Patent Publication Number: US-11658421-B2

Title: Antenna with improved coverage over a wider frequency band

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application claims the benefit under 35 U.S.C. § 119 to French Patent Application No. 2009240, filed on Sep. 11, 2020, the entirety of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     This invention relates to an antenna able to emit with wide coverage in several frequency bands allowing it to fulfil on its own several separate communication functions. 
     PRIOR ART 
     Spacecraft are equipped with antenna which during the flight phases provide communication between these craft and the ground stations. 
     These antennas are particularly used for telemetry, trajectography, or satellite positioning systems (Global Navigation Satellite System or GNSS). The fulfilment of these functions can require the use of a complex system of several antennas, each associated with a particular function, these antennas each emitting in separate frequency ranges. 
     It is consequently desirable to dispose of an antenna able to emit with improved coverage in several separate frequency bands to fulfil several communication functions while remaining of a limited bulk and a simple system. 
     SUMMARY OF THE INVENTION 
     This invention relates to an antenna comprising:
         a radiative antenna element able to emit a signal in at least a first frequency band and in a second frequency band, disjoint from the first band and at a higher frequency than the latter, and   a waveguide covered by the radiative antenna element, comprising at least a first resonant and single mode cavity or mostly single-mode cavity in the first frequency band, and a second resonant cavity separate from the first resonant cavity and located inside the latter, said second resonant cavity being single-mode or mostly single-mode in the second frequency band.       

     The term “single-mode” should be understood to mean that only the fundamental mode of the resonant cavity under consideration can propagate. The term “mostly single-mode” should be understood to mean that the resonant cavity under consideration is single-mode over at least 50%, for example at least 75%, of the frequency band under consideration. In this case, the resonant cavity may not be single-mode over at least one end of the frequency band, and it can be modeless or dual-mode at this end. 
     The use of the first resonant cavity alone makes it possible to widen the emission frequency spectrum but allows the excitation of higher-order resonant modes which can disrupt the radiation diagram in the second frequency band, in particular at low angles of elevation. The addition of the second resonant cavity inside the first resonant cavity advantageously makes it possible to obtain an improved gain in the second frequency band, in particular at low angles of elevation, by not allowing the propagation of these higher-order modes while remaining limited bulk and a simple system, without modification of the dimensions of the antenna with respect to the sole presence of the first cavity. The invention thus provides an antenna with improved coverage in several separate frequency bands for the fulfilment of several communication functions. 
     In an exemplary embodiment, the radiative antenna element is present on a substrate covering the waveguide, and the antenna has one or more openings between a wall delimiting the first cavity and the substrate. 
     Such a feature makes it possible to obtain an improved gain at low angles of elevation in the second frequency band. 
     In particular, an edge of said wall located on the side of the substrate can have a castellation shape thus defining a plurality of openings between said wall and the substrate. 
     Such a feature makes it possible to obtain an improved gain at low angles of elevation in the second frequency band while limiting the drop-in gain in the first frequency band. 
     In an exemplary embodiment, a ratio RA 1  H 1 /H 2  is between 1 and 3.25, where H 1  denotes a height of the first cavity and H 2  a height of the second cavity. 
     Such a feature makes it possible to yet further improve the gain at the low angles of elevation in the second frequency band. 
     An optimum height for the second cavity H 2  providing optimal gain at the low angles of elevation in the second frequency band can be determined by a parametric study. In this regard, the ratio RA 1  is preferably between 1.28 and 2.2. 
     In an exemplary embodiment, the first frequency band corresponds to the frequencies between 1164 MHz and 1591 MHz and the second frequency band corresponds to the frequencies between 2200 MHz and 2290 MHz. 
     According to this example, the first frequency band corresponds to the application relating to satellite positioning systems (GNSS) and the second frequency band to telemetry applications. 
     The invention also relates to a craft equipped with at least one antenna as described above. The craft can be a spacecraft, such as a space launcher, an exploration craft or a satellite. The use of the described antenna is not limited to a space application, as it can be used on other craft such as a train, a motor vehicle or an aircraft. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    represents a first example of an antenna according to the invention. 
         FIG.  2    represents a second example of an antenna according to the invention. 
         FIG.  3    represents a third example of an antenna according to the invention. 
         FIG.  4    is a comparative diagram showing the gain obtained as a function of frequency for antenna according to the invention and an antenna not part of the invention. 
         FIG.  5    is a diagram showing the gain obtained in the second frequency band for an antenna not part of the invention. 
         FIG.  6    is a diagram showing the gain obtained in the second frequency band for an antenna according to the invention. 
         FIG.  7    is a diagram showing the gain obtained in the second frequency band for another antenna according to the invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
       FIG.  1    represents a first example of an antenna  1  according to the invention comprising a substrate  3  on which is present a radiative antenna element  5 . The substrate  3  can be a dielectric substrate. The substrate  3  can be made of a composite material, for example reinforced with glass. A substrate  3  can be used marketed under the reference RO3210® by the company ROGERS Corporation. The substrate  3  can have a planar shape. Note however that the presence of the substrate  3  is not necessary, the radiative antenna element being able, in a variant, to be formed by a self-supporting metallic part. 
     The radiative element  5  is, in the illustrated example, of twin crossed dipole or dual-band crossed asymmetric dipole type and comprises a first pair of dipoles  5   a  and  5   b  which are mutually perpendicular and powered with a phase offset of 90°, and a second pair of dipoles  5   c  and  5   d , separate from the first pair, which are mutually perpendicular and powered with a phase offset of 90°. The dipoles  5   a  and  5   b  of the first pair are of a different length from the dipoles  5   c  and  5   d  of the second pair. The dipoles  5   a - 5   d  of the first and second pairs each have a trapezoid shape in the illustrated example. The dipoles  5   a - 5   d  form in the illustrated example a radiative element  5  having a bow-tie structure. The dipoles  5   a - 5   d  are present on either side of the substrate (on the upper face and on its lower opposite face). The radiative element  5  is able to emit a signal in the radio frequency spectrum, this signal having a circular polarization in at least a first frequency band and in a second frequency band, disjoint from the first band and at a higher frequency than the latter. By way of example, the first frequency band may correspond to frequencies between 1164 MHz and 1591 MHz and the second frequency band to the frequencies between 2200 MHz and 2290 MHz. A coaxial cable  6  powers the radiative element  5 . The radiative element  5  of twin crossed dipole or dual-band crossed asymmetric dipole type is known per se. However, the invention is not limited to this type of radiative element, and in a variant it is possible to use a radiative element formed by a crossed dipole or other types of dual-band or broadband radiative elements such as crossed dipoles coupled to resonators, for example. The radiative element  5  may have a planar shape, as illustrated. The radiative element  5  can be devoid of vertical elements, directed along the direction Z, perpendicular to the plane P containing the radiative element  5  and the substrate  3  in the illustrated example. 
     The radiative element  5  covers a waveguide  7 . The waveguide  7  comprises in its lower part, or at its base, a reflector  9 . In the absence of the waveguide  7  and of the reflector  9  the radiative element  5  emits a signal having a circular polarization upward along the direction Z shown but also downward with an opposite direction of polarization. The reflector  9  makes it possible to obtain a one-directional circular polarization signal along the direction Z, here only directed upward (in the direction opposite the reflector  9 ), by reflecting the signal component emitted downward and reversing its direction of polarization due to this reflection. 
     The waveguide  7  comprises a first resonant cavity  11  which is single-mode or mostly single-mode in the first frequency band. The first resonant cavity  11  may not be single-mode, or mostly single-mode, in the second frequency band. The waveguide  7  further comprises a second resonant cavity  13  which is separate from the first cavity  11  and nested in the latter. The second resonant cavity  13  is single-mode or mostly single-mode in the second frequency band. The second cavity  13  may not be single-mode or mostly single-mode in the first frequency band. The first cavity  11  participates in increasing the gain in the upper half-sphere, owing to the presence of the reflector  9  which reflects the waves upward, thus increasing the gain in the upper half-sphere, and in widening the frequency band in which the antenna  1  emits by allowing the generation of a second circularly polarized signal in addition to the signal generated by the radiative element and corresponding to a separate frequency range. If only the first cavity  11  is used, there is a generation of higher-order modes beyond the cut-off frequency of the second mode TM01 which disrupts the gain in the second frequency band, in particular at low angles of elevation. The addition of the second cavity  13  allows a significant improvement of the gain at the low angles of elevation in the second frequency band by only allowing the excitation of the first modes in the second frequency band. 
     The radiative element  5  is located above the first  11  and second  13  cavities on the side opposite the reflector  9 . The waveguide  7  is, in the illustrated example, closed in its lower part by the reflector  9  which defines a base shared by the first  11  and second  13  cavities and delimits these latters. The reflector  9  is in contact with the first  11  and second  13  cavities. The waveguide  7  is open in its upper part, opposite the reflector  9 , in the absence of the radiative element  5  and the substrate  3 . The first  11  and second  13  cavities are closed in their lower part by the reflector  9  and closed laterally, and are open in their upper part opposite the reflector  9 , in the absence of the radiative element  5  and the substrate  3 . The first  11  and second  13  cavities are located below the radiative element  5 . In the illustrated example, the substrate  3  positioned on the waveguide  7  closes the latter and the first cavity  11  by coming into contact with this latter. The invention does not require such a contact as will be described below. The assembly of the first  11  and second  13  cavities and the reflector  9  can be entirely metallic. The first cavity  11  has greater dimensions than the second cavity  13 . The second cavity  13  has a height H 2  less than or equal to the height H 1  of the first cavity  11 . The greatest dimension D 1  of the first cavity  11  is greater than the greatest dimension D 2  of the second cavity  13 . These greatest dimensions D 1  and D 2  can be diameters in the illustrated example of a circular geometry for the first  11  and second  13  cavities. The second cavity  13  is centered with respect to the first cavity  11 . In the illustrated example, the first and second cavities each have a circular shape, but it does not depart from the scope of the invention when these latters have a different shape, such as a polygonal shape, for example rectangular or octagonal, as will be described below. The walls of the first  11  and second  13  cavities can be solid, i.e. devoid of any slots or material lacks. The coaxial cable  6  extends inside the first  11  and second  13  cavities through these latters. 
     As stated above, the ratio RA 1  H 1 /H 2  can be between 1 and 3.25, for example between 1.28 and 2.2. 
     According to an example, the ratio RA 2  D 1 /D 2  can be between 1.19 and 2.1. The modification of the ratio RA 2  is used to modulate the frequency bands in which the antenna  1  emits as a function of the desired application. 
     The first  11  and second  13  cavities are dimensioned such as to be single-mode or mostly single-mode in the first frequency band and in the second frequency band respectively. The choice of the dimensions to use to realize this is part of the general knowledge of those skilled in the art. For example, the radii of the cavities  11  and  13  can be defined as a function of the cut-off frequencies of a circular waveguide calculated using the formula below. 
     
       
         
           
             
               
                 
                   
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     In the formula above p′ nm  denote the roots of the first-kind Bessel functions, a the radius of the desired waveguide, E and p the dielectric permittivity and the magnetic permeability of the medium respectively. The parameters n and m correspond to the order of the mode guided by the section of the cavity, here circular. 
     By way of example for a first frequency band ranging from 1164 MHz to 1591 MHz and a second frequency band ranging from 2200 MHz to 2290 MHz, one may use a waveguide  7  having a radius R 1  between 125 mm and 155 mm, for example between 135 mm and 150 mm, a radius R 2  between 75 mm and 105 mm, for example between 80 mm and 95 mm, a height H 1  between 35 mm and 60 mm, for example between 45 mm and 55 mm, and a height H 2  between 25 mm and 40 mm, for example between 25 mm and 35 mm. Unless otherwise specified, the radii R 1  and R 2  are respectively taken as being equal to half the greatest dimension of the first and of the second cavity and do not necessarily imply that the waveguide is of circular geometry. These values have been determined taking a dielectric permittivity and a magnetic permeability of the medium filling the cavities equal to 1 (vacuum permittivity and permeability). 
     By way of example for a first frequency band ranging from 1164 MHz to 1591 MHz and a second frequency band ranging from 2200 MHz to 2290 MHz, one can use a waveguide  7  having a radius R 1  of 140 mm, a radius R 2  of 90 mm, a height H 1  between 35 mm and 60 mm, for example between 45 mm and 55 mm, and a height H 2  between 25 mm and 40 mm, for example between 25 mm and 35 mm. 
     Still by way of example for a first frequency band ranging from 1164 MHz to 1591 MHz and a second frequency band ranging from 2200 MHz to 2290 MHz, one can use a waveguide  7  having a height H 1  of 50 mm, a height H 2  of 25 mm, a radius R 1  between 125 mm and 155 mm, for example between 135 mm and 150 mm, and a radius R 2  between 75 mm and 105 mm, for example between 80 mm and 95 mm. 
       FIG.  2    represents a second example of an antenna  10  according to the invention which differs from the example of  FIG.  1    only in that an opening  20  is present between the first cavity  11  and the substrate  3 . The same reference symbols have been re-used for similar elements. The opening  20  extends 360° around the axis of the first  11  and second  13  cavities, corresponding to the axis Z. The height H 3  of the opening  20  can be less than or equal to H 1 -H 2 , for example less than or equal to 25 mm, for example between 0.25 mm and 25 mm. The increase in H 3  makes it possible to further improve the gain for low angles of elevation in the second frequency band. It is however preferable to not increase H 3  too much in order not to decrease the gain in the first frequency band too much. According to the gain requirements of the two frequency bands, this parameter H 3  offers an additional degree of freedom to optimize the antenna. In this example, the substrate  3  is not in contact with the first cavity  11  and is present at a predetermined non-zero distance therefrom. 
       FIG.  3    represents a third example of an antenna  100  according to the invention which differs from the example of  FIG.  1    only in that an edge  22  of the wall  110  of the first cavity has a castellated shape defining a plurality of openings  24  between the substrate  3  and the wall  110 . The openings  24  may each have the same shape and/or the same dimensions. In a variant, the openings  24  differ in terms of shape and/or dimensions. The openings  24  can as illustrated be present all around the axis Z of the first and second cavities (360° around this axis Z). The openings  24  may or may not be regularly distributed around the axis Z of the first and second cavities. As described above, the height H 4  of the openings  24  may be less than or equal to H 1 -H 2 , for example less than or equal to 25 mm, for example between 0.25 mm and 25 mm. As above for the case of the opening  20 , increasing H 4  makes it possible to further improve the gain for the low angles of elevation in the second frequency band. It is however preferable not to increase H 4  too much in order not to decrease the gain in the first frequency band too much. Note that the openings  24  have the same effects as the opening  20  but having a lesser impact on the gain of the first frequency band (increasing the height of the openings  24  provides less of a decrease in gain in the first frequency band). 
     We have just described examples of waveguides having a circular geometry but it does not depart from the scope of the invention when the waveguide has another geometry such as a polygonal shape, for example rectangular or orthogonal. Those skilled in the art know how to dimension resonant cavities for geometries other than circular using other formulae than the formula [Math.  1 ] indicated above for the circular case. Furthermore, the examples that have just been described comprise only two resonant cavities  11  and  13  but it does not depart from the scope of the invention if the waveguide comprises more than two resonant cavities, for example three nested resonant cavities, the third resonant cavity being single-mode or mostly single-mode in a third frequency band disjoint from the first and second frequency bands. This makes it possible to have an antenna emitting with an improved gain in more than two frequency bands. 
       FIG.  4    represents a diagram showing the gain as a function of the frequency for θ=90° with respect to the direction Z, corresponding to the horizon therefore to an angle of elevation of 0°. This figure highlights the minimum value of the gain, all azimuthal angles taken together. The diagram is a comparative diagram showing the effect of the addition of the second cavity  13  into a first cavity  11  or  110 , and showing the influence of the presence of the openings  20  or  24 . The first  11  or  110  and second  13  cavities being both in this test of circular geometry as in  FIGS.  1  to  3   , with a radius R 1 =140 mm, a radius R 2 =90 mm, a height H 1 =50 mm and a height H 2 =25 mm. The opening  20  has a height H 3  of 10 mm and the openings  24  a height H 4  of 15 mm. The curve A 1  corresponds to the gain obtained with the first  11  and second  13  cavities without opening as in  FIG.  1   , the curve A 2  to the gain obtained with the opening  20  as in  FIG.  2   , the curve A 3  to the gain obtained with the opening  24  as in  FIG.  3    and the curve B corresponds to the gain obtained without the second cavity  13 , with the first cavity  11  only. A significant improvement is found in the gain in the second frequency band between 2200 MHz and 2290 MHz when the second cavity  13  is present. Furthermore, an additional improvement of the gain is also found in the second frequency band when the openings  20  and  24  are present. 
       FIG.  5    represents a gain diagram with a frequency of 2300 MHz as a function of the angle θ with respect to the direction Z on the abscissae, θ=90° corresponding to the horizon, so to an angle of elevation of 0°, and of the azimuth on the ordinate. The evaluated antenna included a radiative antenna element of twin crossed dipole type present on a substrate marketed under the reference RO3210® by the company ROGERS Corporation and had only a first resonant cavity (no second cavity) of square shape with a side of 140 mm and a height of 50 mm. 
       FIG.  6    shows the gain diagram obtained at this frequency for an antenna identical to that of  FIG.  5    but which further comprised a second cavity inside the first cavity. The second cavity had a square shape with a side of 90 mm and a height of 25 mm. A significant improvement of the gain was found for low angles of elevation. 
       FIG.  7    shows the gain diagram obtained at this frequency for an antenna which had a first and a second cavity of octagonal shape. The first cavity had a greatest dimension of 140 mm and a height of 50 mm and the second cavity had a greatest dimension of 90 mm and a height of 25 mm. A significant improvement in the gain was also found for low angles of elevation by comparison with the case of  FIG.  5   . 
     The expression “between . . . and . . . ” must be understood as including the bounds.