EP0860894B1 - Miniature resonant antenna in the form of annular microstrips - Google Patents

Description

The invention relates to a transmitting or receiving antenna of the microwave domain. It relates more particularly to a flat antenna made of microstrip technology, which has the general shape of a ring, and is resonant type.

Antennas of this type have a small footprint and a low mass. They are therefore used for embedded applications, especially for space vehicles and satellites.

Especially for space applications, omnidirectional antennas are often needed, that is, capable of transmitting or receiving in a solid angle of great value.

But it has been found that the requirement of omnidirectionality is difficult to reconcile with the requirement of preserving the purity of the polarization of the electromagnetic waves to be emitted or received.

In particular, when the wave to be transmitted (or receive) must have a circular polarization, it is necessary to maintain an ellipticity rate close to 1 in all directions of emission (or reception). This constraint is not easy to respect for flat antennas. Document JP-A-05152829 discloses a resonant antenna for microwave waves comprising a two-slot annular microstrip resonator, which makes it possible to set two resonance frequencies in two orthogonal polarizations. In addition, documents US-A-3,716,861 and US-A-4,804,965 describe wired antennas comprising meanders or crenels of the traveling wave type.

The aim of the invention is to provide a ring-shaped resonant antenna which is of minimal size and which has a maximum angular coverage with a purity of polarization preserved in this angular coverage.

It is characterized in that the flat antenna, of resonant type, has the general shape of a ring with meanders or crenellations.

This form of meandering ring or crenellations makes it possible to maximize the length of the periphery in a predetermined size, that is to say to minimize the space requirement for a given wavelength. Indeed, the wavelength guided in the antenna is proportional to the length of the periphery, for the same wavelength, the size (that is to say the occupied area) of an antenna according to the invention is weaker than the size of an antenna of the same type with a circular ring.

The decrease in the size of the antenna is favorable to the increase of its omnidirectionality.

Moreover, it has been found that, despite the presence of substantially radial portions, with respect to a ring-shaped antenna (without slots or meanders), the purity of the polarization, in particular of the circular polarization, was not not altered. This result is surprising because each radial portion generates a perpendicular electric field, parasite for the polarization. It is believed that the maintenance of the polarization purity is due to the fact that at each radial portion or strand is associated another radial portion or strand creating a field in the opposite direction which compensates for the parasitic field of the first portion.

Thus, according to another embodiment of the invention, two successive radial portions must have an orientation and dimensions such that they generate spurious fields. who compensate each other. It is preferable that the distance between these successive radial portions is small.

More generally, the radial portions are, as a whole, shaped in such a way that they do not produce a disturbing field of the polarization of the signal to be emitted.

In one embodiment of the invention, the excitation of the antenna is performed on the outer section of the ring.

Preferably, the ratio of the largest diameter to the smallest diameter is at most equal to two.

In one example, the ring has eight or sixteen sections in total.

The meandering ring or crenellations is either a metal deposit on a substrate, or a slot provided in a metal deposit.

To minimize the dimensions of the antenna, it is also known that it is advantageous to increase the dielectric permittivity of the substrate because the guided wavelength in the antenna is substantially proportional to the square root of this dielectric permittivity. However, the increase of this permittivity is not, either, favorable to the maintenance of the purity of polarization. A suitable degree of polarization purity could be maintained if the dielectric permittivity was of the order of 1.5. But we do not have a material with this permittivity. However, with a permittivity material 2.5 approximately, a good degree of purity can be maintained provided that the annular antenna is deposited on a substrate which also comprises a housing with metal walls substantially perpendicular to the plane of the substrate, for example cylindrical shape of circular section. Thus, we obtain an increased miniaturization of the radiating element, with preservation of the polarization purity on a large angle, by combining this last arrangement - which consists of a dielectric loading - with the aliasing of the ring.

In one embodiment, for which the number of meanders or crenellations is equal to four, the width of these meanders or crenellations is of the order of 0.2 times the diameter.

• 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 pour 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 pour 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 become apparent with the description of some of its embodiments, this being done 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 highlighting 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 for 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 for feeding 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 for a variant, and
• Figure 9 is a schematic plan of a ring for a variation.

The antenna shown in FIG. 1 is intended to 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 missions measuring or remote control with conventional satellites. Because of this application, it must have a small footprint, a wide angular coverage for the two frequency bands and a circular polarization with a suitable ellipticity rate over this wide angular coverage, especially for the orientations furthest from axis.

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

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

Exemplary embodiments of the conductive rings 22 and 24 will be described hereinafter with reference to FIGS. 2 and 3.

Each of the substrates is enclosed in a cylindrical metal housing of axis 12. The housing for the antenna 14 has the reference 25 and the housing for the antenna 16 has the reference 26. This housing is limited, 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 one quarter of the length of the S-band waves, that is to say 35 mm about. It is open, at 29, on the side where the emission occurs. It constitutes a trap intended to prevent the propagation of the leakage currents of the ring 22 towards the ring 24.

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

The walls 25 and 26 ₂ may 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 ring or metal ring 30.

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

The ring 22 radiates in a cone of axis 12 half-angle at the vertex θ equal to about 60 °. There is however a radiation outside this cone. The purpose of the ring 30 is to diffract the waves deflected outwards in order to increase the omnidirectionality of the antenna 14.

However, it has been found that the ring 30 tends to degrade the circular polarization of the radiation, i.e., to degrade the ellipticity rate. Experience has shown that the skirt 34 made it possible to maintain an ellipticity rate of circularly polarized waves close to 1, especially for the directions forming a wide angle with the axis 12.

The ellipticity rate can be adjusted empirically by varying the orientation of the skirt 34, i.e. the angle it forms with the plane of the ring 30 as well as by varying 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 ring 30.

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

It is believed that the skirt 34 creates an S-band wave diffraction which opposes the negative effect of the diffracting ring 30 on the ellipticity rate of the S-band waves.

It should be noted that the housings or cavities 25 and 26 contribute to symmetrize the radiation pattern around the 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 the dielectric permittivity, the smaller the antenna dimensions can be. However, increasing the dielectric constant is unfavorable to maintaining circular polarization. Therefore, in the example, the constant ε _r does not exceed the value 2.5.

FIGS. 1a, 1b and 1c are diagrams making it possible to highlight the advantages, on the one hand, of the quarter-wave trap constituted by the annular space 28 and, on the other hand, diffractive elements 30 and 34.

On each of these diagrams, the abscissa, the elevation θ (in degrees), that is to say the half-angle of the emission cone axis 12, and the ordinate the amplitudes in decibels normal polarization and cross polarization.

FIG. 1a is a diagram for an antenna similar to that of FIG. 1 but devoid of, on the one hand, the quarter-wave trap 28 and, on the other hand, diffractive elements 30 and 34.

Curve 40 corresponds to normal polarization and curves 41 correspond to cross polarization. The purity of the circular polarization is all the greater the greater the difference between the curves 40 and 41. It is thus seen that for an angle θ of 0 °, that is to say along the axis 12 , the emission is in a circular polarization. By cons, when one moves away from the axis 12, the circular polarization degrades significantly.

In addition, the emission weakens substantially as one moves away from the axis 12.

FIG. 1b corresponds to an antenna similar to that of FIG. 1, with a quarter-wave trap 28, but without the diffracting elements 30 and 34.

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

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

According to one embodiment of the invention, the compactness of the antenna is increased by imparting a crenellated or meandering shape to the 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 in number. Although not shown in FIG. 2, the ring 24 is homothetic of the ring 22.

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

The guided wavelength of the radiation to be transmitted is directly proportional to the electrical length of the ring of the 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 an antenna having a simply circular shape. Indeed, it is found that, with respect to a circular ring having the same diameter as the circle on which the segments 48 are arranged, the electrical length is increased by about the sum of the lengths of the segments 50.

However, it has been found that the longer the length of the segments 50 is, the lower the efficiency of the antenna. The antenna's antenna impedance decreases because the metal ribbon further obscures the aperture; thus, the proportion of energy dissipated in the conductor or the dielectric is larger. It is therefore preferable that the ratio between the outside diameter and the inside diameter is at most of the order of two.

Moreover, it has been observed that the presence of the radial direction segments 50 practically does not alter the ellipticity level of the polarization of the radiation. Indeed, a radial direction segment also has the disadvantage of disturbing the ellipticity rate. However, it is thought that the succession of segments traversed by currents in opposite directions compensates for the negative effect on the ellipticity rate.

We must therefore be careful to arrange these segments in such a way that we obtain this compensation.

FIG. 4 shows, in exploded perspective, the various constituent elements of the antenna combined with rings 22 'and 24' of the type of those of FIG.

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

The rings 24 'and 22' are made by etching on dielectric substrates, respectively 18 and 20, made of a material referred to as "polypenco". In FIG. 4, the rings 22 'and 24' separated from the substrates 18 and 20 are shown; but it goes without saying that 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 connection 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 splitter 54. The role of the latter is to distribute, with appropriate phase shifts, the excitation signal between the four outer segments 48 'of the 'ring 14'.

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

A coaxial cable 62 passes through the bottom 56 to bring the UHF excitation signal to the splitter 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 splitter 54.

The circuits 64, shown in FIGS. 5 and 6, make it possible, from the excitation signal supplied by the coaxial 60, to obtain a circular polarization. For this purpose, they feed the four outer segments 48 'with successive phase shifts of 90 °.

The signal supplied through the coaxial 60 is applied to an input 66 which, as shown in Figure 5, is connected to the input of a phase shifter 70 to 180 ° 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 phase-shifted output 70 ₂ of 180 of the phase-shifter 70 is connected to a another port 80, which is connected to a second phase-shifter 84 of 90 ° via 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. Output 90 ₁ is connected to a first outer segment of ring 22 '.

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

The output without phase shift 84 ₁ of the phase shifter 84 is connected to the third output 90 ₃ via a transformer and an adapter. This output 90 ₃ is connected to a third outer segment of the ring 22 '.

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

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

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

The outputs 90 ₁ to 90 ₄ are at 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 FIG. 7, the metal blanks are sandwiched between distributing dielectrics, respectively 102 and 104.

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

The splitter 64, 102, 104 is enclosed in a metal housing 106 forming a trap preventing the excitation of surface waves on the splitter.

Alternatively, in place of ribbons, or metal cutouts, the circuit 64 is made using metal etchings on a substrate.

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

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

The trap 116 is of length (along the axis) larger than the trap 28 because it is intended to eliminate longer wavelengths, those of the signals emitted by the antenna 112.

Of course, it is possible to provide a number of concentric antennas greater than three.

Although the examples described above relate to resonant ring antennas formed by a metal conductor, it is easy to understand that the invention also applies to an antenna made by a slot in a conductor. For certain applications, especially those for which the heating must be minimized, this slit embodiment will be preferred.

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

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

Although in the examples described above, the excitation is carried out on the outer segments by means of a coaxial cable, it is also possible to provide excitation by proximity coupling with a microstrip line or with a slot in the plane. mass, that is to say in a cavity bottom.

Claims (15)

1. Resonant antenna intended to operate at a predetermined frequency, including a plane conductive ring (22, 24) disposed on a dielectric substrate (18, 20) enclosed in a metal housing (25, 26) having walls (25, 26₁, 26₂) extending parallel to the axis (12) of revolution of the ring, the feeder circuit of the antenna being intended to be connected to the ring via a probe (92₁), the peripheral electrical length of the ring determining the wavelength guided in the antenna, characterised in that the ring (22, 24; 22', 24'; 130) has meanders or crenellations (46, 48; 132) having portions (50) oriented in a substantially radial direction, said meanders or crenellations being created to increase the peripheral electrical length of the ring.
2. Antenna according to claim 1, characterised in that the meanders or crenellations have substantially radial parts (50) such that overall they do not produce any field interfering with the polarisation of a signal to be transmitted.
3. Antenna according to claim 2, characterised in that two successive radial parts create fields interfering with the polarisation that compensate each other.
4. Antenna according to claim 1, characterised in that the meanders or crenellations have rectilinear substantially radial parts (50).
5. Antenna according to any of claims 1 to 4, characterised in that the ring has alternating sections (46, 48) such that the distances from the centre of two successive sections are different and in that the sections at the greatest distance from the centre are all on a common circle.
6. Antenna according to any preceding claim, characterised in that the ring has alternating sections (46, 48) such that the distances from the centre of two successive sections are different and in that the sections nearest the centre are all on a common circle.
7. Antenna according to claim 5 or claim 6, characterised in that the ratio between the diameters of the sections is not greater than 2:1.
8. Antenna according to any preceding claim, characterised in that the meanders or crenellations are equi-angularly distributed about an axis (12).
9. Antenna according to any preceding claim, characterised in that the number of meanders or crenellations is equal to eight or sixteen.
10. Antenna according to any preceding claim, characterised in that, for transmission, it is excited in sections (48) at the greatest distance from the centre.
11. Antenna according to any preceding claim, characterised in that it is adapted to transmit circular polarisation waves, wherein sections of the ring are adapted to be excited with successive phase-shifts of the wave to be transmitted to produce said circular polarisation.
12. Antenna according to claim 11. characterised in that the phase-shift generator circuit (64) is produced by metallic cut-outs or etchings with peripheral outputs (90₁, 90₂, 90₃, 90₄).
13. Antenna according to any preceding claim, characterised in that the ring is a conductive strip.
14. Antenna according to any of claims 1 to 12, characterised in that the ring is a slot (132) in a conductor (134).
15. Antenna according to any preceding claim, characterised in that it is adapted to transmit waves in the UHF band or in the S band.

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