Abstract:
A device for the filtering of electromagnetic waves propagating in a main rotational symmetrical waveguide element comprising at least one filtering section constituted of a rectangular waveguide section inserted in being spliced into the main waveguide element, each transition between said main waveguide element and each of said filtering sections being made by metal walls which are substantially perpendicular to said axis of symmetry, the number and the geometrical and dimensional characteristics of the filtering sections being chosen so as to constitute a filter with a pre-determined filtering profile. The device can be applied especially in dual band filtering, for example for the making of dual band or bi-polarization duplexers.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The field of the invention is that of filtering in electromagnetic waveguides. More specifically, the invention relates to a device for the filtering of waves propagating in rotational symmetrical waveguides, such as the circular waveguides or coaxial waveguides used in TE 11  mode. 
     The invention can be applied in particular to dual band filters. A major use of this invention is indeed the making of dual band and bi-polarization duplexers, notably when the ports of the duplexers are in the same rectangular guide standard. This is, for example, the case of the 10.95-12.5 GHz and 14-14.5 GHz bands in the WR 75 waveguide. As a general rule, the horizontal and vertical polarizations of these duplexers are not identical in the two frequency bands considered. 
     2. Description of the Prior Art 
     FIG. 1 shows a schematic view of a known type of duplexer such as this. The high (for example 14-14.5 GHz) band port 11 and the low (for example 10.95-14.5 GHz) band port 12 are constituted by rectangular waveguides. The output 13 towards the radiating element is constituted by a circular waveguide. 
     For the low band, the excitation is achieved by a coupling, by means of a slot in the duplexer 14, between the rectangular guide and the circular guide. If the wave is to be propagated towards the radiating element and not towards the high band port 11 constituted by a rectangular guide, it is necessary to place a rectangular-to-circular transition 15 and a polarization filter 16 between the high band 11 port and the duplexer 14. 
     There already exists several known types of polarization, such as those shown in FIGS. 2 and 3. FIG. 2 shows a filter with metal plate 21, and FIG. 3 shows a filter with metal wires 31, 32, 33. These metal elements 21 or 31, 32, 33, selectively placed in the waveguide 22, enable the elimination of a polarization, hence the elimination of a given frequency band. 
     An error of one degree in the positioning of this plate 21 or of these wires 31, 32, 33 causes a maximum decoupling (transmission of the electromagnetic wave from the port 11 to the port 12) of 35 dB, which is generally insufficient. In mechanical terms, therefore, the making of these devices calls for high precision in the placing and fixing of the plate 21 or of the wires 31, 32, 33 to the interior of the circular guide 22. Furthermore, the manufacture of such filters calls for several successive and delicate steps. 
     SUMMARY OF THE INVENTION 
     It is an aim of the invention, notably, to overcome the drawbacks of these prior art filters. 
     More specifically, an aim of the invention is to provide a filtering device for rotational symmetrical waveguides that is easy to make from the mechanical viewpoint, and notably a filtering device that does not call for the mounting of elements inside the waveguide. 
     Another aim of the invention is to provide a filtering device such as this providing a satisfactory decoupling of at least 40 to 45 dB. 
     A particular object of the invention is to provide a filtering device such as this enabling a total reflection of one polarization and the total transmission of the other polarization, in a given frequency band. 
     An additional object of the invention is to provide a device such as this, making it possible to pass from a linear polarization to a circular polarization. 
     These aims, as well as others that shall appear hereinafter, are achieved according to the invention by means of a device for the filtering of electromagnetic waves propagating in a main rotational symmetrical waveguide element extending along an axis of symmetry, wherein said device comprises at least one filtering section constituted of a rectangular waveguide section inserted in being spliced into said main waveguide element, each transition between said main waveguide element and each of said filtering sections being made by metal walls which are substantially perpendicular to said axis of symmetry, the number and the geometrical and dimensional characteristics of said filtering sections being chosen so as to constitute a filter with a pre-determined filtering profile. 
     These filtering rectangular waveguide sections introduce a disymmetry in the main waveguide. Depending on their geometrical characteristics, number and spacing, they can be used, for example, to obtain a filter with a reflection coefficient close to 1 for one of the polarizations and a reflection coefficient close to zero for the other polarization, in a given frequency band. For other dimensions, it is also possible that there is no longer any overlapping between the filtered band and the pass band of the filter. 
     The transition between each section is abrupt (i.e. substantially perpendicular to the axis of symmetry of the main waveguide). No particular tapered transition or matching element is needed between the main waveguide and the filtering sections. These sudden transitions are, naturally, closed by a metal conductor on the part of the transition where both cross-sections of the main waveguide and of the rectangular section do not coincide (if not, the waves would no longer be guided). 
     In a preferred embodiment, said main waveguide element is of the circular waveguide type or of the coaxial waveguide type working in TE 11  mode. 
     In a preferred way, the width of said rectangular waveguide sections is greater than or equal to the diameter of said main waveguide element. 
     In an advantageous embodiment of the invention, said main waveguide element and said rectangular waveguide sections are centered on said axis of symmetry. 
     Advantageously, the device of the invention comprises a set of at least two sections of rectangular waveguide sections inserted at locations spaced out in said main waveguide element. 
     It can be seen, in fact, that the quality of the filtering is a function of the number of rectangular waveguides used. It will be noted, furthermore, that it is quite possible to envisage the use of the rectangular sections according to the invention in combination with other known types of elements such as metal plate filters or metal wire filters. 
     In an advantageous embodiment, the number and the geometrical and/or dimensional characteristics of said rectangular waveguide sections are determined by modal analysis. 
     The device of the invention can be used notably for at least one of the following applications: 
     the filtering of a frequency band in a circular waveguide element in TE 11  mode; 
     the filtering of a horizontal or vertical polarization in a circular waveguide element in TE 11  mode; 
     the conversion of a linear polarization into a circular polarization. 
     When the device of the invention is designed for the conversion of a linear polarization into a circular polarization, said linear polarization is advantageously parallel to a diagonal of the cross-section of said rectangular waveguide sections. 
     The invention also relates to duplexers implementing a filtering device as described here above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the invention shall appear from the following description of a preferred embodiment of the invention, given by way of a non-restrictive illustration, and from the appended drawings, of which: 
     FIG. 1 shows a schematic view of a duplexer capable of using a filtering device according to the invention; 
     FIGS. 2 and 3 show two known types of polarization filters, respectively a metal plate filter and a metal wire filter, mounted inside the waveguide and already described in the introduction; 
     FIG. 4 shows a schematic view of a filtering element according to the invention, with a rectangular section inserted in being spliced into a circular main waveguide; 
     FIG. 5 shows the dimensions of the filtering device described as a preferred embodiment, comprising four filtering elements as shown in FIG. 4; 
     FIGS. 6A to 6B illustrate the reflection coefficients of the device of FIG. 5, when the polarizations of the TE 11  mode in circular guide operation are respectively parallel and perpendicular to the small side of the rectangular waveguide sections. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention relates therefore to a filtering device, made by the insertion of rectangular waveguide sections introducing a dissymmetry into a main rotational symmetrical waveguide. 
     In the embodiment described here below in detail, the main waveguide is a circular waveguide. 
     It is clear, however, that the scope of the invention can easily be broadened to include other types of waveguides. Thus, the main waveguide may also be, for example, a coaxial waveguide in TE 11  mode. It is also possible, in conjunction with rectangular filtering elements according to the invention, to use guide elements of a known type, called rotational symmetrical elements, provided that their symmetry is eliminated, for example by placing an dielectrical plate along one of the polarizations. 
     FIG. 4 shows a schematic view in perspective of a filtering element according to the invention. The circular main waveguide is separated into two parts 41 A  and 41 B  between which a rectangular waveguide section 42 is inserted. 
     By associating several elements as shown in FIG. 4, it is possible to make a precise and efficient filter, as illustrated by FIGS. 6A and 6B shown further below. 
     The main waveguide 41 A  and 41 B  is joint to the rectangular section 42 with walls 44 A  and 44 B , which close the part of the transition where both cross-sections of the main waveguide and the rectangular section do not coincide. 
     According to the invention, the walls 44 A  and 44 B  should be abrupt. In other words, the walls 44 A  and 44 B  are substantially perpendicular to the axis of symmetry 45 of the main waveguide 41 A , 41 B . No transition element is inserted. These walls 44 A  and 44 B  are metallic. Naturally they cannot be completely open (i.e. they should not be in the presence of air) nor should they be made of a dielectric. Otherwise, the wave would escape and would no longer be guided. 
     It is seen, therefore, that the machining of a filter such as this is highly simplified as compared with the filters shown in FIGS. 2 and 3. Indeed, there is no element to be placed within the circular guide, nor is there any particular transition to be defined. It is enough to fix the different sections to one another or else to make the filter by means of two half-shells. 
     Advantageously, the circular guide 41 A  is placed at the center of the rectangular guide 42, and the cross-section 43 common to the two waveguides is circular. In this case, the height of the rectangular guide 42 is thus at least equal to the diameter of the circular guide 41 A . 
     In the embodiment described, the sections 41 A , 42, 41 B  are all centered on a same longitudinal axis which is the axis of symmetry 45. In other applications, however, it is possible to arrange for an offsetting of these sections in relation to this axis. 
     The geometry of the rectangular guide section (height, width and thickness) as well as the number of sections and the spacing between these sections are a function of the characteristics desired for the filter. These different parameters may be determined, for example, according to the modal method. 
     There is no limit on the dimensions of the sides of the rectangular guide, so long as these dimensions are greater than the diameter of the circular waveguide. The pass band may therefore be high (for example of the order of 10%). 
     As already mentioned, the invention finds preferred application in dual band and bi-polarization duplexers such as those shown schematically in FIG. 1. In this case, the polarization filter 16 should totally transmit one of the polarizations and should reflect the other polarization. The example, with estimated values, described here below relates to a filter such as this, for the 12-13 GHz frequency band. 
     Since the dimensions of the rectangular waveguide are greater than the diameter of the circular guide, the polarization of the wave getting propagated in the circular guide may be placed along the diagonal of the rectangular guide. 
     Since the pass band is high, it is necessary to connect a circular guide to the output (13) of the polarizer. 
     It is furthermore clear that the device of the invention can find numerous other applications, both in filtering and in polarization. 
     FIG. 5 therefore shows the dimensions of a filter, the performance characteristics of which are illustrated by FIGS. 6A and 6B. This filter is constituted by four rectangular waveguide sections 51 A  to 51 D , inserted in the circular waveguide 52. 
     The circular waveguide has a diameter c=17.5 mm. 
     The rectangular sections have the following dimensions: 
     width: a=28.5 mm; 
     height: b=21.26 mm; 
     length: d=10 mm. 
     The spacing between two rectangular sections is: e=15.8 mm. 
     The excitations are done in TE 11  mode in the circular waveguide 52. 
     It must be noted that this embodiment does not correspond to an optimized filter, but is aimed at enabling the validation of a software computation, as can be seen in FIGS. 6A and 6B. 
     FIG. 6A shows the curve 61 A  of the reflection coefficient of the filtering device of FIG. 5 when the polarization in TE 11  mode in a circular guide is perpendicular to the small side of the rectangular sections 51 A  to 51 D . 
     In this case, the TE 11  mode is completely transmitted on the 12-13 GHz frequency band, the reflection coefficient being close to 0. 
     The purpose of this filter, which is given by way of an example, is to give a filtering result that is as close as possible to the result that is fixed theoretically by computation for a given application, represented by a series 61 A  of + signs. 
     The curve 62 A  of measured reflection shows that it is possible, with the device of the invention, to enforce the filtering characteristics with precision. It is observed, in effect, that the curve 62 A  is very close to the desired results 61 A . 
     FIG. 6B shows the reflection coefficient of the same device, when the polarization of the TE 11  mode in the circular guide is parallel to the small side of the rectangular sections 51 A  to 51 D . The TE 11  mode is then reflected totally for the 12-13 GHz frequency band. Indeed, the reflection coefficient is close to 1 and the transmission is therefore zero. 
     Once again, it is observed that the measured curve 62 B  very closely follows the computed desired characteristics 61 B . 
     With this filtering device, decouplings of the order of 40 to 45 dB are obtained. This corresponds to the values obtained with the standard wire filters or plate filters, when these elements are well positioned. The invention thus provides for the making of filters that are at least as efficient as those of known types, and for making them far more easily from the manufacturing point of view. 
     The geometrical characteristics of this filter have been determined according to the modal method. Other methods of computation can also be determined. Advantageously, the experimental precision tuning of a filter is done by means of an optimizing software implementing, for example, this modal method. 
     The invention is naturally not limited to the embodiment described here above. It is indeed possible to make filters that use rectangular sections with different geometries. These different sections may be attached or not attached, and the spaces between them may have fixed or variable sizes. 
     It is also possible to use rectangular sections according to the invention in conjunction with standard filtering devices, for example metal plate or metal wire filtering devices. 
     Apart from the filtering (horizontal or vertical) in a circular guide in TE 11  mode, when the two linear polarizations coexist, the device of the invention can also be used for the filtering of a frequency band in a circular guide in TE 11  mode, in the case of a rectilinear polarization. 
     Yet another application of the device of the invention lies in the making of polarizers to convert a linear polarization into a circular polarization. 
     A polarizer is a device that enables changing from a linear polarization to a circular polarization. In the case of the invention, the linear polarization should be parallel to a diagonal of the rectangular guide. At output, there is then obtained a circular polarization, since the horizontally polarized waves and the circularly polarized waves do not have the same phase speed in the rectangular guide. A complete polarizer may be made by associating several elements according to the invention, or else by associating them with other already known elements.