EP2912714B1 - Tunable high frequency filter - Google Patents

Description

The invention relates to a high-frequency filter in coaxial design according to the preamble of claim 1.

In radio systems, especially in the mobile sector, a common antenna is often used for transmit and receive signals. The transmit and receive signals each use different frequency ranges, and the antenna must be suitable for transmitting and receiving in both frequency ranges. For the separation of the transmit and receive signals therefore a suitable frequency filtering is required, with the one hand, the transmission signals from the transmitter to the antenna and on the other hand, the received signals are forwarded from the antenna to the receiver. For the distribution of the transmission and reception signals or for the merger or separation of mobile radio bands, inter alia, high-frequency filters are used in coaxial design.

Two interconnected high-frequency filters form a so-called duplex switch, which allows a largely decoupled interconnection of transmitters and receivers to a common antenna. For example, a pair of high-frequency filters can be used, both of which allow a certain frequency band (bandpass filter). Alternatively, a pair of high frequency filters may be used, both of which block a particular frequency band (bandstop filter). Further, a pair of high frequency filters may be used, one of which filters below a frequency between transmit and receive bands and blocks frequencies above that frequency (low pass filter) and the other filter locks frequencies below a frequency between transmit and receive bands and above passing frequencies (high pass filter). Other combinations of the just mentioned filter types are conceivable.

High-frequency filters are often constructed from coaxial resonators, since they consist of milling or casting parts, whereby they are easy to produce. In addition, these resonators ensure a high electrical quality and a relatively high temperature stability.

A temperature-compensated coaxial resonator is out of the WO 2006/058965 A1 known. It comprises according to an embodiment in addition to a coaxial housing with a corresponding inner conductor, which ends at a distance below a lid, an embodiment for adjusting the resonant frequency. As usual, a screw is used which can be turned in and out in different ways in the lid. The actuator is located axially aligned with the inner conductor and has at its the inner conductor facing the front end of a dielectric compensation element, which is designed disk-shaped, on.

A comparable arrangement is also from the JP 62123801 A known.

A band filter for very short electromagnetic waves is from the DE 12 65 316 B known. According to this prior publication, the single-circuit band filter comprises an inner conductor with a cup-shaped capacitive load formed at the free end of the inner conductor whose diameter corresponds to a multiple of the diameter of the inner conductor. In this cup-like capacitive extension, a pot-like punch can be immersed, which is attached to a rod which is slidably mounted in a bushing at the opposite end of the free end of the inner conductor wall of the band filter.

The EP 2 044 648 B1 describes an example of a coaxial RF filter. This filter comprises a resonator with an inner conductor and an outer conductor, wherein in a housing cover of the resonator, a tuning element is provided which has an external thread. In the corresponding housing cover a threaded receptacle is provided with a thread. The thread pitch of the external thread of the tuning element differs from the thread pitch of the internal thread of the threaded receptacle in at least a portion of the internal thread and the external thread, whereby an automatic self-locking of the Abstimmelements is realized. By the thread error between the external thread and the internal thread sets a maximum tension between the external thread of the threaded member and the internal thread of the threaded hole in the resonant filter housing at the axially remote threaded sections, which are generated precisely at these locations due to the high contact forces clearly reproducible electrical conditions, thereby unwanted intermodulation effects can be avoided. The disadvantage of this type of high-frequency filter, especially the critical contact transition from the external thread of the tuning element to the internal thread of the housing cover. Due to metal abrasion between the tuning element and the housing cover, so-called intermodulation products, ie interference frequencies can form.

The US 4,380,747 describes another example of a coaxial high frequency filter. The high frequency filter disclosed in this document comprises a coaxial resonator consisting of an electrically conductive outer conductor and an electrically conductive inner conductor. The outer conductor and the inner conductor are connected to one another via an electrically conductive base plate. The coaxial resonator is terminated by an electrically conductive cover. The frequency tuning is done by a grub screw whose immersion depth in the inner conductor is frequency-determining. If the frequency has been set precisely, the balancing threaded pin is fixed with a counter nut. A disadvantage of this type of coaxial resonator is the critical contact transition from the threaded pin to the lid. Due to metal abrasion and undefined contact points between the threaded pin and the threaded hole, intermodulation products can become form. Another disadvantage is the change of the tuned frequency during the counter process. This is due to a minimal axial movement of the balancing pin during operation of the lock nut. This effect has a negative effect on the overall alignment time, since several correction adjustments are required.

The high-frequency filters described above have the common feature that the tuning elements, which are held in the housing cover variable in position, made of metal. The change in position of the tuning elements is achieved in that the tuning elements have an external thread which is screwed into an internal thread of the housing cover. Consequently, the threads are located in the high-frequency-critical resonator interior, which inevitably causes intermodulation problems. Furthermore, made of aluminum resonator housing for receiving the corresponding tuning element press-in thread, since aluminum is too soft for fine thread, so that the thread of the adjustment can seize. As already mentioned above, the tuning elements in the coaxial high-frequency filters described above are arranged at high-frequency critical points, so that currents also flow over the contact region of the external thread of the tuning element and the internal thread of the resonator housing. In the publication EP 2 044 648 B1 This problem is addressed by strained threads. However, a corresponding coaxial RF filter is expensive to manufacture and therefore expensive.

Furthermore, high-frequency filters known from the prior art have insufficient frequency stabilization with a temperature change. When temperature fluctuations occur, there is a change in the mechanical length of the inner conductor tube. Since the mechanical length is inversely proportional to the frequency, the resonant frequency of the filter decreases as the mechanical length increases with increasing temperature. For example, this effect can cause a change in the resonant frequency of 5.7 MHz for a filter with a resonant frequency of 2.4 GHz at a temperature difference of 120 ° C.

With temperature changes, another second effect occurs. At the free end of the inner conductor, a capacitance between the lid and the inner conductor tube is formed (so-called head capacity). This capacity is also frequency determining. If there is an increase in temperature, the inner conductor tube and the walls of the outer conductor housing expand by the same factor. Since the walls of the outer conductor housing are higher than the inner conductor tube, that is have a greater axial length than the inner conductor tube, there is an increase in the distance between the inner conductor tube and cover, resulting in a decrease in the head capacity and results in an increase in the resonant frequency , This effect thus counteracts the reduction of the resonance frequency due to the greater mechanical length of the inner conductor tube with temperature increases. However, this effect is smaller than the above-described resonance frequency reduction due to the expansion of the resonator, so that there is insufficient temperature compensation.

In order to enhance the effect of decreasing the head capacitance with temperature increases, it is known in the prior art to fabricate parts of the inner conductor tube or the entire inner conductor of a different material having a lower coefficient of thermal expansion than the outer conductor housing. As a result, the head capacity becomes even smaller with an increase in temperature and compensates for the effect of the frequency increase due to the temperature-induced linear expansion. With such filters, a temperature compensation can be achieved in that the resonators in the filter in a certain temperature range have a constant resonance frequency. However, this type of compensation has some disadvantages. By having the inner conductor or parts of the inner conductor made of a different material than the housing, an impurity always occurs between the two materials, even when both are soldered together. This can cause intermodulation problems apart from manufacturing issues.

Furthermore, several different materials must be combined in the high-frequency critical resonator chamber, with mechanical tolerances in this room can have serious effects on the filter. If an inner conductor is e.g. not placed in the filter within a few hundredths of a millimeter, the coupling bandwidth changes to all adjacent resonators, which in turn can cause tuning problems.

From the US Pat. No. 6,407,651 B1 is a high-frequency filter with a temperature compensation device known. This high-frequency coaxial resonator comprises an outer conductor housing with an inner conductor tube arranged axially thereon. The inner conductor tube ends at a distance below a lid closing the outer conductor housing. The inner conductor tube is provided with a longitudinal bore passing through the inner conductor tube into which a screw can be screwed from below. The screw can be screwed into a counterpart, which has a circumferential edge at a distance from the free end of the inner conductor tube, so that a bellows-shaped element can be inserted between this peripheral edge of the counterpart and the free end edge of the inner conductor tube. The screw has a coefficient of thermal expansion which is less than the thermal expansion coefficient of the existing example of aluminum inner conductor tube. The bellows-shaped compensation element further consists of a different material compared to the material of the screw and the inner conductor tube.

In the case of an increase in temperature with a corresponding increase in the axial length of the inner conductor tube is ensured by this compensation device that the bellows-shaped compensation element is further compressed accordingly further, since the overall construction of screw and counterpart changes in the overall length contrast only slightly in length. However, this embodiment also has various disadvantages, since additional elements are necessary, since the bellows-shaped element must be welded to the circumferential end wall of the inner conductor tube, etc. Intermodulation problems can also be caused thereby.

A generic high frequency filter is from the EP 0 068 919 A1 known. It comprises an inner conductor which is anchored to a wall of the filter and extends in the direction of an opposite wall and ends in front of this wall. At the free end of this inner conductor a blind hole is introduced. The inner conductor has a same outer diameter over its entire length.

Opposite the free end of the inner conductor, an adjustment element is anchored in the wall. This adjusting element comprises a screw-threaded metal piece, which is screwed with an external thread in an internally threaded bore of the wall. In this male threaded metal piece, an inner bore is introduced, in which engages an axially displaceable pin. This pin may be made of metal or of a dielectric material. On the side facing the housing wall of the adjustable pin a blind bore is also introduced, in which a dielectric rod is used, which ultimately projects beyond the externally threaded metal piece frontally and engage differently in the blind hole in the inner conductor depending on the setting of the setting ,

By this construction, the propagation of electric currents between the adjusting pin and the male threaded metal part surrounding the adjusting pin should be avoided. In addition, the setting pin with the dielectric rod as a whole can be formed as a dielectric component.

Nevertheless, however, currents may be caused between the piece of metal provided with the external thread, which is screwed into a housing wall provided with an internal thread, and the internally threaded bore.

It is therefore an object of the present invention, starting from the generic state of the art, to provide an improved and simple way of tuning resonators, i. To provide single resonators, high frequency filters, crossovers, bandpass filters, bandstop filters and the like, which is cheaper to implement and does not have the above-described intermodulation problems and also has an improved temperature compensation.

The object is achieved by a high-frequency filter according to claim 1. Advantageous embodiments of the invention are specified in the subclaims.

The high-frequency filter according to the invention is based on a tuning element, which comprises a dielectric material and / or is formed from a dielectric material. This results in the contact points of the Abstimmelements with the housing cover or with a arranged between the housing cover and the tuning element Socket no intermodulation problems, as the use of dielectric material (plastics, ceramics, etc.) for the tuning element avoids the occurrence of current transitions in metallic threaded areas. Thus, it is also possible that the tuning element is operated from the housing cover side, ie from the same side as usually also additional adjustment elements (eg for coupling adjustment between resonators). A vote on two sides of the high-frequency filter, ie on the housing cover side and on the housing bottom side is thereby avoided without an intermodulation problem occurs. It is also advantageous that the housing bottom in the high-frequency filter according to the invention has no adjustment opening, which eliminates additional sealing measures such as sealing films, sealing adhesives or environmental cover in outdoor applications. It is also advantageous that the thermal expansion of the existing of a dielectric material or such comprehensive Abstimmelements in the high-frequency filter has a temperature-compensating effect, ie temperature-induced frequency changes can be significantly minimized. Furthermore, it should be noted that a suitably trained tuning element is particularly inexpensive to manufacture, since due to the choice of material, the tuning can be made very inexpensively, for example by injection molding.

The tuning element according to the invention has a central portion, by means of which the tuning element is held variable in position. Furthermore, the tuning element on a Umlaufwandung, by a circumferential around the central portion recess of the central portion is separated, so that between the central portion and the Umlaufwandung a distance space is formed. The central portion is connected to the Umlaufwandung via a Abstimmelementboden. The housing bottom opposite end face end of the socket is receivable in the space between the central portion and Umlaufwandung the Abstimmelements, so that the Umlaufwandung between the socket and the inner conductor is arranged in the region of the longitudinal recess. The tuning element is thus bell-shaped and is inversely T-shaped in cross section.

By an appropriately trained high-frequency filter, the resonant frequency of the resonator is particularly effective adjustable. In addition, the correspondingly formed high-frequency filter has particularly good temperature compensation properties. Furthermore, an appropriately designed high-frequency filter ensures effective overvoltage protection. Because the distance between the inner conductor tube in the region of the end face and the housing bottom facing the front end of the socket is particularly small, so that in this area the maximum electric field strength occurs at the so-called open end of the inner conductor. At this point there is an increased risk of overturning at higher transmission powers due to resonance effects. The peripheral wall of the tuning element is arranged between the inner conductor tube and the threaded bush, so that the balancing element or the tuning element reliably protects against flashovers due to its insulating effect.

Preferably, the tuning element further comprises a circumferential to the tuning collar, which is connected to the housing cover opposite the front end of the Umlaufwandung and extends radially directed away from the central portion. An appropriately designed high-frequency filter has a further increased flashover protection at the open end of the inner conductor, since the collar spans the frontal end of the inner conductor, so that a flashover between the inner conductor and the inner housing cover side is reliably prevented.

Preferably, the peripheral wall of the tuning element has a peripheral edge, so that the peripheral wall above the peripheral edge, i. directed towards the housing cover has a smaller wall thickness than below the peripheral edge, i. directed towards the case back. An appropriately designed high-frequency filter has again improved temperature compensation properties.

The female part receiving the tuning element can be materially connected to the housing cover. This can for example be achieved in that the housing cover is made of a casting, wherein the socket is an integral part of the cast cover. Alternatively, however, the socket can also be a separate component which is connected to the housing cover. A corresponding connection can be realized, for example, by pressing the socket into the housing cover or by soldering or welding the socket to the housing cover.

Preferably, the inner conductor has a longitudinal recess which extends from the housing cover opposite the front end of the inner conductor in the direction of the housing bottom, wherein the tuning element is insertable into the longitudinal recess of the inner conductor. By means of a corresponding design of the high-frequency filter, its resonant frequency can be set particularly effectively.

Preferably, the bush ends at the level of the front end of the inner conductor or immersed in the longitudinal recess of the inner conductor, wherein the tuning element protrudes from the housing bottom opposite the front end of the socket. A corresponding embodiment of the high-frequency filter allows a particularly effective adjustment of the resonant frequency of the high-frequency filter.

Preferably, the housing wall and the inner conductor of a first material having a first coefficient of thermal expansion, or the housing wall consists of a first coefficient of thermal expansion having first material and the inner conductor consists of a second coefficient of thermal expansion having second material. The tuning element consists of a third material having a third coefficient of thermal expansion. The third thermal expansion coefficient of the third material is greater than the first thermal expansion coefficient of the first material and / or greater than the second thermal expansion coefficient of the second material.

At a temperature increase expands in the axial direction of the Abstimmenements the tuning element stronger than the inner conductor and the housing wall, so that a greater proportion of Umlaufwandung is arranged above the peripheral edge between the inner conductor and the socket, resulting in less dielectric material between the inner conductor and the Socket, which reduces the head capacity of the resonator. Conversely, when the temperature is reduced, the tuning element contracts more in the axial direction than the inner conductor and the housing wall, so that a smaller portion of the peripheral wall is located above the peripheral edge between the inner conductor and the socket, resulting in more dielectric material between the inner conductor and the socket which increases the head capacitance of the resonator.

Consequently, the reduction in the head capacitance is increased with a temperature increase in the correspondingly formed high-frequency filter, so that due to the reduction of the head capacity, the associated increase in the resonance frequency is stronger, resulting in a stronger temperature compensation, because with a temperature increase decreases in parallel the resonance frequency due to mechanical extension of the inner conductor tube. The same applies vice versa for the temperature compensation with a temperature reduction:

In a preferred embodiment of the invention, the height of the sleeve provided on the housing cover with the internal thread in relation to the diameter of the socket to a degree which is greater than or equal to 1.5. Such values will in any case ensure that that no electromagnetic radiation can escape to the outside.

Figur 1:

einen schematischen axialen Querschnitt durch ein erfindungsgemäßes Hochfrequenzfilter gemäß einer ersten Ausführungsform der vorliegenden Erfindung; und

Figur 2:

einen schematischen axialen Querschnitt durch das erfindungsgemäße Hochfrequenzfilter gemäß einer zweiten Ausführungsform der vorliegenden Erfindung.

The invention will be explained in more detail with reference to drawings. In detail:

FIG. 1:

a schematic axial cross section through a high-frequency filter according to the invention according to a first embodiment of the present invention; and

FIG. 2:

a schematic axial cross section through the high-frequency filter according to the invention according to a second embodiment of the present invention.

In the description that follows, like reference numerals designate like components and like features, so that a description with respect to a component once made with respect to a drawing also applies to the remaining drawings or figures, so that a repetitive description is avoided.

In FIG. 1 a high-frequency filter according to the invention is shown, which comprises a resonator 1. However, the high-frequency filter can also comprise a plurality of resonators 1 coupled to one another. Each resonator 1 comprises an inner conductor 10 and an outer conductor housing which in turn comprises a housing bottom 20, a housing cover 22 spaced from the housing bottom 20 and a housing wall 24 encircling the housing bottom 20 and the housing cover 22. Out FIG. 1 It can be seen that the inner conductor 10 is formed integrally with the housing bottom 20 and the housing wall 24. The housing cover 22 lies on the free ends of the housing wall 24 and may be mechanically connected, for example by means not shown screws with the end faces of the housing wall. However, it is also possible that the housing cover 22 is formed integrally with the housing wall. A free end 11 of the inner conductor 10, which is the end face of the inner conductor 10, has a predetermined distance to the inside of the housing cover 22.

Out FIG. 1 It can be seen that the inner conductor 10 has a longitudinal recess 12 which extends from the housing cover 22 opposite the front end of the inner conductor 10 in the direction of the housing bottom 20. In the in the Figures 1 and 2 resonators 1 shown, the inner conductor 10 are formed as inner conductor tubes 10 and as inner conductor cylinder 10.

From the Figures 1 and 2 It can be seen that the high-frequency filter further comprises a bushing 40, which is formed in the illustrated embodiments as a threaded bushing 40 with an internal thread 41. The threaded bushing 40 is galvanically connected to the housing cover 22. The threaded bushing 40 may consequently consist of a metal or may consist of a dielectric material which is coated with a metal layer. The same applies to the housing cover 22, which is formed either from a metal or coated with metal. The bush 40 may also be integrally formed with the housing cover 22, so that the bushing 40 is integrally connected to the housing cover 22. Furthermore, it is possible that the threaded bushing 40 with the housing cover 22, for example by a Pressing is connected. The threaded bushing 40 may be galvanically connected to the housing cover 22 via a soldering or welding.

The threaded bushing 40 dips into the longitudinal recess 12 of the inner conductor 10. However, it is also possible that the threaded bush 40 ends at the level of the front end 11 of the inner conductor 10. It is also possible that the threaded bush 40 ends above the front end 11 of the inner conductor 10. The in the Figures 1 and 2 shown threaded bushing 40 also extends outside the Resonatorinnenraums, so that the housing wall of the threaded bushing 40 extends beyond the housing cover 22 to the outside.

The high-frequency filter according to the invention further comprises a tuning element 30, which is held in its axial position variable in position in the socket 40. The tuning element 30 has for this purpose an external thread 32 on a central portion 31. The external thread 32 is engaged with the internal thread 41 of the threaded bushing 40, so that its axial position can be changed by rotation of the tuning element 30. The tuning element 30 further comprises a circumferential wall 33, which is separated from the central section 31 by a recess 35 running around the central section 31. Thus, a distance space 35 is formed between the central portion 31 and the Umlaufwandung 33. The central portion 31 is connected to the Umlaufwandung 33 via a Abstimmelementboden 36.

The housing bottom 20 opposite the front end of the threaded bushing 40 is in the distance space 35th received between the central portion 31 and the peripheral wall 33 of the Abstimmelements 30. Thus, the Umlaufwandung 33 is disposed between the sleeve 40 and the wall of the inner conductor tube 10. By turning the tuning element 30 into and out of the resonator interior, it is thus possible to set how much of the circumferential wall 33 is arranged between the threaded bushing 40 and the inner conductor 10, so that the head capacitance of the resonator 1 can thereby be adjusted. The tuning element 30 is preferably made of a plastic so of a dielectric. The more material the Umlaufwandung 33 is disposed between the wall of the threaded bushing 40 and the wall of the inner conductor 10, the greater the head capacity of the resonator 1. Consequently, by turning the Abstimmelements 30 into the longitudinal recess 12 of the inner conductor 10, the head capacity of the resonator can be increased. By unscrewing the Abstimmelements 30 from the longitudinal recess 12 of the inner conductor 10 is less dielectric material between the threaded bushing 40 and the inner conductor 10, so that thereby the head capacity of the resonator is lowered.

Since the tuning element 30 is formed of a dielectric material or a dielectric, such as a plastic, occur at the contact point of the external thread 32 with the internal thread 41 no intermodulation problems. By turning the Abstimmelements 30 in the threaded bushing 40 no metal abrasion, which could lead to an intermodulation problem.

In other words, since the tuning element 30 may consist, for example, of a dielectric material such as plastic, that is, including the external thread 32, there can be no current transition to the socket which is made of an electrically conductive material with the associated internal thread 41. In order to avoid such a current transition, it is basically sufficient if, for example, the tuning element 30 in its outer cladding region consists of a dielectric material, so that the entire threads are formed of a dielectric material, so that there is no current transfer with the metal or one with a metallic layer coated internal thread of the sleeve 40 can take place. In this case, so could the axial core in a smaller diameter than the outer diameter of the Abstimmelements 30 also made of metal, since this metal can nowhere contact with the surface of the internal thread 32 of the threaded bushing 40. Otherwise, it is basically noted that ultimately not only the tuning element 30 may consist wholly or partly of a dielectric material so far, but also the threaded bushing. For a threaded thread engagement with an external thread 32 of the tuning element 30 and an internal thread 41 of the threaded bush 40 each made of dielectric material also leads to the fact that no current transitions can take place in the region of the threaded threaded engagement.

The circumferential wall 33, which is arranged between the inner conductor 10 and the threaded bushing 40, is an overvoltage protection of the resonator 1. In the case of the coaxial resonator 1, the maximum field strength occurs at the open end 11 of the inner conductor 10. At high transmission powers, the risk of rollover increases from the inner conductor 10 to the threaded bushing 40. This risk of rollover is considerably reduced by the peripheral wall 33 of the Abstimmelements 30.

From the Figures 1 and 2 it can be seen that the circumferential wall 33 of the Abstimmelements 30 has a so-called peripheral edge 34. The wall thickness of the peripheral wall 33 is smaller than the wall thickness of the peripheral wall below the peripheral edge 34 above the peripheral edge 34. In the illustrated embodiments, the edge 34 faces the threaded bushing 40. However, it is also possible that this edge 34 faces the inner wall of the inner conductor 10.

In FIG. 2 Fig. 12 is a high-frequency filter according to the second embodiment of the present invention. The construction of in FIG. 2 shown high frequency filter is identical to the in FIG. 1 shown high-frequency filter, with the only difference that the tuning element 30 further comprises a circumferential collar 37 which is connected to the housing cover 22 opposite the front end of the Umlaufwandung 33 and radially directed away from the central portion 31. This collar 37 has a further reduction of the risk of rollovers. Because the collar 37 is positioned above the free end 11 of the inner conductor 10, so that the collar 37 is disposed between the free end 11 and the inner wall of the housing cover 22. Thus, a flashover between the inner conductor 10 and the housing cover 22 is also reliably prevented.

The housing bottom 20, the housing wall 24 and the inner conductor 10 are usually made of a metal, i. of a first material having a first thermal expansion coefficient. It is also possible that the housing wall 24 consists of a first material exhibiting a first thermal expansion coefficient and the inner conductor 10 consists of a second material exhibiting a second thermal expansion coefficient. For example, as previously described, the tuning element may be made of a plastic, i. consist of a third material having a third coefficient of thermal expansion. The third thermal expansion coefficient of the plastic is greater than the first thermal expansion coefficient of the first material and / or greater than the second thermal expansion coefficient of the second material. With a temperature increase, this has the consequence that the tuning element 30 expands more strongly than the inner conductor 10 and the housing wall 24, so that a larger proportion of the Umlaufwandung 33 is above the peripheral edge 34 between the inner conductor 10 and the sleeve 40. As a result, there is less dielectric material, from which the tuning element 30 is formed, between the inner conductor 10 and the socket 40, whereby the head capacity of the resonator 1 is reduced.

At a temperature decrease in turn, the tuning element contracts in the axial direction more than the inner conductor 10 and the housing wall 24, whereby a smaller proportion of the Umlaufwandung above the peripheral edge between the inner conductor 10 and the sleeve 40 is located, which in turn has the consequence that more dielectric material is located between the inner conductor 10 and the socket 40. This increases the head capacity of the resonator.

In the high frequency filter described above, the outer conductor housing may be formed of, for example, aluminum, brass, invar steel, cast aluminum or Arnite plastic with glass fiber. From selbigen materials and the housing cover 22 may be formed. Likewise, the housing may be made with the inner conductor, the housing bottom and the housing cover of a dielectric material, which is coated with an electrically conductive layer. Usually, the electrically conductive layer is attached to the lid on the inside, so that at the junction between the housing cover and peripheral housing walls of the outer conductor housing a full-surface galvanic contact is ensured. This electrically conductive layer can also be provided in the region of the bushing 40 and thereby cover the internal thread 41 of the threaded bush 40, so that the internal thread is in turn electrically conductive on its surface. The tuning element may for example be formed from acrylonitrile-butadiene-styrene (ABS plastic). The inner conductor may be formed of the same materials as the outer conductor housing.

In the exemplary embodiment shown, it is shown that the threaded bushing 40 may optionally be attached to the housing cover at different heights. It has proven to be advantageous if the height H, ie the axial length H of the threaded bushing 40 in relation to the inner diameter D of the threaded bushing 40 has a dimension which is ≥ 1.5, preferably ≥ 1.6, 1.7, 1.8, 1.9, 2.0 or even 2.25, 2.5, 2.75, 3.0 and / or more. In general, however, it is sufficient if these values are not greater than 2.0 or 2.5 or even 3.0. In all cases, it is ensured that the overall housing is optimally shielded to the outside and no electromagnetic radiation can escape or enter.

Claims (8)

1. High-frequency filter of a coaxial construction, the high-frequency filter having the following features:

   - the high-frequency filter comprises at least one resonator (1) having an internal conductor (10) and an external conductor housing (24');

   - the external conductor housing (24') comprises a housing base (20), a housing cover (22) spaced apart from the housing base (20), and a housing wall (24) extending around between the housing base (20) and the housing cover (22);

   - the internal conductor (10) is galvanically connected to the housing base (20) and extends in the axial direction from the housing base (20) towards the housing cover (22);

   - the internal conductor (10) ends at a distance from the housing cover (22) and/or is galvanically separated from the housing cover (22);

   - the resonator (1) comprises a tuning element (30), which is arranged opposing the internal conductor (10), and which is movably held in the axial location thereof in the housing cover (22), at least indirectly, and protrudes into the resonator interior;

   - the internal conductor (10) comprises a longitudinal recess (12), which extends from the face end of the internal conductor (10) opposing the housing cover (22) towards the housing base (20);

   - the tuning element (30) can be introduced into the longitudinal recess (12) of the internal conductor (10);

   - an internal thread (41), in which the tuning element (30) provided with an external thread (32) is rotatably arranged, is formed in the housing cover (22) or in a socket (40) which is provided in the housing cover (22) and also connected with the housing cover (22);

   - the tuning element (30) is formed from a dielectric material in such a way that current transitions are prevented between the external thread (32) and the internal thread (41); and

   - the tuning element (30) comprises a central portion (31), by means of which the tuning element (30) is movably held,

   characterised by the following additional features:

   - the tuning element (30) further comprises a peripheral wall (33), the tuning element (30) and the peripheral wall (33) being separated from one another by a recess (35) extending around the central portion (31), in such a way that a separating space (35) is formed between the central portion (31) and the peripheral wall (33), the central portion (31) and the peripheral wall (33) being interconnected via a tuning element base (36); and

   - the face end of the socket (40) opposing the housing base (20) can be received in the separating space (35) between the central portion (31) and the peripheral wall (33) of the tuning element (30) or dips into said space, in such a way that the peripheral wall (33) is arranged between the socket (40) and the internal conductor (10) in the region of the longitudinal recess (12) thereof.
2. High-frequency filter according to claim 1, characterised by the following feature:

   - the housing cover (22) comprises a socket (40), which is galvanically connected to the housing cover (22) and which extends towards the housing base (20).
3. High-frequency filter according to claim 1 or 2, characterised by the following features:

   - the socket (40) ends at the level of the face end of the internal conductor (10) or dips into the longitudinal recess (12) of the internal conductor (10); and

   - the tuning element (30) protrudes out of the face end of the socket (40) opposing the housing base (20) and thus dips even further into the longitudinal recess (12) of the internal conductor (10).
4. High-frequency filter according to any of claims 1 to 3, characterised in that the peripheral wall (33) of the tuning element (30) comprises a rim edge (34), in such a way that the peripheral wall (33) above the rim edge (34) has a smaller wall thickness than below the rim edge (34).
5. High-frequency filter according to any of claims 1 to 4, characterised in that the tuning element (30) further comprises a collar (37), which extends around the tuning element (30), is connected to the face end of the peripheral wall (33) opposing the housing cover (22), and extends radially away from the central portion (31).
6. High-frequency filter according to any of the preceding claims, characterised by the following features:

   - the housing wall (24) and the internal conductor (10) consist of a first material, which has a first thermal expansion coefficient, or the housing wall (24) consists of a first material, which has a first thermal expansion coefficient, and the internal conductor (10) consists of a second material, which has a second thermal expansion coefficient;

   - the tuning element (30) consists of a third material, which has a third thermal expansion coefficient; and

   - the third thermal expansion coefficient of the third material is greater than the first thermal expansion coefficient of the first material and/or greater than the second thermal expansion coefficient of the second material.
7. High-frequency filter according to claim 6 as dependent on claim 6, characterised by the following features:

   - in the event of an increase in temperature, the tuning element (30) expands more than the internal conductor (10) and the housing wall (24) in the axial direction of said tuning element, in such a way that a greater proportion of the peripheral wall (33) above the rim edge (34) is arranged between the internal conductor (10) and the socket (40), meaning that there is less dielectric material between the internal conductor (10) and the socket (40), thus decreasing a head capacitance of the resonator (1); and

   - in the event of a decrease in temperature, the tuning element (30) contracts more than the internal conductor (10) and the housing wall (24) in the axial direction, in such a way that a smaller proportion of the peripheral wall (33) above the rim edge (34) is arranged between the internal conductor (10) and the socket (40), meaning that there is more dielectric material between the internal conductor (10) and the socket (40), thus increasing a head capacitance of the resonator (1).
8. High-frequency filter according to any of the preceding claims, characterised in that the ratio between the axial height or length (H) of the socket (40) and the diameter (D) of the socket (40) has a value of ≥ 1.6, 1.7, 1.8, 1.9, 2.0, 2.25, 2.5, 2.75 and/or 3.0.

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