EP3902059A1 - Directional broadband antenna with longitudinal transmission - Google Patents

Abstract

The present invention relates to a monopolar antenna of the Yagi-Uda type (300) intended to be mounted on the conductive surface of a vehicle, in particular of an aircraft. The antenna comprises a radiating element, in the form of a conductive plate (320), for example in the form of a disc (321), equipped with a conductive return (325), a reflector element (310) and at least one directing element (330) in the form of a monopoly folded in on itself. The various elements are mounted on a substantially flat surface such as the skin of the fuselage of an aircraft. The antenna has both a wide operating band, good compactness and good directivity. It can in particular serve as a global antenna for a plurality of air-ground communication systems of an aircraft.

Description

Technical area

The present invention relates to the general field of antennas and more particularly to longitudinal emission antennas of the Yagi-Uda type. The antenna according to the present invention can advantageously be on board an aircraft to allow air-ground communications in a wide frequency band.

State of the prior art

The growing number of on-board communication systems in vehicles requires the ability to transmit and receive in a plurality of frequency bands, which generally implies having to install on a vehicle as many antennas as there are separate communication systems that it comprises, this multiplication of antennas representing a source of complexity both for installation and for maintenance. It may then be advantageous, a fortiori when the recipients of these communications are collocated or close in terms of viewing angle, to use a global antenna common to all of these communications systems. Thus, for example, on board an aircraft, several air-ground communications systems using distinct frequency bands can share a global broadband antenna. The advantage of such pooling also resides in a smaller protrusion on the surface of the aircraft and consequently a lower drag.

Furthermore, it is often preferable for the on-board antennas to have a high directivity and therefore a high gain, so as to reduce the energy consumption and to increase the signal-to-noise ratio. In general, the gain of an antenna being proportional to the effective cross section of the opening of the antenna, itself proportional to the area of the antenna in the plane orthogonal to the direction of the antenna. main lobe, the search for antennas with strong directivity leads to antennas of large dimensions in the plane orthogonal to that of the direction of emission. In the aforementioned case of communications of an aircraft with the ground, the main lobe of the antenna must have a low angle of elevation and the opening area of the antenna must therefore be large in a plane orthogonal to the longitudinal axis of the aircraft, which increases drag and therefore fuel consumption.

The Yagi-Uda antenna initially developed for aeronautics and then universally used as a TV antenna is an antenna having both good directivity and a relatively small opening area. It is in fact known to those skilled in the art that this type of antenna consists of a half-wave linear dipole, generally folded, of a reflective parasitic element located behind and of one or more directing parasitic elements located. forward of this dipole, all mounted on the same mast, the direction of the main lobe being given by the direction of the mast. The reflector element has a lateral extension greater than that of the dipole, the latter having a lateral extension greater than that of the directing elements. The reflective and directing parasitic elements act like radiating dipoles fed by induction by the half-wave dipole which is the only one fed by wire. The Yagi-Uda antenna can be assimilated in first approximation to a network antenna whose elements would be fed by mutual induction. By properly choosing the position and spacing between the different elements, the waves emitted by the different elements add up constructively in the direction of the mast and destructively in the reverse direction.

However, a major drawback of Yagi-Uda type antennas is their narrow band operation, making them unusable as an overall broadband antenna in the above sense. Indeed, their fractional band, in other words the ratio between their bandwidth on their central frequency is of the order of 10%.

An object of the present invention is therefore to provide an antenna having a small effective aperture section while offering a wide operating band and high directivity.

Presentation of the invention

The present invention is defined by a Yagi-Uda type antenna comprising a radiating element, a reflective parasitic element and at least one directing parasitic element arranged in this order along a longitudinal axis of the antenna, the antenna being specific. in that the radiating element is formed by a conductive plate, arranged substantially orthogonal to the longitudinal axis of the antenna and above a ground plane so as to constitute a monopoly, the plate being provided, on the ground plane side, a power supply terminal for applying or receiving an antenna signal.
The conductive plate is advantageously in circular, ellipsoidal or rectangular form, and is equipped, at one end opposite the ground plane, with a conductive return, the conductive return being electrically connected to the ground plane, so that the assembly formed by the conductive plate and the conductive return form a folded monopoly.
In particular, the conductive plate may appear as a disc with a diameter of the order of λ / 4 where λ is a wavelength corresponding to the lower limit of the operating frequency band of the antenna, the conductive return in the form of a rod or a strip of length substantially identical to the diameter of the disc.
According to a first variant, the conductive return extends parallel to the disc and is located behind the latter, between the disc and the parasitic reflector element.
According to a second variant, the conductive return extends parallel to the disc and is situated in front of the latter, between the disc and the parasitic director element.
Advantageously, the reflective parasitic element has, in the direction perpendicular to the ground plane, a dimension greater than that of the conductive plate in this same direction.
Preferably, the parasitic directing element is configured as a folded monopoly, comprising a first conductive segment and a second conductive segment, parallel to each other and to the conductive plate, said first and second segments. conductors being connected at a first common end, on the side opposite to the ground plane and not being connected at their second ends, on the side of the ground plane.
The conductive plate may be in the form of a disc and the first and second conductive segments have a length less than the diameter of this disc.
The operating bandwidth of the Yagi-Uda type antenna may cover more than one octave.
Finally, the invention also relates to an aircraft on which is mounted a Yagi-Uda antenna as explained above, said antenna being mounted on the lower part of the fuselage of the aircraft, the longitudinal axis of the antenna being substantially parallel. the longitudinal axis of the aircraft and the ground plane being formed by the skin of the fuselage.

Brief description of the figures

• [Fig. 1] représente de manière schématique une antenne plaque monopolaire sous forme de disque ;
• [Fig. 2] représente un diagramme donnant le coefficient de réflexion de l'antenne de la Fig. 1, en fonction de la fréquence ;
• [Fig. 3] représente de manière schématique une antenne à émission longitudinale large bande selon un mode de réalisation de l'invention ;
• [Fig. 4] représente un diagramme donnant le coefficient de réflexion de l'antenne de la Fig. 3 en fonction de la fréquence ;
• [Fig. 5] représente le diagramme de rayonnement tridimensionnel de l'antenne de la Fig. 3;
• [Fig. 6] représente un diagramme de rayonnement bidimensionnel de l'antenne de la Fig. 3 dans un plan d'élévation à 5°.

Other characteristics and advantages of the invention will become apparent on reading a preferred embodiment of the invention, described with reference to the appended figures, among which:

• [ Fig. 1 ] schematically represents a monopolar plate antenna in the form of a disc;
• [ Fig. 2 ] represents a diagram giving the reflection coefficient of the antenna of the Fig. 1 , depending on the frequency;
• [ Fig. 3 ] schematically represents a broadband longitudinal transmission antenna according to one embodiment of the invention;
• [ Fig. 4 ] represents a diagram giving the reflection coefficient of the antenna of the Fig. 3 depending on the frequency;
• [ Fig. 5 ] represents the three-dimensional radiation pattern of the antenna of the Fig. 3 ;
• [ Fig. 6 ] represents a two-dimensional radiation pattern of the antenna of the Fig. 3 in a 5 ° elevation plane.

Description of embodiments

A first idea at the basis of the invention is to modify an antenna of the Yagi-Uda type, by choosing as the radiating element a conductive plate so as to make it broadband without causing it to lose its directivity properties. A second idea at the basis of the invention is to reduce the lateral extension of this antenna by using a ground plane to adopt a monopolar configuration. This monopolar configuration is all the more advantageous as the ground plane is naturally available in the form of a conductive surface of the vehicle itself.

Thus, the linear dipole of the Yagi-Uda antenna supplied by wire is replaced here in an original manner by a monopolar plate antenna, advantageously chosen to have a circular shape.

We will first consider a monopoly in the form of a radiating disc located above a ground plane, as shown schematically in Fig. 1 . This disc is fed at its lower end O 'by an antenna signal via a recess formed through the ground plane P. In a manner known per se, the radiation of such a monopoly is identical to an equivalent dipole consisting of the monopoly and its image in relation to the ground plane.

The operating bandwidth of the circular plate antenna is appreciably greater than that of a monopoly of height equal to the diameter of the antenna in question. By way of example, there is shown in Fig. 2 a diagram giving the reflection coefficient (magnitude in decibels of parameter ^S ₁₁ ) of the antenna of the Fig. 1 depending on the frequency of the antenna signal, for a disc diameter of 20mm. Note that the width of the operating band taken at 10dB extends over a frequency range starting at about 3.3 GHz and going beyond 12 GHz.

The Fig. 3 schematically represents a broadband longitudinal transmission antenna according to one embodiment of the invention.

Advantageously, the antenna is in a monopolar configuration in the sense that it is located above a conductive plane P acting as a ground plane. the The term “above” is here purely relative and the antenna may be located under the conductive plane. For example, if the antenna is mounted under the fuselage of an aircraft for communication with the ground, it will be understood that the antenna in question will be located under the conductive plane formed by the skin of the fuselage.

The antenna shown, 300, is longitudinal emission ( end-fire antenna) in the sense that the signal emitted by the antenna will be in the direction Oz. In the case of mounting on an aircraft, the direction Oz may be substantially parallel to the longitudinal axis of the aircraft and point towards the front or else the rear of the apparatus. Alternatively, the antenna could point in a lateral direction.

The antenna includes a radiating element, 320, in the form of a wire-fed plate. This radiating element is the only element of the antenna to be supplied directly, the other elements being supplied only by induction. Advantageously, the radiating element 320 is in the form of a disc although other shapes can also be envisaged. For example, the radiating element could be in the form of an ellipsoidal or rectangular plate.

In the case of a disc, the diameter will be chosen of the order of λ / 4 where λ is the wavelength corresponding to the lower limit of the operating band of the antenna. In the case of an ellipsoidal or rectangular plate, the dimensions along the axes Ox and Oy orthogonal to the longitudinal axis Oz will be chosen so that the resonance frequencies according to the transverse modes, in the directions in question, are located in the frequency band used.

Advantageously, the radiating element 320 will advantageously be mounted in folded form by means of a conductive return 325 substantially parallel to the plate 321 and having a small transverse dimension in the direction Ox. For example, the conductive return 325 may be formed by a conductive rod of small diameter or a rigid conductive strip of small width. The lower end 326 of the conductive return 325 is electrically connected to a ground plane. On transmission, the antenna signal is applied between the lower end 322 and the ground plane. Similarly, on reception, the antenna signal is taken between end 322 and the ground plane.

The folded form of the radiating element 320 is an advantageous characteristic of the invention. This shape in fact makes it possible to increase the impedance of the radiating element of the monopoly known from the prior art. Indeed, if the impedance of a monopoly disk is about 37 Ohms, that of this monopoly in folded configuration is four times higher.

The conductive return 325 may be located in front of the plate 321 of the monopoly in the direction of the longitudinal direction Oz. For example, the conductive return could extend parallel to the plate and is located in front of the latter, between the plate (for example a disc) and the parasitic director element. Alternatively and preferably, this conductive return will be located behind the plate, between the plate (for example a disc) and a passive reflector element, described below, so as not to hinder propagation in the longitudinal direction.

The antenna also comprises a passive reflector element, 310, also called a parasitic reflector element located at the rear of the radiating element. This reflective element may also be in different forms. In general, the reflective element will have a vertical dimension (that is to say in the direction Oy perpendicular to the ground plane) greater or even simply slightly greater than the vertical dimension of the plate 321. For example, the vertical dimension of the reflective element may exceed that of the radiating plate by 5%. More generally, the reflector element will have transverse dimensions (perpendicular to the axis Oz) greater than that of the radiating plate.

Thus, when the plate 321 is in the form of a disc, the reflector element 310 may be in the form of a disc of larger diameter or even of a paraboloid having an effective section of larger diameter and of which the axis of revolution coincides with the longitudinal axis Oz.

Alternatively, when the plate 321 has an ellipsoidal shape, the reflector element may also be in an ellipsoidal shape, the lengths of the major axis and of the minor axis of which are respectively greater than the lengths of the major axis and of the minor axis of the plate. Also in this case, the reflective element may also be in the form of a flattened paraboloid in the direction of the small axis of the plate and axis of symmetry coincident with the longitudinal axis Oz. In both cases, the major axis of the ellipsoid or of the cross section of the paraboloid will advantageously be chosen orthogonal to the ground plane.

Finally, the plate 321 may be in the form of a cylinder portion, for example in semi-cylindrical form, having an axis of revolution perpendicular to the ground plane, the cylinder portion being open in the direction of the longitudinal axis Oz.

Advantageously, the antenna 300 further comprises one or more directing elements 330. These directing elements can each have the shape of a vertical rod of any diameter or, preferably, of a linear structure folded back on itself presenting the advantage of being stronger and lighter. In this case, such a director element 330 comprises a first segment perpendicular to the ground plane, in the form of a rod or a rigid conductive strip and of a second conductive segment parallel and of the same shape, located at a low distance from the first. The first and second segments are connected together at a first common end 331 on the side opposite to the ground plane. On the other hand, the respective second ends, 332 and 333, of the first and second segments located on the side of the ground plane are not connected to each other.

In general, the fact of using linear elements folded on themselves makes it possible to improve the overall rigidity of the antenna.

The transverse dimensions of the directing elements 330 in a plane orthogonal to the axis Oz are chosen to be less or even slightly less than the respective transverse dimensions of the radiating plate 321. For example, when the plate has a circular, ellipsoidal or rectangular shape, the first and second segments of a directing element have a length of the order of 5% shorter than the diameter of the circle, the short side of the ellipse or the short side of the rectangle in the direction of the Oy axis.

The reflecting element, 310, the radiating element, 320, composed of the plate antenna, and the directing element (s), 330, are advantageously mounted on a substantially flat surface such as for example a ground plane or the skin of an aircraft directed in the direction Oz and form a Yagi-Uda type antenna monopolar.

The relative positions of the elements along the axis Oz and their spacings are chosen so as to optimize the shape of the beam, in particular to reduce its secondary lobes, and to allow impedance matching (generally at 50Ω). The introduction of directing elements and of a reflector element into the field of the radiating element has the consequence of reducing the impedance of the antenna and therefore the non-radiated power. The radiating element has a high impedance, on the order of 150 Ω, which allows the use of director elements 330 and a reflector element 310 while reducing the non-radiated power.

The various elements of the antenna can be produced simply and inexpensively from sheets or metal strips.

The Fig. 4 represents a diagram giving the reflection coefficient (parameter S ₁₁ ) of the antenna of the Fig. 3 depending on the frequency.

The radiating plate consists of a metal disc with a diameter of 20mm. The antenna further includes a semi-cylindrical reflector element and a directing element. We notice on the Fig. 4 that the width of the operating band taken at 10dB extends over an octave of 3 to 6 GHz. It therefore encompasses most of the 4G and 5G frequency bands used around the world.

Thus, the proposed antenna can in particular serve as a global antenna for several on-board air-ground communication systems, in particular when the aircraft is in the approach phase. This antenna can also be used as a relay antenna when cell phones are used by the passengers of the aircraft.

The Fig. 5 represents the three-dimensional radiation pattern of the antenna of the Fig. 3 at a frequency of 4 GHz.

It is noted that the antenna has good directivity at low and medium elevation, a longitudinal emission in the direction of the axis Oz with a gain of nearly 10 dB.

This good directivity at low elevation is confirmed by the two-dimensional radiation pattern of this same antenna, still at a frequency of 4GHz, in a plane of elevation at 5 °, as illustrated in Fig. 6 . This elevation angle corresponds to the case of an antenna mounted on the lower part of the fuselage of the aircraft (the axis Oz being substantially parallel to the longitudinal axis of the latter) and of a typical situation where the aircraft flies at an altitude of 3km and the ground station is about 30 kms away.

The azimuthal angular width of the main lobe is more than 120 ° which allows high quality of service communications even when the ground station is not in alignment with the heading of the aircraft. It is therefore not necessary to perform dynamic beamforming to point in the direction of this station.
In addition, the radiation pattern has few secondary lobes with a strong rejection, which correspondingly reduces the risks of interference on reception.

Claims (10)

Yagi-Uda type antenna comprising a radiating element (320), a reflective parasitic element (310) and at least one directing parasitic element (330) arranged in this order along a longitudinal axis of the antenna, characterized in that that the radiating element is formed by a conductive plate (321), arranged substantially orthogonal to the longitudinal axis of the antenna and above a ground plane (310) so as to constitute a monopoly, the plate being provided, on the ground plane side, with a power supply terminal (322) for applying or receiving an antenna signal. Yagi-Uda type antenna according to Claim 1, characterized in that the conductive plate is in the circular, ellipsoidal or rectangular shape, and that it is equipped, at an end opposite the ground plane, with a conductive return (325), the conductive return being electrically connected to the ground plane, so that the assembly formed by the conductive plate and the conductive return form a folded monopoly. Yagi-Uda type antenna according to claim 1 or 2, characterized in that the conductive plate is in the form of a disc with a diameter of the order of λ / 4 where λ is a wavelength corresponding to the lower limit of the operating frequency band of the antenna and that the conductive return is in the form of a rod or a strip of length substantially identical to the diameter of the disc. Yagi-Uda type antenna according to Claim 3, characterized in that the conductive return extends parallel to the disc and is located behind the latter, between the disc and the parasitic reflector element. Yagi-Uda type antenna according to Claim 3, characterized in that the conductive return extends parallel to the disc and is situated in front of the latter, between the disc and the parasitic director element. Yagi-Uda type antenna according to Claim 1, characterized in that the reflective parasitic element has, in the direction perpendicular to the ground plane, a dimension greater than that of the conductive plate in this same direction. Yagi-Uda type antenna according to one of the preceding claims, characterized in that the parasitic directing element is configured as a folded monopoly, comprising a first conductive segment and a second conductive segment, parallel to each other and to the conductive plate, said first and second conductive segments being connected at a first common end, on the side opposite to the ground plane and not being connected at their second ends, on the side of the ground plane. Yagi-Uda type antenna according to Claim 6 or 7, characterized in that the conductive plate is in the form of a disc and that the first and second conductive segments have a length less than the diameter of this disc. Yagi-Uda antenna according to one of the preceding claims, characterized in that its operating bandwidth covers more than one octave. Aircraft characterized in that it comprises a Yagi-Uda antenna according to one of the preceding claims, said antenna being mounted on the lower part of the fuselage of the aircraft, the longitudinal axis of the antenna being substantially parallel to the axis longitudinal of the aircraft, and the ground plane being formed by the skin of the fuselage.

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