The invention relates to a microwave cavity resonator according to the preamble of claim 1 .

A microwave cavity resonator of this kind comprises a cavity housing forming a cavity, the cavity housing comprising a first housing wall and a second housing wall opposite the first housing wall. A resonator element is arranged in the cavity and extends longitudinally along a longitudinal axis. The resonator element comprises, when viewed along the longitudinal axis, a first end connected to the first housing wall and a second end opposite the first end, the second end being arranged at a distance from the second housing wall. A microwave cavity resonator of this kind may for example be used in a microwave filter, for example a band pass filter or a band stop filter, in a multiplexer or in another radiofrequency (RF) device.

A microwave filter including multiple cavity resonators is for example known from US 6,735,766. A resonator element placed within a cavity of a cavity resonator is herein attached with its first end to a bottom wall of a cavity housing and with a second end is arranged at a distance from a housing cover opposite the bottom wall. The second end of the resonator element hence represents an open end which is not connected to the housing cover.

Within a cavity resonator of this kind the resonator element comprises an electrical length of a quarter wavelength at the resonant frequency of the cavity resonator. This poses a limit for the dimensions of such cavity resonators, which may be in contrast to a desire for a miniaturization and for low production costs. It has been proposed to place a capacitor element at the second, open end of the resonator element in order to increase the capacitance in-between the second, open end of the resonator element and the surrounding cavity housing. This allows to shorten the resonator element.

In EP 1 1 18 134 B1 a cavity resonator is described in which discs are placed in the vicinity of the second, open end of the resonator element, the discs electrically interacting with plates on the cavity housing in order to increase the capacitance in-between the second, open end of the resonator element and the cavity housing.

There is a desire to increase the capacitance between the second, open end of the resonator element and the surrounding cavity housing further. Herein, in known solutions, it is necessary to arrange the second end of the resonator element with respect to the surrounding housing such that a relatively small gap in-between the second end of the resonator element and one or multiple housing walls of the housing is obtained. Such arrangements are sensitive to tolerances and sometimes are mechanically and electrically unstable over the operational range of temperatures.

From US 3,448,412 a miniaturized tunable resonator is known in which a movable assembly, together with a cylindrical member, forms a structure similar to a folded coaxial line and hence a resonator within a cavity.

It is an object of the instant invention to provide a microwave cavity resonator which allows for decreasing the dimensions of the cavity resonator while at the same time exhibiting a mechanically and electrically stable behavior and having a high quality (Q) factor.

This object is achieved with a microwave cavity resonator comprising the features of claim 1 .

Accordingly, the resonator element, at its second end, comprises at least one first capacitor element and the cavity housing comprises at least one second capacitor element reaching into the cavity and arranged at a distance, when viewed along a direction perpendicular to the longitudinal axis, from the at least one first capacitor element such that a gap between the at least one first capacitor element and the at least one second capacitor element is formed. The instant invention starts from the idea to provide one or multiple first capacitor elements at the second, open end of the resonator element. Such first capacitor elements on the second end of the resonator element are associated with one or multiple second capacitor elements on the housing of the cavity resonator such that a capacitance in-between the one or the multiple first capacitor elements and the one or multiple second capacitor elements is formed.

By increasing the capacitance in-between the second, open end of the resonator element and the cavity housing, it becomes possible to shorten the length of the resonator element below the required electrical length of a quarter wavelength in a quarter- wavelength resonator. The physical length of the resonator element can hence be decreased below a quarter wavelength while maintaining the electrical length of the resonator element at a quarter wavelength. The resonator element may comprise one or more first capacitor elements at its second end opposite the first end via which it is connected to the first housing wall for example at a bottom of the cavity housing. The one or multiple first capacitor elements herein extend circumferentially about the longitudinal axis and beneficially are arranged coaxially to the longitudinal axis. The innermost first capacitor element extends about an opening into which, in one embodiment, for example a tuning device such as a tuning screw connected with the second housing wall may reach. At a distance to the innermost first capacitor element and extending (coaxially) about the innermost first capacitor element further first capacitor elements may be arranged at the second end of the resonator element.

The at least one first capacitor element (arranged on the second, open end of the resonator element) and the at least one second capacitor element (arranged on the housing) are placed with respect to one another such that a gap is formed in-between the capacitor elements. Herein, the at least one first capacitor element and the at least one second capacitor element are displaced with respect to each other in a direction perpendicular to the longitudinal axis along which the resonator element extends. In particular, the at least one first capacitor element and the at least one second capacitor element may be arranged coaxially to each other such that a second capacitor element arranged on the housing surrounds a first capacitor element arranged on the second, open end of the resonator element circumferentially about the longitudinal axis. The first capacitor elements of the resonator element as well as the second capacitor elements of the housing generally have the shape of shells arranged for example coaxially and at a radial distance to each other. The capacitor elements hence extend shell-like about the longitudinal axis.

The at least one first capacitor element is distinguished from the shaft of the resonator element. An innermost first capacitor element (which may be the only first capacitor element if only one first capacitor element is present at the resonator element) may axially adjoin the shaft of the resonator element and extend about an inner opening of the resonator element into which for example a tuning device may be inserted. Or the innermost first capacitor element may, at a radial distance, extend about the shaft of the resonator element.

The at least one first capacitor element and the at least one second capacitor element for example may have a cylindrical shape with a circular cross section, wherein the at least one first capacitor element and the at least one second capacitor element are arranged coaxially with respect to each other. Multiple first capacitor elements herein may intermesh with multiple second capacitor elements such that an intermeshed arrangement of capacitor elements is obtained.

The shape of the at least one first capacitor element and the at least one second capacitor element, however, is not limited to a cylindrical shape having a circular cross section. Just as well, the at least one first capacitor element and the at least one second capacitor element can have a quadratic or rectangular shape (when viewed in cross section in a crosssectional plane perpendicular to the longitudinal axis).

In particular, when having multiple first capacitor elements intermesh with multiple second capacitor elements, one second capacitor element is arranged spatially in- between two first capacitor elements and one first capacitor element is arranged spatially in-between two second capacitor elements. When viewed in a direction perpendicular to the longitudinal axis, hence, a first capacitor element connected to the second, open end of the resonator element is followed by a second capacitor element connected to the housing, which again is followed by a first capacitor element connected to the second, open end of the resonator element, and so on. In-between the different capacitor elements a gap is formed such that a capacitance between the capacitor elements is provided. The at least one second capacitor element, in one embodiment, may be arranged on the second housing wall. The at least one second capacitor element hence is connected to the second housing wall of the cavity housing opposite the first housing wall to which the resonator element is connected with its first end. The at least one second capacitor element extends from the second housing wall and reaches into the cavity along the longitudinal axis such that a gap is formed between the at least one second capacitor element on the second housing wall and the at least one first capacitor element on the second, open end of the resonator element. In another embodiment, the at least one second capacitor element may be arranged on a side wall of the cavity housing extending in-between the first housing wall and the second housing wall. From the side wall the at least one second capacitor element reaches into the cavity, wherein the at least one second capacitor element may for example be connected to the side wall via a base such that the at least one second capacitor element is arranged coaxially to the resonator element at a distance from the side wall of the cavity housing.

Multiple first capacitor elements, in one embodiment, are connected to the second, open end of the resonator element via a first base extending in a plane perpendicular to the longitudinal axis. The first base is attached to the resonator element in the vicinity of the second, open end of the resonator element and carries the multiple first capacitor elements, wherein the multiple first capacitor elements are arranged coaxially with respect to each other. In another embodiment, multiple first capacitor elements may be connected to each other via a first base extending along a plane perpendicular to the longitudinal axis, wherein the connection to the resonator element is provided via for example the innermost first capacitor element, but not the base. From the first base, the multiple first capacitor elements may extend towards the second housing wall and/or towards the first housing wall. If the multiple first capacitor elements are arranged coaxially with respect to each other, they may for example be connected to each other via the first base at a side of the multiple first capacitor elements facing away from the second housing wall in which case the multiple first capacitor elements extend from the base towards the second housing wall. Or the base may be arranged at a side of the first capacitor elements facing the second housing wall in which case the multiple first capacitor elements extend from the base towards the first housing wall. It also is conceivable that a portion of at least one first capacitor element extends from the first base towards the second housing wall, whereas a second portion of the at least one first capacitor element extends from the first base towards the first housing wall. A first portion of the at least one first capacitor element hence is arranged on the base to protrude towards the second housing wall, whereas a second portion points towards the first housing wall and hence in an opposite direction. In one embodiment, multiple second capacitor elements are connected to each other via a second base extending along a plane perpendicular to the longitudinal axis. Multiple second capacitor elements connected to the housing hence are carried by a common, second base. Via the second base the second capacitor elements may for example be connected to a side wall or the second housing wall of the housing.

In one embodiment, a tuning device is arranged at the second housing wall, the tuning device having a shaft extending into the cavity along the longitudinal axis. The shaft is arranged coaxially to the resonator element and is adjustable in its position along the longitudinal axis in order to tune the microwave cavity resonator. The tuning device may for example be embodied as a tuning screw which with its shaft can be screwed into or screwed out of the cavity such that the length of the shaft reaching into the cavity may be adjusted. The shaft of the tuning device may, in particular, be arranged coaxially to the at least one first capacitor element and the at least one second capacitor element, wherein the at least one first capacitor element and the at least one second capacitor element are positioned radially outside of the shaft and extend around the shaft.

The cavity housing may for example be fabricated out of a metallic first material, for example aluminum. The resonator element, in contrast, may for example be made of a different, second material, for example brass, wherein it also is conceivable to form the resonator element from a non-metallic material, for example a ceramic material.

It also is conceivable to produce the cavity housing and/or the resonator element from a metalized plastic material, for example a plastic having a metal coating. The idea underlying the invention shall subsequently be described in more detail with regard to the embodiments shown in the figures. Herein:

Fig. 1 A shows a cross-sectional view of a microwave cavity resonator along line II- II according to Fig. 1 B;

Fig. 1 B shows a cross-sectional view of the microwave cavity resonator along line l-l according to Fig. 1 A;

Fig. 1 C shows a cross-sectional view of a modified embodiment of a microwave cavity resonator;

Fig. 2 shows a cross-sectional view of a different embodiment of a microwave cavity resonator;

Fig. 3A shows a cross-sectional view of yet another embodiment of a microwave cavity resonator;

Fig. 3B shows a cross-sectional view along line Ill-Ill according to Fig. 3A;

Fig. 4A shows a cross-sectional view of yet another embodiment of a microwave cavity resonator; and

Fig. 4B shows a cross-sectional view along line IV-IV according to Fig. 4A;

Fig. 5 shows a schematic view of a microwave filter comprising multiple cavity resonators.

A microwave filter 2, as it is schematically shown in Fig. 5, comprises multiple cavity resonators 1 A, 1 B, 1 C, 1 D arranged in a common cavity housing 10. Each cavity resonator 1 A, 1 B, 1 C, 1 D comprises a cavity 1 1 in which a resonator element 12 is located. The cavity housing 10 comprises housing walls 103, 104 and a housing lid 101 and fully encloses the cavity 1 1 of the multiple cavity resonators 1 A, 1 B, 1 C, 1 D.

A microwave filter 2, as schematically shown in Fig. 5, may for example be employed in wireless communication devices and may for example implement a bandpass or bandstop filter. Such microwave filters 2 comprising multiple cavity resonators 1 A, 1 B, 1 C, 1 D shall in general exhibit a high quality (Q) factor leading to a low insertion loss. Further, such microwave filters 2 shall be mechanically and electrically stable and be operable over a wide range of temperatures. Within such microwave filter 2, a radio frequency (RF) signal is fed into an input port 20 and passes through the microwave filter 2 to an output port 21 . Dependent on the design of the microwave filter 2, RF signals in a predefined frequency band are passed (bandpass filter) or suppressed (bandstop filter). Within the cavity 1 1 of the single cavity resonators 1 A, 1 B, 1 C, 1 D, which suitably are electromagnetically coupled for example via openings in the inner housing walls in between the cavities 1 1 of the cavity resonators 1 A, 1 B, 1 C, 1 D, resonator elements 12 in the shape of longitudinally extending rods are placed. Such resonator elements 12 with a first end 120 (see for example Fig. 1 A) are connected to a first, bottom housing wall 100 of the housing 10 and extend within the associated cavity 1 1 along a longitudinal axis L towards a second, top housing wall 101 formed by the housing lid opposite the first housing wall 100. The resonator element 12, together with the cavity housing 10 forming the cavity 1 1 , forms a quarter-wavelength resonator and has an electrical length of a quarter wavelength (at a specified resonant frequency).

The second end 121 opposite the first end 120 of the resonator element 12 herein is not connected to the second housing wall 101 and hence is electrically opened.

In order to shorten the physical length of the resonator element 12 below its electrical length of a quarter wavelength, in the embodiment of Fig. 1 A capacitor elements 123, 124 in the shape of cylindrical rings (see Fig. 1 B) or quadratic or rectangular elements (see Fig. 1 C) are arranged at the second end 121 and intermesh with a capacitor element 106 attached to the second, top housing wall 101 of the cavity housing 10. The capacitor elements 123, 124 of the resonator element 12 as well as the capacitor element 106 of the second housing wall 101 extend circumferentially about the longitudinal axis L and are arranged coaxially with respect to each other and with respect to the longitudinal axis L. The capacitor element 106 attached to the second housing wall 101 herein reaches into an opening formed in between the capacitor elements 123, 124 of the resonator element 12 such that a gap G is formed in-between the capacitor elements 123, 124 of the resonator element 12 and the capacitor element 106 of the second housing wall 101 . The second end 121 of the resonator element 12 with the capacitor elements 123, 124 arranged thereon is spaced apart from the upper, second housing wall 101 of the cavity housing 10 by a distance D such that the second end 121 of the resonator element 12 is not electrically connected to the second housing wall 101 . Because the surfaces of the capacitor elements 123, 124, 106 can be large, a comparatively large capacitance in- between the second end 121 of the resonator element 12 and the surrounding housing 10, namely side walls 102, 103, 104, 105 and the top wall 101 , can be provided.

Because the capacitance in-between the second end 121 of resonator element 12 and the surrounding housing 10 can be large, the physical length of the resonator element 12 can be substantially shortened, such that a reduction of the overall dimensions of the cavity resonator 1 becomes possible while at the same time allowing for a high Q factor and low insertion loss of a corresponding microwave filter 2. With the design described herein, the gap G in between the capacitor element 106 of the second, top housing wall 101 and the inner capacitor element 123 on the one hand and the outer capacitor element 124 on the other hand of the resonator element 12 can be chosen such that a mechanically and consequently electrically stable behavior of the resonator 1 over a wide range of operational temperatures is obtained. In particular, because the gap G can be chosen relatively large (for example in the range of 1 mm), the resonator 1 can be insensitive to tolerances and hence can be easily manufactured without paying particular attention to tight tolerances.

The capacitor elements 123, 124 of the resonator element 12 and the capacitor element 106 of the second housing wall 101 extend about the longitudinal axis L in a ring-like coaxial fashion. The capacitor elements 123, 124 herein are carried by a common base 126 and, via the base 126, are attached to a shaft 128 of the resonator element 12. The base 126 is arranged at a side of the capacitor elements 123, 124 opposite the second housing wall 101 and, together with the capacitor elements 123, 124, forms a groove-like opening into with the capacitor element 106 arranged on the second housing wall 101 extends.

The capacitor elements 123, 124 may be integrally formed with the resonator element 12 and may for example be made of brass. The capacitor element 106 of the second housing wall 101 may be integrally formed with the second housing wall 101 and may be fabricated, as the entire housing 10, for example of aluminum. The radially innermost capacitor element 123 of the resonator element 12 forms an inner, central opening 122, into which a shaft 131 of a tuning device 13 in the shape of a tuning screw extends. The tuning device 13 is arranged on the second housing wall 101 . The shaft 131 reaches through the second housing wall 101 and is rotatable about the longitudinal axis L such that the length of the shaft 131 extending into the cavity 1 1 of the cavity resonator 1 along the longitudinal axis L can be adjusted. The shaft 131 is screwed into a screw nut 132 placed on the housing wall 101 and, at an end 130 outside the cavity 1 1 , can be accessed by using a tool like a screw driver or the like. The shaft 131 is for example made of a metallic material, such as aluminum or brass. As shown in Fig. 1 B, the capacitor elements 106, 123, 124 may have a cylindrical shape extending around the longitudinal axis L and being arranged in a coaxial fashion.

The capacitor elements 106, 123, 124, however, may also have a different shape, for example a quadratic or rectangular shape (when viewed in a cross-sectional plane perpendicular to the longitudinal axis L), as it is illustrated in Fig. 1 C.

Another embodiment of a resonator element 1 is shown in Fig. 2. In this embodiment, the resonator element 12 carries a base 126 with three coaxial capacitor elements 123, 124, 125 attached thereto, the capacitor elements 123, 124, 125 being arranged coaxially with respect to each other and extending circumferentially around the longitudinal axis L along which the resonator element 12 extends and. Two capacitor elements 106, 107 are arranged on the second housing wall 101 of the housing 10, the capacitor elements 106, 107 intermeshing with the capacitor elements 123, 124, 125 of the resonator element 12 such that a gap G is formed in-between neighboring capacitor elements 106, 107, 123, 124, 125.

It is conceivable to increase the number of capacitor elements 123, 124, 125 of the resonator element 12 on the one hand and of the capacitor elements 106, 107 of the housing 10 on the other hand even further. Multiple capacitor elements 123, 124, 125 of the resonator element 12 hence may be arranged to intermesh with multiple capacitor elements 106, 107 of the housing 10. The capacitor elements 123, 124, 125, 106, 107 of the resonator element 12 and of the housing 10 herein alternate when viewed in the radial direction (perpendicular to the longitudinal axis L).

Another embodiment of a resonator element 1 is shown in Fig. 3A, 3B. In this embodiment, two capacitor elements 123, 124 extending circumferentially about the longitudinal axis L of the resonator element 12 are arranged at the second end 121 of the resonator element 12, wherein an outer capacitor element 124 is connected to an inner capacitor element 123 via a base 127 at a side of the capacitor elements 123, 124 facing the second housing wall 101 . The capacitor elements 123, 124 extend from the base 127 towards the first, bottom housing wall 100. Via the inner capacitor element 123 the base 127 is connected to the shaft 128 of the resonator element 12.

The capacitor elements 123, 124 of the resonator element 12 form a groove-like opening in-between them into which a capacitor element 106 of the housing 10 extends. The capacitor element 106 is connected via a circumferential base 108 to the side walls 102, 103, 104, 105 of the housing 10 and hence is carried by the side walls 102, 103, 104, 105 of the housing 10 (see Fig. 3B). The base 108 extends in a plane perpendicular to the longitudinal axis L, and from the base 108 the capacitor element 106 extends upwardly towards the second housing wall 101 into the groove-like opening formed in- between the capacitor elements 123, 124 on the second end 121 of the resonator element 12. The base 108 forms an opening 109 through which the resonator element 12 extends with the capacitor element 123 formed on the second end 121 of the resonator element 12. Another embodiment of a cavity resonator 1 is shown in Fig. 4A, 4B. In the embodiment of Fig. 4A, 4B two capacitor elements 123, 124 are arranged on the second end 121 of the resonator element 12. The capacitor elements 123, 124 are connected to each other via a base 127 extending in a ring-like fashion in a plane perpendicular to the longitudinal axis L of the resonator element 12, as it is shown in Fig. 4B.

The base 127, in the embodiment of Fig. 4A, 4B, divides the capacitor elements 123, 124 of the resonator element 12 into two portions 123A, 123B, 124A, 124B. Namely, an upper portion 123A, 124A of each capacitor element 123, 124 extends from the base 127 towards the second housing wall 101 and forms a groove-like opening extending circumferentially about the longitudinal axis L into which a capacitor element 106 connected to the second housing wall 101 extends, similarly as it has been described for the embodiment of Fig. 1 A, 1 B and 1 C. In addition, a lower portion 123B, 124B of each capacitor element 123, 124 extends from the base 127 towards the first housing wall 100 and hence towards the bottom of the cavity 1 1 , wherein via the lower portion 123B of the inner capacitor element 123 the base 127 is connected to the shaft 128 of the resonator element 12.

Via the outer capacitor element 124 of the resonator element 12 an (increased) capacitance in-between the second, open end 121 of the resonator element 12 and the surrounding side walls 102-105 of the housing 10 in the vicinity of the second, open end 121 is provided. Namely, the outer capacitor element 124 faces with its upper and lower portion 124A, 124B the side walls 102, 103, 104, 105 of the housing 10 with a gap G similar or equal to the gap G in-between the capacitor element 106 of the second housing wall 101 and the capacitor elements 123, 124.

Modifications of the embodiments described above are conceivable.

For example, in the embodiment of Fig. 4A, 4B an additional capacitor element of the housing 10 may be connected via a base to the side walls 102, 103, 104, 105 (similar as shown in Fig. 3A, 3B) and may reach into the opening formed in-between the lower portions 123B, 124B of the capacitor elements 123, 124 of the resonator element 12.

It further is conceivable to use different numbers of capacitor elements on the resonator element 12 as well as on the housing 10. Multiple capacitor elements of the resonator element 12 and the housing 10 herein are arranged to intermesh with each other such that, when viewed in the radial direction radially to the longitudinal axis L a gap G is formed in-between neighboring capacitor elements.

The idea underlying the invention is not limited to the embodiments described above, but may be implemented in an entirely different fashion in entirely different embodiments.

List of reference numerals

1 , 1 A-1 D Microwave cavity resonator 10 Cavity housing

100, 101 Housing wall

102-105 Side wall

106, 107 Capacitor element

108 Base

109 Opening

1 1 Cavity

12 Resonator element

120, 121 End

122 Central opening

123-125 Capacitor element

123A, 123B, 124A, 124B Portion

126, 127 Base

128 Shaft

13 Tuning device

130 End

131 Shaft

132 Screw nut

2 Microwave filter

20 Input port

21 Output port

D Distance

G Gap

L Longitudinal axis