EP3910729B1 - Broadband orthomode transducer - Google Patents

Description

Technical area :

The invention lies in the field of microwave transmissions, and relates more particularly 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 the production of microwave filters or duplexers.

Previous technique :

In order to maximize their spectral efficiency, satellite transmission systems generally use polarization diversity, which consists in 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 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 separate 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 crossed polarization, which leads to coupling phenomena between the signals. A person skilled in the art therefore ensures 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 power supply chain of an antenna, in particular of a satellite antenna. There picture 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 are injected/extracted two signals S ₁ and S ₂ 103 and 104. The orthomode transducer is configured to combine or separate the two signals by applying an orthogonal bias to them. Depending on the embodiment, other signals associated with other frequency bands may be injected/extracted from transducer 102.

Many telecommunications satellites are equipped with array antennas, made up of a large number of transmission chains such as the one shown in picture 1a , making it possible to achieve geographical coverage by beams. There figure 1b schematically represents the components of an array 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 the shape of the waveguides making up the orthomode transducer are chosen according to the frequency of the signals transmitted, so as to allow the propagation of the electromagnetic waves in controlled transverse electric modes.

The array antennas on board satellites can comprise several tens of transmission chains, and therefore as many orthomode transducers. The size and mass of these devices are therefore very dimensioning elements when designing satellite antennas.

In the rest 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 emitted and the other is received.

The 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 polarized orthogonally, and can be transmitted simultaneously. The central core can be rectangular for the propagation of signals in distinct frequency bands. Similarly, 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, such as, for example, the document GB2054974A .

There figure 2a shows a three-dimensional view of a two-pronged orthomode transducer, which is the simplest, most compact, economical, and therefore most common type of orthomode transducer. It is composed of a main waveguide 201 extending along a longitudinal axis zz'. The waveguide is adapted to 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 sized in relation to the minimum frequency of the frequency band considered, but the guide d he wave can take any form 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 realizing the adaptation between the waveguide and the free space. The main waveguide 201 is connected to two guided ports 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 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 access of orthogonally polarized signals). The rear of the main waveguide 201 along the longitudinal axis zz′ can be connected, for example, to other accesses to inject signals into a separate frequency band.

There figure 2b describes the principle of signal polarization in a waveguide with two branches, according to a cross-sectional view in the xy plane. The first signal, intended to be polarized vertically, is injected into the first guided port 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 polarized horizontally, is injected on the second guided port 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 depicted at figure 2c , a two-arm orthomode transducer can be associated with a 90° coupler to circularly polarize the two signals. The 90° coupler 210 is connected to the guided port 202 and to the guided port 203 by two ends. The signal intended to be transmitted according to a polarization, for example the LHCP polarization (English acronym for Left Hand Circular Polarization, or left circular polarization), is injected on the end 211 of the coupler. It then finds itself presented on the guided access 202, and in a delayed and phase-shifted manner of 90° on the guided access 203. In the same way, the signal intended to be transmitted according to the crossed polarization, here the RHCP polarization (abbreviation 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 of 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. .

There 2d figure represents the electric field of the signal injected on the guided port 203 of a two-armed orthomode transducer, in a cross-sectional view in the xy plane orthogonal to zz' at the junction between the guided ports and the main guide 201. The levels of 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, ie 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 port 203 propagates in guided port 202. Since the electric field is always perpendicular to the support, the latter rotates on entering guided port 202. Residues of the transmitted signal on access 203 then find themselves on 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 using a two-arm orthomode transducer is of the order of -20 dB. This level of decoupling may prove to be too low for a certain number of applications, such as for example for satellite antennas, where the losses linked to decoupling result in a degradation of the link budget and therefore of the achievable speeds.

A known way to improve the decoupling between the channels of an orthomode transducer with two guided accesses 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 ports 202 and 203 of a two-branch orthomode transducer, and short-circuited waveguides (in English stub ) 232 and 233 configured to filter the signals. The splitting 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 setting making it possible to cancel the portion of parasitic energy linked to bad 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 setting in phase quadrature of the cross component relative to the principal 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 2f . The accesses are always injected orthogonally on the waveguide 201, but are shifted in the axis zz' of the source. This waveguide makes it possible to achieve high levels of decoupling, around -50 dB, but is bulky.

It is also known from the state of the art for orthomode transducers with four branches, making it possible to achieve greater decouplings than those with two branches. Such an orthomode transducer is shown in picture 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 ports 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.

There figure 3b describes the principle of signal polarization in an orthomode transducer with four branches.

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. Similarly, the second signal, intended to be polarized horizontally, is injected in a synchronized manner and in phase on the main waveguide 301 from the guided ports 304 and 306, opposite with respect to the main waveguide 301. Again , the two injected signals recombine constructively, and the signal propagates in the main waveguide in the propagation mode TE01.

The symmetry of the four-pronged orthomode transducer causes the electric field lines to be straighter than in a two-pronged transducer.

As in the waveguide with two ports, 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. Similarly, 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 horizontally polarized. The signals being injected in phase from the guided ports 303 and 305, the electric field 310 and the electric field 311 of the residues of these signals transmitted in the guide 302 then find themselves in phase opposition (180°). Their recombination is done in a destructive manner, and the residues of the signals injected by the guided accesses 303 and 305 being found in the guided access 302 vanish. The principle is the same in each of the waveguides 302 to 305.

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

However, generating two identical and in-phase signals 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. Furthermore, 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 interleaved, and require a large number of elementary connection components, thus increasing the dispersion. In the end, the performances obtained are limited and the ohmic losses high, 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 implement 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 transmitting a first signal and a second signal in orthogonal propagation modes. The orthomode transducer includes:

• a main waveguide with a square or rectangular section,
• two guided ports having on the one hand a free end via 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 offset 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 on 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 according to 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 among 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, the side ports of which are connected to a common pair of 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 described also relates to a device making it possible to transmit the signals according to orthogonal circular polarizations. He understands :

• an orthomode transducer as previously described, 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 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 better on reading the following description, given without limitation, and thanks to the appended figures which follow, given by way of example, among which:

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

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

Detailed Description :

Although they have good decoupling performance, the four-branch orthomode transducers of the state of the art are complex to implement 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 is oriented perpendicular to the metal walls of the waveguide.

There figure 4a roughly represents the direction of the electric field in the corner 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 according to the distance to the two walls.

The invention proposes to inject the signals not by accesses centered on the sides of the cavity of the main waveguide of the orthomode transducer, but by off-center accesses located on the edges of one or more sides of this waveguide. main wave. With a single off-center injection point, the mode of propagation in the waveguide is not controlled 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-centre ports on one or more sides of the main waveguide, and this symmetrically with respect to an axis of symmetry of the main waveguide. There figure 4b schematically represents the physical principles applying during the injection of a signal by two ports located on the edges of the same side of the main waveguide.

There 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 that this signal propagates according to the TE10 (vertical linear) mode. The solid arrows represent the orientation of the electric field. The signal injected on the guided access 410 is separated into two signals of the same power by a junction 411 acting as a 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, carrying out the division of the signal into two signals in phase opposition and of the same power. The arms of each ports 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 corners of the main waveguide (represented by the solid arrows) are not vertical in the corners. However, the vector recombination of these two injections gives the desired electric field, here a perfectly vertically polarized electric field.

The junction 411 can also be an H-plane T microwave junction, realizing the division of 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 polarized horizontally (mode TE01, horizontal linear). The type of junction is therefore chosen according to the mode of propagation sought in the main waveguide.

By injecting the same signal, in phase or in phase opposition, through two off-center and symmetrical ports in the main waveguide of an orthomode transducer, it is therefore possible to "force" the propagation mode of the wave electromagnetic. In the example of the 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 signals in phase opposition to vertically polarize the signal, or two in-phase signals 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 consists in arranging 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 two-arm guided access in an orthomode transducer according to the invention. THE figures 4c, 4d and 4e 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 an E-plane T, 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 relative to the yy' axis. Therefore, the propagation mode in the main waveguide is the TE01 mode, ie 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 a vertical linear polarization.

In the 4d figure , the two arms are connected to off-center accesses located on two opposite sides of the main waveguide 401. The accesses 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 polarized horizontally (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, since the junction slots between the access arms and the central waveguide create a magnetic coupling (field H), their positioning in the angles optimizing the effectiveness of this coupling.

There figure 5a represents an embodiment of an orthomode transducer with two branches according to the invention. The transducer is configured to output 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 the same way 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 can for example be found. It is suitable for the propagation of signals according to the two fundamental modes TE10 and TE01 in the frequency band(s) considered. There 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 the signal to be transmitted in vertical polarization is injected, a junction 512 configured to dividing the first signal into two identical signals, of the same power, and in phase opposition, such as a plane T junction E, 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 in an off-center and symmetrical manner 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 (whose electromagnetic field is represented by solid arrows in the figure) according to a fundamental mode in the frequency band considered. They can be connected with the main waveguide 501 through irises carrying out 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 comprises 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 (whose electromagnetic field 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 propagation mode 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 ports 510 and 520, to combine two signals with the desired cross polarizations in the main waveguide 501.

There figure 5b represents the electric field of the signal injected on the port 510 of a two-armed orthomode transducer according to an 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 intensity and direction of the electric field.

The electric field in the port 510 evolves 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, i.e. vertically polarized . It is observed that the electric field is oriented much more precisely than in a two-port orthomode transducer shown in Fig. 2d figure , due to the symmetry of the accesses to two branches: the decoupling between the polarizations is therefore greater.

Part of the energy injected from the guided port 510 propagates in the arms 523 and 524 of the guided port 520, where the electric field rotates to find itself oriented horizontally. The in-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 port 521. Consequently, they cancel each other out, and the residues of the signal emitted by the guided port 510 and received in the junction 522 naturally vanish in the waveguide 521. There are therefore no or few coupling effects due to the residues of a signal in the guided access of the cross-polarization signal.

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

Thanks to the properties of symmetry of the off-centre accesses, the orthomode transducer according to the invention as represented in figure 5a makes it possible to improve decoupling performance by a few dB compared to two-arm orthomode transducers such as the one shown in figure 2a , by generating perfectly linear electric fields, and by construction blocking the propagation of the signal from one guided access to the other. In addition, this orthomode transducer is wider band than the two-arm orthomode transducers of the state of the art because its symmetry properties make it build 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.

There figure 5c is a three-dimensional representation of an orthomode transducer according to one embodiment of the invention. Here you can find the guide main waveform 501, to which are connected a first port 510, for injecting the transmitted signal according to one polarization, and a second port 520, for injecting the transmitted signal according to the crossed polarization.

This device has the advantage of being particularly simple and of occupying a volume reduced by almost 75% compared to orthomode transducers with four branches connected two by two such as that of the picture 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 highly appreciated for the production of array antennas embedded in 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 axis zz') remains free. It is therefore possible to add further 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 represented 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 represented in 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 at the figure 6a . This embodiment always involves a guide main wave 601, but the guided accesses of the two cross-polarized signals are injected by the same pair of arms.

For this, the orthomode transducer comprises a device known to those skilled in the art, called magic T junction (in English magic T ) . A magic T-junction is a three-dimensional, four-port microwave component: two side ports, a sum port, and a difference port. It jointly performs the T-junction function plane E and plane H, 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 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 be able to share the arms between the two ports guided in orthogonal polarizations. The positioning of the accesses 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 effects of coupling between the two signals with crossed polarizations.

The waveguide according to the embodiment presented in figure 6a makes it possible to obtain very high levels of decoupling, 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. There figure 6b represents 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 high-performance 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 ports to the main waveguide. However, the invention applies identically for the extraction of signals from the main waveguide to the two guided ports. 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 crossed polarization.

Claims (9)

1. An orthomode transducer for transmitting a first signal and a second signal in orthogonal propagation modes, comprising:

   - a primary waveguide (501, 601) with a square or rectangular cross section, and characterised in that it comprises:

   - two guided access means (510, 520) having, on the one hand, a free end via which the first signal and the second signal are respectively injected or recovered, and, on the other hand, two arms connected to the primary waveguide,

   and wherein each guided access means comprises a junction (512, 522, 602) configured to connect the free end to the two arms of the guided access means, the two arms (513, 514, 523, 524, 610, 611) of each guided access means being connected to the primary waveguide at two off-centred locations on one or more sides of the primary waveguide, the two locations being symmetrical with respect to an axis of symmetry of the primary waveguide.
2. The orthomode transducer according to claim 1, wherein the connection between the primary waveguide and the two arms of a guided access means comprises the two corners of one and the same side of the primary waveguide.
3. The orthomode transducer according to one of the preceding claims, wherein the junction (512, 522, 602) of each guided access means is configured such that the signals transmitted on the pair of arms of a guided access means are in phase or in phase opposition depending on their propagation mode in the primary waveguide.
4. The orthomode transducer according to one of the preceding claims, wherein the two arms of the same guided access means have substantially identical dimensions.
5. The orthomode transducer according to one of the preceding claims, wherein the guided access means are arranged symmetrically with respect to an axis of symmetry of the primary waveguide.
6. The orthomode transducer according to one of the preceding claims, wherein each guided access means comprises a particular junction (512, 522) chosen from among an E-plane T-junction and an H-plane T-junction, and two particular arms (513, 514, 523, 524).
7. The orthomode transducer according to one of claims 1 to 5, wherein the two guided access means comprise one and the same junction (602) in the form of a magic-T junction whose lateral ports are connected to a common pair of arms (610, 611), the first and second signals being transmitted via two separate ports (603, 604) of the magic-T junction.
8. A 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 access means of the orthomode transducer so as to circularly polarise the first and second signals.
9. A 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.

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