Patent Publication Number: US-8981880-B2

Title: Waveguide band-pass filter with pseudo-elliptic response

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/IT2010/000306 filed Jul. 9, 2010, the contents of all of which are incorporated herein by reference in their entirety. 
     FIELD OF THE INVENTION 
     The present invention refers to the field of waveguides and in particular to waveguide band-pass filters. 
     PRIOR ART 
     Waveguide band-pass filters are known which comprise a cascade of waveguide segments whose length is about half of the central wavelength of the filter (λ/2) or multiples of such a value which act as resonators and are coupled to each other (and to input/output guides) by discontinuities such as, typically, diaphragm-structures. These coupling-discontinuities present an equivalent circuit having a shunt reactance. The reactance value, usually inductive, determines the entity of the coupling between the resonating guide segments. 
     The synthesis of such waveguide filters, called “filters with directly-coupled resonator” is analysed in G. L. Matthaei, L. Young e E. M. T. Jones, “Microwave filters, Impedance-Matching Networks, and Coupling Structures” ed. McGraw Hill, 1964. 
     U.S. Pat. No. 7,391,287 discloses a “H-plane” waveguide filter having transmission zeros. The article by W. Maenzel, F. Alessandri, A. Plattner, and J. Bornemann, “Planar integrated waveguide diplexer for low-loss millimeter-wave applications”, in Proc. of the 27th European Microwave Conf., Jerusalem, September 1997, pp. 676-680 illustrates the use of structures comprising rectangular guide segments placed alongside the filter body, which act as a shunt “stubs”, so as to introduce transmission zeros in the response from the guide band-pass filter. 
     US-A-2009-0153272 discloses the use of resonant posts inside the band-stop filter, wherein such posts are spaced by coupling waveguide segments between the resonant posts themselves. The distance between the resonant posts is ¾ of the central wavelength of the band-stop filter stopband. 
     SUMMARY OF THE INVENTION 
     The applicant has noted that, with reference to the waveguide band-pass filters, the prior art does not offer any solutions which enable to achieve an increase in filter selectivity by not complex manufacturing procedures. 
     The problem on which the present invention is based is to provide an alternative waveguide band-pass filter in respect to those known and which, for example, allows an easy manufacturing, while offering good performances in terms of selectivity and keeping compact overall dimensions. 
     The above problem is solved by a band-pass filter as recited in the appended claim  1  and particular embodiments thereof as defined in the dependant claims  2  to  15 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some particular embodiments of the present invention are disclosed in detail below, as a way of example and not a limitation, with reference to the accompanying drawings, wherein: 
         FIG. 1  shows an axonometric and schematic view of the inner structure of an example of a waveguide band-pass filter; 
         FIG. 2  shows an equivalent electrical scheme of the band-pass filter in  FIG. 1  comprising inductive reactances and reactances associated to resonant discontinuities; 
         FIG. 3   a  shows an example of a reduced-height post usable in said filter; 
         FIG. 3   b  shows the equivalent electric circuit of said reduced-height post; 
         FIG. 4  shows behaviours of the reduced-height post reactance depending on frequency and for different values of its height; 
         FIG. 5  shows behaviours of the electric length of the line equivalent to such a reduced-height post depending an frequency and for different values of its height; 
         FIG. 6  shows behaviours of the electric length of the line equivalent to such a reduced-height post depending on frequency and for different values of its lateral dimension; 
         FIG. 7  shows the behaviour of the transmission coefficient S 21  and of the reflection coefficient S 11  experimentally measured on a band-pass filter analogous to that in  FIG. 1  and also shows the behaviours of said coefficients obtained by means of simulation; 
         FIG. 8   a  shows an example of an inductive coupling device made by a full-height septum (asymmetric inductive iris); 
         FIG. 8   b  shows another example of an inductive coupling device made by an individual full-height post with a square base; 
         FIG. 8   c  shows a capacitive coupling device made by a full-width septum (asymmetric capacitive iris); 
         FIG. 9   a  shows an example of a resonant coupling device made by an individual reduced-height post with a square base; 
         FIG. 9   b  shows another example of resonant coupling device made by two reduced-height posts with a square base; 
         FIG. 10  shows an exploded view of a first embodiment of the band-pass filter of  FIG. 1  which can be made by milling, 
         FIG. 11  shows an exploded view of a second embodiment of the band-pass filter of  FIG. 1  which can be made by milling; 
         FIG. 12  shows an exploded view of a third embodiment of the band-pass filter of  FIG. 1  which can be made by the metal insert technique; 
         FIG. 13  shows the behaviour of the transmittance S 21  and reflectance S 11  obtained by simulation relative to a band-pass filter analogous to that in  FIG. 12 ; 
         FIG. 14  shows the inner structure of a fourth embodiment of the band-pass filter of  FIG. 1  of a dielectric type; 
         FIG. 15  shows the behaviour of transmittance S 21  and reflectance S 11  obtained by simulation relative to a band-pass filter analogous to that in  FIG. 14 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically shows the inner structure of an exemplary band-pass filter which can be implemented in a waveguide  100 .  FIG. 2  shows the equivalent scheme  110  of the band-pass filter  100 . To the band-pass filter  100  a pass-band B is associated having a central wavelength designated by λ g0 . The reciprocal band-pass filter  100  of  FIG. 1 , is of the order N=5 and has two transmission zeros. 
     The band-pass filter  100  can be made, according to an example, by means of a metal rectangular waveguide of dimensions a, along an axis x, and b, along an axis y. In more detail, the band-pass filter  100  comprises an input  3  for a signal (i.e. a radiation/electromagnetic wave) to be filtered, a first inductive discontinuity coupling device  4 , connected to input  3 , and a first waveguide  5  resonator segment, coupled to input  3  by the first coupling device  4 . 
     As it is shown in  FIG. 1 , the input  1  is a waveguide segment having an input opening  20  which can be coupled, for example, to a radiation source or to a circuit in a waveguide by means of a flange (components not shown). 
     The first inductive discontinuity coupling device  4  can be made, according to a first embodiment, by means of an iris or inductive diaphragm comprising two metal septums (also referred to by reference numerals  4 ) arranged symmetrically in respect to a median longitudinal plane, which develops parallel to an axis z of the radiation propagation. The metal septums  4  of the first inductive diaphragm identify a first coupling radiation opening  24  of the electromagnetic field. 
     With reference to the equivalent electric scheme of  FIG. 2 , the first inductive diaphragm  4  is represented as an optimal shunt inductor having an inductive impedance jX 4 . The walls of the first inductive diaphragm  4  have the same height as the height b of filter  100 . 
     The first resonator segment of the waveguide  5  has a length, taken on the axis z, approximately equal to half of length of the central wave of the filter: λ g0 /2 and it is coupled to the input  3  by the inductive diaphragm  4 . The resonator segment  5  can also have a length which is multiple of the value λ g0 /2. 
     Moreover, the first resonator segment  5  is coupled to a second resonator segment  7  by a first resonant coupling  6 . The first resonant coupling device is a resonant coupling structure which introduces a discontinuity configured to introduce a zero in the transmission frequency response of the band-pass filter  100 . 
     In more detail, the first resonant coupling device  6  is configured to resonate at a frequency equal to the value of the frequency of the zero being introduced in the transmitting response of the band-pass filter  100 . In particular, such a transmission zero concurs to increase the selectivity of the filter in the higher and lower stop-bands of the filter  100  itself. 
     For different frequencies from the resonance frequency of the first resonant coupling device  6 , the device itself behaves as a coupler. The position on the frequency axis of the transmitting zero can be determined by synthesis procedures known to those skilled in the art. The transmission zero corresponds, in a practical implementation of the filter  100 , to an attenuation peak. 
     As it is visible in the example of  FIG. 1 , the first resonant coupling device  6  can be made by at least a body within the waveguide of the filter  100  and having a reduced height relative to height b of the waveguide itself. In particular, the first resonant coupling device  6  comprises two parallelepiped-shaped (for example, with a square base) posts, parallel oriented to axis y, arranged, for example, symmetrically relative to the median longitudinal plane and having a height h lower than dimension b. 
     Such first reduced-height posts  6  are schematically depicted in  FIG. 2  as a shunt-arranged resonant circuit element and therefore as a series of an inductor and a capacitor with a total reactance X 6  (with impedance jX 6 ). Such an impedance jX 6  results in the presence of a zero at the frequency fz 1  in the transmission response of the band-pass filter  100 . 
     Even if the first reduced-height posts  6  play a role as a resonant body, they act for different frequencies from the resonance frequency as a coupling device which, in conjunction with the first diaphragm  4 , causes the first guide segment  5  to be a resonant cavity. 
     The second resonator segment  7 , with a length equal to approximately half of the central wavelength of the filter (i.e. λ g0 /2) has an end (opposite the first posts  6 ) connected to a second inductive discontinuity coupling device  8 . Such a coupling device  8  is analogous to the first device  4  and comprises a second inductive diaphragm which identifies a second opening  9  for radiating. 
     In the equivalent scheme  110  in  FIG. 2 , the second inductive discontinuity coupling device  8  is represented by another inductive shunt impedance jX 8 . 
     The band-pass filter  100  further comprises a third resonator segment  10  with a length approximately equal to λ g0 /2, coupled to the second resonator segment  7  by the second inductive diaphragm  8 . 
     According to the concerned example, the third resonator segment  10  is connected to a third inductive discontinuity coupling device  11  (analogous to the first coupling device  4 ), implemented by a further inductive diaphragm (impedance jX 11 ) provided with a third aperture  12 . 
     The third resonator segment  10  is further coupled to a fourth resonator segment  13  (of a length λ g0 /2) connected to a second resonator coupling device  14 , comprising two second reduced-height posts, and analogous to the first coupling device  6  and having an impedance jX 14 . 
     The second reduced-height posts  14  are such to resonate, for example, at a different resonance frequency f z2  and therefore they cause the presence of another zero in the transmitting frequency response of the band-pass filter  100 , at the frequency f z2 . For example, the zero placed at frequency f z1  increases the selectivity in the lower stop-band, while the zero at frequency f z2  increases the selectivity of the higher stop-band at the pass-band B of the filter  100 . For different frequencies from the resonance frequency f z2  the second posts with a reduced height  14  act as a coupling device. 
     The fourth resonator segment  13  is coupled to a fifth resonator segment  15  (approximately λ g0 /2 long) by the second posts with a reduced height  14 . The fifth resonant segment  15  is then coupled to an output  17  of the filter  100  by a fourth inductive discontinuity coupling device  18  implemented by a respective fourth inductive diaphragm having a fourth opening  19  and an inductive impedance jX 18 . 
     According to the examples illustrated, the output  17  of the filter  100  is the waveguide segment which has an output opening  25  for providing the filtered signal and for being coupled to a load or to a further waveguide segment or to a further filter. It is to be observed that the resonant coupling devices  6  and  14  are arranged in respective regions of the filter  100  guide wherein the electric field has is at the minimum, in order not to degrade the figure of merit of the resonator guide segments  5 ,  7 ,  13  and  15  adjacent to such resonant coupling devices. 
     Dimensioning and Operation of the Filter 
     The dimensioning of the first, second, third and fourth inductive diaphragm  4 ,  8 ,  11  and  18 , and the first and second reduced-height posts  6  and  14 , is such that each of these devices acts as an impedance inverter around the central frequency of the filter  100 . This causes the first, the second, the third, the fourth and the fifth guide segments  5 ,  7 ,  10 ,  13  and  15 , approximately λgo/2 long, to act as resonant cavities around the central frequency f 0  of filter  100 . 
     Even though in  FIG. 1  only five resonant cavities are shown, the band-pass filter  100  may comprise a number N of cavities, equal to the filter order. In general, the filter  100  may comprise a plurality of resonant coupling structures in generally located (analogous to structures  6  and  14 ), in order to introduce in the band-pass response up to N+1 transmission zeros for a N-order filter. 
     The frequency value f z1  of the first zero (e.g, lower than the mid-band frequency f 0  of the filter  100 ) and the frequency value f z2  of the second zero (e.g, higher than the mid-band frequency f 0  of the filter  100 ) may be suitably selected in the stop-bands within the whole operative band of the waveguide, i.e. from the cut-off frequency f c  up to the value 2f c  and beyond. 
       FIG. 3   a  shows an example of the first resonant coupling device  6 , in the case of an individual square-base, reduced-height post with a side d and a height h, arranged so that it is centred in respect to the transversal cross-section of the waveguide where it is inserted.  FIG. 3   b  shows the circuit equivalent to the first reduced-height post  6 , comprising an impedance jX 6  parallel between two segments of the transmission line having length θ 6 , to which the following parameters are associated: 
     X 6  is the equivalent reactance of the reduced-height post  6 ; 
     Zo is, the characteristic impedance of two segments of the transmission line; 
     θ 6  is the equivalent electric length of the two segments of transmission line; 
     To is the position of the reference sections in respect to which the equivalent circuit is defined. 
     The equivalent reactance X 6  and the electric length θ 6  are related to the transmission parameters of the first reduced-height post  6  according to the following relations: 
     
       
         
           
             
               
                 
                   
                     j 
                     ⁢ 
                     
                       
                         X 
                         6 
                       
                       
                         Z 
                         0 
                       
                     
                   
                   = 
                   
                     
                       - 
                       
                         1 
                         2 
                       
                     
                     ⁢ 
                     
                       
                         S 
                         21 
                       
                       
                         S 
                         11 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                     θ 
                     6 
                   
                   = 
                   
                     - 
                     
                       
                         π 
                         + 
                         
                           ∠ 
                           ⁡ 
                           
                             ( 
                             
                               
                                 S 
                                 11 
                               
                               - 
                               
                                 S 
                                 21 
                               
                             
                             ) 
                           
                         
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     wherein S 11  is the reflectance and S 21  is the transmittance, both evaluated in respect to the To sections. 
     For example, taking into account a guide having a=30 mm and b=a/2, the frequency dependence on the ratio X 6 /Z 0  (normalised reactance) for the first reduced-height post  6  of  FIG. 3   a , was diagrammatically depicted in  FIG. 4  for several values of height h=9, 11, 13 and 15 mm (side d=3 mm). For each value of h, the behaviour of the normalized reactance of the first reduced-height post  6  corresponds to a LC-series resonator around its own resonance frequency. Such a resonance frequency decreases when height h increases. 
     Considering the same dimensional values, exemplarily denoted above, the frequency dependence of the equivalent length θ 6  is diagrammatically depicted in  FIG. 5 , where both reference sections are arranged at the longitudinal symmetry plan of the first reduced-height post  6  (To in  FIG. 3   a ). As it is seen in  FIG. 5 , the behaviour of the equivalent electric length θ 6  is analogous to the one of a full-height post having an inductive behaviour; the slope of the electric length θ 6  increases upon the increase of height h. 
       FIG. 6  shows the frequency dependence of the ratio X 6 /Z 0  for several values of the side d=2, 3, 4 and 5 mm (height h=13 mm). However the resonance frequency and the behaviour of the ratio X 6 /Z 0  with the frequency depend on both the height h and on the side d. This behaviour allows to use the reduced-height post  6  as a coupler between waveguide resonators and allows also the introduction of transmission zero. 
     By properly dimensioning the components of the band-pass filter  100  a transmission response may be obtained by the filter which is, for example, of the Chebyshev type, with transmission zeros (pseudo-elliptical response). Due to the presence of zeros, thus the band-pass filter selectivity can be increased (i.e. the attenuation in the higher and lower stop-bands at the pass-band) with the same number of resonators. 
       FIG. 7  illustrates the behaviour of transmittance S 21  and reflectance experimentally measured on a band-pass filter analogous to that in  FIG. 1 , schematically depicted in  FIG. 2 , implemented with a waveguide and having two transmission zeros (corresponding in the practice to attenuation peaks). The experiments were carried out on a filter made by the Applicant in a R70/WR137 guide, having inner dimensions equal to 34.85 mm×15.799 mm, using silvered aluminium. The project was carried out according to the following specifications 
     a central frequency f 0 =7.070 GHz; 
     a bandwidth B=28 MHz; 
     a level of the band return loss of 22 dB; 
     order N=5; 
     two transmission zeros (corresponding in practice to attenuation peaks) located at frequencies f z1 =7.020 GHz (in the lowest stop-band) and f z2 =7.120 GHz (in the highest stop-band). 
     The experimental results shown in  FIG. 7  (solid lines) perfectly match those provided by the simulation (dashed lines). 
     As to the operation, an electromagnetic wave in the form of, the mode TE 10  (basic mode in a rectangular guide) affects the input  3 . The electromagnetic wave propagates along the axis z of the filter  100 , being partially reflected at the input  3  and partially transmitted at the output  17 , according to the frequency of the wave itself. 
     When passing through the filter  100  the electromagnetic wave with a frequency comprised within the pass-band B of the filter itself interacts with the resonances of the resonant segments  5 ,  7 ,  10 ,  13 , and  15  and, due to the coupling devices  4 ,  6 ,  8 ,  11 ,  14  and  18 , it is transmitted to the output  17  with a reduced reflection at the input  3 . The electromagnetic wave with a frequency outside the pass-band of the filter  100 , instead, undergoes reflections within the filter and therefore it is substantially stopped, to an extent which depends on the difference between the wave frequency and the filter central frequency. 
     The electromagnetic wave having a frequency equal to one of the resonance frequencies of the two resonant coupling devices  6  and  14 , in particular, is totally reflected at input  3  (with a null transmission at the output  17 , giving rise to an attenuation peak) as the effect of the short-circuit created along the guide by the resonant coupling devices. 
     Further Embodiments 
     It is to be observed that according to other embodiments, each of the inductive diaphragms described above may be made not by the pairs of symmetrical septums  4 ,  8 ,  11  and  18  shown in  FIG. 1 , but by the following alternative modes: 
     an asymmetrical inductive iris, comprising an individual full-height septum  50  ( FIG. 8   a ); 
     a full-height inductive post  51  ( FIG. 8   b ): the post may be centred, or not, have a rectangular, circular or other base; there can be one or more full-height inductive posts  51 . 
     Moreover, instead of an inductive diaphragm, an asymmetrical capacitive iris can be used as a (non-resonant) coupling device, comprising a reduced-height, full-width septum  52  ( FIG. 8   c ). Also a symmetrical capacitive iris may be used, comprising another reduced-height, full-width septum. 
     Moreover, each of the resonant coupling devices  6  and  14  may be implemented, as an alternative to the embodiment in  FIG. 1 , by one or more full-height posts having different forms (for example, with a rectangular, square, circular or other base). For example,  FIG. 9   a  shows an individual reduced-height post  53  with a square plan, while  FIG. 9   b  shows a pair of reduced-height posts  54  and  55 . 
     It is to be observed that the illustrated geometries are only exemplary; and also a pair of reduced-height posts may be used wherein one is secured to the top wall of the filter  100  guide and the other is secured to the bottom wall of the same guide, or wherein the post are differently shaped and sized in respect to each other. 
       FIG. 10  shows a first embodiment of the filter of  FIG. 1  comprising a waveguide  200  provided with a top wall  21  facing a bottom wall  22  and a first side wall  23 , facing a second side wall  27 . In the Figures, the same reference numerals refer to the same or analogous components or devices. 
     The waveguide  200  of  FIG. 10  can be obtained by processing an individual metal slug (corresponding to the bottom of the waveguide  200 , which comprises the bottom wall  22 ), for example, by milling steps (which can be carried out by Numeric Controlled machines) which allow, by removing the material, to form the bodies which form the inductive/capacitive or resonant discontinuities present in the waveguide  200 . The rounding offs within the waveguide  200 , shown as a way of example in  FIG. 10 , refer to the particular use of a candle-mill. 
       FIG. 11  illustrates an analogous embodiment to that of  FIG. 10 , which requires, however, two slugs, one for the top wall  21  integral with the reduced-height posts  6  and  14 , and one for the bottom of the guide  200 , integral with the bottom wall  22 . The embodiment of  FIG. 11  has an advantage, in respect to that of  FIG. 10 , in terms of manufacturing process when the distances between the reduced-height posts  6  and  14  and the side walls  23  and  27  are smaller than the minimum diameter of the mill. 
       FIG. 12  refers to another embodiment  300  of the “metal insert-type” band-pass filter  100 , which is an alternative to those of  FIGS. 10 and 11 . In  FIG. 12 , the metal-insert type band-pass filter  300  is made by assembling (for example by welding or the like) a first guide shell  31 , a structure  32  and a second guide shell  33 , all made of metal. The first and the second guide shells  31  and  33 , when assembled, form a rectangular wave guide. 
     The structure  32 , intended to be placed in the middle of the wave guide and parallel to the axis of propagation z comprises a carrying longitudinal top laminar rod  34  and a carrying longitudinal bottom laminar rod  35 , between which a plurality of laminar discontinuity bodies extend. 
     In particular, the structure  32  comprises a first reduced-height laminar body  36 , a first full-height laminar body  37 , a second reduced-height laminar body  38 , a second full-height laminar body  39  and a third reduced-height laminar body  40 . 
     The operation and the equivalent electric scheme of the filter in  FIG. 12  are analogous to those disclosed above and therefore the full-height laminar bodies  37  and  38  have the function of inductive coupling devices, while the reduced-height laminar bodies  36 ,  38  and  40  are resonant coupling devices (from the circuit perspective, analogous to the reduced height posts  4 ) and adapted for introducing a respective transmission zero. 
     The four guide segments interposed between consecutive laminar bodies  36 ,  37 ,  38 ,  39  and  40  are segments intended to operate as resonators within the pass-band. It is to be noted that also two plates, analogous to plate  32 , may be used, each one having the plurality of discontinuities indicated above, which will be arranged, preferably, symmetrically in respect to a longitudinal middle plane of the assembled waveguide. 
     The embodiment shown in  FIG. 12  is particularly advantageous since it provides a simple and quite inexpensive manufacturing method based on working the plate  32 , which provides removing metal portions, for example, by laser cutting or electro-erosion. 
     The metal-insert band-pass filter  300  of FIG.  12  is a four-resonator filter with three transmission zeros.  FIG. 13  shows the behaviours of the reflectance S 11  and the transmittance S 21  obtained by numerical simulation, referring to an example of the metal insert filter  300  of  FIG. 12  with a guide dimension of 30×15 mm; pass-band 7.50-7.75 GHz, return losses 20 dB, three zeros at the following frequencies: 7 GHz, 8.25 GHz and 9 GHz. 
     As it is evident to those skilled in the art, the alternating inductive coupling devices in respect to the resonant coupling devices may follow a different order from those disclosed and designated as a way of example in the accompanying Figures. Furthermore, it is to be noted that according to a variant of the filter  300  of  FIG. 12 , instead of metal lamina  32  a thin metallised dielectric plate may be used, from the processing thereof the above disclosed coupling devices being obtained (“E-plane filters” technique). 
       FIG. 14  refers to an embodiment  400  of the band-pass filter  100 , which may be implemented by processing the low-loss dielectric slug, and suitable for the guided propagation of electromagnetic waves, obtaining hollow geometrical shapes which reproduce as a negative both the shape of the inductive coupling devices such as the diaphragms  4 ,  8 ,  11  and  18  and the resonant coupling devices (such as the two posts  6 ). 
     These cavities obtained in the dielectric slug are then coated with a metal material by a metallization step, which enables to obtain the four external walls of the waveguide of the dielectric filter  400 . In particular, the dielectric-type filter  400  of  FIG. 14  is a four-resonator band-pass filter with a transmission zero. For the sake of clarity of the depiction in  FIG. 14 , they are not shown. 
       FIG. 15  shows the behaviours of the reflectance S 11  and transmittance S 21  obtained by a numerical simulation with reference to an example of the dielectric filter  400  of  FIG. 14 , made of quartz, of 15×7.5 mm; pass-band 7.5-8.00 GHz, return loss 20 dB, a zero at 8.85 GHz. 
     The band-pass filter  100  and its different embodiments disclosed above, with reference to the several appended figures, may further comprise adjusting screws (not shown since they are known to those skilled in the art) which allow to carry out a fine calibration by compensating possible process tolerances. 
     The band-pass filter  100  may be used in waveguides which operate at the typical microwave frequencies, for example at frequencies ranging from 100 MHz and 40 GHz. 
     The disclosed band-pass filter is advantageous since it allows to obtain a remarkable increase in the selectivity in respect to the prior art filters, with the same number of resonators, and at the same time it may be implemented quite simply, with similar size and losses, and according to the different technologies currently available. A particular advantage is due to the possibility to implement also the resonant coupling devices by bodies within the guide itself. 
     Finally, the present invention is capable of a number of modifications and variants, all of which fall within the appended claims, whereas the technical details can change according to specific needs.