Abstract:
The invention relates to a tunable resonator filter. In each resonator cavity of the filter there is a movable dielectric tuning element ( 728; 748 ) to adjust the resonator&#39;s natural frequency. The tuning elements are advantageously arranged to be moved by a common control implemented by a rod ( 708 ) joining them together, to shift the filter&#39;s band through equal displacements of the natural frequencies of the resonators. When the tuning element is moved horizontally sidewards from the resonator ( 710; 720; 730; 740; 750; 760 ) axis, the electrical length and natural frequency of the resonator change. In that case, when sub-bands are used it is not necessary to tune the filters separately for each sub-band in the stage of manufacture, as the sub-band can be chosen when the filter is put into use. The tuning elements can be movable also in each resonator separately, to implement the basic tuning in connection with the manufacture of the filter. The basic tuning can be automated, in other words it can be made without inconvenient handwork.

Description:
[0001]     The invention relates to a filter consisting of resonators, which filter can be tuned after its manufacture. A typical application of the invention is an antenna filter in a base station.  
       BACKGROUND OF THE INVENTION  
       [0002]     In order to obtain a frequency response of a resonator filter, which meets the specifications, it is necessary to have right coupling strengths between the resonators, and a certain resonator frequency, or natural frequency for each resonator. In series production the variation of the natural frequencies of resonators made in the same way is usually too large to keep all natural frequencies at a sufficiently right value. Therefore each resonator in each filter must be tuned individually. Here such tuning is called basic tuning. If the filter is intended to be used as a part in a system, where the transmitting and receiving bands are divided into sub-bands, then the width of the passband of the filter must equal the width of a sub-band. Further the passband of the filter must be located at the desired sub-band. An adjusting of the natural frequencies of the resonators is sufficient to shift the passband; it is not necessary to change the couplings between the resonators.  
         [0003]     Previously it is known the use of tuning screws in the adjusting of the natural frequency of a resonator. The lid of e.g. coaxial resonator is equipped with a metal screw at the inner conductor of the resonator. When the screw is turned, the capacitance between the inner conductor and the lid changes, in which case also the natural frequency of the resonator changes. A disadvantage in the use of tuning screws is that in a multi-resonator filter it may be necessary to manually turn the screws in many stages to obtain the desired frequency response. Thus the tuning is time consuming and relatively expensive. The screw accessories increase the number of components in the filter, and the threaded screw holes mean an increased number of work steps. These facts will raise the manufacturing costs on their part. In addition the electrical contact in the threads may deteriorate in the course of time, which results in tuning drift and in increased losses in the resonator. Moreover, there is a risk of electric breakdown in high-power filters of the transmitting end if the point of the screw is close to the end of the inner conductor.  
         [0004]     Regarding a coaxial resonator, the capacitance between its inner conductor and the surrounding conductive parts can be changed by means of bendable elements, too. In a known structure there is a planar extension at the end of the inner conductor, in parallel with the lid. At the edges of the extension there is at least one projection parallel with a side wall, which functions as a tuning element. By bending the tuning element said capacitance and, at the same time, the natural frequency of the resonator will be changed. A disadvantage of that kind of solution is that in a multi-resonator filter it can be necessary to manually bend the tuning elements in several stages to obtain the desired frequency response. The filter&#39;s lid must be opened and closed for each tuning stage. Thus also in this case the tuning is time-consuming and relatively expensive. This is emphasized by the use of sub-bands, as the filters must be tuned for each sub-band during the manufacturing.  
         [0005]      FIG. 1  presents a tuning way of resonator filter using a dielectric tuning element, the way being known from the publication JP 62123801. In the figure there is a longitudinal section of one coaxial resonator  110  of the filter, the resonator comprising a bottom  111 , an inner conductor  112 - 113 , an outer conductor  114  and a lid  115 . The outer conductor surrounds the inner conductor over its whole length, as normally. In addition, the resonator comprises in this example a cylindrical dielectric block having an axial, vertical hole. The block has been coated by conductive material apart from its upper surface. The inner conductor consists of that coating material  112  and a cylindrical conductor  113  having a firm contact with the wall of the hole. The widened upper end of the cylindrical conductor extends above the upper surface of the dielectric block. The bottom  111  shorts the transmission line formed by the inner and outer conductors at its lower end. At the upper end of the structure the inner conductor does not extend to the conductive lid, so the transmission line is open at the top. A result of this is that the structure functions as a quarter wave resonator.  
         [0006]     For the tuning of the resonator  110  there is a conductive screw  117  in its lid  115 . A cylindrical dielectric tuning element  118  has been attached on the lower surface of the screw, the tuning element being made of material, which has relatively high dielectricity, such as ceramics. That dielectric tuning element is located in the resonator cavity above the inner conductor of the resonator, at certain distance d from the upper surface of the inner conductor. When the screw  117  is turned deeper, for instance, the distance d is decreased. In that case the effective dielectricity between the inner conductor and the lid increases, because the ceramics fills greater part of the space therebetween, in the proportion. The capacitance between the inner conductor  116  and the lid is increased for the increase of the dielectricity and for the approach of the conductive screw, on the other hand. The increase of the capacitance results in increase of the resonator capacitance results in increase of the resonator electric length and lowering in the resonator natural frequency. Disadvantages of this solution, too, are that in a multi-resonator filter it may be necessary to manually turn the screws in many stages to obtain the desired frequency response, and that the electrical contact in the screw joint may deteriorate in the course of time.  
         [0007]      FIGS. 2   a,b  present another example of a known resonator filter, for tuning of which a dielectric tuning element is used. In  FIG. 2   a  there is a longitudinal section of the filter  200  and in  FIG. 2   b  it is seen from above the lid  205  cut open. The filter comprises a conductive housing formed by a bottom, outer walls and the lid, the space of the housing being divided into the resonator cavities by conductive partition walls. In each cavity there is, to reduce the resonator size, a fixed cylindrical dielectric block, such as the dielectric block  216  of the first resonator, visible in the figure. The bases of the cylinder are parallel with the bottom and the lid of the resonator, and the block has been raised over the resonator bottom  211  by a dielectric support piece SU. The support piece has substantially lower dielectricity than the dielectric block  216 . The dielectric block has been dimensioned so that a transverse electric wave TE 01  is excited in it at the operating frequencies of the filter. The resonators then are half wave cavity resonators by type.  
         [0008]     For the tuning of the first resonator of the filter  200 , in the resonator&#39;s lid there is a screw  217  made of a dielectric material such as a plastic. A cylindrical, e.g. ceramic tuning element  218  has been attached on the lower surface of the screw. That tuning element is located in the resonator cavity above the dielectric block  216 , at certain distance from the upper surface of the block. When the screw  217  is turned e.g. deeper the tuning element  218  approaches the dielectric block  216 . In that case the electric size of the dielectric block increases, and the natural frequency of the block and the whole resonator lowers. Disadvantages of this solution, too, are that in a multi-resonator filter it may be necessary to manually turn the screws in many stages to obtain the desired frequency response.  
       SUMMARY OF THE INVENTION  
       [0009]     The object of the invention is to reduce the mentioned disadvantages relating to prior art. A resonator filter according to the invention is characterised in what is presented in the independent claims  1  and  25 . The other claims present some advantageous embodiments of the invention.  
         [0010]     The basic idea of the invention is as follows: In each resonator cavity of a resonator filter there is a movable dielectric tuning element to adjust the resonator&#39;s natural frequency. The tuning elements are advantageously arranged to be moved by a common control, to shift the filter&#39;s band through equal displacements of the natural frequencies of the resonators. When the tuning element is moved e.g. horizontally sidewards from the resonator axis, the electrical length and natural frequency of the resonator change. The tuning elements can be movable in each resonator separately, too, to implement the basic tuning of a filter.  
         [0011]     An advantage of the invention is that when sub-bands are used it is not necessary to tune the filters separately for each sub-band in the stage of manufacture, as the sub-band can be chosen when the filter is put into use, by a single tuning motion the common control being applied. Another advantage of the invention is that the tuning mechanism consists of few parts, even only of one object, which brings savings in production costs. A further advantage of the invention is that the basic tuning of the filter can be automated, in other words it can be made without inconvenient handwork. Then an actuator and a device measuring the response of the filter are programmed to cooperate so that the tuning elements are moved programmably until an optimal response is obtained. A further advantage of the invention is that the tuning does not change in the course of time, as there are no metallic junctions between the tuning element and the rest of the structure. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The invention is described in detail below. The description refers to the enclosed drawings, in which  
         [0013]      FIG. 1  shows an example of a prior art tunable filter,  
         [0014]      FIGS. 2   a,b  show another example of a prior art tunable filter,  
         [0015]      FIGS. 3   a,b  show an example of a tunable filter according to the invention,  
         [0016]      FIG. 4  shows another example of a tunable filter according to the invention,  
         [0017]      FIGS. 5   a - d  show examples of tuning elements according to the invention,  
         [0018]      FIG. 6  shows an example of a tuning element according to the invention and of moving of the tuning element,  
         [0019]      FIGS. 7   a - c  show a further example of a filter according to the invention,  
         [0020]      FIG. 8  shows a further example of a filter according to the invention,  
         [0021]      FIG. 9  shows a further example of a filter according to the invention,  
         [0022]      FIGS. 10   a,b  show a further example of a filter according to the invention,  
         [0023]      FIGS. 11   a,b  show a further example of a filter according to the invention,  
         [0024]      FIG. 12  shows an example of a shifting of the pass band of a filter according to the invention,  
         [0025]      FIG. 13  shows an example of a filter, for which it can be done both the basic tuning and the common tuning, according to the invention,  
         [0026]      FIG. 14  shows another example of a filter, for which it can be done both the basic tuning and the common tuning, according to the invention, 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]      FIGS. 1 and 2  were described already in connection with the description of prior art.  
         [0028]      FIGS. 3   a  and  3   b  show an example of a tunable filter according to the invention. The filter  300  consists of quarter wave coaxial resonators, from which the first resonator and partly the second resonator are seen in the figure.  FIG. 3   a  shows the structure in a longitudal section from one side. The first resonator has a bottom  311 , an inner conductor  312 , an outer conductor  314  and a lid  305 . The tuning element  318  is a right-angled prismatic dielectric piece located at the open end of the resonator. In the vertical direction it extends from the lid about halfway to the top of the inner conductor  312 . In this example the tuning element is attached to the lower surface of the resonator&#39;s lid with the aid of the guide rails GU 1  and GU 2  shown in the accompanying figure so that it can be moved back and forth in the horizontal plane. In order to be able to move it manually from the outside the lid  305  has an elongated hole HO. In the hole there is a vertical tap TA, which extends at the lower end into the tuning element and at the upper end into a control rod  308  being located on the filter&#39;s lid. That rod is linked in the same way with the tuning elements of the other resonators, too. Thus the whole filter can be tuned in one go by moving the rod.  
         [0029]     In  FIG. 3   b  the first resonator is seen from above with the lid cut open. When the tuning element  318  is at one end of the adjusting range, at the right end in  FIG. 3   b , it is located at the centre of the resonator, seen from above. In this example the size of the tuning element is such that it wholly covers the top surface of the inner conductor  312 . When the tuning element  318  is at the other end of the adjusting range, at the left end in  FIG. 3   b , then seen from above it is located on one side so that the whole top surface of the inner conductor is visible. When the tuning element is at the right end of its adjusting range the effective dielectric factor in the upper part of the resonator cavity has a maximum, because in that case the dielectric piece  318  is located at a point where the strength of the electric field has a maximum while the structure resonates. Further, when the effective dielectric factor is at its maximum the capacitance between the top of the inner conductor and the surrounding conductor surfaces is at its maximum, the electric length of the resonator has a maximum, and the natural frequency has a minimum. Correspondingly, when the tuning element  318  is at the left end of its adjusting range the natural frequency of the resonator is at its maximum.  
         [0030]     The tuning element can be e.g. round instead of rectangular, seen from above. Its motion direction can be, seen from above, e.g. from the centre of the resonator towards some of the corners or from the side of the inner conductor towards a corner.  
         [0031]      FIG. 4  shows another example of a coaxial resonator filter according to the invention. In the figure one resonator of the filter  300  is seen whole, from above with the lid cut open. Its basic structure is similar to that of the  FIGS. 3   a ,  3   b . Also in this case the tuning element  418  is a movable dielectric piece at the open end of the resonator. The difference to the solution of  FIGS. 3   a ,  3   b  is that instead of a linear motion the tuning element  418  is now moved by rotating it in the horizontal plane. To this end the tuning element is provided with an axis to the resonator lid  415 , at a point P close to its end. Seen from the point P of rotation towards the opposite end the tuning element has a broadening form, as seen in the horizontal plane. The axis of rotation is slightly outside the inner conductor  412 . When the tuning element  418  is at one end of the adjusting range, at the clockwise end as seen in  FIG. 4 , its broad end covers the upper surface of the inner conductor, as seen from above. When the tuning element  418  is at other end of the adjusting range, at the anticlockwise end as seen in  FIG. 4 , then the whole tuning element is located on one side of the inner conductor, as seen from above. Due to reason mentioned before the natural frequency of the resonator reaches its minimum when the tuning element has been turned clockwise so that it is above the inner conductor, and the natural frequency increases as the tuning element is turned anticlockwise.  
         [0032]     The axis AX, going through the lid, is seen in the small accompanying figure. At its top the axis is fixedly fastened to an arm LEV, the other end of which is fastened by a shaft locking to a control rod  408  located on the lid. That rod is connected in the same way to the tuning elements of the other resonators, too. Thus all tuning elements can be turned in one go by moving the rod. Instead of the arm structure shown in the figure, for instance a small cog would be at the top of the axes, and as the control rod would be a rack bar fit to the cogs.  
         [0033]     When the common tuning is not used or the control rod with the arms has not yet mounted, the tuning element of a resonator can be turned, for the basic tuning, by a tool fit to the shape of the axis AX.  
         [0034]      FIGS. 5   a  to  5   d  show other examples of a tuning element according to the invention. In the examples the resonators are coaxial quarter wave resonators. In  FIG. 5   a  the dielectric tuning element  518  extends from the lower surface of the lid  515  to the top surface of the inner conductor  512 . Compared with  FIG. 3   a  the tuning element has a greater effect on the electric length of the resonator, and the adjusting range of the natural-frequency is wider, if the materials are the same. In  FIG. 5   b  the tuning element  528  is wedge-like, seen from the side in a direction perpendicular to its motion direction. Then the resonator&#39;s natural frequency as a function of the motion of the tuning element changes in a manner different from that of the structure according to  FIG. 3   a . It is for instance possible to obtain a more linear change of the natural frequency. In  FIG. 5   c  the resonator&#39;s tuning element has two parts on top of each other. The dielectric constant ε 1  of the first part  538  does not equal the dielectric constant ε 2  of the second part  539 . The resonator&#39;s tuning element in  FIG. 5   d  has also two parts, with different dielectric constants. In this case the first part  548  and the second part  549  of the tuning element are located side by side, as seen from the side in a direction perpendicular to its motion direction. By suitably varying the dielectricity within the tuning element it is possible to vary the width of the adjusting range and the sensitivity of the tuning in a desired way.  
         [0035]      FIG. 6  shows a further example of a tuning element according to the invention and of the tuning. The tuning element  618  has bowl-like form, and it encloses the top of the inner conductor  612 . Now the tuning element can be moved back and forth in the vertical direction. For moving the resonator has in this example an internal actuator ACT, to vertically movable shaft of which the tuning element has been attached. The actuator is arranged to receive its control electrically from the outside of the resonator. By an electric control first the basic tuning of the filter can be performed in each resonator separately, and later the common tuning to place the band. In the latter tuning all resonators get the same control, of course. The electric control can be implemented by a cable or a radio way.  
         [0036]      FIGS. 7   a  to  7   c  show an example of a filter, the passband of which can be shifted according to the invention. The filter of the example is a duplex filter  700  with six resonators. In  FIG. 7   a  the structure is seen from above, with the lid removed. The resonators are in two rows, three resonators in each row. The first  710 , the second  720  and the third  730  resonator form the receiving side of a duplex filter, and the fourth  740 , the fifth  750  and the sixth  760  resonator form the transmitting side of the duplex filter. The third and the fourth resonators are located side by side in the 2×3 formation, and both of them have a coupling to the antenna connector ANT. The sixth resonator has a coupling to the receiving connector RXC and the first resonator to the transmitting connector TXC. The structure comprises further a unitary dielectric tuning body, which consists of resonator-dedicated tuning elements, such as the tuning element  728  of the second resonator, and of a rod part  708 . The rod part has a shape as a rectangular U; it has a first section extending from the first resonator to the third resonator, a transversal section extending from the third resonator to the fourth resonator and a third section extending from the fourth resonator to the sixth resonator. Each resonator-dedicated tuning element is an extension of the rod part of the tuning body, in a way. The unitary tuning body can be moved in the horizontal plane, back and forth in the longitudinal direction of the filter, so that the tuning elements are moved to a position above the inner conductors of the resonators, or away from a position above the inner conductors. In that case, on the basis of what was described above both the transmitting and receiving bands of the duplex filter will be shifted simultaneously. The lid of the filter has a slot to enable motion of the tuning body, the length of the slot corresponding to the width of the adjusting range. Alternatively the tuning body will be moved through the end of the filter housing at the third and fourth resonators. Then a relatively small hole in the end wall is sufficient for the tuning.  
         [0037]      FIG. 7   b  shows the lid  705  and the tuning body seen from the side of the filter&#39;s  700  transmitting side. In the example of the figure the extensions in the tuning body, or the tuning elements, extend deeper into the resonators than the rod part  708  interconnecting the tuning elements. For instance the tuning element  728  of the second resonator extends close to the top of the second inner conductor  722  drawn in the figure. Naturally the tuning body can be also monotonous in the horizontal direction.  
         [0038]      FIG. 7   c  shows a cross-section X-X of the filter  700  at those partition walls, which separate the first and second resonators and the fourth and fifth resonators. In the upper part of the former partition wall, immediately below the filter&#39;s lid  705  there is a small notch, through which the rod part  708  travels from the second tuning element to the third tuning element. Correspondingly there is a notch in the partition wall separating the fourth and fifth resonators, through which the rod part  708  travels from the fourth tuning element to the fifth tuning element.  FIG. 7   c  shows also a coupling hole H 23  between the second and third resonators, and a coupling hole H 45  between the fourth and fifth resonators. These holes are marked also in  FIG. 7   a  by dotted lines.  
         [0039]      FIG. 8  shows another example of a filter, the passband of which can be shifted according to the invention. The filter  800  is presented from above the lid removed. It is a duplex filter like the filter  700  in  FIG. 7 . The first  810 , the second  820 , the third  830  and the fourth  840  resonators are located in a square, and they form the receiving side of the duplex filter. The receiving filter comprises a first tuning body  808  in a form resembling a rectangular letter U, the first tuning body being an unitary and plate-like dielectric object. It has a first section extending from the first resonator to the second resonator, a transversal second section extending from the second resonator to the third resonator, and a third section extending from the third resonator to the fourth resonator. A projection directed toward the centre of the U-form joins to the second section in the same plane, an external tool being fit into this projection during the tuning of the filter. The tuning body  808  has at each resonator a hole, which corresponds to an individual tuning element. For instance, in the space of the third resonator the tuning body has a hole  838 , and in the space of the fourth resonator it has a hole  848 . A part of the top surface of the inner conductor  846  of the fourth resonator is seen through the hole  848 . At the other three resonators the situation is similar. The tuning body  808  can be moved back and forth in the horizontal plane, so that its holes move to a position above the inner conductors of the resonators, or away from the position above the inner conductors.  
         [0040]     Seen in the direction of motion one end of the holes is clearly wider than the other end. At the one end of the adjusting range the wide ends are above the inner conductors, and at the opposite end of the adjusting range the narrow ends of the holes are above the inner conductors. The effective dielectricity in the upper part of the resonators will increase when moving from the former state to the later, whereby the natural frequencies will be reduced, and the passband will shift downwards.  
         [0041]     In the duplex filter  800  the fifth  850 , the sixth  860 , the seventh  870 , the eighth  880  and the ninth  890  resonators form the transmitting filter. The fifth, sixth, eighth and the ninth resonators are located in a square, and the seventh is on the side of the square, in the propagation direction of the signal between the sixth and the eighth resonators. The location of the transmitting filter&#39;s passband is changed by a second tuning body  809 , which is similar to the first tuning body  808 . The only differences are that the second tuning body is longer due to the higher number of resonators, and its transversal section has a hole  878  for adjusting the natural frequency of the seventh resonator.  
         [0042]      FIG. 9  shows a further example of a filter, the passband of which can be shifted according to the invention. The filter  900  has six resonators  910  to  960  in a similar 3×2 matrix as in  FIG. 7   a , as seen from above. The unitary tuning body has a longitudinal rod part, from which there is a transversal branch to each resonator. The end of each branch carries an extension a tuning element, such as the tuning element  948  in the fourth resonator. The tuning body can be moved in the horizontal plane, back and forth in the transversal direction, so that the tuning elements are moved to a position above the inner conductors of the resonators, or away from the position above the inner conductors. In  FIG. 9  the tuning body is in an intermediate position where the top surfaces of the inner conductors of the resonators are seen about half below the tuning elements. The motion of the tuning body can be arranged also vertical by direction, in principle, in which case the height of the holes in the partition walls for the transversal branches must correspond to the width of the adjusting range.  
         [0043]      FIGS. 10   a, b  show a further example of a filter, the passband of which can be shifted according to the invention. The resonators of the filter A 00  are similar dielectric cavity resonators to that of  FIG. 2 , in regard to the basic structure. So there is, in each cavity, a fixed cylindrical dielectric block, such as the dielectric block A 16  in the resonator A 10 , separated from the bottom and other walls of the resonator by means of a support piece SU. The support piece has substantially lower dielectricity than the dielectric block A 16 , for which reason it has only an minor influence on the characteristics of the resonator. In the upper base of each dielectric block there is a rectangular recess having constant breath and the same direction as the longitudinal direction of the filter. In that kind of recess there is a rectangular tuning element, such as the tuning element A 18  in the resonator A 10 , having almost same breath as the recess. The tuning elements have been connected to each other by a rod part A 08  resulting in an unitary tuning body. The rod part goes through the holes in the partition walls of the resonators, and the end of the rod part sticks out through the hole in the end wall of the filter. When the tuning body is moved using the end of the rod, the tuning elements slide in the recesses of the dielectric blocks. In that case the natural frequencies of the resonators change by the same amount and the filter&#39;s passband will be shifted.  
         [0044]     Said recesses of the dielectric blocks are not necessary, of course. The upper bases then can be also even, in which case the tuning body is moved along the surfaces of the upper bases or above the upper bases. In the latter case the tuning body has been supported only to the holes in the partition walls and end wall of the filter housing. The tuning body can be located also below the dielectric blocks, as well, if the dielectric blocks have been attached to the resonator walls by support pieces. The shape of the tuning elements, seen from above, can be e.g. triangular instead of rectangular, to work up the adjusting effect. Regardless of the shape of the broadening, the tuning elements can be also as thick as the rod part of the tuning body, in the vertical direction.  
         [0045]      FIGS. 11   a, b  show a further example of a filter, the passband of which can be shifted according to the invention. The resonators of the filter B 00  are dielectric cavity resonators, as in  FIG. 10 . However the basic structure is different such that the axis of a fixed cylindrical dielectric block in a resonator cavity now is horizontal being united with the axes of the other dielectric blocks in the successive resonator cavities. Also in this example the dielectric block has been arranged approximately to the middle of the resonator cavity by a support piece SU having low dielectricity. Each dielectric block has an axial hole. In this kind of hole there is a cylindrical tuning element, such as the tuning element B 18  of the resonator B 10 , the diameter of the tuning element being close to the diameter of the hole. The tuning elements have been connected to each other by a rod part B 08  resulting in an unitary dielectric tuning body. The rod part goes through the holes in the partition walls of the resonators, and the end of the rod part sticks out through the hole in the end wall of the filter. When the rod is pushed or pulled at its end, the tuning elements move in the holes of the dielectric blocks. In that case the natural frequencies of the resonators change by the same amount and the filter&#39;s passband will be shifted. In  FIG. 11   b  the exemplary filter B 00  is seen from the side of an end as a cross section. From the figure it appears, that the resonators are in two parallel rows, the end resonators being B 50  and before-mentioned B 10 . The tuning bodies for each of two rows can be united by a transversal rod to single enlarged tuning body. The all resonators of the filter also can be in one row, of course.  
         [0046]      FIG. 12  shows an example of the shifting of the passband of a filter according to the invention. The figure shows the propagation coefficient S 21  as a function of frequency, i.e. the amplitude response, in four situations. All four amplitude response curves have the same form. The width of the passband, appearing from the curves, is about 28 MHz. The first curve  11  shows a situation where the centre frequency of the pass band is about 1.937 GHz. The second  12 , the third  13 , and the fourth  14  curves show situations, where the pass band has been shifted upwards in steps of about 14 MHz. The curves has been measured from a filter similar to the transmitting part of the duplex filter in  FIG. 8 , its tuning body being in different positions.  
         [0047]      FIG. 13  shows an example of a filter, in which both the basic tuning and the shifting of the passband can be implemented according to the invention. The filter D 00  comprises in a row a first D 10 , a second D 20 , a third D 30  and a fourth D 40  coaxial resonator. The upper part of the first resonator houses a first tuning element D 18 , the upper part of the second resonator houses a second tuning element D 28 , the upper part of the third resonator houses a third tuning element D 38 , and the upper part of the fourth resonator houses a fourth tuning element D 48 . The dielectric tuning elements are attached to a dielectric control rod D 08  so that they can be moved separately with regard to the control rod along a certain line segment. In the longitudinal direction of the filter the control rod extends through notches in the top edges of the filter&#39;s partition walls D 14 , D 24 , and D 34  from the first resonator to the fourth resonator. The size of the notches in the partition walls corresponds to the cross-section of the control rod. Thus the control rod will be pressed against the lower surface of the filter&#39;s lid D 05 , however so that the control rod can be moved in its longitudinal direction. In the situation shown in  FIG. 13 a  suitable position has been found for each tuning element within its adjusting range, so that the shape of the filter&#39;s frequency response is optimised, and all tuning elements then have been locked in their places. Now the tuning elements and the control rod together form an unitary tuning body. When the tuning body is then moved all tuning elements move the same distance, and the location of the filter&#39;s passband is shifted on the frequency scale. Also the  FIG. 13  does not show the couplings, by which the electromagnetic energy of a signal is supplied to the filter, from one resonator to the next, and out from the filter.  
         [0048]      FIG. 14  shows another example of a filter, in which both the basic tuning and the shifting of the passband can be implemented according to the invention. The filter E 00  has four resonators, also in this example. The upper part of the first resonator houses a first tuning element E 18 , the upper part of the second resonator houses a second tuning element E 28 , the upper part of the third resonator houses a third tuning element E 38 , and the upper part of the fourth resonator houses a fourth tuning element E 48 . The dielectric tuning elements are now in recesses of a dielectric tuning rod E 08 , so that each element can be separately moved within its own recess. The shape of the recess REC allows a motion of the tuning element in the horizontal plane, in transversal direction with regard to the longitudinal direction of the filter. The tuning shaft extends through notches in the top edges of the filter&#39;s partition walls E 14 , E 24 , and E 34  from the first resonator to the fourth resonator. The size of the notches in the partition walls corresponds to the cross-section of the tuning rod. Thus the tuning rod will be pressed against the lower surface of the filter&#39;s lid E 05 , however so that the tuning rod can be moved in its longitudinal direction. When the basic tuning has been completed by moving the tuning elements, these are locked in their places. Then an unitary tuning body is formed. After that a location is set for the filter&#39;s passband on the frequency scale by moving the tuning body.  
         [0049]     In  FIGS. 13 and 14  the tuning elements are provided with a first dielectric constant ε 1 , and the tuning rod is provided with a second dielectric constant ε 2 . Advantageously the constant ε 1  is greater than the constant ε 2 .  
         [0050]     In this description and in the claims the epithets “lower”, “upper” or “top”, “above”, “below”, “horizontal”, “vertical”, “from one side”, from above”, “on top of each other” and “side by side” refer to a position of the resonators where the inner conductors and/or are vertical, and these epithets have nothing to do with the operating position of the devices.  
         [0051]     Above filter structures based on resonators are described, which structures have movable dielectric elements for the tuning of a filter. The moving is realized with an electrically controlled regulation unit, such as a step motor or a actuator based on piezoelectricity or piezomagnetism. The shape of the tuning elements and the way to attach them can of course differ from those presented above. Neither does the invention restrict the manufacturing methods of the resonators and their tuning elements. The inventive idea is applicable in different ways within the scope of the independent claims  1  and  25 .