This application claims priority to foreign French patent application No. FR 1800260, filed on Mar. 29, 2018, the disclosure of which is incorporated by reference in its entirety.
The invention relates to a circularly polarized radiating element, in particular for a planar antenna, intended to be used in particular in space communications, on board satellites or in user terminals. The invention also relates to an array antenna comprising at least one such radiating element.
Various types of radiating elements have recently been developed, which meet the constraints and specificities of space communications.
Radiating elements of the type said to be “compact”, such as for example Fabry-Perot resonator antennas, in particular allow a good compromise to be achieved between a number of specifications: a good effective aperture in the entire operating band, sufficiently wide matching and emission passbands, a low bulk and a low mass. Bulk is particularly critical in low-frequency bands: L band (1 to 2 GHz), S band (2 to 4 GHz) and C band (from 3.4 to 4.2 GHz in reception and from 5.725 to 7.075 GHz in emission), which are penalized by significant wavelengths. Thus, compact wideband elements are being sought in a particularly active way for multispot antennas, which combine a reflector and a focal array made up of many sources. The Fabry-Perot resonator antennas currently used in space communications are linearly polarized. To obtain a circular polarization with such antennas, a device allowing a circularly polarized emission to be obtained must be added without degrading the compactness of the radiating element.
Radiating elements that have continuous linear radiating apertures, such as for example quasi-optical beamformers, for their part allow a plurality of planar wavefronts to be radiated over a large angular sector. They are formed from a parallel-plate waveguide terminated by a longitudinal horn that forms the transition between the parallel-plate waveguide and free space. A focusing/collimator device is inserted on the propagation path of the radiofrequency waves, between the two metal parallel plates, allowing the cylindrical wavefronts generated by the sources to be converted into planar wavefronts. These continuous linear radiating apertures operate over a very wide band (for example at 20 and at 30 GHz) because of the absence of resonant propagating modes. They are moreover capable of radiating over a very large angular sector. However, in nominal operation the polarization of the radiated wave is that of the wave that propagates through the parallel-plate waveguide, namely a linear polarization.
To obtain identical beam widths in two planes, it is moreover known to enlarge the continuous linear radiating aperture using a parallel-plate divider. These arrays of linear apertures also radiate in linear polarization, just like each linear radiating aperture.
There is therefore currently a need to develop devices that are capable of converting a linear polarization into circular polarization, that are compatible with existing radiating apertures and that are moreover able to function as a circularly polarized radiating element.
A first known solution consists in covering the radiating element with a polarizing radome made up of a plurality of frequency selective surfaces (FSS), the characteristics of which are optimized so as to generate a phase difference of 90° between the two orthogonal polarizations, without disrupting the operation of the antenna. Polarizing radomes in which quarter wave layers are arranged in cascade perform well in terms of passband and at oblique angles of incidence but are thick (thickness of the order of one wavelength in vacuum), decreasing the compactness of the antenna. Thin polarizers have also been developed, but their performance in terms of passband and at oblique angles of incidence is limited.
One solution consisting in combining a polarizer and a Fabry-Perot cavity is described in document “Self polarizing Fabry-Perot antennas based on polarization twisting element” (S. A. Muhammad, R. Sauleau, G. Valerio, L. L. Coq, and H. Legay, IEEE Trans. Antennas Propag., vol. 61, no. 3, pp. 1032-1040, Mar. 2). The solution is illustrated in
The invention therefore aims to obtain a radiating element that is compact heightwise, very wideband and that is able to generate a circular polarization from a linear excitation.
One subject of the invention is therefore a circularly polarized radiating element comprising:
Advantageously, the metasurface comprises a ground plane on which are placed a substrate and the array of metasurface cells, which cells are arranged in rows, the centres of each metasurface cell of a given row being aligned along an alignment axis, the alignment axis being oriented by a rotation angle (P) with respect to the excitation polarization, the rotation angle (P) being defined so as to make the matrix [S′] diagonal, where:
[S′]=t[R][S][R],
[S] being the scattering matrix of the metasurface, and [R] the rotation matrix of a rotation of angle ψ.
Advantageously, the metasurface cells of a given row are coupled by a metasurface interconnect line that is elongate along the alignment axis.
Advantageously, the rows are connected to one another by way of metasurface cells, forming with the metasurface interconnect lines a rectangular grid.
As a variant, the metasurface cells of a given row are mutually isolated.
Advantageously, the metasurface cells of a given row are all periodically spaced.
Advantageously, all the metasurface cells of the metasurface have the same dimensions.
Advantageously, the frequency selective surface comprises an array of parallel metal wires that are periodically spaced and aligned with the excitation polarization.
As a variant, the frequency selective surface comprises a two-dimensional array of metal dipoles that are arranged periodically.
Advantageously, the excitation aperture comprises at least one waveguide aperture opening into the resonant cavity.
Advantageously, the excitation aperture comprises a dual feed formed by two waveguides that open symmetrically into the resonant cavity, and that are connected to an impedance matching network.
Advantageously, the excitation aperture is a horn of a linear radiating aperture.
Advantageously, the radiating element comprises a plurality of excitation apertures, the excitation apertures being formed by an array of linear radiating apertures.
Advantageously, the radiating element comprises at least one second cavity arranged in cascade on the frequency selective surface.
Advantageously, the metasurface cells are of rectangular shape.
The invention also relates to an array antenna comprising at least one aforesaid radiating element.
Other features, details and advantages of the invention will become apparent on reading the description given with reference to the appended drawings, which are given by way of example and show, respectively:
A wave polarized linearly with a first excitation polarization is produced in the excitation aperture OE. The excitation aperture OE is represented by a rectangular waveguide that penetrates the metasurface S1 but that does not extend beyond the metasurface S1, or if it does extends therebeyond only slightly. The linearly polarized wave propagates into the cavity, which is bounded by the metasurface S1 and by a frequency selective surface S2 comprising an arrangement of metal wires or dipoles that have a periodic distribution. The metasurface S1 and the frequency selective surface S2 are spaced apart from each other by a distance D1. The frequency selective surface S2 partially reflects the excitation polarization Ex (also called the transverse-electric (TE) polarization) and is transparent to a second polarization Ey, referred to as the orthogonal polarization (also called the transverse-magnetic (TM) polarization), that is orthogonal to the excitation polarization Ex and to the direction of propagation of the wave. The frequency selective surface S2 is therefore characterized by reflection and transmission coefficients r2x and t2x, respectively. The wave produced by the excitation aperture is partially radiated (Etx) and partially reflected. The reflected portion is called the incident wave Eix
The metasurface S1 is completely reflective. It acts as a ground plane, facing the frequency selective surface S2. The metasurface S1 is characterized by reflection coefficients r1xx and r1yx, respectively, which express the components of the reflected wave with the polarizations Ex and Ey, resulting from the incident wave Eix.
A resonance of the type typically observed in Fabry-Perot resonators is established between the two surfaces for the wave having the excitation polarization Ex. The incident wave Eix, which propagates through the cavity, undergoes a series of reflections from the frequency selective surface S2 and from the metasurface S1. On each reflection from the frequency selective surface S2, some of the incident wave Eix is radiated. On each reflection from the metasurface S1, one portion of the incident wave Eix undergoes a rotation of polarization, also referred to as a depolarization, producing a polarized wave Er1y having the orthogonal polarization Ey. The amplitude of the polarized wave Er1y having the orthogonal polarization Ey is determined by the reflection coefficient r1yx. Another portion of the incident wave Eix preserves its polarization, producing a polarized wave Er1x having the excitation polarization Ex. The amplitude of the polarized wave Er1x having the excitation polarization Ex is determined by the reflection coefficient r1xx. A circularly polarized emission is obtained when the wave E′tx radiated by the frequency selective surface S2, and generated from the polarized reflected wave Er1x having the excitation polarization Ex, corresponds in amplitude to the polarized wave Er1y having the orthogonal polarization Ey, with a phase shift of ±90°. The amplitude of the wave E′tx radiated by the frequency selective surface S2 is determined by the transmission coefficient t2x. Since the frequency selective surface S2 is transparent to the orthogonal polarization Ey, the polarized wave Er1y having the orthogonal polarization Ey is radiated without being attenuated. The polarized wave Er1y having the orthogonal polarization Ey is denoted E′ty. A first circularly polarized emission is therefore composed of the waves E′tx and E′ty.
The reflected wave Er1x undergoes a new reflection from the frequency selective surface S2, with a reflection coefficient r2x, and, according to the same principle, a second circularly polarised emission is composed of the waves E″tx and E″ty, then a third circularly polarized emission, composed of the waves E′″tx and E′″ty.
Thus, a circularly polarized beam that is increasingly attenuated with distance from the excitation aperture OE is obtained.
This radiating element may be pre-dimensioned on the basis of ray theory, which is conventionally used for this category of radiating element. It is assumed that:
the size of the cavity is infinite in the xy plane;
the frequency selective surface S2 is characterized respectively by reflection and transmission coefficients r2x and t2x. It is completely transparent to the polarised wave Ey;
It follows from the above that, in the far field, the transfer functions Tx and Ty of the polarised transmitted waves Etrans (x) and Etrans (y) may be written as the sum of all the transmitted fields:
From (1) the transfer function Tx may be determined:
where k0 is the wave number in free space, namely 2π/λ0, and θ the angle of incidence of the excitation wave.
From (2) the transfer function Ty may be determined:
The condition for resonance is met when:
where ∠r1xx is the phase component of the reflection coefficient r1xx, ∠r2x is the phase component of the reflection coefficient r2x, and N is any integer.
Using the transfer functions calculated with (5) and (8) for the two polarizations, it is possible to calculate the axial ratio (AR) for the whole antenna, using the following relationship:
Starting with relationships (12) and (13), and using the transfer functions calculated with (5) and (8), it is therefore possible to write the condition of production of a pure circular polarization with the following relationships:
By combining equation (9), which describes the condition for resonance, and equation (15), which describes the condition for circular polarization, the following relationship may be obtained:
where N′ is any integer.
Equation (16) does not depend to the first order on frequency (the wave number k0 is not found in the equation), but solely relates the components of the reflection and transmission matrices of the frequency selective surface S2 and of the metasurface S1. The passband is no longer limited by the mechanism of generation of the circular polarization, but by the operating mechanism of the Fabry-Perot cavity. Techniques for widening the passband of the latter may thus be used, without affecting the circular polarization. In particular, arranging a second cavity in cascade above the frequency selective surface S2 allows the passband to be widened without degrading the quality of the circular polarization.
The phase component of the transmission coefficient t2x of the frequency selective surface S2 sets the directivity of the radiating element; it is therefore preset and known, depending on the desired directivity. Thus, from equation (16), to produce a pure circular polarization, all that is required is to suitably select the phase components of the reflection coefficients r1yx and r1xx.
The scattering matrix [S] of the metasurface S1 may be written in the conventional way in the form:
However, the metasurface S1 receives no incident wave of orthogonal polarization Ey, in so far as the frequency selective surface S2 is transparent to the orthogonal polarization. The reflection coefficients r1xy and r1yy, which respectively express the reflection coefficient of the excitation polarisation Ex and of the orthogonal polarisation Ey for an incident wave of orthogonal polarisation Ey, may therefore be neglected when dimensioning the metasurface S1. Only the reflection coefficients r1xx and r1yx need be taken into consideration when dimensioning the metasurface S1, and are determined from relationship (16).
A coordinate system Ox′y′z is defined as being the result of the rotation by an angle Ψ about the axis Oz of the coordinate system Oxyz (the axis Ox is defined by the excitation polarization Ex, and the axis Oy by the orthogonal polarization Ey).
It is therefore sought to obtain, from the scattering matrix [S] in the coordinate system Oxyz, a diagonal scattering matrix [S′] in the coordinate system Ox′y′z able to be written in the form:
where the diagonal reflection coefficients ejφ
Under the condition of normal incidence (θ=0°, there is thus a congruence relationship between the scattering matrix [S] in the plane Oxy, and the scattering matrix [S′] in the plane Ox′y′, which may therefore be written in the form:
where [R] is the rotation matrix of a rotation of angle Ψ:
It is therefore necessary to identify the angle Ψ that allows the required scattering matrix [S] to be converted into a diagonal matrix. For this calculation, which is not detailed here, only the reflection coefficients r1xx and r1yx have an effect on the operation of the antenna, the reflection coefficients r1xy and r1yy merely being fitting coefficients. Thus, once the angle Ψ required to obtain a diagonal matrix has been identified, the diagonal reflection coefficients ejφ
Because of the misalignment of the metasurface S1 with respect to the excitation polarization Ex, each linearly polarized incident wave is reflected with a component of excitation polarization Ex and with a component of orthogonal polarization Ey. In the case of a metasurface S1 consisting of an arrangement of rectangular conductive planar elements (or “patches”), the phase response as a function of the polarization Ex or Ey is controlled to the first order by the dimensions of the conductive planar element.
The metasurface S1 may comprise an array of metasurface cells MS such as illustrated in
The metasurface cells may advantageously be rectangular. The metasurface S1 may therefore consist of a plurality of rows RA of metasurface cells MS.
As illustrated in
The metasurface cells MS may all have the same length (dimension ly in
According to one variant, illustrated in
As illustrated in
The frequency selective surface S2, which is partially reflective, consists of an array of metal wires FI that are periodically spaced and that are oriented according to the excitation polarization Ex. As a variant, the frequency selective surface S2 may consist of slot or patch dipoles. The slots may be produced in a metal plate, and the patches placed on an electrically transparent substrate.
The array of metasurface cells MS is placed on a substrate SUB1, itself placed on a ground plane PM. The ground plane PM is passed through by the excitation aperture OE. The substrate SUB1 may for example be composed of a layer of nidaquartz sandwiched between two layers of Astroquartz™.
According to one variant, illustrated in
The wideband behaviour may be even further improved by arranging a second cavity in cascade on the frequency selective surface S2. To achieve this cascade arrangement, at least one second resonant cavity is placed on the cavity that is the subject of the invention. The second resonant cavity has as lower surface the frequency selective surface of the lower cavity, and as upper surface a partially reflective surface. The transverse cross section of the upper cavity may be larger than that of the lower first cavity, as described in document FR2959611, or, alternatively, its transverse cross section may be substantially identical to that of the lower cavity. This so-called “two-cavity” embodiment makes it possible to decrease the reflectivity of the frequency selective surface of the lower cavity, this promoting the wideband behaviour of the radiating element, without however having an influence on the quality of the circular polarization.
Number | Date | Country | Kind |
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1800260 | Mar 2018 | FR | national |