EP0108463B1 - Radiating element for cross-polarized microwave signals and planar antenna consisting of an array of such elements - Google Patents

Description

The invention relates to a radiating element of orthogonal linear polarized microwave signals, comprising a first plate, comprised between a said second plate and a said third plate, the faces of which face towards the external medium are conductive, and further comprising, arranged on one side and on the other side of the first plate along perpendicular axes, microstrip conductors, the ends of which are coupled with the external medium to each transport a linear polarization.

The invention also relates to a planar antenna comprising an array of such juxtaposed elements, and finds in particular an application in the field of reception of 12 gigahertz television signals retransmitted by means of satellites. It goes without saying that, given the reciprocity of an antenna, a receiving element (or an antenna composed of a network of receiving elements) is capable of operating as a transmitting element (as a transmitting antenna) without any modification of his characteristics. This remark remains valid without exception throughout the description which follows, and the words reception, receive, receiver may always be replaced by the words send, send, transmitter.

A planar antenna comprising such elements is described in the article "New wideband high-gain stripline planar array for 12 GHz satellite TV" by E. Rammos, published in the review Electronics letters, Volume 18, n ° 6, 18 March 1982, pages 252 and 253.

Another radiating element is known from the French document FR-A-2 408 921. This document describes a radiating element of microwave signals with low bandwidths, centered on two lengths, one corresponding to the frequency 1.2 GHz and the other at the frequency 1.5 GHz, these signals being moreover circularly polarized and the element consisting of three plates: a first dielectric plate on the faces of which microstrip lines are made on either side, a second plate on the external face of which circular and concentric slots are formed in a conductive layer, and a third plate whose external face is conductive and constitutes a ground plane.

The production of the microstrip lines of the radiating element according to the cited document is such that their ends are arranged along radii of the circular slots, and orthogonally to one another. The sections of microstrips extending these ends are then placed opposite one another, on either side of the first dielectric plate to form a 90 ° coupler. The microstrip conductor produced on the side closest to the ground plane is then connected to a coaxial line, while the microstrip conductor produced on the side closest to the circular slots is terminated by an impedance matching device.

This radiating element, although allowing the realization of a flat antenna, for the reception or the emission of microwave signals with circular polarizations is not proposed for the reception of television broadcasts relayed by artificial satellites. Indeed, the structure including concentric circular slots of the radiating element described in the cited document only allows operation in a narrow band around given frequencies.

In addition, in the radiating element considered as state of the art, the microstrip lines are produced on the faces of a thick dielectric plate whose dielectric properties must be excellent to avoid losses as well as possible.

With regard to the second and third plates, which are also dielectric, their quality also plays a role in the quality of the antenna.

The object of the invention is to propose a radiating element and an antenna (composed of a network of such elements) by means of which the problems posed by the application for the reception of television broadcasts relayed by satellites are resolved.

To this end, the invention relates to a radiating element as defined in the preamble, furthermore conforming to the characteristics of claim 1.

In the structure thus proposed, the use of transmission lines with suspended substrate and the possibility, mainly resulting from the use of such lines, to carry out an adaptation of the excitation probes by a different choice of their lengths according to the distance between these probes help to significantly increase the radiation characteristics. Furthermore, this structure allows a very simple mechanical production while allowing the planes in which the two excitation probes are located to be fairly widely spaced, which in particular allows the installation, in the layers, of grooves forming with the conductors. the transmission lines (this guidance in the air then makes it possible to use a dielectric of ordinary quality from the point of view of its microwave properties, without its losses becoming troublesome).

The invention relates, on the other hand, to a planar microwave antenna composed of a whole network of such elements and produced with similar characteristics.

• - la figure 1 montre un mode de réalisation de l'élément récepteur selon l'invention;
• - la figure 2 montre une disposition des sondes excitatrices permettant d'obtenir un gain élevé pour l'élément récepteur;
• - la figure 3 est une vue en coupe partielle suivant l'axe AA de la figure 1 et met en évidence la disposition des lignes de transmission selon la structure dite à substrat suspendu.

The features and advantages of the element and the antenna thus concerned will now appear more precisely in the description which follows and in the appended drawings, given by way of nonlimiting example and in which:

• - Figure 1 shows an embodiment of the receiver element according to the invention;
• - Figure 2 shows an arrangement of the excitation probes to obtain a high gain for the receiving element;
• - Figure 3 is a partial sectional view along the axis AA of Figure 1 and highlights the arrangement of the transmission lines according to the so-called suspended substrate structure.

This element comprises the following structure: on either side of a first layer 10, in which a first recess 11 (in this example, circular) of metallized interior surface is provided, a first transmission line 20 and a second transmission line 30 consisting of conductive strips 21 and 31 carried in the median plane of grooves 22 and 32 by a thin dielectric sheet 23 and 33 providing mechanical support for the conductors. The end of the central conductors of these suspended ribbon microwave transmission lines referenced 24 and 34 penetrates along two perpendicular axes inside the recesses, thus constituting two excitation probes which achieve coupling with the propagation medium allowing reception of the signals microwave; these two ends have a penetration length opposite the recess which is distinct, as specified below. The other end of each line constitutes its output, in the case of reception.

On the other side of the line 20 is provided a second layer 40 also comprising a second recess 41 of metallized interior surface located opposite the first recess 11, and similarly, on the other side of the line 30 is provided a third layer 50 with a third recess 51 of metallized interior surface located opposite the other two. This recess 51 is short-circuited in a plane parallel to the faces of the layers, at a distance from the line 30 of course less than the thickness of the layer 50, so as to constitute a single reflective plane for the received microwave signals. The element thus described behaves like a waveguide-line transition with a suspended substrate, in which the axis of the guide is perpendicular to the plane of the lines.

The first, second and third layers 10, 40 and 50 may be metallic, or else made of a dielectric material with metallization of the walls of the recesses 11, 41 and 51 which pass through them respectively. Furthermore, the diameter of the recesses must be both sufficiently small, relative to the wavelength associated with the frequency of the microwave signals, to avoid the appearance or attenuate the propagation of the undesirable higher modes and sufficiently large to allow the propagation of the main mode in the considered bandwidth. Finally, the recess 41 ends with a tapered flare 61 possibly covered with a polyurethane type screen, these arrangements helping to enhance the gain and improve the characteristics of the radiation.

The tests carried out with a receiving element having the structure which has just been described have led to studying the influence, on the performances obtained, of the end length of the lines 20 and 30 actually situated opposite the recesses 11,41, 51 aligned. These experimental measurements, essentially relating to the coupling between these ends of lines 20 and 30 and the propagation medium, that is to say the cavity formed by all of the aligned recesses, have led to an optimization of this coupling when said two ends, or excitation probes, have a different length. More precisely, for a determined length of one of the excitation probes, the distance from this probe to the single reflective plane (formed by the bottom of the layer 50) is sought which provides a satisfactory and if possible maximum adaptation in the frequency band concerned. (here substantially from 11.7 to 12.5 gigahertz): Figure 2 shows an example of arrangement of the two probes of different lengths.

It is thus possible to have tables of correspondence between the probe lengths and the distance to the reflector, giving the best possible adaptations. The distance between the probes is then fixed by the thickness of the layer 10 (chosen according to the electromechanical requirements imposed: mechanical production of the layer, installation on the one hand in the layers 10 and 40 and on the other hand in the layers 10 and 50 of the grooves of the transmission lines 20 and 30, ...), we look in such correspondence tables for two length values for which the associated values of distance to the single reflective plane differ from this value of the thickness of layer 10.

• -côté du carré égal à 0,31 À g soit ici 15 millimètres (la longueur d'onde λ, g étant celle dans la partie guide de l'élément récepteur) et rayon de courbure des sommets arrondis égal à 3 millimètres;
• -distance sonde de la ligne 20-plan réflecteur: 0,27 À g;
• -distance sonde de la ligne 30-plan réflecteur: 0,17 À g;
• - longueur d'extrémité de la sonde de la ligne 20 dépassant dans l'évidement: 0,12 À g;
• - longueur d'extrémité de la sonde de la ligne 30 dépassant dans l'évidement: 0,10 À g;
• - distance verticale entre ces deux sondes: 0,10 λ g;

(soit, à 12 gigahertz, 5 millimètres, ce qui est suffisant pour la mise en place, par usinage, des cannelures des lignes de transmission 20 et 30).In the context of tests carried out with square receiving elements with rounded tops, it was possible to obtain at the end of the transmission line, the end of the central conductor of which constitutes the excitation probe, a standing wave rate of less than 1 , 6 (which corresponds to transmission losses of less than 0.25 dB) under the following conditions:

• -side of the square equal to 0.31 λ g, here 15 millimeters (the wavelength λ, g being that in the guide part of the receiving element) and radius of curvature of the rounded vertices equal to 3 millimeters;
• -distance probe from the line 20-plane reflector: 0.27 g g;
• -distance probe from line 30-reflector plane: 0.17 g g;
• - end length of the probe of line 20 protruding into the recess: 0.12 À g;
• - end length of the probe of line 30 protruding into the recess: 0.10 g g;
• - vertical distance between these two probes: 0.10 λ g;

(or, at 12 gigahertz, 5 millimeters, which is sufficient for the installation, by machining, of the grooves of the transmission lines 20 and 30).

These values, corresponding as indicated above to the example of square elements with rounded vertices, are valid for a line impedance of approximately 70 ohms, with a width of the central conductors of 1.4 millimeters, in grooves of dimensions 2.5 x 1.8 millimeters.

For the establishment of lines 20 and 30 between layers 10 and 40 on the one hand, 10 and 50 on the other On the other hand, it will be noted that the grooves mentioned above, of rectangular shape in general, are known for example from FIG. 4 of the patent of the United States of America no. 3,587,110 issued on June 22, 1971 in the name of the company RCA Corporation, a figure the principle of which is reproduced in Figure 3 of this application (see also the article "Careful MIC design prevents waveguide modes", published in the review Microwaves, May 1977, p. 188 et seq., figure 1). It will also be noted that at the output of these lines 20 and 30, to allow the reconstruction of the signals with right circular polarization and with left circular polarization, a 3 dB hybrid coupler can be provided, with its two inputs connected respectively to the outputs of lines 20 and 30 and its two outputs providing said right and left circularly polarized signals. It is also possible, instead of the coupler, to provide a depolarizing structure in front of the receiving element. Finally, without a coupler or depolarizing structure, signals are obtained having two perpendicular linear polarizations.

Of course, the present invention is not limited to the receiving or radiating element described above, from which variants can moreover be proposed without thereby departing from the scope of the invention. In particular, the invention also relates to a planar microwave antenna which is composed of a whole network of such receiving elements, the conditions seen previously and relating to the diameter of the recesses then being supplemented by the fact that, for placing the elements in such a way satisfactory next to each other, this diameter must be sufficiently small (compared to the wavelength in vacuum associated with the frequency of the microwave signals) so that the distance between the elements can be less than said wavelength . It is indeed only with this last condition that the appearance of undesirable secondary lobes, called lobes of the network, is avoided.

The structure of this radiating or receiving antenna is quite similar to that of the radiating or receiving element, and everything that has been said above about it can be transposed as is in the case of the antenna , except for transmission lines. In fact, the antenna no longer comprises only two transmission lines leading from the receiving element to two output connections but, more precisely, two networks of microwave transmission lines, electrically independent like lines 20 and 30 and intended , like them, to ensure the transmission of the microwave signals received towards the electronic circuits external to the antenna. In this case, it is now at the output of these two networks that a 3 dB hybrid coupler can be provided (or, instead of the coupler, a depolarizing structure in front of the entire antenna) for the reconstruction of the polarized signals. left and right circular.

These networks are each composed, in a manner well known in numerous embodiments (see in particular the network structure represented in FIG. 1 of French patent application FR-A-2 050 408, of a succession of combination stages. If the antenna includes n radiating elements, the first n ends of each network serve, as already described for a single radiating element, for coupling with the propagation space of the signals to be received, while the opposite opposite end of each of the two networks, the point of convergence of all the transmission lines across the successive combination stages, is connected to the electronic reception circuits external to the antenna (and, for example, in the first place to one and the other of the two inputs of the 3 dB coupler which allows the reconstruction of signals with right and left circular polarizations).

An antenna produced in this way lends itself particularly well to a low-cost modular embodiment, in which elementary blocks forming subsets of radiating elements can be used in an appropriate number and by joined assembly for the constitution of antennas of dimensions , gain and directivity diagram well determined, either for example a symmetrical antenna of square shape, or more generally asymmetrical antennas, in particular of rectangular shape, having different radiation patterns in two orthogonal planes. This last characteristic is particularly interesting for the antennas of reception of television signals with 12 gigahertz retransmitted by satellites, since an opening to 3 dB lower than 2 ° is in this case essential only in the equatorial plane to separate the signals of two “distant” satellites, in this plane, of 3 ° (see the recommendations of the CCIR, Geneva, 1977).

It is clear, finally, that the application of the invention to the reception of 12 gigahertz television signals retransmitted by satellites is not limiting, although the antenna described is in fact mainly intended for coupling with one or more heads reception of such signals (an example of these reception heads is described in particular in the review “l'Onde Electrique”, Volume 62, N ° 3, March 1982, pages 39 and 40). On the one hand the invention is applicable to all kinds of purely terrestrial microwave transmission networks, and on the other hand the choice of an example of application at the frequency of 12 gigahertz is not exclusive of any other possible frequency operating in the microwave range, linked to the intended application.

Claims (11)

1. A transmitting or receiving element for orthogonal linearly polarised microwave frequency signals, containing a first layer (10) confined between a second layer (40) and a third layer (50) whose faces turned towards the external medium are conducting and also containing on both sides of the first layer along perpendicular axes stripline (21 and 31) whose ends (24 and 34) are coupled to the external medium for transmitting a linear polarisation, characterized in that it has a first cavity (11) provided in the first layer (10) facing the ends (24 and 34) of the conducting striplines and whose sides are conducting, a second cavity (41) provided in the second layer (40) connected with a horn-like cavity (61), facing the first cavity (11) and whose sides like those of the cone (61) are conducting, a third cavity (51) provided in the third layer (51) facing the first cavity (11), whose sides are conducting and short-circuited on the side facing the first cavity (11) by a conducting plane called reflecting plane, in that the conducting striplines (21 and 31) are carried by thin dielectric films (23 and 33), kept between the first and the second layer (10 and 40) and between the first and the second layer (10 and 40) and between the first and the third layer (10 and 50), respectively, in that the ends (24 and 34) of the stripline conductors (21 and 31) form exciting probes for the transmitting element whose penetration lengths in the cavities (11, 41, 51) are determined as functions of the distance between these probes (24, 34) and the reflecting plane, and characterized in that it is fed by two transmission lines each one of which being formed by a central conductor constituted by the extension of one of the stripline conductors (21 or 31) carried by one of the thin dielectric films (23, 33) and by a casing constituted by grooves in rectangular section having conducting sides and being provided symmetrically on both sides of the central conductor in the first and second layer (10 and 40) and also in the first an third layer (10 and 50).

2. An element as claimed in Claim 1, characterized in that the section of the cavities (11, 41, 51, 61) has a square shape with rounded-off tops when observed in parallel with the reflecting plane.

3. An element as claimed in Claim 1, characterized in that the section of the cavities (11, 41, 51, 61) has a circular form when observed in parallel with the reflecting plane.

4. An element as claimed in Claims 1 to 3, characterized in that the first, second and third layers (10, 40, 50) are made of a dielectric material, whilst the sides of the cavities extending through them are metal-plated.

5. An element as claimed in Claims 1 to 3, characterized in that the first, second and third layers (10, 40, 50) are metal plated.

6. An element as claimed in Claims 1 to 5, characterized in that it comprises a polyurethane screen.

7. An element as claimed in Claims 1 to 6, characterized in that it comprises a depolarising arrangement of the randome type for transmitting circularly polarised microwave frequency signals.

8. An element as claimed in any one of the Claims 1 to 6, characterized in that it comprises an arrangement of the coupler 3 dB type for transmitting the circularly polarised signals.

9. A microwave frequency planar antenna for receiving or transmitting, orthogonal linearly polarised microwave frequency signals, characterized in that it is composed of a plurality of elements as claimed in Claims 1 to 8, juxtaposed for forming a network, in that this network of elements is fed by a first and second network of microwave frequency transmission lines (20 and 30) carried by the thin dielectric films (23 and 33) and each ensuring a connection between the elements and a single distinct output connection via a series of combining stages, and in that the dimension of the elements arranged in parallel with the planar antenna is sufficiently small with respect to the wavelength associated with the frequency of the microwave frequency signals so that the distance between the elements is smaller than the said wavelength.

10. An antenna as claimed in Claim 9, characterized in that it comprises for a modular embodiment, elementary blocks forming sub-assemblies and capable of being used in adequate numbers and joined assemblies to form antennas having predetermined dimensions, gain and directivity pattern, and more specifically asymmetrical antennas exhibiting different radiation patterns according to the planes considered.

11. An antenna as claimed in Claims 9 or 10, chracterised in that it comprises a single arrangement selected from a depolarising radome, or a 3 dB coupler connected to two distinct output terminals of the supply networks, for transmitting the circularly polarised signals.

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