators, they require less dielectric material and they are easy to manufacture. For example, the resonator element 4 of Figure 19 may be easily produced by only pressing ceramic material. No additional steps for providing holes or bores are required, as is the case in some prior art resonator elements. In the same way as the resonator element 4 of Figure 16, the resonator elements 4 of Figures 17 to 19 may also be regarded as being comprised of two interconnected bifurcated portions.
In US 5,325,077 a TE triple mode resonator is disclosed in which each the three fundamental resonance modes is guided by one of three interconnected perpendicular rings having a common center. To achieve similar triple mode operation, such a ring could be added to the resonator element of Figure 18. However, it is already possible to achieve similar results by just interconnecting the longitudinal ends 5b, connected to bottom wall 2a, of the resonator element 4 of Figures 10a and 10b with such a ring. With other words, three further elongate portions 5 are added to the resonator element 4 of Figures 10a and 10b, wherein each of the three further elongate portions 5 interconnects the longitudinal ends 5b of adjacent ones of the elongate portions 5 shown in Figures 10a and 10b. In this way, three further branching regions 6 are created. The resulting resonator element 4 has no free longitudinal ends 5b to be connected to a wall. A topologically identical resonator element 4, having a tetrahedral structure, is shown in Figure 20. Each of the four corners of the tetrahedral structure is a branching region 6 from which exactly three elongate portions 5 extend in different directions. Due to the symmetry of this resonator element 4, all three resonance modes have the same resonance frequency.
However, since the electric field is not able to follow bends that are too sharp, the topologically identical resonator element 4 shown in Figure 21 in which the elongate portions 5 extend on the outer surface of an imaginary sphere enclosed by the resonator element 4 achieves a better performance. In order to facilitate manufacturing, a topologically identical, planar version of the resonator element 4 shown in Figure 21 could be utilized. Such a resonator element 4 is shown in Figure 22.
As is best seen from Figure 21, the resonator elements 4 shown in Figures 20 to 22 may also be regarded as being comprised of four bifurcated portions, wherein each elongate portion of each bifurcated portion is shared with a different one of the remaining three bifurcated portions.
In Figures 23a, 23b and 23c the distribution of the electric field for the three orthogonal resonance modes supported by the resonator element 4 shown in Figure 22 is illustrated by the open arrows. In the first resonance mode illustrated in Figure 23a, the electric field distribution in the three inner, straight elongate portions 5 is identical to the field distribution shown in Figure 3a. However, since no longitudinal end of an elongate portion is connected to a sidewall, the electric field extends in the two outer, curved elongate portions 5 connecting the upper inner elongate portion 5 with the lower left inner elongate portion 5 and the lower right inner elongate portion 5, respectively, to form two closed electric field loops. No electric field is present in the lowermost outer elongate portion 5. In the second resonance mode illustrated in Figure 23b, the electric field distribution in the three inner, straight elongate portions 5 is identical to the field distribution shown in Figure 3b. However, since no longitudinal end of an elongate portion is connected to a sidewall, the electric field extends in the outer, curved elongate portions 5 to form two closed electric field loops. In the third resonance mode illustrated in Figure 23c, the electric field is only present in the three outer curved elongate portions 5, in which it is guided annularly in a closed loop. The latter resonance mode is a TE mode. These field distributions are identical in all of the topologically identical resonator elements 4 shown in Figures 20 to 23. By suitably choosing the width, length and shape of the individual outer and inner elongate portions 5, the two TM modes and the TE mode can be tuned to the same frequency. Furthermore, a topologically identical structure that is similar to the resonator element 4 of Figure 19 may be considered, which is not planar, but nevertheless easier to fabricate than the resonator elements 4 of Figures 20 and 21 by pressing ceramic material. Also, with regard to the resonator element 4 of Figure 20, since the electric field already tends to escape from the tetrahedral structure at the corners, the corners could be connected to the ground to produce additional fundamental frequencies.
With the electric fields shown in Figures 23a, 23b and 23c, another set of non-orthogonal modes can be obtained by superposition in case of the respective resonance modes being degenerated. The resulting set of non-orthogonal modes is depicted in Figures 24a, 24b and 24c. As can be seen from the figures, for each mode in this set, the electric field distribution is concentrated in a different pair of two adjacent inner elongate portions 5 and the outer elongate portion 5 connecting the respective two inner elongate portions 5. Thus, in the ideal case in which the electric field does not penetrate into other portions of the resonator element 4, the non-orthogonal modes are decoupled with respect to the outer elongate portions 5, but are coupled adjacently in pairs by the inner elongate portions 5. In this case, it is advantageously possible to provide an input and an output coupling means (not shown), each of which is arranged such that it essentially only interacts with a different outer elongate portion 5. For example, each of the input and the output coupling means could comprise an electrically conductive plate located beneath only one of the outer elongate portions. It should then be taken care of that the electrical connections between an input connector and an output connector and an associated one of the plates are arranged and constructed such that coupling to additional portions of the resonator element 4 are avoided. In any case, such coupling plates constitute inductive coupling means. Preferably, they are adjustable - primarily in their width and distance to the dielectric resonator element - to render the coupling strength adjustable. Electromagnetic energy is only coupled to one of the three outer elongate portions 5, and thus to only one of the three non-orthogonal modes shown in Figures 23a, 23b and 23c. The electromagnetic energy is then coupled in series from one mode to the next via the overlapping fields at the inner elongate portions 5 common to two adjacent modes until the output mode is reached.
In the alternative, it is, of course, also possible to arrange the input and the output coupling means such that energy is simultaneously coupled into and/or out of two or all of the resonance modes of Figures 24a to 24c. The electromagnetic energy is then coupled (at least in part) in parallel from the input coupling means to the output coupling means.
In Figure 25 a mirrored version of Figure 13 is shown, in Figure 26 a mirrored version of Figure 25 is shown, and in Figure 27 a mirrored version of Figure 26 is shown. It should be noted that the resonator element 4 of Figure 25 is topologically identical to the resonator element 4 shown in Figure 14. Thus, the resonator element 4 likewise is capable of supporting triple mode operation. The resonator element 4 shown in Figure 26 is capable of supporting quadruple mode operation. The resonator element 4 shown in Figure 27 has a cubical structure, wherein each of the corners of the cube constitutes a branching region 6 from which exactly three elongate portions 5 extend in three different directions. In principle, this resonator element 4 is capable of supporting at least quintuple mode operation with at least five orthogonal resonance modes. As in the case of the tetrahedral resonator element 4 of Figure 20 and the modified version shown in Figure 21, a topologically identical resonator element 4, the elongate portions 5 of which essentially extend on the surface of an imaginary sphere enclosed by the resonator element 4, will generally achieve an improved performance. Such a resonator element 4 is depicted in Figure 28.
As can be best seen in Figure 28, the resonator elements 4 shown in Figures 27 and 28 may also be regarded as being comprised of eight bifurcated portions, wherein each elongate portion of each bifurcated portion is shared with a different one of the remaining seven bifurcated portions.
It should be noted that for all resonator elements 4 having no free longitudinal end connected to a wall, no means for supporting the resonator element spaced from the walls within the resonator cavity is shown in the Figures for reasons of simplicity. However, suitable means and arrangements are readily known from the state of the art. For example, a low loss dielectric material having a much lower dielectric constant than the resonator element, e.g. a dielectric constant ε_r of between 8 and 12, and preferably of about 10, could be used to suspend the resonator element in the cavity. Advantageous materials are alumina, forsterite or quartz.

Claims

Dielectric multimode resonator comprising
- walls (2a, 2b, 2c) enclosing a resonator cavity (3), and

- a resonator element (4) made of dielectric material and disposed in the resonator cavity (3), wherein the resonator element (4) comprises a plurality of interconnected elongate portions (5), each extending longitudinally either
- from a connection to a wall (2a, 2b, 2c) to an adjacent branching region (6) from which the elongate portion (5) and at least two further of the elongate portions (5) extend longitudinally in different directions or

- from one branching region (6), from which the elongate portion (5) and at least two further of the elongate portions (5) extend longitudinally in different directions, to an adjacent branching region (6), from which the elongate portion (5) and at least two further of the elongate portions (5) extend longitudinally in different directions,
wherein at least a section of each of the elongate portions (5) between its two longitudinal ends (5a, 5b) is spaced from the walls (2a, 2b, 2c) and from connections to the walls (2a, 2b, 2c), wherein the elongate portions (5) are only joined at longitudinal ends (5b) thereof at the branching regions (6), and wherein each two adjacent branching regions (6) are separated by an elongate portion (5) extending longitudinally from one branching region (6) to the other,
characterized in that the plurality of elongate portions (5) includes a group of at least three interconnected elongate portions (5), wherein each of the two longitudinal ends (5a, 5b) of each elongate portion (5) included in this group is either connected to a wall (2a, 2b, 2c) or is directly joined to exactly one of the two longitudinal ends (5b) of each of exactly two of the other elongate portions (5) included in this group, so that there is at least one branching region (6) from which exactly three elongate portions (5) extend in three different directions.
Dielectric multimode resonator according to claim 1, wherein each of the two longitudinal ends (5a, 5b) of each elongate portion (5) is either connected to a wall (2a, 2b, 2c) or is directly joined to exactly one of the two longitudinal ends (5b) of each of exactly two of the other elongate portions (5) at one of the branching regions (6), so that exactly three elongate portions (5) extend from each branching region (6) in three different directions.
Dielectric multimode resonator according to claim 1 or claim 2, wherein the elongate portions (5) are straight, curved or angled.
Dielectric multimode resonator according to any of claims 1 to 3, wherein for at least one branching region (6) from which exactly three elongate portions (5) extend in three different directions the three elongate portions (5) longitudinally extending from it in three different directions extend from the branching region (6) with an angular distance of 120° between each two of the three elongate portions (5).
Dielectric multimode resonator according to any of claims 1 to 4, wherein the resonator element (4) comprises exactly three elongate portions (5).
Dielectric multimode resonator according to claim 5, wherein the resonator element (4) consists of three elongate portions (5).
Dielectric multimode resonator according to claim 5 or claim 6, wherein the three elongate portions (5) are symmetrically arranged about the branching region (6) from which they extend in different directions.
Dielectric multimode resonator according to any of claims 5 to 7, wherein each of the three elongate portions (5) is connected to a wall (2a, 2b, 2c) with the longitudinal end (5a) that is disposed opposite the branching region (6) from which the three elongate portions (5) extend in different directions.
Dielectric multimode resonator according to any of claims 5 to 8, wherein the resonator element (4) is planar with the three elongate portions (5) extending along a common plane.
Dielectric multimode resonator according to claim 9, wherein the three elongate portions (5) are straight, and the angle between each two adjacent elongate portions (5) is 120°.
Dielectric multimode resonator according to claim 9, wherein the three elongate portions (5) are straight, and the angle between one of the three elongate portions (5) and each of the two adjacent elongate portions (5) is 90°.
Dielectric multimode resonator according to any of claims 5 to 11, wherein the walls (2a, 2b, 2c) include a base wall (2a), an upper cover wall (2b) and either a cylindrical sidewall (c) to define a cylindrical resonator cavity (3) or three or more sidewalls (2c) to define a resonator cavity (3) having the shape of a right prism, and wherein the three elongate portions (5) are connected with their ends (5a), that are located opposite the branching region (6), to the same wall (2a, 2b, 2c).
Dielectric multimode resonator according to claim 12, wherein the walls (2a, 2b, 2c) include a base wall (2a), an upper cover wall (2b) and a cylindrical sidewall (2c) to define a cylindrical resonator cavity (3), and wherein each of the three elongate portions (5) is connected with its end (5a), that is located opposite the branching region (6), to the sidewall (2c).
Dielectric multimode resonator according to claim 13, wherein the resonator element (4) is planar and extends parallel to the base wall (2a) and perpendicular to the cylindrical sidewall (2c).
Dielectric multimode resonator according to any of claims 5 to 11, wherein the walls (2a, 2b, 2c) include a base wall (2a), an upper cover wall (2b) and either a cylindrical sidewall (2c) to define a cylindrical resonator cavity (3) or three or more sidewalls (2c) to define a resonator cavity (3) having the shape of a right prism, and wherein two of the three elongate portions (5) are connected with their ends (5a), that are located opposite the branching region (6), to the same wall (2a, 2b, 2c) and the third elongate portion (5) is connected with its end (5a), that is located opposite the branching region (6), to a different wall (2a, 2b, 2c).
Dielectric multimode resonator according to any of claims 5 to 11, wherein the walls (2a, 2b, 2c) include a base wall (2a), an upper cover wall (2b) and either a cylindrical sidewall (2c) to define a cylindrical resonator cavity (3) or three or more sidewalls (2c) to define a resonator cavity (3) having the shape of a right prism, and wherein each of the three elongate portions (5) is connected with its end (5a), that is located opposite the branching region (6), to a different wall (2a, 2b, 2c).
Dielectric multimode resonator according to any of claims 5 to 7, wherein the three ends (5a) of the three elongate members (5) opposite the branching region (6) are likewise directly joined such that the three elongate portions (5) are directly joined at both of their two ends (5a, 5b) at two branching regions (6) separated by each of the three elongate portions (5).
Dielectric multimode resonator according to any of claims 5 to 17, wherein the three elongate portions (5) are constructed and arranged such that the resonator element (4) supports two orthogonal resonance modes including
- a first resonance mode in which the electric field is concentrated in all three elongate portions (5) and is guided longitudinally in the three elongate portions (5) in such a manner that, with respect to the branching region (6), the direction of extension of the electric field in one of the three elongate portions (5) is opposite to the direction of extension of the electric field in the other two elongate portions (5), and

- a second resonance mode in which the electric field is concentrated in only two of the three elongate portions (5) and is guided longitudinally in the two elongate portions (5) in such a manner that, with respect to the branching region (6), the direction of extension of the electric field in one of the two elongate portions (5) is opposite to the direction of extension of the electric field in the other two elongate portions (5),
and such that the resonant frequencies of the first and the second resonance mode are within the same pass band of the dielectric multimode resonator.
Dielectric multimode resonator according to any of claims 5 to 18, wherein the three elongate portions (5) have an arcuate shape, and each elongate portion (5) is extending along a plane.
Dielectric multimode resonator according to any of claims 1 to 4, wherein the plurality of elongate portions (5) includes at least four elongate portions (5).
Dielectric multimode resonator according to claim 20, wherein the resonator element (4) consists of the plurality of interconnected elongate portions (5).
Dielectric multimode resonator according to claim 20 or claim 21, wherein each of the two longitudinal ends (5a, 5b) of each elongate portion (5) is directly joined to exactly one of the two longitudinal ends (5a, 5b) of each of exactly two of the other elongate portions (5) at one of the branching regions (6), so that the elongate portions (5) form a closed structure without free longitudinal ends (5a, 5b).
Dielectric multimode resonator according to claim 22, wherein the resonator element (4) consists of six elongate portions (5), wherein a first, a second and a third of the elongate portions (5) are directly joined with one of their two longitudinal ends (5a, 5b) at a branching region (6) from which they extend longitudinally in different directions, and wherein the longitudinal ends (5a, 5b) of the first, the second and the third elongate portion (5) opposite the branching region (6) from which they extend in different directions are interconnected by the remaining three elongate portions (5) which together form a closed loop.
Dielectric multimode resonator according to claim 23, wherein the resonator element (4) is planar, or three dimensional defining an interior space.
Dielectric multimode resonator according to claim 23 or claim 24, wherein the six elongate portions (5) are constructed and arranged such that the resonator element (4) supports three orthogonal resonance modes including
- a first resonance mode in which the electric field is concentrated in the first, the second and the third elongate portion (5) and in only two of the three remaining elongate portions (5), and is guided longitudinally in these five elongate portions (5) in such a manner that, with respect to the branching region (6) at which the first, the second and the third elongate portion (5) are directly joined, the direction of extension of the electric field in the second elongate portion (5) is opposite to the direction of extension of the electric field in the first and the third elongate portion (5), and such that, following the closed loop, the direction of extension of the electric field in the elongate portion (5) connecting the first and the second elongate portion (5) is opposite to the direction of extension of the electric field in the elongate portion (5) connecting the second and the third elongate portion (5),

- a second resonance mode in which the electric field is concentrated only in the first and the third elongate portion (5) and in the three remaining elongate portions (5), and is guided longitudinally in these five elongate portions (5) in such a manner that, with respect to the branching region (6) at which the first, the second and the third elongate portion (5) are directly joined, the direction of extension of the electric field in the first elongate portion (5) is opposite to the direction of extension of the electric field in the third elongate portion (5), and such that, following the closed loop, the direction of extension of the electric field in the elongate portion (5) connecting the first and the third elongate portion (5) is opposite to the direction of extension of the electric field in the two remaining elongate portions (5) connecting the first and the second elongate portion (5) and the second and the third elongate portion (5), respectively, and

- a third resonance mode in which the electric field is only concentrated in and guided annularly closed in the closed loop,
and such that the resonant frequencies of the first, the second and the third resonance mode are within the same pass band of the dielectric multimode resonator.
Dielectric multimode resonator according to any of claims 23 to 25, wherein the elongate portions (5) are arranged such that they form a tetrahedral structure.
Dielectric multimode resonator according to any of claims 20 to 26, wherein the elongate portions (5) extend on the outer surface of an imaginary three dimensional body, so that they define an interior space.
Dielectric multimode resonator according to any of the preceding claims, wherein the resonator element (4) is integrally formed in one piece
Dielectric multimode resonator according to any of the preceding claims, wherein all elongate portions (5) have the same length and diameter or at least one of the elongate portions (5) has a different length and/or diameter as compared to another elongate portion (5).
Microwave filter comprising a plurality of coupled resonators including at least one of the dielectric resonators (1) according to any of claims 1 to 29, wherein the coupling to and/or from the at least one dielectric resonator (1) to the adjacent resonators is effected by means of coupling loops and/or coupling apertures.