Patent Publication Number: US-2022231417-A1

Title: Antenna network with directive radiation

Description:
The present application claims priority from French Patent Application number FR 20 13099 filed Dec. 11, 2020, the entire disclosure of which is incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The present invention relates to an antenna network with directive radiation adapted to operate in at least one predetermined frequency band. 
     The invention is in the communications field in which directive radiation is desired, and more particularly in the field of satellite geolocation and navigation communication. 
     BACKGROUND OF THE INVENTION 
     A global navigation satellite systems (GNSS) system comprises a satellite signal receiver with a receiving antenna or antenna network that has good directivity, maximum gain in the zenith direction for receiving satellite signals, and right-hand circular polarization, also called RHCP. For various practical applications, GNSS receivers are carried on a carrier such as a motor vehicle or any other type of vehicle. 
     In the state of the art, ceramic antennas are known, such as patch antennas on a ceramic substrate, which are miniature and make directive radiation and right-hand circular polarization radiation possible, which makes them suitable for application in GNSS systems. Nevertheless, the cost of the material forming such antennas is incompressible, which limits the large-scale deployment of multi-band GNSS that requires ceramic patches superimposed on each other. 
     Multi-wire antennas with helical geometry are also known, where the wires are wound around a cylinder of dielectric material and rest on a reflector plane. In such an antenna structure, the number of antenna wires makes operation in several frequency bands possible, for communication with several satellites. However, the antenna takes up a lot of space and can reach heights of about 20 cm. 
     The object of the invention is to overcome the disadvantages of the state of the art by proposing an antenna network with directive radiation, in circular polarization, in particular in right circular polarization, which is both compact and low cost. 
     SUMMARY OF THE INVENTION 
     To this end, the invention proposes an antenna network with directive radiation, adapted to operate in at least one predetermined frequency band, which comprises:
         at least one pair of metal antennas formed of a first metal antenna and a second metal antenna, the second metal antenna being positioned in sequential rotation by a predetermined angle of rotation relative to the first metal antenna,   a load circuit, each metal antenna being connected to said load circuit,   a monopole antenna, having a central position in the antenna network, connected to said load circuit,   said metal antennas and said monopole antenna being arranged on a ground plane and coupled, the loading circuit being parameterized to make radiation in which the monopole antenna has a destructive contribution of a magnetic transverse radiation mode, making it possible to obtain a radiation of selected circular polarization by said at least one pair of metal antennas.       

     Advantageously, the proposed antenna network is made from metal antennas with a low manufacturing cost, and the proposed arrangement makes it possible to achieve the directivity and circular polarization while making it possible to make a compact antenna. 
     The antenna network according to the invention can also have one or more of the following features, taken independently or in any technically feasible combination. 
     The circular polarization chosen is a straight circular polarization. 
     The antenna network comprises two pairs of metal antennas, each pair of metal antennas being adapted to operate in an associated frequency band, so as to make a dual frequency band antenna. 
     A first pair of metal antennas is formed by two antennas each having a radiating element of a first length, and a second pair of resonant metal antennas is formed by two antennas each having a radiating element of a second length, the second length being different from the first length. 
     For the or each pair of metal antennas, the predetermined rotation angle is a 90° (90 degrees) angle. 
     The antenna network comprises four pairs of metal antennas, symmetrically arranged about a center of rotation of said sequential rotation. 
     Each metal antenna is an inverted F planar antenna. 
     Each pair of metal antennas has two inverted F planar metal antennas of the same dimensions, each inverted F planar metal antenna having a folded capacitive roof connected to the ground plane by a short circuit and a metal feed strand connected to said load circuit. 
     Each metal antenna of a pair of metal antennas is made by printing on a board. 
     The load circuit is composed of passive components of capacitive, inductive, resistive nature or a combination of these components. 
     According to another aspect, the invention relates to a satellite geolocation system comprising an antenna network as briefly described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the invention will be apparent from the description given below, by way of indication and not in any way limiting, with reference to the appended Figures, among which: 
         FIG. 1  shows schematically an antenna network according to a first embodiment; 
         FIG. 2  illustrates the geometry of a part of the antenna network according to the first embodiment; 
         FIG. 3  illustrates the geometry of the antenna network according to the first embodiment; 
         FIG. 4  illustrates a reference radiation pattern for an application in a satellite-based geolocation system; 
         FIG. 5  illustrates a radiation pattern made by an antenna network arrangement according to the first embodiment; 
         FIG. 6  shows schematically an antenna network according to a second embodiment; 
         FIG. 7  shows schematically an antenna network according to a third embodiment; 
         FIG. 8  shows schematically an antenna network according to a fourth embodiment; 
         FIG. 9  shows schematically a four pair arrangement of sequentially rotating antenna. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     A first embodiment of an antenna network according to the invention, forming a micro antenna network, is illustrated with reference to  FIGS. 1 to 3 . 
       FIG. 1  shows schematically, viewed from above, an antenna network  2  according to a first embodiment of the invention. 
       FIGS. 2, 3  show schematically the antenna network  2  in perspective, in an orthogonal 3D reference frame (X, Y, Z). 
     The antenna network  2  has a ground plane  4 , on which a load circuit  6  of the antenna network is printed. 
     The antenna network  2  is configured to operate in a predetermined frequency band centered on a given center frequency. For a GNSS system, the satellite transmission frequencies are L1=1,575.42 MHz and L2=1,227.60 MHz. For example, the antenna network  2  has a center frequency of 1575 MHz. 
     The antenna network  2  in the embodiment of  FIG. 1  includes a pair of metal antennas formed by a first metal antenna  8  and a second metal antenna  10 . 
     Each metal antenna  8 ,  10  comprises a radiating element whose central resonant frequency belongs to the selected frequency band. 
     In one embodiment, each said first metal antenna  8  and second metal antenna  10  is a planar inverted F-antenna antenna (PIFA). PIFA antennas are classically used in the field of radio communications. 
     In this embodiment, the two PIFA antennas  8 ,  10  are structurally identical. 
     The second PIFA antenna  10  is sequentially rotated relative to the first PIFA antenna  8 , orthogonally to the first PIFA antenna  10 . 
     In this embodiment, each PIFA antenna  8 ,  10  extends along a respective axis A 1 , A 2 , the antennas being positioned so that the axes A 1 , A 2  are perpendicular. 
     Sequential rotation is defined as rotation in a predetermined direction of rotation, about a predetermined center of rotation and by an associated selected angle of rotation. Preferably, the center of rotation is a point located substantially at the center of the antenna array, such as a point located on an axis perpendicular to the ground plane  4 , which intersects the ground plane at the center of the antenna array. 
     Thus, the second PIFA antenna placed orthogonally to the first PIFA antenna corresponds to a sequential rotation of equal rotation angle 90° from the initial position of the first PIFA antenna  8 . The center of rotation is referenced O in  FIG. 1 ; it is a point located substantially at the center of the antenna network  2 . 
     According to variants, it is possible to arrange a larger number of metal antenna pairs in this way, such as PIFA antennas with each antenna pair comprising two antennas sequentially rotating an associated rotation angle, forming several rotation sequences around the center O of the antenna network. 
     The antenna network  2  further comprises a monopole antenna  12 , which is placed at the center of the antenna network. In other words, the monopole antenna  12  has the point O as its center of symmetry, which is placed substantially at the center of the antenna network  2 . 
     Each PIFA antenna  8 ,  10  comprises a folded capacitive roof  14 ,  16 , and a metal feed strand  18 ,  20 . The capacitive roof  8 ,  10  is connected to the ground plane  4  by a short circuit  22 ,  24 . 
     In one embodiment, the dimensions of the PIFA antennas  8 ,  10  are as follows: length L=20 mm; width l=6 mm and height h=10 mm. 
     The monopole antenna  12  comprises a capacitive roof  26  and a metal feed strand  28 , which extends in the vertical direction when the ground plane  4  is horizontal in the illustrated embodiment. 
     In the illustrated example, the capacitive roof  26  of the monopole antenna  12  has a square or rectangular geometric shape in the plane of the antenna network  2 . In variants, the capacitive roof  26  of the monopole antenna  12  has a different geometric shape, such as a disk shape or any other chosen geometric shape. 
     According to an alternative embodiment, the metal antennas  8 ,  10  are patch type antennas (also called “microstrip antennas”), which operate in an analogous manner. In this embodiment, each antenna  8 ,  10  comprises a capacitive roof and a feed strand  18 ,  20 . Differently from PIFA antennas, in the embodiment with patch antennas, there is no short circuit  22 ,  24 . 
     Each of the feed strands  18 ,  20 ,  28  is connected to the load circuit  6  which is printed on the ground plane  4 . The load circuit  6  is illustrated schematically in  FIG. 3 , in dashed lines. 
     The metal antennas  8 ,  10 ,  12  are coupled, and the load circuit  6  is optimized to obtain an adequate radiation. 
     In the antenna network  2 , the metal antennas  8 ,  10  are resonant and the monopole antenna  12  is non-resonant, its radiation being used to cancel the unwanted radiation generated by the metal antennas of the antenna pair  8 ,  10 , as explained below. 
     Preferably, the load circuit  6  is a load circuit with load impedances calculated by a constrained calculation method, as described in patent EP2840654 B1, to achieve a radiation target shown in  FIG. 4 . This method is based on the use of the spherical wave decomposition principle, which decomposes the electromagnetic field radiated by each antenna into a series of modes, taking into account the coupling between the different antennas of the antenna array. This optimization tool makes it possible to apply a weighting on the series of radiation modes by amplifying the desired modes and attenuating the undesired modes. The optimal weighting obtained is then converted into complex impedances, making it possible to make the antenna loading circuit. 
     The antenna network  2  is configured for operation in a frequency band for receiving signals from satellites for application in a GNSS receiver. It is desired that the antenna network has a directional operation in a given direction, i.e. at the zenith, in right circular polarization. 
     The desired radiation is broken down into two radiation modes, the transverse electric mode TE −11  and the transverse magnetic mode TM −11  respectively. In an operation suitable for the intended application, these two radiation modes have the same amplitude and have phases of 0° and 180° respectively, or, in other words, are in phase opposition. The other radiation modes, TE 10  and TE 11  and TM 10  and TM 11  respectively, are zero. 
     The association of the transverse electric, TE −11  and transverse magnetic, TM −11  modes of radiation result in a radiation pattern with maximum right-hand circular polarization (RHCP) gain at the zenith and minimum left-hand circular polarization (LHCP) gain at the zenith. 
     A radiation pattern  30 , referred to as a baseline radiation pattern, is shown in  FIG. 4 . The radiation pattern shows the angular distribution of radiated power depending on the azimuth Φ. The power is expressed in circular isotropic decibels (dBic). 
     The radiation pattern  30  comprises the right-hand circularly polarized (RHCP) gain  32 , maximum at Φ=0° (zenith) and the left-hand circularly polarized (LHCP) gain  34 , maximum at Φ=180°. 
     A PIFA metal antenna fed by an electric current generates the electric transverse modes TE −11 , TE 11  and the magnetic transverse modes TM −11 , TM 10  and TM 11 . 
     Advantageously, due to the geometric arrangement, in sequential rotation, of the two metal PIFA antennas  8 ,  10  of the antenna network pair  2 , the electric transverse mode radiations TE 11  of two antennas are in phase opposition, and thus cancel each other out when they are at the same amplitude. Similarly, the magnetic transverse mode TM 11  radiations of two antennas are in phase opposition, and thus cancel each other out when they are at the same amplitude. 
     The transverse electric TE −11  and transverse magnetic TM −11  modes of the two sequentially rotating PIFA antennas are in phase and are added. 
     There remains a magnetic transverse mode radiation TM 10 , which has a phase at 90° for the first metal PIFA antenna  8 , for example, and at 180° for the second metal PIFA antenna  10 . Advantageously, the monopole antenna  12  emits a magnetic transverse mode radiation TM 10 , which, thanks to the load circuit optimization, is oriented to compensate the magnetic transverse mode radiation TM 10  of the metal PIFA antennas  8 ,  10 . Thus, the monopole antenna  12  has a destructive contribution; the magnetic transverse mode radiation TM 10  is cancelled. 
     The adjustment of the amplitudes and phases of the radiation modes generated by the metal antennas  8 ,  10 ,  12  is done by parameterization of the load circuit  6 . In one embodiment, this load circuit is composed of passive components of a capacitive, inductive or resistive nature, or of a combination of these components. The load circuit parameters are calculated using the method described in patent EP 2 840 654 B1. 
     For example, in one concrete embodiment, an antenna network  2  is developed for a GNSS geolocation and navigation system, for an on-board receiver on a motor vehicle. The antenna network has the following dimensions: a height of 10 mm and a square support of side 35 mm, for operation at the center frequency of 1.575 GHz. The antenna network is optimized to radiate with a maximum gain of 2 dBic at the zenith, with an axial ratio of 1 dB and a RHCP polarization in the L1 frequency band around 1.575 GHz. 
     The loading circuit  6  is such that the first metal PIFA antenna  8  is fed by a radio frequency (RF) source of impedance  500 , the second metal PIFA antenna  10  is loaded with a capacitance of 2.7 pF and the monopole antenna  12  is loaded with a capacitance of 10 pF. These load values are determined for the adjustment of the amplitudes and phases of the radiation modes present in the antenna array, so as to keep the TE −11 , TM −11  modes and to cancel the TM 10  magnetic transverse mode radiation, as explained above. 
       FIG. 5  shows the radiation pattern  35  obtained by the antenna network  2  made according to this concrete embodiment, this pattern including the right-hand circularly polarized (RHCP) gain  36  and the left-hand circularly polarized (LHCP) gain  38 . As can be seen, the RHCP gain is maximum at=0° (at zenith), and is comparable to the RHCP gain  32  of the reference radiation pattern  30 . 
     According to variants, the antenna network comprises more than one pair of resonant metal antennas. 
     For example, as illustrated in  FIG. 6 , an antenna network  40  according to a second embodiment of the invention includes two pairs  42 ,  44  of metal antennas, a first pair  42 , consisting of two orthogonally positioned, sequentially rotating metal antennas  46 ,  48 , and a second pair  44 , consisting of two metal antennas  50 ,  52 . In the illustrated embodiment, the two metal antennas  50 ,  52  of the second pair  44  have different dimensions than the dimensions of the antennas  46 ,  48  of the first pair  42 , and are respectively positioned above the antennas of the first pair. Thus, the two metal antennas of the first pair have a resonant element of a first length, and the two metal antennas of the second pair have a resonant element of second length, smaller than the first length, to target a lower frequency band dedicated to GNSS, for example, such as the L2 or L5 band. For example, the metal antennas  46 ,  48 ,  50 ,  52  are PIFA antennas, as described in the first embodiment. 
     The antenna network also includes a monopole antenna  54 , centered relative to the center of symmetry O of the antenna network  40  and non-resonant. 
     Advantageously, in this embodiment, the first pair  42  of antennas is configured to operate in a first frequency band, such as the L1 band, and the second pair  44  of antennas is configured to operate in a second frequency band, such as the L2 band. The load circuit (not visible in  FIG. 6 ) is set up to perform dual frequency band operation of these antenna pairs. 
     According to a third embodiment, shown in  FIG. 7 , an antenna network  60  comprises two pairs  62 ,  64  of metal antennas, a first antenna pair  62 , consisting of two orthogonally positioned, sequentially rotating metal antennas  66 ,  68 , and a second antenna pair  64 , consisting of two sequentially rotating metal antennas  70 ,  72 , also positioned with a rotation angle equal to 90°. 
     In the illustrated embodiment, the two metal antennas  70 ,  72  of the second pair  64  have different dimensions than the dimensions of the antennas  66 ,  68  of the first pair  62 , and are respectively positioned at a translational offset from the antennas  66 ,  68  of the first pair  62 . 
     The two metal antennas  66 ,  68  of the first pair  62  have a resonant element of a first length, and the two metal antennas  70 ,  72  of the second pair  64  have a resonant element of a second length, less than the first length for example, to target a lower frequency band dedicated to GNSS, such as the L2 or L5 band. For example, the metal antennas  66 ,  68 ,  70 ,  72  are PIFA antennas, as described in the first embodiment. 
     The antenna network also includes a monopole antenna  74 , centered relative to the center of symmetry O of the antenna network  60 , and non-resonant. 
     The load circuit (not visible on  FIG. 7 ) is parameterized to conduct a dual frequency band operation of these antenna pairs. 
     According to a fourth embodiment, illustrated in  FIG. 8 , the antenna network  80  comprises two pairs of antennas  82 ,  84 , arranged symmetrically relative to the point O which is located substantially at the center of the antenna network. In this embodiment, the first pair of antennas  82  is composed of two metal antennas  86 ,  88  sequentially rotated at 90° rotation angle, and the second pair of antennas  84  is composed of two metal antennas  90 ,  92 , also positioned sequentially rotated at 90° rotation angle. In other words, the two pairs of antennas are arranged so that the second pair of antennas is rotated 180° relative to the first pair of antennas. The metal antennas  86 ,  88 ,  90 ,  92  are structurally identical; they are PIFA antennas, for example. The resulting Antenna network  80  is a centrally symmetrical antenna array. The antenna network  80  further comprises a monopole antenna  94 . Depending on the chosen parameterization of the associated load circuit (not shown), the antenna network  80  is adapted to operate in one or two frequency bands. 
       FIG. 9  schematically illustrates an arrangement of four sequentially rotating antenna pairs, forming a circularly rotating antenna network structure. This arrangement comprises a first pair of antennas  100 ,  102 , sequentially rotated at an angle of 180° about point O, a second pair of antennas  104 ,  106 , sequentially rotated at an angle of 180° about point O, a third pair of antennas  108 ,  110 , sequentially rotated at an angle of 180° about point O, a fourth pair of antennas  112 ,  114 , sequentially rotated at an angle of 180° about point O. In this arrangement, the second pair of antennas is rotated 45° relative to the first pair of antennas, the third pair of antennas is rotated 45° relative to the second pair of antennas, and the fourth pair of antennas is rotated 45° relative to the third pair of antennas. The antennas  100 ,  102 ,  104 ,  106 ,  108 ,  110 ,  112  and  114  are metal PIFA antennas, for example, and their dimensions are chosen to form a substantially circular structure. Advantageously, by adding a monopole antenna substantially centered on point O and a suitably parameterized load circuit, an antenna network suitable for providing directive radiation in straight circular polarization is formed. The size and shape of the monopole antenna (not shown in  FIG. 9 ) is chosen to suit the type of radiation required. 
     The example in  FIG. 9  has 4 pairs of sequentially rotating antennas. More generally, a larger number N of antenna pairs, for example metal PIFA antennas, is used. 
     According to another embodiment, the antenna network is composed of metal antennas printed on a dedicated board or printed circuit board (PCB). Advantageously, in this embodiment, the dimensions of the antenna network are further reduced depending on the permittivity or permeability value of the substrate. 
     Of course, combinations of the above-described embodiments are possible. 
     The invention has been described above according to several embodiments, more particularly including metal PIFA antennas, since the use of such antennas makes it possible to obtain a particularly compact antenna network. 
     More generally, the invention applies with other types of metal antennas, such as for example patch antennas, which operate similarly and can be optimized for a similar operation as described above, by parameterizing the load circuit to provide radiation in which the monopole antenna has a destructive contribution of a magnetic transverse radiation mode, making it possible to obtain a radiation of selected circular polarization by said at least one pair of metal antennas. 
     Advantageously, an antenna network according to the invention makes it possible to make circularly polarized directive radiation with a small footprint and low manufacturing cost.