EP3910729A1 - Broadband orthomode transducer - Google Patents

Abstract

The invention relates to an orthomode transducer and a satellite transmission chain comprising the orthomode transducer, for transmitting a first signal and a second signal in orthogonal propagation modes. The transducer comprises: - a main waveguide (501, 601) with a square or rectangular section, - two guided accesses (510, 520) having on the one hand a free end through which the first signal is respectively injected or recovered and the second signal, and on the other hand two arms connected to the main waveguide.Each guided access includes a junction (512, 522, 602) configured to connect the free end with the two arms of the guided access, the two arms (513, 514, 523, 524, 610, 611) of each guided access being connected to the main waveguide at two off-center locations on one or more sides of the main waveguide symmetrically with respect to a axis of symmetry of the main waveguide.

Description

Technical area :

The invention relates to the field of microwave transmissions, and more particularly relates to an orthomode transducer used to transmit two signals in orthogonal polarizations.

If the proposed solution is particularly useful in the field of antenna sources, and in particular satellite antennas, it is not limited to these applications, and the orthomode transducer according to the invention can also be used for other devices. , such as for example for the production of filters or microwave duplexers.

Prior technique :

In order to maximize their spectral efficiency, satellite transmission systems generally use polarization diversity, which consists of transmitting on the same frequency band two orthogonally polarized signals (for example a vertical polarization and a horizontal polarization, or a circular polarization right and a left circular polarization). When the polarizations between the two signals are perfectly orthogonal, the signals can be recovered independently, which makes it possible to transmit or receive two signals simultaneously in the same frequency band, or to transmit and receive simultaneously in the same band. frequency, from a single antenna.

In the theoretical case, the decoupling between the two signals is infinite, which makes it possible to dissociate them perfectly. In practice, the asymmetries of the transmission equipment create an angle of the electric fields. In this case, a sub-component of each polarization merges with the cross polarization, which leads to coupling phenomena between the signals. Those skilled in the art therefore ensure that the two orthogonally polarized signals are transmitted with the strongest possible decoupling.

Orthomode transducers, or signal duplexers (better known under the English name of Orthogonal Mode Transducer , or OMT) are devices belonging to the supply chain of an antenna, in particular a satellite antenna. The figure 1a very schematically represents a transmission chain for an antenna. It comprises a source, generally a horn 101, through which the satellite signals are transmitted / received, and an orthomode transducer 102, in the form of a waveguide through which two signals S ₁ and S ₂ 103 are injected / extracted. and 104. The orthomode transducer is configured to combine or separate the two signals by applying orthogonal polarization to them. Depending on the embodiment, other signals associated with other frequency bands can be injected / extracted from transducer 102.

Many telecommunications satellites are equipped with network antennas, made up of a large number of transmission chains such as the one shown in figure 1a , making it possible to achieve geographic coverage by beams. The figure 1b represents very schematically the components of a network antenna. It comprises a plurality of sources 111 to 116, each associated with one or more beams. An orthomode transducer 121 to 126 is associated with each source, thus allowing the transmission of two orthogonally polarized signals in the beam or beams concerned, generally a signal in transmission and a signal in reception. The dimensions and shape of the waveguides making up the orthomode transducer are chosen as a function of the frequency of the signals transmitted, so as to allow the propagation of the electromagnetic waves in controlled transverse electric modes.

Network antennas on board satellites can include several dozen transmission chains, and therefore as many orthomode transducers. The size and mass of these devices are therefore very sizing elements when designing satellite antennas.

In the remainder of the description, and in order to simplify the understanding of the physical phenomena which apply, the explanations are given by considering the case of application of two signals injected on the transducer orthomode in order to be orthogonally polarized and combined and then emitted by the source of the satellite antenna. However, the invention applies identically in the case of two signals of orthogonal polarizations received from the source of the satellite antenna, and transmitted and separated by the orthomode transducer, or in the case where a signal is transmitted and the other is received.

Orthomode transducers have a square central core configured so as to allow the transmission of a first signal according to a TE10 propagation mode, in which the electric field of the signal is linear and vertical, and of a second signal according to a propagation mode TE01, in which the electric field of the signal is linear and horizontal. The two signals are then orthogonally polarized, and can be transmitted simultaneously. The central core can be rectangular for the propagation of signals in distinct frequency bands. Likewise, the signals can be transmitted according to circular polarizations by associating for example a coupler with the orthomode transducer, so that each signal is transmitted on the one hand according to a first mode and on the other hand in a delayed and phase-shifted manner according to a second mode. The resulting electric field is then rotating, which creates a circularly polarized signal.

Several different structures of orthomode transducers are known from the state of the art.

The figure 2a shows a three-dimensional view of an orthomode transducer with two branches, which constitutes the simplest, most compact, most economical and therefore the most widespread type of orthomode transducer. It is composed of a main waveguide 201 extending along a longitudinal axis zz '. The waveguide is suitable for the propagation of the two fundamental electromagnetic modes in the frequency band considered. In practice, the two signals being in the same frequency band, this result is achieved by using a waveguide of square section whose size is dimensioned with respect to the minimum frequency of the frequency band considered, but the guide d The wave can take any shape allowing the propagation of the two signals in the desired modes.

The main waveguide is connected by a first side along its longitudinal axis zz 'to a source, a radiating element performing the adaptation between the waveguide and the free space. The main waveguide 201 is connected to two guided accesses 202 and 203 through which the two signals to be transmitted are injected. The junctions between the guided accesses and the main waveguide are made at the same level of the main waveguide, in an xy plane orthogonal to the zz 'axis, through slots made in the middle of orthogonal walls of the main guide , which has the effect that the signals injected by the two guided accesses are combined according to orthogonal polarizations in the main waveguide before being transmitted to the source (and conversely, makes it possible to extract on each of the orthogonally polarized signal access). The rear of the main waveguide 201 along the longitudinal axis zz 'can be connected, for example, to other accesses to inject signals in a separate frequency band.

The figure 2b describes the principle of signal polarization in a two-branch waveguide, in a sectional view in the xy plane. The first signal, intended to be vertically polarized, is injected into the first guided access 202. The solid arrows give the direction of the electric field of the first signal, perpendicular to the direction of propagation of the electromagnetic wave. In the main waveguide 201, the first signal propagates according to the fundamental propagation mode TE10, corresponding to a vertical polarization. The second signal, intended to be horizontally polarized, is injected on the second guided access 203. The dotted arrows give the direction of the electric field of the second signal, perpendicular to the direction of propagation of the electromagnetic wave. In the main waveguide 201, the second signal propagates according to the fundamental propagation mode TE01, corresponding to a horizontal polarization. Within the main waveguide 201, the two signals propagate according to orthogonal propagation modes.

As shown in the figure 2c , a two-branch orthomode transducer can be associated with a 90 ° coupler to circularly polarize the two signals. The 90 ° coupler 210 is connected to the guided access 202 and the guided access 203 by two ends. The signal intended to be transmitted according to a polarization, for example the LHCP polarization (acronym for Left Hand Circular Polarization , or left circular polarization), is injected on the end 211 of the coupler. It is then presented on the guided access 202, and in a delayed manner and out of phase by 90 ° on the guided access 203. In the same way, the signal intended to be transmitted according to the cross polarization, here the RHCP polarization (acronym English for Right Hand Circular Polarization , or right circular polarization), is injected on the end 212 of the coupler. It is then presented on the guided access 203, and in a delayed and phase-shifted version by 90 ° on the guided access 202. The delays and phase shifts applied have the effect of rotating the electric field, and therefore of circularly polarizing the signals. .

The figure 2d represents the electric field of the signal injected on the guided access 203 of a two-branch orthomode transducer, in a sectional view in the xy plane orthogonal to zz 'at the junction between the guided accesses and the main guide 201. The levels in gray represent the intensity, and the arrows the direction, of the electric field.

The signal injected by the guided access 203 propagates inside the main waveguide 201 according to the propagation mode TE01, that is to say it is linear and horizontal. Within the main waveguide, the electric fields are not perfectly aligned. These slight distortions are linked to the sensitivity of the electric field to the asymmetries present on a centered access, and have the effect of producing coupling phenomena between the two orthogonally polarized signals.

In addition, a small part of the signal injected by the access 203 propagates in the guided access 202. The electric field being always perpendicular to the support, the latter rotates on entering the guided access 202. Residues of the transmitted signal on the access 203 are then found on the guided access 202 with the same polarization as the signal transmitted on this access (vertical linear), which is the cause of additional parasitic coupling phenomena. For this reason, the decoupling generally achieved by means of an orthomode transducer with two branches is of the order of -20 dB. This level of decoupling may turn out to be too low for a certain number of applications, such as for example for satellite antennas, where the losses associated with decoupling result in a degradation of the link budget and therefore of the achievable speeds.

A known manner making it possible to improve the decoupling between the channels of an orthomode transducer with two guided ports is described in the patent. EP 2.202.839 B1 and represented at the figure 2e , in sectional view, for circularly polarized signals. The device comprises a coupler with unbalanced branches 231 making it possible to transmit each of the signals in controlled proportions on the guided accesses 202 and 203 of an orthomode transducer with two branches, and short-circuited waveguides (in English stub ) 232 and 233 configured to filter signals. The partition coefficients of the coupler 231 are adjusted so that a portion of a polarization of the signal injected on one channel is injected on the other channel, with a very precise phase adjustment making it possible to cancel the portion of related parasitic energy. to the wrong decoupling. This operation is managed by the action on the short-circuited waveguides of the filters 232 and 233 of the transmission channel which allow, in addition to the rejection of the reception band, the phase quadrature of the crossed component. compared to the main component.

This solution makes it possible to achieve high levels of decoupling, but is complex to implement and cumbersome.

Another way to improve the decoupling of a two port orthomode transducer is shown in Figure figure 2f . The ports are always injected orthogonally on the waveguide 201, but are offset in the axis zz 'of the source. This waveguide makes it possible to achieve significant decoupling levels, of around -50 dB, but is bulky.

It is also known from the state of the art of orthomode transducers with four branches, making it possible to achieve greater decoupling than those with two branches. Such an orthomode transducer is shown in figure 3a . It is composed of a main waveguide 301 extending longitudinally along an axis zz 'and connected to two pairs of waveguides (302/304 and 303/305) constituting accesses through which are injected the two signals to be transmitted. The two waveguides of the same pair are positioned face to face in a same plane orthogonal to the zz 'axis, The two waveguides of the other pair are connected to the other two sides of the main waveguide.

The figure 3b describes the principle of signal polarization in a four-branch orthomode transducer.

The signal intended to be vertically polarized is injected onto the main waveguide 301 from the guided ports 303 and 305, opposite with respect to the main waveguide 301. The signals injected from the two guided ports are identical, synchronized, in phase and have the same power level. They then recombine constructively in the main waveguide, and the signal propagates according to the TE10 mode. Likewise, the second signal, intended to be horizontally polarized, is injected in a synchronized manner and in phase on the main waveguide 301 from the guided accesses 304 and 306, opposite with respect to the main waveguide 301. There too , the two injected signals recombine constructively, and the signal propagates in the main waveguide in the TE01 propagation mode.

The symmetry of the four-branch orthomode transducer results in the electric field lines being more straight than in a two-branched transducer.

As in the two-port waveguide, part of the signal injected from the guided port 303 is found in the guided port 302 with an electric field 310 rotated by 90 °, and therefore horizontally polarized. Likewise, part of the signal injected from the guided access 305 is found in the guided access 302 with an electric field 311 rotated by 90 °, and therefore polarized horizontally. The signals being injected in phase from the guided accesses 303 and 305, the electric field 310 and the electric field 311 of the residues of these signals transmitted in the guide 302 are then found in phase opposition (180 °). Their recombination takes place in a destructive manner, and the residues of the signals injected by the guided accesses 303 and 305 found in the guided access 302 disappear. The principle is the same in each of the waveguides 302 to 305.

The symmetry properties of orthomode transducers with four branches therefore make it possible to obtain a perfectly linear electric field, the cross polarization naturally disappearing in cross accesses. They generally exhibit significant decoupling levels, of the order of -40 dB.

However, generating two identical signals and in phase for each polarization transfers the complexity upstream since it is then necessary to duplicate the generation of the signals, the signals transmitted to an access pair having to be perfectly identical and synchronized. In addition, the orthomode transducer having four independent ports is not optimal in terms of compactness.

Alternatively, the guided accesses used to inject a given signal can be recombined two by two, taking care that the paths to each injection point are of the same length so that the signals are injected simultaneously and in phase. The recombination circuits are then complex, especially since the two guided accesses are interlaced, and require a large number of elementary connection components, thus increasing the dispersion. In the end, the performance obtained is limited and the ohmic losses are significant, for a bulky and heavy device.

An object of the invention is therefore to describe an orthomode transducer having a high level of decoupling, which is both simple to use and compact.

Summary of the invention:

• un guide d'onde principal à section carrée ou rectangulaire,
• deux accès guidés ayant d'une part une extrémité libre par laquelle sont respectivement injectés ou récupérés le premier signal et le deuxième signal, et d'autre part deux bras reliés au guide d'onde principal.

To this end, the present invention describes an orthomode transducer, for the transmission of a first signal and of a second signal in orthogonal propagation modes. The orthomode transducer includes:

• a main waveguide with square or rectangular section,
• two guided accesses having on the one hand a free end through which the first signal and the second signal are respectively injected or recovered, and on the other hand two arms connected to the main waveguide.

Each guided access includes a junction configured to connect the free end to the two arms of the guided access, the two arms of each guided access being connected to the main waveguide at two off-center locations on one or more sides of the main waveguide, symmetrically with respect to an axis of symmetry of the main waveguide.

Advantageously, the connection between the main waveguide and the two arms of a guided access comprises the two angles of the same side of the main waveguide.

According to the embodiment of the orthomode transducer according to the invention, the junction of each guided access is configured so that the signals transmitted on the two arms of a guided access are in phase or in phase opposition depending on their mode of propagation in the main waveguide.

Advantageously, the two arms of the same guided access are of substantially identical dimensions.

Advantageously, the guided accesses are arranged symmetrically with respect to an axis of symmetry of the main waveguide.

In one embodiment of the orthomode transducer described, each guided access comprises a particular junction chosen from an E-plane T junction and an H-plane T junction, and two particular arms.

In an alternative embodiment, the two guided accesses comprise the same junction in the form of a magic T-junction whose side ports are connected to a pair of common arms, the first and the second signal being transmitted by two separate ports. of the magic T-junction.

• un transducteur orthomode tel que décrit précédemment, et
• un coupleur 90° relié aux extrémités libres des accès guidés du transducteur orthomode de manière à polariser circulairement le premier et le deuxième signal.

The invention also relates to a device for transmitting the signals in orthogonal circular polarizations. He understands :

• an orthomode transducer as described above, and
• a 90 ° coupler connected to the free ends of the guided ports of the orthomode transducer so as to circularly polarize the first and the second signal.

Finally, the invention addresses a transmission chain for a satellite antenna comprising a source connected to an orthomode transducer as described above, or a device as described above for the transmission of signals according to orthogonal circular polarizations.

Brief description of the figures :

• la figure 1a représente très schématiquement une chaîne de transmission pour antenne, par exemple une antenne satellite,
• la figure 1b représente très schématiquement les composantes d'une antenne réseau embarquée dans un satellite,
• la figure 2a représente une vue en trois dimensions d'un transducteur orthomode à deux branches selon l'état de l'art,
• la figure 2b décrit le principe de polarisation des signaux dans un guide d'onde à deux branches,
• la figure 2c représente un montage permettant de polariser circulairement et combiner des signaux dans un transducteur orthomode à deux branches,
• la figure 2d représente le champ électrique du signal injecté sur l'accès guidé 203 d'un transducteur orthomode à deux branches,
• la figure 2e représente un montage permettant d'améliorer le découplage d'un transducteur orthomode à deux branches,
• la figure 2f représente un transducteur orthomode à deux branches décalées,
• la figure 3a représente une vue en trois dimensions d'un transducteur orthomode à quatre branches selon l'état de l'art,
• la figure 3b décrit le principe de polarisation des signaux dans un transducteur orthomode à quatre branches,
• la figure 4a représente grossièrement le champ électrique dans l'angle d'un guide d'onde de section carrée ou rectangle,
• la figure 4b représente schématiquement les principes physiques s'appliquant lors de l'injection d'un signal par deux accès situés sur les bords d'un côté du guide d'onde principal,
• la figure 4c représente une configuration permettant d'injecter un signal de manière décentrée sur les côtés d'un guide d'onde,
• la figure 4d représente une configuration permettant d'injecter un signal de manière décentrée sur les côtés d'un guide d'onde,
• la figure 4e représente une configuration permettant d'injecter un signal de manière décentrée sur les côtés d'un guide d'onde,
• la figure 5a représente un mode de réalisation d'un transducteur orthomode à deux branches selon l'invention,
• la figure 5b représente le champ électrique du signal injecté sur l'accès 510 d'un transducteur orthomode à deux branches selon un mode de réalisation de l'invention,
• la figure 5c est une représentation en trois dimensions d'un transducteur orthomode selon un mode de réalisation de l'invention,
• la figure 5d distingue les différentes parties d'un transducteur orthomode selon un mode de réalisation de l'invention pour une fabrication par fraisage,
• la figure 6a représente un mode de réalisation d'un transducteur orthomode à deux branches selon l'invention,
• la figure 6b distingue les différentes parties d'un transducteur orthomode selon un mode de réalisation de l'invention pour une fabrication par fraisage.

The invention will be better understood and other characteristics, details and advantages will appear more clearly on reading the description which follows, given without limitation, and thanks to the appended figures which follow, given by way of example, among which:

• the figure 1a very schematically represents a transmission chain for an antenna, for example a satellite antenna,
• the figure 1b very schematically represents the components of a network antenna on board a satellite,
• the figure 2a represents a three-dimensional view of an orthomode transducer with two branches according to the state of the art,
• the figure 2b describes the principle of signal polarization in a two-branch waveguide,
• the figure 2c represents an assembly making it possible to circularly polarize and combine signals in an orthomode transducer with two branches,
• the figure 2d represents the electric field of the signal injected on the guided access 203 of an orthomode transducer with two branches,
• the figure 2e represents an assembly making it possible to improve the decoupling of an orthomode transducer with two branches,
• the figure 2f represents an orthomode transducer with two offset branches,
• the figure 3a shows a three-dimensional view of an orthomode transducer with four branches according to the state of the art,
• the figure 3b describes the principle of signal polarization in a four-branch orthomode transducer,
• the figure 4a roughly represents the electric field in the angle of a waveguide of square or rectangle section,
• the figure 4b schematically represents the physical principles applying during the injection of a signal by two accesses located on the edges of one side of the main waveguide,
• the figure 4c represents a configuration allowing a signal to be injected off-center on the sides of a waveguide,
• the figure 4d represents a configuration allowing a signal to be injected off-center on the sides of a waveguide,
• the figure 4e represents a configuration allowing a signal to be injected off-center on the sides of a waveguide,
• the figure 5a represents an embodiment of an orthomode transducer with two branches according to the invention,
• the figure 5b represents the electric field of the signal injected into the port 510 of an orthomode transducer with two branches according to one embodiment of the invention,
• the figure 5c is a three-dimensional representation of an orthomode transducer according to one embodiment of the invention,
• the figure 5d distinguishes the different parts of an orthomode transducer according to one embodiment of the invention for manufacturing by milling,
• the figure 6a represents an embodiment of an orthomode transducer with two branches according to the invention,
• the figure 6b distinguishes the different parts of an orthomode transducer according to one embodiment of the invention for manufacturing by milling.

Identical references are used in different figures when the designated elements are identical.

Detailed description :

If they have good decoupling performance, the orthomode transducers with four branches of the state of the art are complex to use and bulky. The invention is therefore naturally oriented towards orthomode transducers with two branches.

It is based on the properties of the electromagnetic field, which orients itself perpendicular to the metal walls of the waveguide.

The figure 4a roughly represents the direction of the electric field in the angle of a waveguide 401 of square or rectangular section. The electromagnetic field being always perpendicular to the support, in the angle of the waveguide, it is inclined as a function of the distance from the two walls.

The invention proposes to inject the signals not through ports centered on the sides of the cavity of the main waveguide of the orthomode transducer, but through off-center ports located on the edges of one or more sides of this guide. main wave. With a single off-center injection point, the propagation mode in the waveguide is not mastered since it is not certain that the electric field in the waveguide will be perfectly linear and oriented in the direction wanted. The invention proposes to inject each signal not by one, but by two off-center accesses of one or more sides of the main waveguide, and this symmetrically with respect to an axis of symmetry of the main waveguide. The figure 4b schematically represents the physical principles applying during the injection of a signal by two accesses located on the edges of the same side of the main waveguide.

The figure 4b takes the example of the injection of a first signal into the main waveguide 401 of an orthomode transducer through a guided access 410, with the aim of this signal propagating according to the TE10 (vertical linear) mode. Solid arrows represent the orientation of the electric field. The signal injected into the guided access 410 is separated into two signals of the same power by a junction 411 playing the role of signal separation means. The junction is connected to two arms 412 and 413 of the same length. The junction can for example be a plane E microwave T, dividing the signal into two signals in phase opposition and of the same power. The arms of each accesses are connected to the main waveguide 401 by two off-center slots located at the ends of the right edge of the main waveguide 401, symmetrically with respect to the axis xx '. The electric fields applied in this way in the angles of the main waveguide (represented by the solid arrows) are not vertical in the angles. However, the vectorial recombination of these two injections gives the desired electric field, here an electric field polarized perfectly vertically.

Junction 411 can also be a T-plane H-plane microwave junction, dividing the signal into two signals in phase and of the same power. In this case, the electric field of the signals (represented by the dotted arrows) at the output of the junction 411 is in phase. The signal in the main waveguide 401, resulting from the vector combination of the signals injected by the arms 412 and 413, is then horizontally polarized (TE01 mode, horizontal linear). The type of junction is therefore chosen according to the propagation mode sought in the main waveguide.

By injecting the same signal, in phase or in phase opposition, through two off-center and symmetrical accesses in the main waveguide of an orthomode transducer, it is therefore possible to "force" the wave propagation mode. electromagnetic. In the example of figure 4b , where the arms 412 and 413 of the guided access 410 are positioned in the corners of a vertical wall of the main waveguide 401, the junction 411 separates the signal into two out-of-phase signals to vertically polarize the signal, or two signals in phase to polarize it horizontally.

The use of arms of the same dimensions (same length, same width and same height) makes it possible to inject the signal into the main waveguide in a synchronized manner and with the same power level. A simple way to obtain arms of the same length is to arrange the whole of the guided access symmetrically with respect to the axis of symmetry xx 'of the main waveguide 401.

The arrangement described in figure 4b is not the only one possible for a guided access with two arms in an orthomode transducer according to the invention. The figures 4c, 4d and 4th describe other configurations for injecting a signal off-center to the sides of the main waveguide 401.

In the figure 4c , the junction 421 is a T plane E, which generates two signals in phase opposition on the two arms 422 and 423, which inject the signals into the two angles of a horizontal side of the main waveguide 401, symmetrically with respect to the yy 'axis. Consequently, the propagation mode in the main waveguide is the TE01 mode, that is to say a horizontal linear polarization. By using a junction 421 configured to generate in-phase signals, such as an H-plane T junction, the propagation mode obtained is the TE10 mode, ie vertical linear polarization.

In the figure 4d , the two arms are connected to off-center ports located on two opposite sides of the main waveguide 401. The ports are always symmetrical with respect to the axis xx '. The electric field evolves as on the figure 4b , in TE10 mode, although the injection points of the arms 432 and 433 in the main waveguide are different from those of the arms 412 and 413 of the figure 4b . By using an H-plane T-junction instead of an E-plane T junction, the signal is horizontally polarized (TE01 mode).

In the figure 4e , the two arms are connected to the same horizontal side of the main waveguide 401, and are offset symmetrically with respect to the axis yy 'but without encompassing the angles. The electric field evolves in the same way as on the figure 4c , although the arrangement of the arms and their positioning relative to the angles of the main waveguide differ.

The arms of a guided access therefore do not necessarily join the main waveguide 401 in one of its angles, provided that the injection points in the main waveguide are symmetrical with respect to an axis of symmetry of the main waveguide 401, so that the combination of the signals injected from the two arms generates a perfectly rectilinear electric field. However, the proximity of the angles improves the performance of the orthomode transducer according to the invention, because the junction slots between the access arms and the central waveguide create a magnetic coupling (H field), their positioning in the angles optimizing the effectiveness of this coupling.

The figure 5a represents an embodiment of an orthomode transducer with two branches according to the invention. The transducer is configured to emit a first signal in vertical linear polarization, and a second signal in horizontal linear polarization.

It comprises a main waveguide 501, with a square section, but the invention would apply in an identical manner for a waveguide with a rectangular section, in the case of two injected signals operating in different frequency bands. The main waveguide 501 extends along an axis zz 'in which a source for an antenna system may for example be located. It is suitable for the propagation of signals according to the two fundamental modes TE10 and TE01 in the frequency band or bands considered. The figure 5a represents the orthomode transducer in a sectional view at the level of the intersections with the guided accesses, according to an xy plane orthogonal to the axis zz 'in which the main waveguide 501 extends.

A first guided access 510 is configured to inject the first signal into the main waveguide 501. It comprises a waveguide 511 having a free end through which is injected the signal to be transmitted in vertical polarization, a junction 512 configured for divide the first signal into two identical signals, of the same power, and in phase opposition, such as an E-plane T junction, and two arms 513 and 514, connected on the one hand to the junction 512 and on the other hand on the same side of the main waveguide off-center and symmetrical with respect to its axis xx '. The elements constituting the guided access 510 are dimensioned so as to allow the propagation of the first signal (the electromagnetic field of which is represented by solid arrows in the figure) according to a fundamental mode in the frequency band considered. They can be linked with the main waveguide 501 through irises performing the impedance matching. The vector combination of the electric fields of the signals injected by the two arms 513 and 514 into the waveguide 501 forms the propagation mode of the signal in the waveguide, that is to say here the TE10 mode, corresponding to vertical linear polarization.

Similarly, a second guided access 520 is configured to inject the second signal into the main waveguide 501, at the same level as the first guided access. It includes a waveguide 521, through which is injected the signal, connected to a junction 522, configured to divide the second signal into two identical signals, of the same power and in phase opposition. The two outputs of the junction 522 give onto the arms 523 and 524. The two arms are respectively connected to the edges of the same side of the main waveguide, symmetrically with respect to its axis of symmetry yy '. The side of the waveguide chosen here is the side orthogonal to that where the arms of the first guided access are connected. However, in the orthomode transducer according to the invention, any other side could have been selected since the final polarization of the signal is a function of the combination of the positions where the signal is injected by the two arms and of the type of junction chosen. The elements constituting the guided access 520 are dimensioned so as to allow the propagation of the second signal (the electromagnetic field of which is represented by dotted arrows in the figure) according to a fundamental mode in the frequency band considered. They can be connected with the main waveguide 501 by slots provided with irises for impedance matching. The vector combination of the electric fields of the signals injected by the two arms 523 and 524 makes it possible to form the mode of propagation of the signal in the waveguide, here the TE01 mode corresponding to a horizontal linear polarization.

The orthomode transducer according to the invention therefore makes it possible, from two accesses 510 and 520, to combine two signals with the desired cross polarizations in the main waveguide 501.

The figure 5b represents the electric field of the signal injected into the port 510 of an orthomode transducer with two branches according to one embodiment of the invention, in a sectional view in the xy plane at the intersection between the guided ports and the guide main 501. The length and direction of the arrows represent the strength and direction of the electric field.

The electric field in the access 510 changes so that the vector combination of the signal injected in phase through the arms 513 and 514 propagates in the main waveguide according to the TE10 mode, that is to say vertically polarized . We observe that the electric field is oriented much more precisely than in a two-port orthomode transducer shown in figure 2d , by the symmetry of the accesses to two branches: the decoupling between the polarizations is therefore greater.

Part of the energy injected from the guided access 510 propagates into the arms 523 and 524 of the guided access 520, where the electric field rotates to be oriented horizontally. The phase junction 522 (E-plane T junction) then acts as a means of combining the signals in phase opposition. The position of the two arms being symmetrical with respect to the axis of symmetry yy 'of the main waveguide 501, the signals transmitted in the two arms are identical and of the same power. The orientation of the electric field causes them to be in phase opposition (180 °) in the access 521. Consequently, they cancel each other out, and the residues of the signal emitted by the guided access 510 and received in the junction 522 naturally vanish into waveguide 521. There is therefore little or no coupling effects due to residues of a signal in the guided access of the cross-polarized signal.

The phenomenon is the same in the other direction, where residues of the signal emitted by the access 520 are found in phase opposition in the arms 513 and 514. Their recombination by the junction 511 in phase opposition causes the signal to be polarized. horizontally fades out. There are therefore no or few coupling effects in this sense either.

Thanks to the symmetry properties of the off-center accesses, the orthomode transducer according to the invention as shown in figure 5a improves the decoupling performance of a few dB compared to orthomode transducers with two arms such as the one shown in figure 2a , by generating perfectly linear electric fields, and by blocking by construction the propagation of the signal from one guided access to the other. In addition, this orthomode transducer is wider band than the two-branch orthomode transducers of the state of the art because its symmetry properties mean that it builds polarization alignments that are always well oriented, regardless of the frequency band considered. . This is not the case with two-arm orthomode transducers, which are not symmetrical and therefore must be optimized for a given frequency band.

The figure 5c is a three-dimensional representation of an orthomode transducer according to one embodiment of the invention. We find the guide there of main wave 501, to which are connected a first port 510, for the injection of the signal emitted according to a polarization, and a second port 520, for the injection of the signal emitted according to the cross polarization.

This device has the advantage of being particularly simple and of occupying a volume reduced by nearly 75% compared to orthomode transducers with four branches connected in pairs, such as that of the figure 3a , which is one of the aims sought by the invention. This compactness is important, in particular for the production of array antennas involving a large number of orthomode transducers arranged in a restricted mesh. The reduction in mass is in the same proportions, which is also very appreciated for the production of network antennas on board the payload of satellites.

Another advantage of the orthomode transducer according to the invention is that the bottom of the cavity of the orthomode transducer (the rear of the main waveguide along the zz 'axis) remains free. It is therefore possible to add afterwards other accesses making it possible to process the polarizations of signals transmitted in another frequency band, or a load playing the role of termination of the main waveguide.

If the orthomode transducer according to the invention, where each of the ports comprises a pair of distinct arms, makes it possible to polarize signals according to orthogonal linear polarizations, it can be associated with a coupler so as to circularly polarize the signals, in a manner comparable to which is done with orthomode transducers with two branches known from the state of the art such as that shown in figure 2c .

Finally, the realization of the orthomode transducer according to the invention can be envisaged in additive manufacturing (metallic three-dimensional printing) for a reduced cost or in a milled technique, in only three parts 531, 532 and 533 shown in figure figure 5d , the part 533 representing a step allowing adaptation of the orthomode transducer to the source of the antenna.

Another embodiment of an orthomode transducer according to the invention is given in figure 6a . This embodiment always involves a guide main wave 601, but the guided ports of the two cross-polarized signals are injected by the same pair of arms.

For this, the orthomode transducer includes a device known to the skilled person named magic tee junction (English magic T). A magic T-junction is a three-dimensional, four-port microwave component: two side ports, one sum port, and one difference port. It jointly performs the function of T-plane and H-plane junction, the side ports and the sum port forming the T plane H, the side ports and the difference port forming the T plane E.

The first access to the main waveguide is formed by a waveguide 603 having a free end through which the first signal is injected, and connected to the difference port of the magic T-junction. The two side ports of the magic T-junction are connected to two arms 610 and 611, themselves connected to the main waveguide 601 by off-center accesses positioned on the edges of the same side of the main waveguide, symmetrically with respect to its axis of symmetry yy '.

The second access to the main waveguide is formed by a waveguide 604 having a free end through which the second signal is injected, and connected to the sum port of the magic T-junction. The arms of this port are the arms 610 and 611 connected to the side ports of the magic T-junction, just like the first port.

The use of a magic T-junction makes it possible to share the arms between the two guided accesses in orthogonal polarizations. The positioning of the ports makes it possible to obtain orthogonal propagation modes in the main waveguide 601 with perfectly formed electric fields. Finally, the positioning and the structure of the accesses, associated with the magic T-junction, make it possible to avoid the coupling effects between the two signals with crossed polarizations.

The waveguide according to the embodiment presented in figure 6a makes it possible to obtain very high decoupling levels, of the order of -70 dB, with an extremely compact device. Compared to the embodiments presented previously, however, it operates on a reduced frequency band, given by the operating band of the magic T-junction.

Its realization is very simple since it can be generated by additive manufacturing, or by milling requiring only the assembly of two parts. The figure 6b shows the two parts 621 and 622 required for the production by milling of an orthomode transducer according to the invention.

The embodiments presented above for an orthomode transducer according to the invention make it possible to combine signals in orthogonal polarizations in a simple, compact and very efficient manner.

The orthomode transducer according to the invention has been described in the case of application of the injection of two signals from the free ends of the guided accesses to the main waveguide. However, the invention applies in an identical manner for the extraction of signals from the main waveguide to the two guided accesses. In this case, the T-junctions play the role of means of combining the signals received by the arms from the main waveguide. The invention also applies in the same way for the injection of a first signal and the simultaneous extraction of a second signal in cross-polarization.

Claims (9)

Orthomode transducer, for the transmission of a first signal and a second signal in orthogonal propagation modes, characterized in that it comprises: - a main waveguide (501, 601) with a square or rectangular section, - two guided accesses (510, 520) having on the one hand a free end through which the first signal and the second signal are respectively injected or recovered, and on the other hand two arms connected to the main waveguide, and wherein each guided access includes a junction (512, 522, 602) configured to connect the free end with the two arms of the guided access, the two arms (513, 514, 523, 524, 610, 611) of each guided access being connected to the main waveguide at two off-center locations on one or more sides of the main waveguide, the two locations being symmetrical with respect to an axis of symmetry of the main waveguide. An orthomode transducer according to claim 1, in which the connection between the main waveguide and the two arms of a guided access comprises the two angles of the same side of the main waveguide. Orthomode transducer according to one of the preceding claims, in which the junction (512, 522, 602) of each guided access is configured so that the signals transmitted on the pair of arms of a guided access are in phase or out of phase according to their mode of propagation in the main waveguide. Orthomode transducer according to one of the preceding claims, in which the two arms of the same guided access are of substantially identical dimensions. Orthomode transducer according to one of the preceding claims, in which the guided ports are arranged symmetrically with respect to an axis of symmetry of the main waveguide. Orthomode transducer according to one of the preceding claims, in which each guided access comprises a particular junction (512, 522) chosen from an E-plane T junction and an H-plane T junction, and two particular arms (513, 514, 523 , 524). Orthomode transducer according to one of claims 1 to 5, in which the two guided accesses comprise the same junction (602) in the form of a magic T-junction, the side ports of which are connected to a pair of common arms (610, 611), the first and second signal being transmitted through two separate ports (603, 604) of the magic T-junction. Device comprising: - an orthomode transducer (102) according to one of claims 1 to 7, and - A 90 ° coupler (210) connected to the free ends of the guided ports of the orthomode transducer so as to circularly polarize the first and the second signal. Transmission chain for a satellite antenna comprising a source (101) connected to an orthomode transducer (102) according to one of claims 1 to 7 or to a device according to claim 8.

Images