EP2202839A1 - Compact feed system for the generation of circular polarisation in an antenna and method of producing such system - Google Patents

Abstract

The compact excitation unit for generating a circular polarization in an antenna comprises a diplexant orthomode transducer and a branch coupler, and is characterized in that the orthomode transducer (21), called OMT, is asymmetrical and comprises a main waveguide (22) with a square or circular cross-section of longitudinal axis ZZ 'and two branches coupled to the main waveguide (22) by two parallel coupling slots (25, 26) respectively, the two slots of coupling (25, 26) being formed in two orthogonal walls of the waveguide, the two branches of the OMT being respectively connected to two waveguides (35, 36) of an unbalanced branch coupler (40), the branch coupler (40) having two different partition coefficients (±, ²) and optimized to compensate for orthogonal spurious components ('y, x) of the electric field generated by the dissymmetry of the OMT (21). Application in particular to transmitting and / or receiving antennas such as, for example, multibeam antennas.

Description

The present invention relates to a compact excitation unit for generating a circular polarization in an antenna, to an antenna comprising such a compact excitation assembly and to a method for producing such a compact excitation assembly. It applies in particular to the field of transmitting and / or receiving antennas and more particularly to antennas comprising an array of elementary radiating elements connected to an orthomode transducer device associated with a coupler, such as for example multibeam antennas.

The development of a large number of contiguous beams involves producing an antenna comprising a large number of elementary radiating elements, placed in the focal plane of a parabolic reflector, the spacing of which depends directly on the angular difference between the beams. The volume allocated for the location of an RF radio frequency chain responsible for performing the circular bipolarization transmission and reception functions is limited by the radiative surface of a radiating element, in the case of a multibeam application.

In the most common configuration where each source, consisting of a radiating element coupled to a radiofrequency chain, produces a beam, also called a spot, each formed beam is emitted for example by a dedicated horn constituting the elementary radiating element and the radiofrequency chain performs, for each beam, transmission / reception functions in mono-polarization or bi-polarization in a frequency band chosen according to the needs of users and / or operators. Generally, a radiofrequency chain mainly comprises an exciter and waveguide paths, also called recombination circuits, for connecting the radio frequency components. To develop a circular polarization, it is known to use an exciter comprising an orthomode transducer known by the acronym OMT (in English: OrthoMode Transducer) connected to an elementary radiating element by example of cornet type. The OMT feeds the horn (in transmission), or is fed by the horn (in reception), selectively either with a first electromagnetic mode having a first polarization, or with a second electromagnetic mode having a second polarization orthogonal to the first. The first and second polarizations, to which two electric field components are associated, are linear and called respectively horizontal polarization H and vertical polarization V. Circular polarization is achieved by associating the OMT with a branch coupler (in English: branch line coupler ) responsible for placing the electric field components H and V in quadrature phase. The search for a compact solution leads to grouping the radio frequency components and the recombination circuits of the radiofrequency chain on several levels stacked one below the other, as represented for example on the Figures 1a and 1b described below. However, the higher the number of beams, the greater the complexity, the mass and the cost of the radiofrequency chain. To further reduce the weight and cost of a radio frequency chain, it is therefore necessary to modify its electrical architecture.

The object of the present invention is to remedy this problem by proposing a new excitation unit operating in bi-polarization, not requiring adjustment and making it possible to simplify and make more compact the radiofrequency chain and thereby reduce its mass. and the cost.

For this, the invention relates to a compact excitation unit for generating a circular polarization in an antenna comprising a diplexant orthomode transducer and a branch coupler, characterized in that the orthomode transducer, called OMT, is asymmetrical and comprises a main waveguide with a square or circular cross section of longitudinal axis ZZ 'and two branches coupled to the main waveguide respectively by two parallel coupling slots, the two coupling slots being made in two orthogonal walls of the waveguide; wave, the two branches of the OMT being respectively connected to two waveguides of an unbalanced branch coupler, the branch coupler having two different partition coefficients and optimized to compensate for orthogonal spurious electric field components generated by the dissymmetry of the OMT.

Advantageously, the section of the main waveguide of the OMT downstream of the coupling slots is smaller than the section of the main waveguide of the OMT upstream of the coupling slots, the section breaking forming a plane of short circuit.

Advantageously, the coupling slots of the OMT, having a length L1 and a width L2, are connected to the branch coupler by means of two stub filters placed at a distance D1 from the coupling slots, the distance D1, the length L1 and width L2 are chosen so as to achieve an orthogonality between the parasitic components of the electric field generated by the dissymmetry of the OMT.

• α² + β² = 1
• $\mathrm{\alpha }\mathrm{.}\mathrm{Ex}\mathrm{-}\mathrm{\beta }\mathrm{.}\mathrm{\delta y}\mathrm{=}{}^{\mathrm{1}}{\mathrm{/}}_{\sqrt{\mathrm{2}}}\mathrm{volt}\mathrm{/}\mathrm{mètre}\mathrm{,}$
  
• $\mathrm{\beta }\mathrm{.}\mathrm{Ey}+\mathrm{\alpha }\mathrm{.}\mathrm{\delta x}\mathrm{=}{}^{\mathrm{1}}{\mathrm{/}}_{\sqrt{\mathrm{2}}}\mathrm{volt}\mathrm{/}\mathrm{mètre}\mathrm{.}$
  
  volt/mètre.

Advantageously, the partition coefficients of the branch coupler are determined from the following three relationships:

• α ² + β ² = 1
• $\mathrm{\alpha }\mathrm{.}\mathrm{Ex}\mathrm{-}\mathrm{\beta }\mathrm{.}\mathrm{.DELTA.Y}\mathrm{=}{}^{\mathrm{1}}{\mathrm{/}}_{\sqrt{\mathrm{2}}}\mathrm{volt}\mathrm{/}\mathrm{metre}\mathrm{,}$
  
• $\mathrm{\beta }\mathrm{.}\mathrm{Ey}+\mathrm{\alpha }\mathrm{.}\mathrm{.delta.x}\mathrm{=}{}^{\mathrm{1}}{\mathrm{/}}_{\sqrt{\mathrm{2}}}\mathrm{volt}\mathrm{/}\mathrm{metre}\mathrm{.}$
  
  voltmeter.

The invention also relates to an antenna characterized in that it comprises at least one such compact excitation unit.

Finally, the invention also relates to a method for producing a compact excitation unit for generating a circular polarization in an antenna, characterized in that it consists in coupling an asymmetric OMT orthomode transducer to two branches with an unbalanced branch coupler having two different partition coefficients, to size the OMT so as to establish a phase quadrature between two components electrical field sparks generated by the dissymmetry of the OMT, and optimize the partition coefficients of the branch coupler to compensate for the two parasitic components of the electric field.

Advantageously, the dimensioning of the OMT consists in determining a length L1 of the coupling slots of the OMT, in determining a distance D1 separating the coupling slots of two stub filters placed between the coupling slots and the branch coupler. placing a short-circuit plane in the main waveguide of the OMT downstream of the coupling slots, the distance D1, the length L1 and the width L2 being chosen so as to achieve orthogonality between the parasitic components of electric field generated by the dissymmetry of the OMT.

• α² + β² = 1
• $\mathrm{\alpha }\mathrm{.}\mathrm{Ex}\mathrm{-}\mathrm{\beta }\mathrm{.}\mathrm{\delta y}\mathrm{=}{}^{\mathrm{1}}{\mathrm{/}}_{\sqrt{\mathrm{2}}}\mathrm{volt}\mathrm{/}\mathrm{mètre}\mathrm{,}$
  
• $\mathrm{\beta }\mathrm{.}\mathrm{Ey}+\mathrm{\alpha }\mathrm{.}\mathrm{\delta x}\mathrm{=}{}^{\mathrm{1}}{\mathrm{/}}_{\sqrt{\mathrm{2}}}\mathrm{volt}\mathrm{/}\mathrm{mètre}\mathrm{.}$
  

Advantageously, the partition coefficients of the branch coupler are determined from the following three relationships:

• α ² + β ² = 1
• $\mathrm{\alpha }\mathrm{.}\mathrm{Ex}\mathrm{-}\mathrm{\beta }\mathrm{.}\mathrm{.DELTA.Y}\mathrm{=}{}^{\mathrm{1}}{\mathrm{/}}_{\sqrt{\mathrm{2}}}\mathrm{volt}\mathrm{/}\mathrm{metre}\mathrm{,}$
  
• $\mathrm{\beta }\mathrm{.}\mathrm{Ey}+\mathrm{\alpha }\mathrm{.}\mathrm{.delta.x}\mathrm{=}{}^{\mathrm{1}}{\mathrm{/}}_{\sqrt{\mathrm{2}}}\mathrm{volt}\mathrm{/}\mathrm{metre}\mathrm{.}$
  

• figure 1a: un schéma en vue de dessus d'un exemple d'OMT diplexant, selon l'art antérieur ;
• figure 1b : une vue en perspective d'un exemple de chaîne RF comportant un OMT diplexant de la figure 1 a;
• figure 2 : une vue en coupe d'un exemple d'architecture simplifiée d'une chaine RF comportant un ensemble d'excitation compact, selon l'invention;
• figures 3a et 3b : deux vues, respectivement en perspective et en vue de dessus, d'un exemple d'OMT diplexant dissymétrique, selon l'invention;
• figure 4: un exemple de couplage entre les deux ports couplé et isolé obtenu avec un OMT dissymétrique avant optimisation de la forme de l'OMT, selon l'invention ;
• figure 5 : un exemple de dispersion de phase entre les ports couplé et isolé d'un OMT avant optimisation de la forme de l'OMT, selon l'invention ;
• figure 6 : un exemple de dispersion de phase entre les ports couplé et isolé d'un OMT après optimisation des paramètres de forme de l'OMT, selon l'invention ;
• figure 7 : une vue schématique de dessus de l'OMT montrant les composantes de champ parasites après optimisation des paramètres de forme de l'OMT, selon l'invention.
• figures 8a et 8b : une vue en perspective et une vue en coupe longitudinale, d'un exemple de coupleur à branches déséquilibré, selon l'invention ;
• figures 9a et 9b : un exemple montrant le taux d'ellipticité obtenu en associant un OMT à deux branches et un coupleur à branches déséquilibré pour former un ensemble d'excitation compact, selon l'invention.

Other features and advantages of the invention will become clear in the following description given by way of purely illustrative and non-limiting example, with reference to the attached schematic drawings which represent:

• figure 1a : a diagram in top view of an example of OMT diplexant, according to the prior art;
• figure 1b : a perspective view of an exemplary RF chain including a diplexant OMT of the figure 1 at;
• figure 2 : a sectional view of an example of simplified architecture of an RF chain comprising a compact excitation assembly, according to the invention;
• Figures 3a and 3b two views, respectively in perspective and in top view, of an example of OMT diplexant asymmetrical, according to the invention;
• figure 4 an example of coupling between the two coupled and isolated ports obtained with an asymmetrical OMT before optimizing the shape of the OMT, according to the invention;
• figure 5 an example of phase dispersion between the coupled and isolated ports of an OMT before optimizing the form of the OMT, according to the invention;
• figure 6 an example of phase dispersion between the coupled and isolated ports of an OMT after optimization of the shape parameters of the OMT, according to the invention;
• figure 7 : a schematic view from above of the OMT showing the parasitic field components after optimization of the shape parameters of the OMT, according to the invention.
• Figures 8a and 8b : a perspective view and a longitudinal sectional view of an example of unbalanced branch coupler according to the invention;
• Figures 9a and 9b an example showing the ellipticity ratio obtained by combining a two-branch OMT and an unbalanced branch coupler to form a compact excitation unit according to the invention.

The orthomode transducer 5 with four branches shown in FIG. figure 1a comprises a main waveguide 10 having a longitudinal axis ZZ ', with a square or circular cross-section for example, having a first end intended to be connected to a horn, not shown, and a second outlet end, the two ends being situated in the longitudinal axis of the body of the main waveguide. A group of four parallel, or parallel, longitudinal slots 11, 12, 13, 14 are formed in the wall of each of the four lateral faces of the main waveguide and arranged diametrically opposite in pairs. Between the horn and the coupling slots, the dimensions of the main waveguide 10 are adapted to the propagation of the fundamental electromagnetic modes associated with the H and V field components of the main waveguide in the transmit and transmit frequency bands. reception. Beyond the coupling slots, the section of the main waveguide decreases which generates a short circuit plane for the low frequency band. At the cut-off frequency, the waveguide behaves like a high-pass filter that passes only the high frequency band. The H and V field components associated with the fundamental electromagnetic modes TE01 and TE10 of the square section waveguide, or the TE11H and TE11 V modes of the circular section waveguide, are coupled in the low frequency band by for example, the transmission band, by the four parallel coupling slots 11, 12, 13, 14. The high frequency band, for example the reception band, is rejected by four stub filters 15, 16, 17, 18 connected to the four parallel access slots and propagates in the main waveguide to its output end. The OMT and filters unit, called OMT diplexant, thus has six physical ports and its operation is compatible with an application in linear polarization or circular polarization. The low frequency band may, for example, be reserved for the transmission of RF radio frequency signals and the high frequency band may be reserved for receiving the RF signals. As shown on the figure 1b , on transmission, the development of a circular polarization is provided by a 3 dB balanced branch coupler 19 which supplies the four coupling slots 11, 12, 13, 14 in pairs in quadrature phase. Opposite slots are phase-fed via in-phase recombination circuits. The different components of the excitation assembly consisting of the diplexant OMT and the branch coupler are optimized separately and the overall transfer function results from the intrinsic performance of each component. The geometry of the four-pointed OMT 5 imposes, at the location of the coupling slots, a plane of symmetry to the electric field which propagates in the OMT which minimizes the amplitude of the cross-components of the electric field. Thus the circular polarization purity does not depend on the OMT 5 but only the branch coupler 19 and recombination circuits 20 which perform power sharing and phase quadrature between the coupling slots. A septum polarizer, not shown, is connected to the output end of the main waveguide of the OMT, the septum polarizer realizing the development of circular polarization on reception.

The radio frequency components and recombination circuits of the radio frequency chain are stacked on several levels, two levels 1, 2 are represented on the figure 1b but there are usually three, arranged one below the other. The integration of the components is then maximal and to further reduce the mass, the volume and the cost of the radiofrequency chain, it is necessary to modify its architecture.

The figure 2 represents an example of a simplified architecture of an RF chain comprising a compact excitation unit, according to the invention. The RF chain essentially comprises a diplexant orthomode transducer 21 with two branches represented on the Figures 3a and 3b and an unbalanced branch coupler 40. The OMT 21 comprises a main waveguide 22, for example of square or circular section, and of longitudinal axis ZZ ', comprising two ends 23, 24, the first end 23 coupled to a circular access 31 being intended to be connected to a horn, not shown, and having two parallel access coupling slots 25, 26 formed in the wall of the main waveguide and opening into the respective two branches of the OMT. The two parallel access slots 25, 26 are formed in two orthogonal sidewalls of the main waveguide and arranged, for example and preferably at the same height relative to the two ends 23, 24 of the waveguide main. The low frequency band may for example be reserved for the transmission of RF signals and the high frequency band may be reserved for receiving the RF signals. On transmission, each of the two coupling slots 25, 26 is connected to the branch coupler 22 via a stub filter 27, 28 and recombination circuits 29, 30. The circular access 31 constitutes the common input and output port with two electric field components, respectively horizontal H and vertical V, corresponding to two orthogonally polarized electromagnetic modes propagating on transmission and reception. Each parallel access slot associated with a stub filter constitutes an input and output port of one of the electric field components, called a coupled port for this component, the other port being called an isolated port. For example, on the figure 3a the vertical electric field component H passes through the coupled port 32, the port 33 being the isolated port for this component H. For the vertical electric field component V, the coupled port is the port 33 and the isolated port is the port 32. The coupler 40 has two rectangular waveguides 35, 36 forming two main branches connected respectively by a first end at one of the OMT ports 32, 33, and at a second end at a respective power port 37, 38, the power ports 37, 38 having the same electrical length. Each supply port is connected to each of the two main branches 35, 36 of the branch coupler 40 to supply it with an electric field. The two main branches of the branch coupler are coupled together by means of coupling slots, not shown, opening into at least one transverse waveguide 39 constituting a transverse branch. The length of the transverse guides 39, in a predetermined number, for example equal to three on the figure 2 , is equal to λ _g / 4 so as to produce, at the output of the branch coupler 40, a phase shift of 90 ° between the two electric field components, λ _g being the guided wavelength of the fundamental mode propagating in the main branches 35, 36 of the coupler 40.

On reception, a septum polarizer, not shown, may be connected to the second end 24 of the main waveguide of the OMT.

From a geometrical point of view, the diplexant OMT with two branches does not allow the natural decoupling of the horizontal and vertical electric field components V due to the absence of symmetry at the location of the coupling slots 25, 26 The analysis of the parameters of the energy dispersion matrix between the common port 31 and the coupled port 32 corresponding to one of the components of the electric field, then between the common port and the isolated port 33 of the same component of the electric field shows, as shown on the Figures 4 and 5 , that there is an energy coupling, of the order of -20 dB, between the coupled port and the isolated port and that there is a frequency-dispersive phase difference between the two ports, the phase quadrature being obtained only for a particular frequency, although physically the lengths from the common port 31 to the two coupled and isolated ports 32, 33 are identical. This means that, because of the dissymmetry of the OMT, the fundamental mode energy propagating in the main waveguide does not pass entirely into the coupled port but partly to the isolated port. The power distribution between the two ports is due to the fact that in addition to the coupling of the TE10 fundamental mode at -20 dB, there is a -20 dB coupling of the TE20 mode (or TE02 depending on whether the component is considered H or V of the electric field) between the coupled port and isolated port. The TE20 (or TE02) mode interferes with the power sharing and induces a phase insertion different from the electric field on the port coupled to the isolated port.

According to the invention, the two-branched OMT does not allow to completely decouple the two components of the electric field when it is associated with a 3 dB balanced branch coupler which realizes the sharing of power in equal parts and the quadrature of phase between the coupling slots, it is not possible to obtain a circular polarization. The polarization obtained is elliptic, with an ellipticity ratio of the radiated field equal to 1.7 dB.

However, by acting on the shape parameters of the OMT such as the length L1 and the width L2 of the coupling slots 25, 26, the distance between the slot and the short circuit plane for the low frequency band corresponding to the changes of section of the main guide, the distance D1 between the slots 25, 26 and the beginning of the stub filters 27, 28, it is possible, as shown in the example of the figure 6 , to put the field component on the isolated port in phase quadrature with the field component on the coupled port and to make the differential phase behavior between these two coupled and isolated aperiodic field components over a bandwidth greater than 7% of the low total frequency band. The distance D1 acts on the frequency dispersion of the phase of the main field component on the coupled port relative to the parasitic cross-field component on the isolated port. The length L1 and the width L2 make it possible to adjust the absolute phase at -90 ° between the field component on the coupled port and the parasitic field component on the isolated port. The distance between the slot and the short circuit plane may for example be zero. However, the optimization of the OMT shape parameters is a multivariate optimization for which other parameters act in the second order, creating for example energy beats between radiofrequency discontinuities, and that it does not It is possible to optimize only by successive iterations and by an analysis of the electromagnetic modes that propagate.

The figure 7 shows that the electric field resulting from a supply on the port of access 32, 33 of the horizontal polarization H, respectively vertical V, is then breaks down into two out of phase components of -90 °. Thus, for the access port 33 of the vertical component V of the electric field Ey is added a parasitic horizontal component δy that is out of phase by -90 ° with respect to Ey and for the access port 32 of the horizontal component H of the field Ex electric is added a parasitic vertical component δx phase-shifted by -90 ° with respect to Ex. The spurious components δy and δx are attenuated by 20 dB with respect to the amplitude of Ex and Ey.

The asymmetrical OMT, according to the invention, associated with an unbalanced branched coupler, allows the compensation of the defect induced by the dissymmetry of the OMT and an operation of the antenna in mono-polarization and bi-polarization with excellent purity of polarization.

chargés de répartir l'énergie du champ électrique appliqué sur l'un de ses ports 1 ou 4 entre les ports 2 ou 3, avec un déphasage de 90° en valeur absolue entre les ports 2 et 3. Ainsi lorsqu'un champ électrique est appliqué sur le port 1, il se propage dans la branche du coupleur reliée au port 1 jusqu'au port 2 avec un coefficient de couplage α et se propage en diagonale, en traversant les fentes de couplage et les différents guides transversaux, jusqu'au port 3 avec le coefficient de couplage β. Le retard de phase de 90° entre les deux composantes de champ électrique en sortie du coupleur à branches sur les ports 2 et 3 correspond aux longueurs des guides transversaux égales à un quart de la longueur d'onde λ_g/4. Les guides transversaux ont des longueurs identiques mais des largeurs différentes. Le nombre de branches transversales est choisi en fonction du besoin en bande passante. Les largeurs des branches transversales sont définies en fonction des valeurs de coefficient de couplage α et β à réaliser. Réciproquement, lorsqu'un champ électrique est appliqué sur le port 4, il se propage dans la branche principale du coupleur reliée au port 4 jusqu'au port 3 avec un coefficient de couplage α et se propage en diagonale en traversant les fentes de couplage et les différents guides transversaux, jusqu'au port 2 avec le coefficient de couplage β et un déphasage de -90°.To have good circular polarization purity, the H and V components of the electric field must have the same amplitude and be in phase quadrature. The Figures 8a and 8b show a perspective view and a longitudinal sectional view of an example of unbalanced branch coupler 40, according to the invention. The branch coupler 40 has four ports 1 to 4 located at the four ends of the two main branches. Ports 1 and 4 are intended to be connected to the two power ports, the two ports 2 and 3 are respectively intended to be connected to the ports coupled and isolated from the OMT. The branch coupler has two partition coefficients α and β, with $\mathrm{\beta }\mathrm{=}\sqrt{\mathrm{1}\mathrm{-}{\mathrm{<figure-callout\; id="2"\; label="\alpha "\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">\alpha </figure-callout>}}^{\mathrm{2}}}\mathrm{,}$

charged to distribute the energy of the applied electric field on one of its ports 1 or 4 between the ports 2 or 3, with a phase shift of 90 ° in absolute value between the ports 2 and 3. Thus when an electric field is applied on the port 1, it propagates in the branch of the coupler connected to the port 1 to the port 2 with a coupling coefficient α and propagates diagonally, crossing the coupling slots and the various transverse guides, until port 3 with the coupling coefficient β. The phase delay of 90 ° between the two electric field components at the output of the branch coupler on the ports 2 and 3 corresponds to the lengths of the transversal guides equal to a quarter of the wavelength λ _g / 4. The transverse guides have identical lengths but different widths. The number of transverse branches is chosen according to the bandwidth requirement. The widths of the transverse branches are defined according to the values of the coupling coefficient α and β to be produced. Conversely, when an electric field is applied on port 4, it propagates in the main branch of the coupler connected to port 4 to port 3 with a coupling coefficient α and propagates diagonally across the coupling slots and the different transverse guides, up to port 2 with the coupling coefficient β and a phase shift of -90 °.

According to the invention, the sharing coefficients α and β are chosen so as to compensate for the parasitic defect related to the dissymmetry of the OMT. Thus the α and β coefficients will no longer be equal as is the case in the balanced couplers usually used with a four-branch OMT, but will be different.

The partition coefficients are optimized in the presence of the OMT and compensate the horizontal and vertical parasitic components δy and δx so as to obtain on each port 2 and 3 of output, half of the power received on the input port 1.

Since the operation of the coupler is symmetrical in reception and transmission, the optimization of the sharing coefficients can be performed in reception, so as to compensate for the horizontal and vertical parasitic components δy and δx related to the dissymmetry of the OMT.

Thus, in reception, at the crossing of the coupler, the field components entering on the port 2, Ex and δy.e ^{-j90 °} respectively become, in output on the port 1: α.Ex and α.δx.e ^{-j90 °} .

Similarly, the input field components on port 3, Ey and δy.e ^{-j90 °} , become respectively output on port 1: β.Ey.e ^{-j90 °} and β.δy.e ^{-j180 °} .

• Suivant l'axe X : α.β.Ex +β.δy.e^-j180°
• Suivant l'axe Y : β.Ey.e^-j90° et α.δy.e^-j90°

The projections of these field components along the orthogonal axes X and Y are then as follows:

• Along the X axis: α.β.Ex + β.δy.e ^{-j180 °}
• Along the Y axis: β.Ey.e ^{-j90 °} and α.δy.e ^{-j90 °}

• α² + β² = 1
• $$\mathrm{\alpha }\mathrm{.}\mathrm{Ex}-\mathrm{\beta }\mathrm{.}\mathrm{\delta y}=1/\sqrt{2}\mathrm{volt}/\mathrm{métre},$$
  
  ce qui correspond à -3dB en puissance
• $$\mathrm{\beta }\mathrm{.}\mathrm{Ey}+\mathrm{\alpha }\mathrm{.}\mathrm{\delta x}=1/\sqrt{2}\mathrm{volt}/\mathrm{métre},$$
  
  ce qui correspond à -3dB en puissance

Along the X axis the field components Ex and δy are in phase opposition and the compensation is destructive. Along the Y axis, the Ey and δx field components are in phase and the compensation is constructive. For the compensation to obtain on each output port 2 and 3, half of the power received on the input port 1, the sharing coefficients α and β are such that the following three relationships are respected:

• α ² + β ² = 1
• $$\mathrm{\alpha }\mathrm{.}\mathrm{Ex}-\mathrm{\beta }\mathrm{.}\mathrm{.DELTA.Y}=1/\sqrt{2}\mathrm{volt}/\mathrm{metre},$$
  
  which corresponds to -3dB in power
• $$\mathrm{\beta }\mathrm{.}\mathrm{Ey}+\mathrm{\alpha }\mathrm{.}\mathrm{.delta.x}=1/\sqrt{2}\mathrm{volt}/\mathrm{metre},$$
  
  which corresponds to -3dB in power

The Figures 9a and 9b show that the ellipticity rate obtained by combining a two-branch OMT and an unbalanced branch coupler according to the invention, is less than 0.1 dB on the Ka band between 19.7GHz and 20.2 GHz. The ellipticity rate is less than 0.4 dB on 1.5 GHz bandwidth, which allows a use of this structure for a mission users but also for other applications regardless of the frequency bands.
The new architecture has the advantages of being very compact, the bulk of the sources, consisting of the RF chain and the horn of emission and reception, thus realized is of 60mm of diameter and 100mm of height. For comparison, an equivalent source assembly according to the prior art has a footprint of 150mm in height and 72mm in diameter. The cost of implementation is optimal compared to the number of components. Indeed, reducing the number of mechanical parts allows a gain in preparation time. The mass of the RF chain out of the horn is reduced by 60%. The structure is simplified and the number of electric layers is reduced to one instead of three since the OMT, the branch coupler and the recombination circuits are on the same level. The length of the guide paths is reduced by 50%, which allows a reduction in ohmic losses of 0.1 dB compared to the prior art with four-branched OMT whose ohmic losses were 0.25 dB.

Although the invention has been described in connection with a particular embodiment, it is obvious that it is not limited thereto and that it comprises all the technical equivalents of the means described and their combinations if they are within the scope of the invention.

Claims (8)

Compact excitation unit for generating a circular polarization in an antenna comprising a diplexant orthomode transducer and a branch coupler, characterized in that the orthomode transducer (21), called OMT, is asymmetrical and comprises a waveguide main section (22) having a square or circular cross-section of longitudinal axis ZZ 'and two branches coupled to the main waveguide (22) by respectively two parallel coupling slots (25, 26), the two coupling slots (25, 26) being made in two orthogonal walls of the waveguide, the two branches of the OMT being respectively connected to two waveguides (35, 36) of an unbalanced branch coupler (40), the branched coupler (40) having two different partition coefficients (α, β) and optimized to compensate for orthogonal spurious components (δy, δx) of the electric field generated by the dissymmetry of the OMT (21). Excitation assembly according to claim 1, characterized in that the section of the main waveguide (22) of the OMT downstream of the coupling slots (25, 26) is smaller than the section of the main waveguide (22) of the OMT upstream of the coupling slots (25, 26), the section breaking forming a short circuit plane. Excitation assembly according to one of claims 1 or 2, characterized in that the coupling slots (25, 26) of the OMT (21) having a length L1 and a width L2 are connected to the branch coupler (40) via two stub filters (27, 28) located at a distance D1 from the coupling slots (25, 26) and in that the distance D1, the length L1, the width L2 are selected so to achieve an orthogonality between the parasitic components (δy, δx) of the electric field generated by the dissymmetry of the OMT. Excitation assembly according to one of the preceding claims, characterized in that the partition coefficients (α, β) of the branch coupler (40) are determined from the following three relationships: - α ² + β ² = 1 - $\mathrm{\alpha }\mathrm{.}\mathrm{Ex}\mathrm{-}\mathrm{\beta }\mathrm{.}\mathrm{.DELTA.Y}\mathrm{=}{}^{\mathrm{1}}{\mathrm{/}}_{\sqrt{\mathrm{2}}}\mathrm{volt}\mathrm{/}\mathrm{metre}\mathrm{,}$

- $\mathrm{\beta }\mathrm{.}\mathrm{Ey}+\mathrm{\alpha }\mathrm{.}\mathrm{.delta.x}\mathrm{=}{}^{\mathrm{1}}{\mathrm{/}}_{\sqrt{\mathrm{2}}}\mathrm{volt}\mathrm{/}\mathrm{metre}\mathrm{.}$

Antenna characterized in that it comprises at least one compact excitation unit according to one of the preceding claims. A method of producing a compact excitation unit for the generation of a circular polarization in an antenna, characterized in that it consists in coupling an asymmetric OMT orthomode transducer (21) with two branches, respectively by two slots ( 25, 26), with an unbalanced branch coupler (40) having two partition coefficients (α, β) different, to size the OMT (21) so as to establish a phase quadrature between two parasitic components (δy, δx) of electric field generated by the dissymmetry of the OMT, and to optimize the partition coefficients (α, β) of the branch coupler (40) to compensate for the two spurious electric field components (δy, δx) . A method according to claim 6, characterized in that the dimensioning of the OMT consists of determining a length L1 and a width L2 of the coupling slots (25, 26) of the OMT (21), to place a short plane of circuit in the main waveguide of the OMT downstream of the coupling slots, determining a distance D1 separating the coupling slots (25, 26) of two stub filters (27, 28) placed between the coupling slots and the branch coupler (40), the distance D1, the length L1 and the width L2 being chosen so as to achieve an orthogonality between the components parasitic (δy, δx) electric field generated by dissymmetry of the OMT. Method according to one of Claims 6 or 7, characterized in that the partition coefficients (α, β) of the branch coupler (40) are determined from the following three relationships: - α ² + β ² = 1 - $\mathrm{\alpha }\mathrm{.}\mathrm{Ex}\mathrm{-}\mathrm{\beta }\mathrm{.}\mathrm{.DELTA.Y}\mathrm{=}{}^{\mathrm{1}}{\mathrm{/}}_{\sqrt{\mathrm{2}}}\mathrm{volt}\mathrm{/}\mathrm{metre}\mathrm{,}$

- $\mathrm{\beta }\mathrm{.}\mathrm{Ey}+\mathrm{\alpha }\mathrm{.}\mathrm{.delta.x}\mathrm{=}{}^{\mathrm{1}}{\mathrm{/}}_{\sqrt{\mathrm{2}}}\mathrm{volt}\mathrm{/}\mathrm{metre}\mathrm{.}$

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