EP0860893A1 - Concentric set of microwave antennas - Google Patents

Abstract

Each ring is arranged in a location 25,26 of conductive material, with the quarter wave trap 28 fitted between the two locations. the quarter wave trap is limited on the one hand by the external wall of the location 25 of the closest antenna to the centre 14 and on the other hand by an internal wall 262 of the location 26 of the most distant antenna. these two walls are joined on a first side by a base and form an opening 29 issuing on the path of a leak current from the closest antenna to the centre. the external walls 25 of the location of the closest antenna to the centre 14 and those internal 262 of the location of the most distant antenna have an essentially cylindrical shape centred on the axis 12 of the assembly.

Description

The invention relates to a set of antennas transmission or reception of waves in the microwave domain.

For a number of applications it is necessary to transmit or receive microwave waves according to several bands. In general, the antennas provided for a band frequencies are not optimized for another band of frequencies. This is why we usually provide an antenna by frequency band. However, this multiplication of antennas poses problems of congestion, in particular for the space applications. To reduce clutter, it is known to arrange the antennas concentrically, the central antenna being intended for the highest frequencies.

The invention starts from the observation that the purity of signals emitted by an assembly with at least two concentric antennas is not always satisfactory and that the origin of the disturbance is found in the antenna signal transmission central to the peripheral antenna.

The invention is characterized in that, between a indoor antenna, for example central, and an antenna concentric further from the center, a means is provided for prevent or attenuate the propagation of indoor antenna waves to the other antenna.

Said means is for example a quarter wave trap, tuned to the wavelength of the signals provided for the antenna interior.

In a particularly simple embodiment, each antenna has a conductive housing having walls extending substantially parallel to the axis of the antenna, the trap being formed in the interval separating the outer wall of the inner antenna housing of the wall inside the annular housing of the peripheral antenna. In this case, it is enough that the interval has a length, in the direction of the axis, about a quarter of the wavelength of signals to be transmitted by the indoor antenna.

Thus, with the invention, the propagation of waves from the cavity housing the indoor antenna towards the cavity housing the other antenna. This limits the origin of the radiation of the upper band antenna.

The exterior wall of the interior antenna housing forms, in one embodiment, a single piece with the inner wall of the peripheral antenna housing. These two single-piece walls define a closed toroidal volume of side and open on the other. At the bottom of this toric volume, we can have a conductive crown to adjust the length of the trap.

The invention is not limited to the combination of two concentric antennas. In one embodiment, provision is made for additional concentric antennas, and between two antennas adjacent, means are provided to prevent transmission of signals at the frequency of the innermost antenna to the antenna the outermost.

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 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 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 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 for 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 located on small satellites, such as satellites assigned to the location of objects or for missions measurement or remote control with conventional satellites. Because of this application, it must present a small footprint, wide angular coverage for both frequency bands as well as circular polarization with a suitable ellipticity rate over this wide angular coverage, especially for the most distant orientations 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 to 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 figure 1, with a quarter-wave trap 28, however deprived 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 the radial direction segment also has the disadvantage of disturbing the ellipticity rate. However, it is believed to be the succession of segments traversed by currents in direction 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 supply the four exterior 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 on the figure 6. On this last one, we indicated the same elements 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 some applications, especially those for which overheating must be minimized, this slotted embodiment will be preferred.

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 (9)

Set of two concentric antennas for two frequency bands of the microwave domain, characterized in that it comprises, between the two concentric antennas (14, 16), means (28) for eliminating or mitigating the spread of waves from the antenna closest to the center (14) towards the antenna more distant (16). Assembly according to claim 1, characterized in that that the mitigation means is a quarter-wave type trap for the waves from the antenna closest to the center (14). Assembly according to claim 1 or 2, characterized in that each antenna being arranged in a housing (25, 26) made of conductive material, the attenuation means (28) is arranged between the two dwellings. Assembly according to claims 2 and 3, characterized in that the attenuation means is limited, on the one hand, by an external wall of the housing (25) of the antenna closest to the center (14), and on the other hand, by an inner wall (26 ₂ ) of the housing (26) of the antenna more distant, and in that these two walls are connected on one side by a bottom (27), and provide an opening (29 ) on the other side of this opening leading to the path of a leakage current from the antenna closest to the center. Assembly according to claim 4, characterized in that said external walls (25) of the housing of the antenna closest to the center (14) and internal (26 ₂ ) of the housing of the more distant antenna have a substantially cylindrical shape centered on the axis (12) of the assembly. Assembly according to claim 4 or 5, characterized in that at the bottom (27) of the gap (28) separating said walls, is arranged a conductive ring (36) so as to adjust the length of this interval to about a quarter of the wavelength of the signals to be emitted by the antenna (14) most close to the center. An assembly according to any of claims 3 to 6, characterized in that each antenna (14, 16) is of the type resonating with a radiating element (22, 24) arranged on a dielectric (18, 20) arranged inside the corresponding housing (25, 26). Assembly according to claim 7, characterized in that that said radiating elements are substantially in the same plan. Assembly according to claim 7 or 8, characterized in that each of the radiating elements has substantially the shape of a ring.

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