FR2518828A1 - Frequency spatial filter for two frequency microwave antenna - comprising double sandwich of metallic grids and dielectric sheets - Google Patents

Abstract

The filter can either separate or combine two electromagnetic waves of determinate mean angle of incidence in different frequency bands. A central sheet (2) of low-loss dielectric material (e.g. polyphenylene) carries on both faces (4,5) identical metallic grids of rectangles symmetrical about the plane of incidence (6). The central sheet is sandwiched between two others (1,3) of the same material or of other materials having the same characteristics. The combination reflects quasi-optically a beam of W-band (94 GHz) waves, and transmits without deflection a beam of X-band (10 GHz) waves arriving at the same angle of incidence on the opposite face. Also claimed are a Gregory offset antenna and a Cassegrain antenna, each incorporating such a filter.

Description

251882 a

FREQUENCY SPACE FILTER

AND ANTENNA COMPRISING SUCH A FILTER

  The present invention relates to a spatial frequency filter and its application to an antenna operating simultaneously on two different frequency bands. It is known that such filters are used to combine in a single beam of electromagnetic waves at least two beams of different frequencies from generally from two separate sources Conversely, such filters are also used to separate, in a composite beam, the beams of electromagnetic waves of different frequencies and direct them towards distinct receivers The behavior of these filters is to be compared to that of dichroic mirrors used in optics whose property is to be transparent to light beams of a certain wavelength and to be reflective

  for beams of light of another wavelength.

  Spatial frequency filters are used, in particular, in microwave antennas and more particularly in parabolic reflector antennas. It is indeed advantageous to make the most of such antennas by increasing the amount of information transmitted or

  One of the known means is to operate the antenna simultaneously-

  ment on several frequency bands which implies the use of frequency multiplexing and demultiplexing means, which, in particular,

  can be spatial frequency filters Such multi-antenna

  bands are used in telecommunications in ground links or between the ground and generally geostationary satellites Simultaneous reception on several frequencies is also very advantageous in radio astronomy where the cost of multiple antennas is particularly high Finally, the simultaneous use of second frequency in a tracking radar increases its performance and in particular makes it possible to distinguish

  more clearly the real target of his image in the ground or in the sea.

  The spatial filters used in the antennas and produced according to the known art are generally multilayer dielectric filters, filters using the selective properties of metallic networks and more rarely

  filters using sets of waveguides.

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  Multilayer dielectric space filters consist of a

  superposition of layers of dielectric materials of different thicknesses.

  Two types of dielectric materials of different characteristics are used. Pun for the layers of even rank, the other for the layers of odd rank. A dielectric material of a third type must be used for the first and the last layer in order to achieve the impedance matching between the filter and the medium This type of multilayer dielectric filter must, in order to satisfactorily maintain the purity of polarization of the electromagnetic waves, operate at a low angle of incidence which generally complicates the problems of implantation of the filter in the chain of elements which connects the fixed sources (or detectors) to the

  generally movable main reflector Furthermore, these dielectric filters

  multilayer cudgel require materials of three different natures which, given the scarcity of dielectric materials with low losses, limits the possible combinations of realization Finally, the bonding of

  different layers of dielectric materials pose technological problems

  giques because these filters can reach up to three meters in diameter.

  Spatial filters using the selective properties of metal networks consist of a flat or curved surface on which were made, generally by photoengraving, thin copper drawings whose shape depends on the desired result. A mesh metal network presents the properties of a high-pass filter while the complementary network, that is to say a set of conductive rectangles isolated from each other, presents the properties of a low-pass filter Finally, a set of conductive crosses Greek or potent (Jerusalem cross) has properties of the "band pass" type. Several disadvantages are specific to this type of filter. The metal network, the thickness of which is a few tens of micrometers, is carried by an insulating substrate, the thickness of which is appreciably more large, however remains less than a hundred micrometers; such a device, generally of large dimensions, poses a problem of mechanical rigidity In addition, the substrate carrying the conductive elements is a dielectric material which, although of small thickness, introduces very annoying parasitic phenomena of the resonance type, difficult

to predict and reduce.

  Spatial filters using waveguides comprising a certain number of these guides stacked so as to form a volume whose entry face corresponds to the dimensions of the global beam which strikes it. The high frequencies are transmitted by these guides while the frequencies bass are reflected, the cutoff frequency being determined by the dimensions of the waveguides These filters are rarely used because they limit the bandwidth and especially because of their weight and

high volume.

  To overcome the drawbacks of the known art, the invention proposes a spatial filter consisting of thin metallic networks associated with layers of dielectric materials. The association in these same device of these two known filter techniques allows, according to the essential characteristic of the invention of retaining the advantages specific to these techniques while

dismissing the drawbacks.

  The subject of the invention is therefore a spatial frequency filter capable of separating or combining electromagnetic waves of determined average angle of incidence and located in different frequency bands, characterized in that it comprises at least two layers of materials superimposed dielectrics and metallic networks interposed between

  successive layers of dielectric material.

  The invention also relates to an antenna comprising such a filter.

  The invention will be better understood and the details of embodiment appear.

  more clearly using the description below, with reference to

attached figures.

  FIG. 1 illustrates an exemplary embodiment of a spatial filter according to the invention. Figure 2 is an enlarged view of part of the metal network

  used in the previous example.

  Figure 3 is a side view of the device according to the invention

  showing the action of the filter on the electromagnetic wave beams.

  FIG. 4 illustrates an example of application of a filter according to the invention.

  tion to an off-set Gregory type antenna.

  FIG. 5 represents a first variant of the Cassegrain antenna

  using a filter according to the invention.

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  4: FIG. 6 represents a second variant of the Cassegrain antenna

  using a filter according to the invention.

  FIG. 1 illustrates an exemplary embodiment of a spatial filter according to the invention A central plate 2 ern low loss dielectric material carries on its two faces identical networks 4 and 5 made up of metallic rectangles admitting the plane of incidence -6 as a plane of

  symmetry The metallic networks 4 and 5 can be obtained by photo-

  engraving or by milling using a dielectric strip 2 metallized on its two faces This central strip 2 is placed between two other strips 1 and 3, the three strips being made of the same dielectric material or of materials having the same characteristics Layers 1, 2 and 3 of dielectric material and respective thicknesses 51, 52 and 53

  are, in this example, polyphenylene The beam of electro-

  magnetic is directed on the filter in a direction parallel to the line 7 1 located in the plane of incidence 6 This line 7 is made with the line 8 also located in the plane of incidence and perpendicular to the planes of the layers of dielectric material a angle of incidence Q which in this preferred variant of operation of the filter according to the invention is

of 45.

  FIG. 2 is an enlarged view of part of the metallic network used in the previous example Small metallic rectangles 10 a few tens of micrometers thick and generally made of copper are arranged according to FIG. 2 on each of the faces of the layer of central dielectric material 2 These rectangles of length a, of width b and separated from each other - by a distance d can be made, by way of,

  example by photogravure or milling of a layer of dielect material

  metallized stick on both sides.

  FIG. 3 is a side view of the device according to the invention showing the action of the filter on the electromagnetic wave beams A first wave beam located in the W band (94 G Hz), directed towards the spatial filter 11 under an angle of incidence 6 in a direction parallel to the right is reflected, according to the laws of optics, in a direction parallel to the right 14 A second wave beam located in the band X (r 10 G Hz) , directed towards the other face of the filter 11 at the same angle of incidence 6 and in

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  a direction parallel to the right 14 is transmitted, without change of direction, through said filter in a direction parallel to that of

  beam of the band W Such a description corresponds to the case of a

  frequency multiplexing where two separate beams X and W are combined to form a single composite beam In Figure 3, it is enough to reverse the direction of the arrows to understand how the filter works in demultiplexing mode The composite beam then directed towards a face of the filter II in a direction parallel to the right 14 sees its component W reflected in a direction parallel to the right 15 and its component X propagates through the filter while preserving its initial direction The two frequency bands X and W can thus to be led

to separate receivers.

  Table I at the end of this description summarizes the

  characteristics and performance obtained in an exemplary embodiment

typical.

  FIG. 4 illustrates an example of application of a filter according to the invention.

  tion to an antenna of the "Gregory off-set" type A corrugated horn 21 emits a beam of electromagnetic waves 22 located, for example, in the X band This beam 22 crosses, without changing direction and without significant losses, a filter II according to the invention before being reflected a first time by an ellipsoid element 23 then a second time by a paraboloid element 24 before being transmitted into space A second corrugated horn 24 emits a beam of electromagnetic waves 25 located, for example, in the band W This beam 25 is directed at an angle of incidence of 450 towards the filter II which reflects it in a direction parallel to the beam 22 towards the reflectors 23 and 24 beams 22 and 25 form a composite beam transmitted in the same direction of

  space by an antenna called in this case "bi-frequencies".

  FIG. 5 represents a first variant of the Cassegrain antenna using a filter according to the invention The filter 51 is a spatial filter in accordance with the variant described in relation to FIG. 1 but curved,

  as shown in Figure 5, according to a hyperbolic law.

  A first source of electromagnetic waves emitting in the X band and represented by the corrugated horn 53 emits a beam of waves 54 which crosses, without changing direction, the filter 51 before being reflected towards space by the main reflector 50 A second wave source emitting in the W band and represented by the horn 52 emits a wave beam 55 which is directed towards space in a direction parallel to the beam 54 after two successive reflections on the filter 51 then on the

main reflector 50.

  Figure 6 shows a second variant of Cassegrain antenna

  using a filter according to the invention.

  A main parabolic reflector 30, an auxiliary mirror 31 and a deflection mirror 34 form an assembly 41 which can move in elevation around an axis 33 A set 42 comprising an ellipsoidal mirror 35 can move around an axis 32 ensuring therefore a displacement in azimuth

of the set 41.

  The fixed part 43 comprises a spatial filter 38 according to the invention, two

  ellipsoidal mirrors 36 and 37, a primary source of electromagnetic waves

  genetic emitting in the W band represented by the corrugated horn 39 and a second source emitting in the X band represented by the

horn 40.

  The ellipsoldic mirrors 35, 36 and 37 are arranged to each have one of its focal points located on the filter 38; the horns 39, 40 and the mirror 34 are

  respectively in coincidence with the other focal point of mirrors 36, 37 and 35.

  The electromagnetic wave located in the W band and emitted by the horn 39 is directed, after reflection by the mirror 36, at an angle of incidence of 450 towards the spatial filter 38; the wave is fully reflected by the filter 38 and directed towards space by successive reflections on the elements 35, 34, 31 and 30 The electromagnetic wave located in the X band and emitted by the horn 40 is directed, after reflection by the mirror 37, at an angle of incidence of 45 towards the spatial filter 38; the wave is completely transmitted without change of direction, by the filter 38 and directed towards

  space by successive reflections on elements 35, 34, 31 and 30.

  The spatial filter according to the invention is not limited to the examples of

  achievements which have just been described.

  The filter can be determined to operate at another angle of incidence, this angle being able to be chosen from a range between

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  ten and fifty degrees The frequency bands to be separated or combined can be different from those mentioned by way of illustration The filter according to the invention can be produced so as to have high-pass or band-pass properties Metal networks can be of different configurations from those mentioned; the same filter can comprise more than two metallic networks, each network, separated from the others by a dielectric material, being able to have its own configuration In addition, each metal network can be indifferently attached to one or the other of the layers which are adjacent, it can still be independent and held in place by any known means The filter can comprise at least two layers of dielectric material of different thicknesses and be curved according to any predetermined shape Other dielectric materials having properties

  similar to polyphenylene can be used.

  The spatial filter according to the invention therefore has the advantages, compared to the multilayer dielectric filters of the prior art, of preserving the purity of polarization of the electromagnetic wave for an angle of incidence

  much larger, requiring only one type of dielectrical material

  stick instead of three, to simplify mechanical assembly problems

  by considerably reducing the number of layers of dielectric material

  In comparison with metal grating filters of the known art, the filter according to the invention does not pose problems of mechanical stiffness and does not introduce very annoying parasitic phenomena linked to the existence in the known art of a dielectric material intended only to serve as a mechanical support for the metal network Finally, compared to known filters using sets of waveguides, the filter according to the invention is much lighter, more economical and can operate with a width

wider band.

TABLE I

  Characteristics and performances obtained in an example of reali-

typical location.

  Thickness of dielectric materials Sil 52, 53 between 3 and 4 mm

  Nature of the dielectric material: polyphenylene.

  Dimensions of the rectangles of the metal network: a lmm b ^ 0.8 mm

d = from O 1 to O 2 mm.

  Frequencies used: X band: 9 to 10 G Hz

W band: 94 G Hz.

  Losses in X-band transmission: 0.5 d B Losses in W-band reflection: 0.5 d B Rate of ellipticity of the reflected wave for a perfectly circular incident wave: 0.1 d B Rate of ellipticity of the wave having passed through the filter for a perfectly circular incident wave: O, 1 d B.

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Claims (12)

  1 spatial frequency filter capable of separating or combining electromagnetic waves of determined average angle of incidence and located in different frequency bands, characterized in that it comprises at least two layers (1 3) of dielectric material superimposed and metallic networks (4 5) interposed between the successive layers of dielectric material. 2 Filter according to claim 1, characterized in that the metal networks have the configuration of a set of conductive rectangles (10) isolated from each other.   3 Filter according to claim 1, characterized in that the metal networks have the configuration of a grid of filiform conductors   parallels in contact at their intersection points.   4 Filter according to claim 1, characterized in that the metal networks have the configuration of a set of insulated conductive crosses between them.   Filter according to any one of claims 2 to 4, characterized   in that the metal networks consist of a metallization of at least one face of the layers of dielectric material resulting from a photoengraving or milling.   6 Filter according to any one of claims 2 to 5, characterized   in that it comprises at least two different configurations of metal networks.   7 filter according to any one of claims 1 to 6, characterized   in that it comprises at least two layers of dielectric material of different thicknesses.   8 Filter according to any one of claims 1 to 7, characterized   in that its structure is planar (11).   9 Filter according to any one of claims 1 to 7, characterized   in that its structure is in the form of hyperboloid elements (51).   10 Filter according to any one of claims 1 to-9, characterized in   that the determined average angle of incidence of the electron beam   magnetic is chosen in a range between ten and fifty degrees. 11 Filter according to claim 10, characterized in that the angle   average incidence chosen is forty five degrees.   12 Filter according to any one of claims 1 to 10, characterized   that the dielectric material is polyphenylene.   13 Antenna operating simultaneously on two different frequency bands and characterized in that it includes a filter according to   any one of claims I to 12.   14 Antenna according to claim 13, characterized in that it is "Gregory off-set" type.   15 antenna according to claim 13, characterized in that it is "Cassegrain" type.