WO2018091539A1

WO2018091539A1 - Resonator, resonator assembly and filter

Info

Publication number: WO2018091539A1
Application number: PCT/EP2017/079342
Authority: WO
Inventors: Efstratios Doumanis
Original assignee: Alcatel Lucent
Priority date: 2016-11-17
Filing date: 2017-11-15
Publication date: 2018-05-24
Also published as: EP3324480A1; WO2018091539A9; EP3324480B1

Abstract

A resonator (40) for a filter comprises a cavity (49) defined by first (41) and second (42) opposing conductive end walls and a conductive sidewall (43). A dielectric element (44) extends into the cavity from the first conductive end wall and a conductive element (48) extends into the cavity from the second conductive end wall. The free end of the conductive element is an open end thereof and the dielectric rod extends into the free end, forming a gap between the periphery of the dielectric element and the interior surface of the free end of the conductive element. For a given volume, the resonator has a lower resonant frequency compared to dielectric or dielectric combline resonators of the prior art, and a smaller volume for a given resonant frequency.

Description

RESONATOR, RESONATOR ASSEMBLY AND FILTER

FIELD OF THE INVENTION

The invention relates to filters for use in telecommunications applications, particularly RF filters.

BACKGROUND

For medium to high power filtering applications within telecommunications applications, particularly at the lower end of the microwave spectrum (e.g. -700 MHz), stringent performance requirements relating to insertion loss, isolation, etc, mean that there is currently no practical alternative to the use of cavity filters. However the physical size and weight of cavity filters with the necessary performance characteristics leads to significant disadvantages in terms of cost and difficulties in manufacturing, tuning and deployment. Cavity filters are typically the bulkiest and heaviest components within mobile cellular base stations, and in this regard are rivalled only by power-amplifier heat- sinks.

One of the current standard 'building blocks' for a cavity filter is the combline resonator, which comprises a cavity or enclosure having first and second opposing conductive end walls, a conductive side wall or conductive side walls, and a conductive or dielectric post which extends into the cavity from the first end wall. A tuning element may be provided which extends into the cavity from the second end wall, the length of the tuning element within the cavity being adjustable in order to allow tuning of the resonant frequency of the cavity. Resonance corresponds to a high transmission state of the filter and occurs when the length of the conductive or dielectric post is approximately one quarter- wavelength. The end of the conductive or dielectric post remote from the first end wall is an open end and the tuning element extends to the vicinity of the open end. Use of a dielectric post with a high relative permittivity leads to a (dielectric combline) resonator with a higher unloaded Q-factor compared to an equivalent resonator having a conductive post, however such a resonator nevertheless has significant disadvantages relating to its physical size, weight and very limited spurious-free windows. Other known dielectric resonators also have these disadvantages, especially those designed to operate at frequencies at the lower end of the microwave spectrum.

SUMMARY A first aspect of the present invention provides a resonator for a filter, the resonator comprising a cavity having first and second opposing conductive end walls and a conductive side wall or conductive side walls, a dielectric element extending into the cavity from the first conductive end wall, a conductive element extending into the cavity from the second conductive end wall, the end of the conductive element remote from the second end wall being an open end of the conductive element, wherein the end of the dielectric element remote from the first conductive end wall extends into the open end of the conductive element forming a gap between the periphery of the dielectric element and the interior surface of the conductive element.

For a given resonant frequency, a resonator of the invention has a smaller physical size and improved spurious-free performance compared to a resonator of the prior art employing a dielectric element. Equivalently, for a given physical size, a resonator of the invention has a lower resonant frequency compared to a resonator of the prior art employing a dielectric element.

Conveniently the dielectric element is a dielectric rod or a dielectric post and the conductive element is a conductive cylinder. The cylinder may have any of a variety of forms, and its form is not limited to a right-cylinder of circular cross-section; similar considerations apply to the dielectric rod or dielectric post.

The conductive cylinder may be hollow and the dielectric post may extend across the cavity from the first conductive wall to the second conductive wall. The dielectric post may have longitudinal bore; the presence of a hollow bore can lead to improved performance (higher Q-factor) and a lower electric field intensity in use of the resonator, and may also ease manufacture of the resonator. In this case, the resonator may further comprise a conductive post extending into the longitudinal bore from either the first conductive end wall or the second conductive end wall, the length of the conductive post within the longitudinal bore being adjustable to allow tuning of the resonant frequency of the resonator.

Alternatively the dielectric post may extend partially across the cavity from the first conductive end wall towards the second conductive end wall. In this case, the conductive cylinder may be hollow and the resonator may further comprise a conductive post extending into the cavity from the second conductive end wall and within and spaced apart from the conductive cylinder, the length of the conductive post within the cavity being adjustable to allow tuning of the resonant frequency of the resonator. In order to provide more effective tuning by improving the capacitive coupling between the dielectric post and the conductive post, a terminal portion of the dielectric post at the end thereof remote from the first conductive end wall may be hollow with an internal diameter greater than the external diameter of the conductive post, the conductive post aligning with the interior of the terminal portion of the dielectric post.

The dielectric post may extend partially across the cavity from the first conductive end wall towards the second conductive end wall, and have a longitudinal bore. A terminal portion of the dielectric post at the end thereof remote from the first conductive end wall may be hollow, the longitudinal bore communicating with the interior of the terminal portion of the dielectric post. A conductive post may extend into the longitudinal bore from the first conductive end wall, the length of the conductive post within the longitudinal bore being adjustable to allow tuning of the resonant frequency of the resonator.

A second aspect of the invention provides a resonator assembly for a filter, the resonator assembly comprising first and second resonators each of which is a resonator according to the first aspect of the invention, the first and second resonators being coupled such that the cavity of the first resonator communicates with the cavity of the second resonator and energy may be transferred from one resonator to the other. The resonator assembly may comprise a serial arrangement of three or more resonators each of which is a resonator according to the first aspect of the invention, and wherein the cavities of each pair of adjacent resonators are coupled such that the cavities of adjacent resonators communicate and energy may be transferred from one resonator to the other.

When used in filtering applications, a resonator assembly according to the second aspect of the invention may provide additional stop band attenuation and/or more flexible tuning (where tuning means are provided) compared to a single resonator. In order to ensure that harmonic resonant frequencies of individual resonators of the resonator assembly do not coincide, preferably the conductive element and the dielectric element of each resonator define a respective gap between the interior surface of the conductive element and the periphery of the dielectric element, and the gaps differ in at least one dimension.

A third aspect of the invention provides a filter comprising either a resonator according to the first aspect of the invention, or a resonator assembly according to the second aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described below, by way of example only, and with reference to the accompanying drawings in which:

Figure 1 shows a longitudinal section of a dielectric combline resonator of the prior art; gures 2 & 3 shows longitudinal sections of two dielectric resonators of the prior art;

Figures 4 & 5 show transverse and longitudinal sections respectively of a

example resonator of the present invention;

Figures 6 & 7 show transverse and longitudinal sections respectively of a second example resonator of the invention; Figures 8 & 9 show longitudinal sections of third and fourth example resonators of the invention, respectively;

Figures 10 & 11 show longitudinal sections of fifth and sixth example resonators of the invention, respectively;

Figure 12 shows a longitudinal section of a first example resonator assembly of the invention;

Figures 13 & 14 show perspective views of parts of a second example resonator

assembly of the invention; and

Figure 15 shows a longitudinal section of a third example resonator assembly of the invention.

DESCRIPTION

Figure 1 shows a dielectric combline resonator of the prior art, indicated generally by 10, in longitudinal section. The resonator 10 comprises a cavity 19 defined by first 11 and second 12 opposing conductive end walls connected by a conductive side wall or conductive side walls 13. The shape of cavity 19 in transverse section (i.e. in the plane parallel to the plane of Figure 1) may vary, for example the shape may be circular or rectangular. A dielectric post 14 extends into the cavity from the first conductive end wall 11 and has a hollow terminal portion 15. A conductive post 16 extends into the cavity from the second conductive end wall 12, the respective lateral positions (i.e.

positions in a direction generally parallel to the end walls 11, 12) of the conductive post 16 and the dielectric post 14 generally coinciding. The length of the conductive post 16 within the cavity 19 may be adjusted thereby allowing tuning of the resonant frequency of the resonator 10, for example the conductive post 16 may be mounted on the second conductive end wall 12 by means of screw threads. The resonator 10 has a higher unloaded Q than a like resonator of the prior art in which a conductive post replaces the dielectric post 14, however the physical size of the resonator 10 and the limited spurious- free windows are significant disadvantages. Figure 2 shows dielectric resonator 20 of the prior art having a cavity 29 defined by first 21 and second 22 opposing conductive end walls and a conductive side wall 23. A dielectric post 24 having a longitudinal bore 27 extends across the cavity 29 from the first conductive end wall 21 to the second conductive end wall 22. A conductive post 26 extends into the longitudinal bore 27; the length of the conductive post 26 within the cavity 26 is adjustable to allow tuning of the resonant frequency of the resonator.

Figure 3 shows a dielectric resonator 30 of the prior art having first 31 and second 32 opposing conductive end walls and a conductive side wall 33 defining a cavity 39. A first (non-ceramic) dielectric electric element 44 A in contact with the first conductive end wall 31 supports a second ceramic dielectric element 44B of higher relative permittivity. A tuning screw 46 extends into the cavity 39 from the second conductive end wall 32. The length of the tuning screw 46 within the cavity 39 may be adjusted to tune the resonant frequency of the resonator 30. The resonators 20, 30 also have the disadvantages of relatively large physical size when designed to operate at low microwave frequencies, and limited spurious-free windows.

Certain examples of the prior art having been described above, examples of the invention will now be described.

Figures 4 and 5 show transverse and longitudinal sections respectively of a first example resonator 40. Figure 4 shows a transverse section of the resonator 40 taken along plane AA in Figure 5. The resonator 40 comprises a cavity 49 defined by first 41 and second 42 opposing conductive end walls and a conductive side wall or walls 43. In Figure 4, the shape of the cavity 29 in transverse section is shown as square, however in alternative embodiments this shape may be other than square for example circular, elliptical or any one of a number of different shapes. A dielectric rod or post 44, for example of ceramic material, extends into the cavity 49 from the first conductive end wall 41 and has a hollow terminal length portion 45 at the end of the dielectric rod 44 remote from the first conductive end wall 41. A hollow conductive cylinder 48 extends into the cavity 49 from the second conductive end wall 42 such that the dielectric rod 44 extends into the open end of the hollow conductive cylinder 48 remote from the second conductive end wall 42. The external diameter of the dielectric rod 44 is less than the internal diameter of the hollow conductive cylinder 48. The dielectric rod 44 is therefore spaced apart from the hollow conductive cylinder 48: an annular gap exists between the dielectric rod 44 and the interior surface of the hollow conductive cylinder 48. The natural frequencies of the resonator 40 depend at least in part on the dimensions of the annular gap, for example its length (defined by the overlap of the hollow conductive cylinder 48 and the dielectric rod 44) and its thickness (defined by the difference between the internal diameter of the hollow conductive cylinder 48 and the external diameter of the dielectric rod 44). A conductive post 46 extends into the cavity 49 from the second conductive end wall 42 and lies within the hollow conductive cylinder 48 and is generally aligned with the terminal end portion of the dielectric rod 44.

The performance characteristics of two variants of the resonator 40, each having a cylindrical cavity of circular transverse cross section, but which are otherwise structurally similar to the resonator 40, were simulated using modelling software (CST Microwave Studio ®). The variants each have the dimensions and properties shown in Table 1. The variants differ in that in the first variant, the loss tangent of the material of the dielectric rod is 1.0 x 10^"4 and in the second variant the loss tangent of the material of the dielectric rod is 4.0 x 10^"5. The performance characteristics of an equivalent prior art dielectric combline resonator (as shown in Figure 1 , with a cylindrical cavity of circular transverse cross-section) were also modelled. The performance characteristics of the first and second variants and of the equivalent prior art dielectric combline resonator are shown in Table 2.

Resonator Dimensions Values

Circular Cavity (Diameter x Length) 3.2cm x 1.2 cm (9.65 cm³)

Conductive cylinder -outer diameter 11.70 mm

Dielectric Rod - outer diameter 8.90 mm

Conductive cylinder - length; thickness 6.2 mm; 0.8 mm

Dielectric rod - length; thickness of

9.5 mm; 0.8 mm hollow terminal end portion

Dielectric rod and conductive cylinder:

0.8 mm; 3.7mm gap size; length overlap

Electrical length at 2400 MHz (125mm) -34.56 degrees

Dielectric rod: relative permittivity 36.0

Table 1

Table 2 The two variants both have a spurious-free window which is more than four times the frequency-width of the spurious-free window of the equivalent dielectric combline resonator. The power handling capability of the variants of the resonator 40 of Figures 4 and 5 depends strongly on the gap between the dielectric post 44 and the hollow conductive cylinder 48, and also on the length overlap of these elements. These dimensions also determine the extent of miniaturisation compared to the equivalent prior art dielectric combline resonator, in the case of any particular single -pole resonator of the invention. There is therefore a trade-off to consider in the design of a resonator such as the resonator 40: increased miniaturisation results in lower power-handling capability. The two variants of the resonator 40 each have a resonant frequency which is lower than that of the equivalent dielectric combline resonator. In order for the equivalent dielectric combline resonator to have a resonant frequency equal to that of the two variants of the resonator 40, it would need to be increased in size. The resonator 40 therefore has an architecture which is able to achieve a lower resonant frequency compared to an equivalent dielectric combline resonator such as 10.

Figures 6 and 7 show a second example resonator 50. Figure 6 is a transverse section through the resonator 50 taken along the plane BB in Figure 7. The resonator 50 comprises a cavity 59 defined by first 51 and second 52 opposing conductive end walls and a conductive side wall 53. A dielectric (e.g. ceramic) post 54 extends into the cavity 59 from the first conductive end wall 51 and a hollow conductive cylinder 58 extends into the cavity from the second conductive end wall 52. The dielectric post 54 extends into the end of the hollow conductive cylinder 58 remote from the second conductive end wall 52, these two elements being spaced apart by an annular gap. The annular gap has a length dimension defined by the overlap of the hollow conductive cylinder 58 and the dielectric rod 54, and a thickness dimension defined by the difference between the internal diameter of the hollow conductive cylinder 58 and the external diameter of the dielectric rod 54. The dimensions of the annular gap influence the natural resonant frequencies of the resonator 50. A terminal end portion 55 of the dielectric rod, at the end thereof remote from the first conductive end wall 51, is hollow. A conductive post 56 extends into the cavity 59 from the second conductive end wall 52 and coincides laterally with the interior of the terminal end portion 55, i.e. the conductive post 56 and terminal end portion 55 generally coincide in a direction generally parallel to the first 51 and second 52 conductive end walls. The length of the conductive post 56 which extends into the cavity 59 may be adjusted to allow tuning of the resonator 50; for example the conductive post 56 may be mounted on the second conductive end wall 52 by means of screw threads. The dielectric rod 54 has a central longitudinal bore 57 extending from the end of the rod 54 adjacent the first conductive end wall 51 and communicating with the interior of the hollow terminal end portion 55 of the dielectric rod 54. The longitudinal bore 57 results in the resonator 50 having a higher Q and a lower electric field intensity in use, compared to the resonator 40. The bore 57 may also result in easier manufacture of the resonator 50 compared to that of the resonator 40.

Figure 8 shows a longitudinal section of a third example resonator 60. The resonator 60 comprises a cavity 69 defined by first 61 and second 62 opposing conductive end walls, a conductive side wall 63, a dielectric rod 64, a hollow conductive cylinder 68 and a conductive post 66 which has a length within the cavity 69 which is adjustable to allow tuning of the resonant frequency of the resonator 60. The hollow conductive cylinder 68 has a hollow terminal end portion at the end remote from the second conductive end wall 62; the remainder of the hollow conductive cylinder having an internal bore which accommodates the conductive post 62.

Figure 9 shows a longitudinal section of a fourth example resonator 70, the resonator 70 having a cavity 79 defined by first 71 and second 72 opposing conductive ends walls and a conductive side wall 73, a dielectric (e.g. ceramic) rod 74 extending into the cavity 79 from the first conductive end wall 71, and a conductive cylinder 78 extending into the cavity 79 from the second conductive end wall 72. The end of the dielectric rod 74 remote from the first end wall 71 extends into an open end of the conductive cylinder 78 remote from the second end wall 72; the remainder of the conductive cylinder 72 is solid. The dielectric rod 74 and the open end of the conductive cylinder 78 form an annular gap. The resonator 70 has a fixed resonant frequency, i.e. the resonator 70 is not tuneable.

Figure 10 shows a longitudinal section of a fifth example resonator indicated generally by 80. The resonator 80 comprises a cavity defined by first 81 and second 82 opposing conductive end walls and a conductive side wall 83. A hollow conductive cylinder 82 extends into the cavity 89 from the second conductive end wall 82. A dielectric post 84, which has a longitudinal bore 87, extends across the cavity 89 from the first conductive end wall 81 to the second conductive end wall 82, passing into the interior of the hollow conductive cylinder 88 and forming an annular gap with it. A conductive post 86 is mounted by screw threads through the second conductive end wall 82 from which it extends into the longitudinal bore 87 of the dielectric post 84. The length of the conductive post 86 which extends into the longitudinal bore 87 (and hence also into the cavity 89) is adjustable, allowing the resonant frequency of the resonator 80 to be tuned.

Figure 11 shows a longitudinal section of a sixth example resonator indicated generally by 90. The resonator 90 is similar to the resonator 80; parts of the resonator 90 are labelled with reference signs differing by 10 from those labelling corresponding parts of the resonator 80 of Figure 10. A conductive post 96 is mounted by screw threads through first conductive end wall 92 from which it extends into longitudinal bore 97 of the dielectric post 94. The length of the conductive post 96 which extends into the longitudinal bore 97 (and hence also into the cavity 99) is adjustable, allowing the resonant frequency of the resonator 90 to be tuned.

Referring now to Figure 12, a first example resonator assembly 100 is shown in longitudinal section. The resonator assembly 100 comprises a first cavity 109 A defined by first 101 A and second 102 A conductive end walls and a first conductive side wall 103 A. The resonator assembly 100 has a first hollow conductive cylinder 108 A extending into the first cavity 109 A from the second conductive end wall 102A, a first dielectric (e.g. ceramic) rod 104A extending into the first cavity 109A from the first conductive end wall 101 A and into an open end of the first hollow conductive cylinder 108A which is remote from the second conductive end wall 102A. The first dielectric rod 104A and the first hollow conductive cylinder 108A are spaced apart, defining an annular gap. A first conductive post 106 A extends into the first cavity 109 A from the second conductive end wall 102A, within the first hollow conductive cylinder 108 A, the length of the first conductive post 106 A within the first cavity 109 A being adjustable. A second dielectric rod 104B, a second hollow conductive cylinder 108B and a second conductive post 106B extend between first 101B and second 102B conductive end walls of a second cavity 109B, which is also defined by a second conductive side wall 103B, in a similar arrangement to that of parts 104A, 106 A and 108 A within the cavity 109 A. The resonator assembly 100 is made up of two resonators which are coupled together, i.e. cavities 109 A, 109B communicate and electromagnetic energy can pass between one resonator and the other. The respective lengths of the conductive posts 106 A, 106B within the cavities 109 A, 109B may be adjusted to provide tuning of the resonant frequency of the resonator assembly 100, and control of the resonator assembly's pass band and attenuation in its stop-bands.

Figures 13 and 14 show perspective views of a base part 110A and lid part 110B of a second example resonator assembly which is similar to the resonator assembly 100 of Figure 12 except that it is a serial arrangement of five coupled resonators. Each resonator has a cavity which communicates with the cavity or cavities of its nearest neighbour(s). The base part 110A comprises a conductive end wall 112 having five upstanding hollow conductive cylinders 118 A-E each having a respective adjustable conductive post (not shown) within it. The lid part HOB comprises another conductive end wall 111 having five dielectric rods 114 A-E upstanding from the end wall 111. Application of the lid part 110B to the base part 110A such that the dielectric rods 114 A-E extend into the hollow conductive cylinders 118 A-E respectively results in formation of the resonator assembly. The conductive end walls 111, 112 provide the end walls for each of the five resonators. A conductive side wall 113 is shaped to define five cavities with a respective coupling slot (iris) between adjacent pairs of cavities to control energy transmission between individual resonators of the resonator assembly. Electromagnetic energy may be transmitted serially along the resonator assembly from one resonator to the next.

Figure 15 shows a third example resonator assembly 120 in longitudinal section. A first cavity 129 A is defined by first 121 A and second 122B conductive end walls and a first conductive side wall 123 A and communicates with a second cavity 129B defined by first 121B and second 122B conductive end walls and a second conductive side wall 123B. The resonator assembly 120 has a first hollow conductive cylinder 128 A extending into the first cavity 129A from the second conductive end wall 122A, a first dielectric (e.g. ceramic) rod 124A extending into the first cavity 129 A from the first conductive end wall 121 A and into an open end of the first hollow conductive cylinder 128A remote from the second conductive end wall 122A. The first dielectric rod 124A and the first hollow conductive cylinder 128 A define an annular gap. A first conductive post 126 A extends into the first cavity 129 A from the second conductive end wall 122 A, within and spaced apart from the hollow conductive cylinder 128 A, the length of the conductive post 126 A within the first cavity 129A being adjustable. A second dielectric rod 124B, a second hollow conductive cylinder 128B and a second adjustable conductive post 126B extend between the first 121B and second 122B conductive end walls of a second cavity 129B, which is also defined by a second conductive side wall 123B, in an order opposite to that in which elements 124A, 126A, 128A extend between the conductive ends walls 121 A, 122A of the first cavity 129 A. The resonator assembly 120 is made up of two resonators having respective cavities which communicate with each other. Electromagnetic energy may be transmitted between one resonator and the other. The lengths of the conductive posts 126 A, 126B within the cavities 129 A, 129B may be adjusted to provide tuning of the resonant frequency of the assembly, and control of its attenuation and pass bands.

Each of the example resonators and resonator assemblies described above may used as a filter; typically input and output excitation conductors would be included within the cavity of a resonator, or within the first and last cavity of a resonator assembly, to provide input and output paths.

The resonator assembly 100 of Figure 12 is made up of two, coupled, resonators each having the structure of the resonator 40 of Figures 4 and 5. Any of the individual resonators 40, 50, 60, 70, 80, 90 may form the basis of a resonator assembly of the invention having a serial arrangement of two or more resonators in which the cavities of adjacent resonators communicate. In order to provide a resonator assembly having a transmission function which has clearly defined pass and stops bands, it may be desirable to ensure that harmonics of the natural resonant frequencies of the individual resonators do not coincide in frequency. This can be achieved by designing the individual resonators so that the annular gaps defined by the conductive cylinders and the dielectric posts of the various resonators differ in at least one dimension. For example the internal diameters of the conductive cylinders and/or the diameters of the dielectric posts may differ so that the thicknesses of the annular gaps defined by these elements differ between the individual resonators. Additionally, or instead, the length overlap between the conductive cylinder and the dielectric post within each resonator may differ so that the annular gaps differ in length between the individual resonators. The conductive walls of the cavities of resonators and resonator assemblies of the invention may each be formed of metal or alternatively of a non-conductive substrate, e.g. plastic, having a conductive coating (e.g. a silver coating) on its interior surface. The interior surfaces of the conductive cylinders of each of the resonators and resonator assemblies of Figures 4 to 15 each define a gap with the periphery of a respective dielectric post, however the length of any given hollow conductive cylinder need not be uniform in azimuth.

Claims

A resonator for a filter, the resonator comprising:

a cavity having first and second opposing conductive end walls and a conductive side wall or conductive side walls;

a dielectric element extending into the cavity from the first conductive end wall; and

a conductive element extending into the cavity from the second conductive end wall, the end of the conductive element remote from the second conductive end wall being an open end of the conductive element;

wherein the end of the dielectric element remote from the first conductive end wall extends into the open end of the conductive element forming a gap between the periphery of the dielectric element and the interior surface of the conductive element.

A resonator according to claim 1 wherein the dielectric element is a dielectric rod or a dielectric post and the conductive element is a conductive cylinder.

A resonator according to claim 2 wherein the conductive cylinder is hollow and the dielectric post extends across the cavity from the first conductive end wall to the second conductive end wall.

A resonator according to claim 3 wherein the dielectric post has a longitudinal bore.

A resonator according to claim 4 further comprising a conductive post extending into the longitudinal bore from either the first conductive end wall or the second conductive end wall, the length of the conductive post within the longitudinal bore being adjustable to allow tuning of the resonant frequency of the resonator.

6. A resonator according to claim 2 wherein the dielectric post extends partially across the cavity from the first conductive end wall towards the second conductive end wall.

A resonator according to claim 6 wherein the conductive cylinder is hollow and the resonator further comprises a conductive post extending into the cavity from the second conductive end wall and within and spaced apart from the conductive cylinder, the length of the conductive post within the cavity being adjustable to allow tuning of the resonant frequency of the resonator.

A resonator according to claim 8 wherein a terminal portion of the dielectric post at the end thereof remote from the first conductive end wall is hollow with an internal diameter greater than the external diameter of the conductive post, the conductive post aligning with the interior of the terminal portion of the dielectric post.

9. A resonator according to claim 6 wherein the dielectric post has a longitudinal bore.

A resonator according to claim 9 wherein a terminal portion of the dielectric post at the end thereof remote from the first conductive end wall is hollow and the longitudinal bore communicates with the interior of the terminal portion of the dielectric post.

A resonator according to claim 9 or claim 10 further comprising a conductive post which extends into the longitudinal bore from the first conductive end wall, the length of the conductive post within the longitudinal bore being adjustable to allow tuning of the resonant frequency of the resonator. A resonator assembly for a filter, the resonator assembly comprising first and second resonators each of which is a resonator according to any preceding claim and wherein the first and second resonators are coupled such that the cavity of the first resonator communicates with the cavity of the second resonator and energy may be transferred from one resonator to the other.

A resonator assembly according to claim 12 and comprising a serial arrangement of three or more resonators each of which is a resonator according to any of claims 1 to 11, wherein the cavities of each pair of adjacent resonators are coupled such that the cavities of adjacent resonators communicate and energy may be transferred from one resonator to the other.

A resonator assembly according to claim 12 or claim 13 wherein the conductive element and the dielectric element of each resonator define a respective gap between the interior surface of the conductive element and the periphery of the dielectric element and wherein the gaps differ in at least one dimension.

A filter comprising either:

(a) a resonator according to any of claims 1 to 11 ; or

(b) a resonator assembly according to claim 13 or claim 14.