EP0860895A1 - Resonant antenna for emitting or receiving polarized waves - Google Patents

Abstract

The antenna has a second diffractive device (34) correcting the purity of polarisation of the waves at least for certain directions. The second diffractive device increases the purity of polarisation in the angular directions distant from the axial direction (12) of the antenna. The first diffractive device incorporates a crown (30) enclosing the radiating component and the second diffractive device comprises a skirt of the radiation of the radiating component (22). The skirt has the shape of an essentially flattened cone, with an outside edge (341) of greater diameter than that (301) of the crown (30).

Description

The invention relates to an antenna for transmitting or reception of the microwave domain. It relates more particularly a flat resonant antenna, for example made in microstrip technology.

Antennas of this type have a small footprint and a low mass. They are therefore used for applications on-board, in particular for space vehicles and the satellites.

We often need, especially for applications spatial, omnidirectional antennas, that is to say capable of transmitting or receiving in a solid angle of large value.

But we found that the requirement of omnidirectionality is difficult to reconcile with the conservation requirement of the purity of the polarization of the electromagnetic waves to be emitted or receive.

In particular, when the wave to transmit (or receive) must have a circular polarization, a ellipticity rate close to 1 in all emission directions (or reception).

Most often, the degradation of polarization purity is highest for the most distant directions of the antenna axis.

The invention aims to provide a resonant antenna which has maximum angular coverage with a purity of polarization preserved in this angular coverage.

The antenna according to the invention has a radiating element resonant central and it is characterized in that it comprises, preferably around this element, a first means diffracting to increase the emission angle of the radiating element and a second diffracting means for making a correction polarization purity at least for inclined directions relative to the axis of the antenna.

Each diffracting element has a dimension at more of the same order of magnitude as the wavelength to be emitted (or receive).

In one embodiment, the first diffracting means, intended to increase the opening angle of the beam to be emitted, has a conductive ring centered on the axis of the antenna and surrounding the radiating element, this crown being advantageously substantially in the same plane as the element radiating, and the second diffracting means comprises a skirt conductor arranged near the crown and on the side which is opposite to the direction of the radiation, the inclination of the skirt relative to the crown determining the direction in which polarization correction is mainly performed.

In one embodiment, the internal rim of the skirt is integral with the internal rim of the crown, this skirt and this crown forming, for example, a single piece holding. It has been found that, with regard to the correction of polarization purity, better results were obtained if the largest diameter of the skirt is greater than the outside diameter of the crown.

The resonant radiating element is either an element solid conductor ("patch"), for example square or circular, either a conductive crown or a slot provided in a conductive element. Anyway, for a wavelength given to send (or receive), we have an interest, to maximize omnidirectionality, to provide a ring-shaped antenna, these shapes make it possible to minimize the bulk. The ring is either conductive or in the form of a slot. The minimization of the size of the resonant element, and therefore the maximization of omnidirectionality, can also be obtained by depositing the element conductor resonating on a permittivity dielectric important. However, the increase in permittivity is not not favorable to polarization purity.

la figure 1 est un schéma en coupe d'une antenne selon l'invention, utilisable pour deux bandes de fréquences, les figures 1a, 1b et 1c sont des diagrammes mettant en évidence des avantages de l'antenne de la figure 1,

la figure 2 est un schéma en plan d'un anneau d'une antenne conforme à l'invention,

la figure 3 est un schéma en plan des deux anneaux d'une antenne selon l'invention, mais un autre mode de réalisation,

la figure 4 est un schéma en perspective éclatée d'une antenne du type de celle de la figure 1,

la figure 5 est un schéma électrique d'alimentation d'un anneau de l'antenne de la figure 4,

la figure 6 est un schéma correspondant à un mode de réalisation de la figure 5,

la figure 7 est un schéma correspondant aussi à un mode de réalisation de la figure 5,

la figure 8 est un schéma simplifié correspondant à celui de la figure 1, mais une variante, et

la figure 9 est un schéma en plan d'un anneau pour une variante.

Other characteristics and advantages of the invention will appear with the description of some of its embodiments, this being carried out with reference to the attached drawings in which:

FIG. 1 is a sectional diagram of an antenna according to the invention, usable for two frequency bands, FIGS. 1a, 1b and 1c are diagrams showing the advantages of the antenna of FIG. 1,

FIG. 2 is a plan diagram of a ring of an antenna according to the invention,

FIG. 3 is a plan diagram of the two rings of an antenna according to the invention, but another embodiment,

FIG. 4 is an exploded perspective diagram of an antenna of the type of that of FIG. 1,

FIG. 5 is an electrical diagram of the supply of a ring of the antenna of FIG. 4,

FIG. 6 is a diagram corresponding to an embodiment of FIG. 5,

FIG. 7 is a diagram also corresponding to an embodiment of FIG. 5,

FIG. 8 is a simplified diagram corresponding to that of FIG. 1, but a variant, and

Figure 9 is a plan diagram of a ring for a variant.

The antenna shown in Figure 1 is intended for receive or transmit microwave signals in two bands, namely, on the one hand, the S band at 2 GHz and, on the other hand, the UHF band at 400 MHz.

This antenna is mainly intended to be implanted on small satellites, such as satellites assigned to the location of objects or for measurement or remote control with conventional satellites. Of made of this application it should have a bulk reduced, wide angular coverage for the two bands of frequencies as well as a circular polarization with a rate of suitable ellipticity on this wide angular coverage, especially for the directions furthest from the axis.

The antenna 10 shown in FIG. 1 is of the type combined. It is formed by the association of two planar antennas concentric, respectively 14 and 16. Each of the antennas 14 and 16 and the assembly 10 have an axis 12 of symmetry of rotation. The central antenna 14, of smaller dimensions, is for the 2 GHz S-band and the outdoor antenna 16, of larger dimensions, is intended for the UHF band at 400 MHz.

Each of the individual antennas 14, 16 has a dielectric substrate, respectively 18 and 20, on which is deposited a conductive ring, respectively 22 and 24. Both rings 22 and 24 are centered on axis 12.

Examples of embodiment of the conducting rings 22 and 24 will be described below in relation to FIGS. 2 and 3.

Each of the substrates is enclosed in a metal housing of cylindrical shape with an axis 12. The housing for the antenna 14 has the reference 25 and the housing for the antenna 16 has the reference 26. The latter housing is limited, of a on the one hand, by a cylindrical outer wall 26 ₁ and, on the other hand, by an inner cylindrical wall 26 ₂ at a short distance from the wall of the housing 25.

The space 28 formed between the wall of the housing 25 and the wall 26 ₂ has a length (in the direction of the axis 12) equal to a quarter of the length of the S-band waves, that is to say 35 mm about. It is open, in 29, on the side where the emission occurs. It constitutes a trap intended to prevent the propagation of leakage currents from the ring 22 to the ring 24.

A metal filling ring 36 can be arranged at the bottom of the space 28 adjust the length (parallel to axis 12) of this space 28 so that it is equal to the quarter of the wavelength of the S band.

The walls 25 and 26 ₂ can be formed from the same sheet of metal.

Around the housing 26, substantially in the plane of the ring 24, and therefore perpendicular to the axis 12, is a metal ring or crown 30.

The inner rim 32 of the crown 30 is connected to a skirt 34 moving away, on the one hand, from the crown 30 in the direction from the bottom of the housing 26 and, on the other hand, from the axis 12. In an example the angle formed, in the plane of Figure 1, by the plane of the crown 30 and the skirt 34 is of the order of 45 °.

The ring 22 radiates in a cone of axis 12 of half angle at the top  equal to about 60 °. However, there is still a radiation outside this cone. The purpose of crown 30 is to diffract waves deflected outward to increase omnidirectionality antenna 14.

However, it was found that the crown 30 tended to degrade the circular polarization of the radiation, that is, to degrade the rate of ellipticity. The experience has shown that the skirt 34 made it possible to maintain a rate of ellipticity waves with circular polarization close to 1, especially for directions forming a large angle with axis 12.

The ellipticity rate can be adjusted empirically by varying the orientation of the skirt 34, that is to say the angle that it forms with the plane of the crown 30 as well as by making vary its dimensions.

The outer edge 34 ₁ of the skirt 34 is further from the axis-12 than the outer edge 30 ₁ of the crown 30.

In one example, the inside diameter of the crown 30 is 256 mm, its outside diameter 300 mm, while the outer diameter of the skirt 34 - which has a generally frustoconical shape - is 348 mm.

Skirt 34 is thought to create wave diffraction in S-band which opposes the negative effect of the diffracting crown 30 on the ellipticity rate of S-band waves

It should be noted that the housings or cavities 25 and 26 contribute to symmetrizing the radiation diagram around axis 12 and to improve the ellipticity rate.

In the example, the dielectric substrates 18 and 20 have a relative dielectric permittivity ε _r of the order of 2.5. As indicated above, the higher this dielectric permittivity, the more the dimensions of the antennas can be reduced. However, the increase in the dielectric constant is unfavorable for maintaining the circular polarization. This is why, in the example, the constant ε _r does not exceed the value 2.5.

Figures 1a, 1b and 1c are diagrams allowing to highlight the advantages, on the one hand, of the quarter trap wave formed by the annular space 28 and, on the other hand, diffracting elements 30 and 34.

On each of these diagrams, we have plotted on the abscissa, elevation  (in degrees), i.e. the half-angle of the cone axis 12 emission, and on the ordinate, the amplitudes in decibels radiation in normal polarization and in cross polarization.

Figure 1a is a diagram for a similar antenna to that of Figure 1 but lacking, on the one hand, the quarter trap wave 28 and, on the other hand, diffracting elements 30 and 34.

Curve 40 corresponds to normal polarization and the curves 41 correspond to the cross polarization. The purity of circular polarization is all the greater as large difference between curves 40 and 41. We can see that for an angle  of 0 °, that is to say along the axis 12, the emission is according to a circular polarization. On the other hand, when we move away from axis 12, the circular polarization degrades notably.

In addition, the emission weakens appreciably as soon as moves away from axis 12.

Figure 1b corresponds to an antenna similar to that of FIG. 1, with a wave trap 28, however devoid of diffracting elements 30 and 34.

It is noted that the omnidirectionality as well as the purity of circular polarization are improved compared to the case of FIG. However, the purity of circular polarization is not entirely satisfactory between 30 ° and 60 °, the distance between the curves 41 ₁ and 40 ₁ remaining relatively small.

The diagram in FIG. 1c corresponds to the antenna shown in FIG. 1, with a quarter-wave trap 28, the crown 30 and the skirt 34. It can be seen, compared with FIG. 1b, that omnidirectionality is everything quite satisfactory up to an angle  of 60 °. In addition, the purity of circular polarization is significantly improved between the angles 30 ° and 60 °, the distance between the curves 40 ₂ and 41 ₂ being significantly greater.

According to a provision of the invention, the compactness of the antenna is increased by giving a crenellated shape or by meanders at rings 22 and 24.

In the example of FIG. 2, the ring 22 comprises, regularly distributed around the axis 12, eight internal segments 46 ₁ to 46 ₈ alternated with eight external segments 48 ₁ to 48 ₈ . These segments 46 and 48 in the form of arcs of circles are connected at their ends by rectilinear segments 50, of radial directions. Thus, in this example, the radial segments are sixteen. Although not shown in FIG. 2, the ring 24 is homothetic with the ring 22.

In the example of FIG. 3, provision is made for S 22 'and UHF 24' antennas, four internal segments and four external segments.

The guided wavelength of the radiation to be transmitted is directly proportional to the electrical length of the ring resonant antenna 14 (14 ') or 16 (16'). This length electric is equal to the sum of the lengths of all segments 46, 48 and 50.

Thus, for the same guided wavelength, that is to say for the same frequency, an antenna according to the invention has a smaller footprint than a shaped antenna simply circular. Indeed, we note that, compared to a circular ring with the same diameter as the circle on which are arranged the segments 48, the electrical length is increased by approximately the sum of the lengths of the segments 50.

However, it was found that the longer the length of the segments 50 is large and the more the efficiency of the antenna decreases. The antenna radiation impedance decreases because the ribbon metallic further obscures the opening; so the proportion of energy dissipated in the conductor or the dielectric is more important. It is therefore preferable that the ratio between the diameter outside and inside diameter be at most around of two.

Furthermore, it was observed that the presence of the segments 50 of radial directions practically did not alter the rate of ellipticity of the polarization of the radiation. Indeed, a radial direction segment also has the disadvantage of disturbing the ellipticity rate. However, it is believed to be the estate of segments traversed by currents in opposite directions which compensates for the negative effect on the ellipticity rate.

Care must therefore be taken to arrange these segments of in such a way that this compensation is obtained.

Figure 4 shows, in exploded perspective, the various components of the antenna combined with 22 'rings and 24 ′ of the type of those in FIG. 3.

As can be seen in this figure, the crown 30 and the skirt 34 inclined at 45 ° constitute a single piece holding 50.

The 24 'and 22' rings are made by engraving on dielectric substrates, respectively 18 and 20, of a material called "polypenco". In Figure 4, the rings 22 'and 24' separated from the substrates 18 and 20; but it goes from these rings are deposited on the respective substrates 18 and 20.

Between the bottom 52 of the housing 25 and the substrate 18 is disposed a distributor 54 which will be described later in relation with Figures 5 to 7.

A coaxial cable 60 passes through the bottom 52 of the housing 25 to bring the excitation signal to the distributor 54. The role of the latter is to distribute, with appropriate phase shifts, the excitation signal between the four exterior segments 48 'of the 14 'ring.

Similarly, between the bottom 56 of the housing 26 and the dielectric 20, a distributor 58 is arranged.

A coaxial cable 62 crosses the bottom 56 to bring the UHF excitation signal to the distributor 58 which distributes, with appropriate phase shifts, this excitation signal between the four outer segments of the ring 24 '.

Figures 5, 6 and 7 show the distributor 54.

The circuits 64, shown in FIGS. 5 and 6, allow, from the excitation signal provided by the coaxial 60, to obtain a circular polarization. To this end, they feed the four outer segments 48 'with phase shifts successive 90 °.

The signal brought by the coaxial 60 is applied to an input 66 which, as shown in FIG. 5, is connected to the input of a 180 ° phase shifter 70 via a transformer 68. The output 70 ₁ without phase shift of the phase shifter 70 is connected to a port 74 which is itself connected to a 90 ° phase shifter 78 via a transformer 76. The output 70 ₂ with 180 ° phase shift from the phase shifter 70 is connected to a another port 80, which is connected to a second 90 ° phase shifter 84 by means of a transformer 82.

The output 78 ₁ without phase shift of the phase shifter 78 is connected to a first output 90 ₁ of the circuit 64 via a transformer 86 and an adapter 88. The output 90 ₁ is connected to a first external segment of the 22 'ring.

Likewise, the 90 ° phase shift output 78 ₂ of the phase shifter 78 is connected to a second output 90 ₂ , via another transformer and another adapter. The outlet 90 ₂ is connected to a second outer segment of the ring 22 '.

The phase-free output 84 ₁ of the phase shifter 84 is connected to the third output 90 ₃ via a transformer and an adapter. This outlet 90 ₃ is connected to a third outer segment of the ring 22 '.

Finally, the output 84 ₂ of 90 ° phase shift from the phase shifter 84 is connected to the fourth output 90 ₄ of the circuit 64 by means of a transformer and an adapter. This outlet 90 ₄ is connected to a fourth outer segment of the ring 22 '.

The signal on output 90 ₁ is in phase with the input signal on the first port 66, while the signals on outputs 90 ₂ , 90 ₃ and 90 ₄ are phase shifted by 90 °, 180 ° and 270 ° respectively. relative to the input signal.

The various elements of the circuit of Figure 5 are made using metal cutouts shown in the figure 6. On the latter, the same elements have been indicated as those of FIG. 5, with the same reference numbers.

The outlets 90 ₁ to 90 ₄ are located on the periphery of the cutouts and regularly distributed; these outputs are in line with the outer segments of the ring 22 'to which they are connected.

As can be seen in Figure 7, the cutouts metallic are sandwiched between dielectric distributors, 102 and 104 respectively.

The connection of each output 90 of circuit 64 to the corresponding external segment of the ring is effected by means of a probe 92. Four probes are therefore provided. In FIG. 7, the probe 92 _{1 is shown} .

The distributor 64, 102, 104 is enclosed in a housing metallic 106 constituting a trap preventing excitation of surface waves on the distributor.

Alternatively, instead of ribbons, or metal cutouts, circuit 64 is made using metal engravings on a substrate.

In the example shown in FIG. 8, provision is made three concentric antennas, respectively 110, for the antenna central, 112 for the intermediate antenna and 114 for the antenna the outermost.

As in the embodiment shown in FIG. 1, a diffraction ring 30 surrounds the outermost antenna and this crown 30 is integral with a skirt 34 oriented substantially at 45 ° relative to the plane of the crown 30. Also as in the embodiment of figure 1, a quarter trap wave 28 prevents the propagation of a leakage current from the cavity excited towards the surrounding cavities. Similarly, a quarter wave trap 116 prevents the propagation of a current of leak to antenna 114.

The trap 116 is of greater length (along the axis) that the trap 28 because it is intended to eliminate lengths wave, those of the signals emitted by the antenna 112.

Of course, a number of antennas can be provided concentric greater than three.

Although the examples described above relate to resonant ring antennas formed by a metallic conductor, it is easy to understand that the invention also applies to an antenna produced by a slot in a conductor. For certain applications, in particular those for which overheating must be minimized, this slotted realization will be favorite.

The variant shown in Figure 9 represents a resonant annular cavity which applies more particularly to a slot antenna. However, this example could apply also to a resonant ring antenna formed by a conductor metallic.

The ring 130 is constituted by a slot 132 in a metallic conductor 134. This ring 130 forms meanders each having substantially the shape of a petal. Number of petals is, in this embodiment, equal to 8.

Although in the examples described above, the excitation be performed on the outer segments using a coaxial cable, it is also possible to provide excitation by coupling proximity with a microstrip line or with a slit in the ground plane, that is to say in a bottom of the cavity.

Claims (12)

Antenna comprising a radiating resonant element (22, 22 ') for emitting polarized microwave waves, characterized in that it comprises a first diffracting means (30) radiate the waves at an angle greater than the angle of emission of the radiating element (22, 22 '), and a second means diffractant (34) correcting the polarization purity of the waves at least for some directions. Antenna according to claim 1, characterized in that that the second diffracting means (34) increases the purity of polarization in angular directions away from the direction axial (12) of the antenna. Antenna according to claim 1 or 2, characterized in that the first diffracting means comprises a crown (30) surrounding the radiating element (22) and in that the second means diffractant comprises a skirt (34) disposed near the crown (30) and opposite to the direction of radiation of the radiating element (22). Antenna according to claim 3, characterized in that the skirt has an internal rim integral with the rim internal (32) of the crown (30). Antenna according to claim 3 or 4, characterized in that the skirt, which has a substantially frustoconical shape, has an outer edge (34 ₁ ) of larger diameter than the outer edge (30 ₁ ) of the crown (30). Antenna according to any one of claims 3 to 5, characterized in that the inclination of the skirt relative to to the axis (12) of the antenna determines the direction in which polarization correction is preferred. Antenna according to any one of claims 3 à 6, characterized in that the dimensions of the skirt determine the direction in which polarization correction is favored. Antenna according to any one of claims 3 7, characterized in that the crown (30) is located substantially in the same plane as the radiating element (22, 22 '). Antenna according to any one of claims 3 to 8, characterized in that between the radiating element (22, 22 ') and the first and second diffracting means, there is at least one other antenna (16). Antenna according to any one of the claims previous, characterized in that the radiating element (22, 22 ') is arranged on a dielectric substrate (18) enclosed in a conductive housing (25) having walls extending from substantially parallel to an axis (12) perpendicular to the surface of the radiating element (22, 22 '). Antenna according to any one of the claims previous, characterized in that it is intended to emit S-band waves Antenna according to any one of the claims previous, characterized in that it is intended to emit waves with circular polarization.

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