WO2019024082A1

WO2019024082A1 - Bandpass filters and associated methods.

Info

Publication number: WO2019024082A1
Application number: PCT/CN2017/095989
Authority: WO
Inventors: Qing Zhou; Yong Gan SONG; Hongjun Zhao
Original assignee: Nokia Solutions And Networks Oy
Priority date: 2017-08-04
Filing date: 2017-08-04
Publication date: 2019-02-07

Abstract

A bandpass filter having a bandpass at a first resonant frequency comprising: a housing comprising a resonant cavity and an aperture extending through the housing to the resonant cavity; and a moveable conductive resonator element comprising a conductive positioning portion and a conductive resonator head portion, wherein the conductive positioning portion extends through the aperture of the housing into the resonant cavity and positions the resonator head portion within the resonant cavity.

Description

Bandpass filters and associated methods.

TECHNOLOGICAL FIELD

Embodiments of the present invention relate to bandpass filters and associated methods. In particular, they relate to bandpass filters for radio telecommunications, and, in particular, cellular communications.

BACKGROUND

A bandpass filter is a filter that ‘passes’ frequencies within a certain range and ‘rejects’ frequencies outside that range. Typically, a bandpass filter has a frequency dependent impedance that is high outside the range and a much lower impedance within the range. The range is typically defined by a resonant frequency where the impedance is a minimum and an operational resonant bandwidth where the impedance stays below a threshold.

It is common practice in radio telecommunications to use frequency division to separate radio channels. It is therefore possible for different reception channels to be separated in frequency； for different transmission channels to be separated in frequency； and for reception and transmission channels to be separated in frequency. A bandpass filter may be channel-selective and configured to select a signal that relates only to a particular channel.

BRIEF SUMMARY

According to various, but not necessarily all, embodiments of the invention there is provided a bandpass filter having a bandpass at a first resonant frequency comprising:

a housing comprising a resonant cavity and an aperture extending through the housing to the resonant cavity； and

a moveable conductive resonator element comprising a conductive positioning portion and a conductive resonator head portion, wherein the conductive positioning portion extends through the aperture of the housing into the resonant cavity and positions the resonator head portion within the resonant cavity.

According to various, but not necessarily all, embodiments of the invention there is provided a method comprising:

providing a moveable conductive resonator element comprising a conductive positioning portion and a conductive resonator head portion within a bandpass filter, by positioning the conductive resonator head portion within a resonant cavity defined by a housing of the bandpass filter and by extending the conductive positioning portion through an aperture extending through the housing between the resonant cavity and an exterior of the bandpass filter.

According to various, but not necessarily all, embodiments of the invention there is provided examples as claimed in the appended claims.

BRIEF DESCRIPTION

For a better understanding of various examples that are useful for understanding the detailed description, reference will now be made by way of example only to the accompanying drawings in which:

Fig 1 illustrate an example of a bandpass filter having a bandpass at a first resonant frequency；

Fig 2 illustrates an example of a frequency response of a bandpass filter；

Fig. 3 illustrates an example of an electrical model that may be used to understand, in part, the operation of a bandpass filter；

Figs. 4A, 4B, 5A, 5B, 6A, 6B and 7A illustrate different examples of a bandpass filter 10 and the corresponding moveable conductive resonator elements 30 of those bandpass filters；

Figs. 8A and 8B illustrate examples of a bandpass filter that comprises multiple filter cavities；

Figs. 9A, 9B and 9C illustrate examples of how a bandpass filter may be used； and

Fig 10 illustrates an example of a method for manufacturing or repairing a bandpass filter.

DETAILED DESCRIPTION

In the examples and figures below, a filter cavity 22 is configured to operate as a resonant cavity that is capacitively loaded, typically a resonant microwave cavity.

Fig 1 illustrate an example of a bandpass filter 10 having a bandpass at a first resonant frequency.

Fig 2 illustrates an example of a frequency response of a bandpass filter 10. The Figure illustrates a variation of an impedance dependent parameter (y-axis) with increasing frequency of electromagnetic radiation (x-axis, left to right) or increasing wavelength of electromagnetic radiation (x-axis, right to left ) .

The bandpass filter 10 is a filter that ‘passes’ frequencies within a certain range 2 and ‘rejects’ frequencies outside that range. The impedance dependent parameter, e.g. the reflection coefficient S11, is high outside the range 2 and much lower within the range 2. The range 2 is typically defined by a resonant frequency f_o where the impedance dependent parameter is a minimum and an operational resonant bandwidth Δf＝f₂-f₁ where the impedance stays below a threshold T where T is normally greater than 3dB.

The bandpass filter 10 may be configured to selectively ‘pass’ one or more operational resonant frequency bands (channels) . For example, the operational frequency bands may include (but are not limited to) Long Term Evolution (LTE) (US) (734 to 746 MHz and 869 to 894 MHz) , Long Term Evolution (LTE) (rest of the world) (791 to 821 MHz and 925 to 960 MHz) , amplitude modulation (AM) radio (0.535-1.705 MHz) ； frequency modulation (FM) radio (76-108 MHz) ； Bluetooth (2400-2483.5 MHz) ； wireless local area network (WLAN) (2400-2483.5 MHz) ； hiper local area network (HiperLAN) (5150-5850 MHz) ； global positioning system (GPS) (1570.42-1580.42 MHz) ； US –Global system for mobile communications (US-GSM) 850 (824-894 MHz) and 1900 (1850 –1990 MHz) ； European global system for mobile communications (EGSM) 900 (880-960 MHz) and 1800 (1710 –1880 MHz) ； European wideband code division multiple access (EU-WCDMA) 900 (880-960 MHz) ； personal communications network (PCN/DCS) 1800 (1710-1880 MHz) ； US wideband code division multiple access (US-WCDMA) 1700 (transmit: 1710 to 1755 MHz , receive: 2110 to 2155 MHz) and 1900 (1850-1990 MHz) ； wideband code division multiple access (WCDMA) 2100 (transmit: 1920-1980 MHz, receive: 2110-2180 MHz) ； personal communications service (PCS) 1900 (1850-1990 MHz) ； time division synchronous code division multiple access (TD-SCDMA) (1900 MHz to 1920 MHz, 2010 MHz to 2025 MHz) , ultra wideband (UWB) Lower (3100-4900 MHz) ； UWB Upper (6000-10600 MHz) ； digital video broadcasting -handheld (DVB-H) (470-702 MHz) ； DVB-H US (1670-1675 MHz) ； digital radio mondiale (DRM) (0.15-30 MHz) ； worldwide interoperability for microwave access (WiMax) (2300-2400 MHz, 2305-2360 MHz, 2496-2690 MHz, 3300-3400 MHz, 3400-3800 MHz, 5250-5875 MHz) ； digital audio broadcasting (DAB) (174.928-239.2 MHz, 1452.96-1490.62 MHz) ； radio frequency identification low frequency (RFID LF) (0.125-0.134 MHz) ； radio frequency identification high frequency (RFID HF) (13.56-13.56 MHz) ； radio frequency identification ultra high frequency (RFID UHF) (433 MHz, 865-956 MHz, 2450 MHz) .

Referring back to Fig 1, the bandpass filter 10 comprises: a housing 20 comprising a filter cavity 22 and aperture 24 extending through the housing 20 to the filter cavity 22； and a moveable conductive resonator element 30.

The moveable conductive resonator element 30 comprises a conductive positioning portion 32 and a conductive resonator head portion 34. The conductive positioning portion 32 extends through the aperture 24 of the housing 20 into the filter cavity 22 and positions the resonator head portion 34 within the filter cavity 22.

In some but not necessarily all examples, the filter cavity 22 is a resonant cavity that enables a resonant transverse-electromagnetic (TEM) mode.

In some but not necessarily all examples, the filter cavity 22 is a resonant cavity that has a coaxial electro-magnetic configuration that enables a resonant transverse-electromagnetic (TEM) mode. The coaxial electro-magnetic configuration comprises a conductive exterior boundary of the filter cavity 22 provided by the housing 20 and a central longitudinal axis 11. A central conductor, the conductive positioning portion 32, extends along the central longitudinal axis. The coaxial electro-magnetic configuration satisfies the boundary conditions for the TEM mode according to Maxwell’s equations for conductors. The electric (E) field is orthogonal to the central longitudinal axis 11 (the parallel component of E at the boundary is zero) and the magnetic field (B) is orthogonal to the electric field and circumferential to the central longitudinal axis 11 (the perpendicular component of B at the boundary is 0) . There are only transverse, no longitudinal, components of the electric field and the magnetic field.

In some but not necessarily all examples, conductive positioning portion 32 extends at least partially along the central longitudinal axis 11. This creates a coaxial geometry that supports TEM modes in which the electric field and the magnetic field are only radial (orthogonal) to the central longitudinal axis 11 of the filter cavity 22 and the electric field is perpendicular to the magnetic field.

In some but not necessarily all examples, conductive resonator head portion 34 is positioned on the central longitudinal axis 11. In some but not necessarily all examples, the conductive resonator head portion 34 has rotational symmetry about the central longitudinal axis 11.

In some but not necessarily all examples, the coaxial electro-magnetic configuration may be a symmetrical configuration in which the conductive exterior boundary of the filter cavity 22 has rotational symmetry about a central longitudinal axis 11.

Although the exterior boundary of the filter cavity 22 (interior surface of the housing) has, in some examples, rotational symmetry, it may be cylindrical in shape or a regular polygon such as for example a hexagon.

In other examples, the coaxial electro-magnetic configuration may be an asymmetric configuration in which the conductive exterior boundary of the filter cavity 22 does not have rotational symmetry about a central longitudinal axis 11.

The electro-magnetic configuration is electrically terminated via variable capacitive coupling that tunes a resonant frequency of the resonant mode.

The capacitive coupling between the conductive resonator head portion 34 and a conductive closure 26 of the filter cavity 22 capacitively loads the resonant cavity changing its electrical length L. A gap 50 between the conductive resonator head portion 34 and the conductive closure 26 of the filter cavity 22 controls the capacitive loading. Reducing the size of the gap 50 by bringing the conductive resonator head portion 34 closer to the conductive closure 26 increases capacitance and decreasing the size of the gap 50 increases capacitance.

The boundary conditions for an electrically terminated coaxial electro-magnetic configuration support a TEM mode at quarter wavelength resonance. The quarter wavelength resonance has a standing quarter wavelength wave within the electrical length L of the filter cavity 22. The resonant wavelength λ_o (the wavelength equivalent to the resonant frequency f_o) is 4*L.

In some but not necessarily all examples, the filter cavity 22 has a physical length between a base 23 and the conductive closure 26, parallel to the longitudinal axis 11, that is less than half a resonant wavelength λ_o. That is, the length of the filter cavity 22 is, for example, sufficient to support a quarter wavelength resonant mode at the resonant frequency f_o but is of insufficient length to support a half wavelength resonant mode at the resonant frequency f_o. In some examples, the length of the filter cavity 22 is, for example, λ_o/8 or between λ_o/8 and λ_o/4.

Fig. 3 illustrates an example of an electrical model that may be used to understand, in part, the operation of the bandpass filter 10. At a simple level, the bandpass filter 10 may be modelled as a serially connected inductance L and variable capacitance C. The variable capacitance C may be varied by varying the gap 50 between the conductive resonator head portion 34 and the conductive closure 26 of the filter cavity 22. As is well known in the art, the resonant frequency of a series LC circuit is 2π (LC) ^-1/2, varying the capacitance therefore varies the resonant frequency according to this model. The gap 50 and capacitance C may be controlled by a user by positioning the resonator head portion 34 of the moveable conductive resonator element 30 at an appropriate position within the filter cavity 22. This may be achieved by controlling the degree to which the conductive positioning portion 32 extends through the aperture 24 of the housing 20 into the filter cavity 22.

There is a fixed spatial relationship between the conductive resonator head portion 34 and the conductive positioning portion 32, therefore moving the conductive positioning portion 32 further into the filter cavity 22 necessarily moves the conductive resonator head portion 34 closer to the conductive closure 26 of the bandpass filter 10, reducing the gap 50 and increasing the associated capacitance. It will therefore be appreciated that the resonant frequency of the bandpass filter 10 may be controlled by controlling the extent to which the conductive positioning portion 32 extends through the aperture 24 of the filter cavity 22.

In some, but not necessarily all, examples, the moveable resonator element 30 may be a one-piece element in which the resonator head portion 34 and the conductive positioning portion 32 are portions of a single conductive element.

It may, in some examples, be desirable to not only control the position of the conductive resonator head portion 34 but also to prevent the position of the conductive resonator head portion 34 changing except under user-control. In some, but not necessarily all, examples, the aperture 24 extending through the housing 20 to the filter cavity 22 is a threaded aperture 24 and the conductive positioning portion 32 is a threaded screw portion 32 configured to engage with the threaded aperture 24. In these examples, the position of the conductive resonator head element 34 within the filter cavity 22 is controlled by rotating the moveable conductive resonator element 30 so that the threads of the threaded screw portion 32 via engagement with the threads of the threaded aperture 24 cause the threaded screw portion 32 to move along the central longitudinal axis 11. Typically, if the moveable conductive resonator element 30 is rotated clockwise, looking from the direction of the conductive positioning portion 32 towards to the conductive resonator head portion 34, the conductive resonator head portion 34 moves upwards towards the conductive closure 26 of the filter cavity 22. When the moveable conductive resonator element 30 is rotated counter-clockwise, the resonator head portion 34 moves away from the conductive closure 26 of the filter cavity 22 increasing the gap 50.

In some, but not necessarily all, examples, the threaded screw portion 32 may have an end 35 that is outside the housing 20 and is configured to enable clockwise or counter-clockwise rotation of the threaded screw portion 32 using a tool.

In some, but not necessarily all, examples, the moveable conductive resonator element is configured to fixedly position the conductive resonator head portion 34 at one of multiple different positions along a length of the filter cavity 22. For example, the position of the threaded screw portion 32 relative to the threaded aperture 24 may be retained so that the position is fixed without further user input. This may be, for example, achieved by using self-locking threads on the threaded aperture 24 and/or threaded screw portion 32, for example using Nylok threads, or it may alternatively be achieved by using a fixing nut that passes over the threaded screw conductive positioning portion 32 and abuts an exterior of the housing 20 adjacent the aperture 24.

In some examples, but not necessarily all examples, an interior surface of the conductive closure 26 of the filter cavity 22, which define a part of the conductive exterior boundary of the filter cavity 22, is a flat surface. The conductive closure 26 closes the housing and forms a part of the closed housing. An interior surface of the conductive closure 26 defines that part of the exterior conductive boundary of the filter cavity 22 that opposes the aperture 24. That is, there are no extensions from the surface of the conductive closure 26 that extend into the filter cavity 22. In particular, no extensions from the conductive closure 26 of the filter cavity 22 overlap the conductive resonator head portion 34 of the moveable conductive resonator element 30 in a transverse direction nor overlap, in a transverse direction, with the post 28 (if any) circumscribing the aperture 24 and extending parallel to the longitudinal axis 11.

As the post 28 defining the aperture 24 has limited extension into the filter cavity 22, in some but not necessarily all examples, a length of the conductive positioning portion 32 that extends beyond the aperture 24 of the housing 20 into the filter cavity 22 is exposed within the filter cavity 22. For example, the length of the threaded screw portion 32 between the end of the post 28 defining the aperture 24 and the conductive resonator head portion 34 of the moveable conductive resonator element 30 is, in some examples, greater than a length of the threaded screw portion 32 that is retained within the threaded aperture 24.

Figs. 4A, 4B, 5A, 5B, 6A, 6B and 7A illustrate different examples of a bandpass filter 10 (Figs. 4A, 5A, 6A, 7A) and the corresponding moveable conductive resonator elements 30 of those bandpass filters 10 (Figs. 4B, 5B, 6B) .

It will be appreciated from these examples that there may be a lateral gap 52 between the conductive resonator head portion 34 of the moveable conductive resonator element 30 and the side wall of the filter cavity 22. That is, the conductive resonator head portion 34 is not necessarily close-fitting within the filter cavity 22.

In each of these examples the conductive resonator head portion 34 has a circular cross-section in the radial plane (orthogonal to the longitudinal axis 11) and the conductive positioning portion 32 has a smaller circular cross-section, in the radial plane.

In the example of Fig. 4, the resonator head portion 34 is a hollow cup with an opening towards the conductive closure 26. The cupped conductive resonator head portion 34 is illustrated in perspective view in Fig. 4B

In the example of Fig. 5A, the conductive resonator head portion 34 is a hollow cup with a fluted rim edge. The fluted cup conductive resonator head portion 34 is illustrated, in perspective view, in Fig. 5B.

In the example of Fig. 6A, the conductive resonator head portion 34 comprises a disc mounted partially along the conductive positioning portion 32. The disc-like conductive resonator head portion 34 is illustrated, in perspective view, in Fig. 6B.

In some, but not necessarily all examples, the bandpass filer 10 comprises an adjustment element 80 for controlling capacitive coupling between the conductive resonator head portion 34 and a conductive closure 26 of the filter cavity 22. For example, in the example of Fig 6B the adjustment element 80 is a part of the moveable conductive resonator element 30. In Fig 6B, the adjustment element 80 is positioned at a terminal end of the moveable conductive resonator element 30 on a terminal end of that part of the conductive positioning portion 32 that extends through the disk-like conductive resonator head portion 34.

In Fig. 7A the conductive resonator head portion 34 is a disc mounted at a terminal end of the conductive positioning portion 32.

It will be appreciated from Figs. 4A, 5A, 6A and 7A, that each of the filter cavities 22 comprises only a single aperture 24 and comprises only a single moveable conductive resonator element 30 that passes through the single aperture 24. The filter cavities 22 do not comprise any additional apertures used for the tuning of the resonant frequency. That is there is a single user controllable input namely the conductive positioning portion 32 that is used to tune the resonant frequency of the bandpass filter.

In these examples, the length of the filter cavity 22 may be of a length that supports microwaves in a λ/4 resonant mode. The electrical length of the filter cavity 22 may be λ/4 at resonance, however, the physical length of the filter cavity 22 may be less.

The physical length of the filter cavity may, for example, be of the order of 12mm (or less) and may enable a resonant frequency of the order of 2.4GHz or similar. The radius of the circular cross-section of the conductive head portion 34 may be of the order of 5mm, for example, providing a diameter of 10mm with a tolerance of 0.05mm. The depth of the conductive head portion 34 parallel to the longitudinal axis 11 may be of the order of 1.2mm. The radial cross-section of the conductive positioning portion 32 may be of the order of 1.5mm (diameter 3.0mm) with a tolerance of 0.05mm.

Figs. 8A and 8B illustrate examples of a bandpass filter 10 that comprises multiple filter cavities 22. Each filter cavity 22 comprises one or more moveable conductive resonator elements 30 as previously described and the previous description of the bandpass filter 10 given above is also relevant to this example.

In this example, but not necessarily all examples, each of the multiple filter cavities 22 does not have rotational symmetry about a longitudinal axis. The bandpass filter 10 may not therefore operate only in a λ/4 resonant TEM mode. The individual capacitive couplings between each of the moveable conductive resonator elements 30 and the conductive closure 26 of the filter cavities tunes the characteristics of the bandpass filter such as resonant frequency and operational resonant bandwidth.

In this example, the housing 20 of the bandpass filter 10 comprises multiple filter cavities 22 and each of the filter cavities 22 has one or more apertures 24 extending through the housing 20 to the filter cavity 22. Each of the apertures 24 is associated with a different moveable conductive resonator element 30. Each moveable conductive resonator element 30 comprises a conductive positioning portion 32 and a conductive resonator head portion 34. The conductive positioning portion 32 extends through the associated aperture 24 of the housing 20 into the filter cavity 22 and positions the resonator head portion 34 within the filter cavity 22.

The position of each of the multiple different moveable conductive resonator elements 30 may be separately controlled. The bandpass filter 10 is arranged so that there is cross-coupling between the filter cavities 22. Coupling elements 60 between the filter cavities 22 provide for electro-magnetic coupling between adjacent filter cavities 22.

The arrangement of multiple filter cavities 22 and coupling elements 60 provides for multiple different routes for electro-magnetic energy at the resonant frequency f_o to travel through the bandpass filter 10 between an input port 102A and an output port 102B. Each of the different multiple routes travels through a different sequence of one or more of the filter cavities 22 between the input port 102A and the output port 102B. This can be designed to provide an asymmetric frequency response.

A common conductive closure 26 may be used to close all of the filter cavities 22.

The coupling elements between the filter cavities 22 may, for example, be windows between the cavities 22.

In some but not necessarily all examples, the filter cavities 22 may have the same orientation, such that the multiple conductive positioning portions 32 of the multiple moveable conductive resonator elements 30 extend from the housing 20 from multiple apertures 24 on the same external face of the housing 20.

Figs. 9A, 9B and 9C illustrate examples of how a bandpass filter 10, as previously described, may be used.

In Fig. 9A, a network element 100, such as for example a base station, comprises transmission circuitry 102 which is connected to antenna 104 via at least the bandpass filter 10.

In Fig. 9B, a network element 100, such as for example a base station, comprises receiver circuitry 106 which is connected via at least the bandpass filter 10 to an antenna 104.

In Fig. 9C, a network element 100, such as for example a base station, comprises transmission circuitry 102 connected to an antenna 104 via at least a first bandpass filter 10₁ and reception circuitry 106 connected to the same antenna 104 via at least a second bandpass filter 10₂.

Fig 10 illustrates an example of a method 200 for manufacturing a bandpass filter 10. The method 200 comprises: at block 202, providing a moveable conductive resonator element 30 comprising a conductive positioning portion 32 and a conductive resonator head portion 34 within a bandpass filter 10, by positioning the conductive resonator head portion 34 within a filter cavity 22 defined by a housing 20 of the bandpass filter 10 and by extending the conductive positioning portion 32 through an aperture 24 extending through the housing 20 between the filter cavity 22 and an exterior of the bandpass filter 10.

The method 200 also comprises at block 202, a stage for tuning the bandpass filter 10.The conductive resonator head portion 34 is positioned within the filter cavity 22 by moving the conductive positioning portion 32 relative to the aperture 24.

The above described bandpass filter 10 can be configured as a compact, high-performance bandpass filter.

Where a structural feature has been described, it may be replaced by means for performing one or more of the functions of the structural feature whether that function or those functions are explicitly or implicitly described.

As used here ‘module’ refers to a unit or apparatus that excludes certain parts/components that would be added by an end manufacturer or a user. the bandpass filter 10 as illustrated in Figs 1, 4A, 5A, 6A, 7A, 8A, 8B, 9A, 9B, 9C may be a module. The moveable conductive resonator element 30 as illustrated in Figs 1, 4A, 4B, 5A, 5B, 6A, 6B, 7A, 8A, 8B may be a module.

The term ‘comprise’ is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising Y indicates that X may comprise only one Y or may comprise more than one Y. If it is intended to use ‘comprise’ with an exclusive meaning then it will be made clear in the context by referring to “comprising only one. . ” or by using “consisting” .

In this brief description, reference has been made to various examples. The description of features or functions in relation to an example indicates that those features or functions are present in that example. The use of the term ‘example’ or ‘for example’ or ‘may’ in the text denotes, whether explicitly stated or not, that such features or functions are present in at least the described example, whether described as an example or not, and that they can be, but are not necessarily, present in some of or all other examples. Thus ‘example’ , ‘for example’ or ‘may’ refers to a particular instance in a class of examples. A property of the instance can be a property of only that instance or a property of the class or a property of a sub-class of the class that includes some but not all of the instances in the class. It is therefore implicitly disclosed that a feature described with reference to one example but not with reference to another example, can where possible be used in that other example but does not necessarily have to be used in that other example.

Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed.

Features described in the preceding description may be used in combinations other than the combinations explicitly described.

Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not.

Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not.

Whilst endeavoring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

I/we claim:

Claims

A bandpass filter having a bandpass at a first resonant frequency comprising:

a housing comprising a resonant cavity and an aperture extending through the housing to the resonant cavity； and

a moveable conductive resonator element comprising a conductive positioning portion and a conductive resonator head portion, wherein the conductive positioning portion extends through the aperture of the housing into the resonant cavity and positions the resonator head portion within the resonant cavity.
A bandpass filter as claimed in claim 1, wherein the moveable conductive resonator element is configured to fixedly position the conductive resonator head portion at one of multiple different positions along a length of the resonant cavity, wherein a gap between the conductive resonator head portion and a conductive closure of the resonant cavity is controlled by repositioning the conductive resonator head portion within the resonant cavity.
A bandpass filter as claimed in claim 1 or 2, wherein the aperture extending through the housing to the resonant cavity is a threaded aperture and wherein the conductive positioning portion is a threaded screw portion configured to engage with the threaded aperture.
A bandpass filter as claimed in any preceding claim, wherein there is a fixed spatial relationship between the conductive positioning portion and the conductive resonator head portion of the moveable conductive resonator element.
A bandpass filter as claimed in any preceding claim, wherein the moveable conductive resonator element is a one-piece resonator element defining the conductive positioning portion and the, differently shaped, conductive resonator head portion.
A bandpass filter as claimed in any preceding claim, wherein the resonant cavity has a physical length less than half of a wavelength at the first resonant frequency.
A bandpass filter as claimed in any preceding claim, wherein an interior surface of a conductive closure is flat, wherein the interior surface of the conductive closure defines part of a conductive boundary of the resonant cavity and opposes the aperture.
A bandpass filter as claimed in any preceding claim having a coaxial electro-magnetic configuration in which the resonant cavity has a central longitudinal axis and the conductive positioning portion extends at least partially along the central longitudinal axis.
A bandpass filter as claimed in any preceding claim configured to support a transverse electromagnetic mode in which the electric field and the magnetic field are only radial to a central longitudinal axis of the resonant cavity.
A bandpass filter as claimed in any preceding claim, wherein the moveable conductive element is configured to change a capacitance of a resonant circuit defining the first resonant frequency wherein the conductive resonator head portion comprises an adjustment element for controlling capacitive coupling between the conductive resonator head portion and a conductive closure of the resonant cavity.
A bandpass filter as claimed in any preceding claim, wherein the resonant cavity comprises only a single aperture.
A bandpass filter as claimed in any preceding claim comprising:

multiple resonant cavities, each resonant cavity comprising an aperture extending through the housing to the resonant cavity； and multiple moveable conductive resonator elements each comprising a conductive positioning portion and a conductive resonator head portion, wherein the conductive positioning portion extends through a respective aperture of the housing into a respective resonant cavity and positions the resonator head portion within the respective resonant cavity wherein there are multiple different routes by which electro-magnetic energy may pass through the multiple resonant cavities from an input port to an output port.
A bandpass filter as claimed in claim 12 further comprising a common conductive closure for all of the multiple resonant cavities.
A bandpass filter as claimed in claim 12 or 13 further comprising electro-magnetic coupling elements between adjacent ones of the multiple resonant cavities to provide routes for the transmission of electro-magnetic energy from cavity to cavity.
A bandpass filter as claimed in any one of claims 12 to 14, wherein the multiple resonant cavities have the same orientation wherein the multiple apertures to the multiple resonant cavities are located on a common external face of the bandpass filter.
A base station for radio telecommunications comprising a bandpass filter as claimed in any preceding claim.
A method comprising:

providing a moveable conductive resonator element comprising a conductive positioning portion and a conductive resonator head portion within a bandpass filter, by positioning the conductive resonator head portion within a resonant cavity defined by a housing of the bandpass filter and by extending the conductive positioning portion through an aperture extending through the housing between the resonant cavity and an exterior of the bandpass filter.