EP3902059B1 - Directional broadband antenna with longitudinal transmission - Google Patents

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 communications systems on board vehicles requires the ability to transmit and receive in a plurality of frequency bands, which generally involves having to install on a vehicle as many antennas as there are distinct communications systems 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 less protrusion on the surface of the aircraft and consequently less drag.

Furthermore, it is often preferable for the on-board antennas to have high directivity and therefore high gain, so as to reduce energy consumption and increase the signal-to-noise ratio. In general, the gain of an antenna being proportional to the effective aperture cross section of the antenna, itself proportional to the area of the antenna in the plane orthogonal to the direction of the lobe principal, the search for antennas with high 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 aperture surface 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 aperture surface. 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 parasitic reflector element located behind and of one or more parasitic director elements located in front 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 greater lateral extension than that of the dipole, the latter having a greater lateral extension than that of the directing elements. The parasitic reflector and director elements act as radiating dipoles powered by induction by the half-wave dipole which is the only wire-powered one. The Yagi-Uda antenna can be assimilated in a first approximation to an array antenna whose elements would be fed by mutual induction. By suitably choosing the position and the 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 opposite direction.

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

The document US8228254 B2 describes reduced size dipole and monopole antennas, printed on one side of a substrate with split loading patches at the ends of the antenna, and conductive tape on the other side to form a folded dipole or monopole structure.

The document US 2019/280365 A1 has an antenna embedded in or on glass structures having apparatus extending a ground plane onto an antenna near a dielectric structure, such as a windshield, to couple a radiated wave into the dielectric structure to control the tilt of the antenna radiation pattern.

The paper "A Dual-Band Microstrip fed Monopole Quasi-Yagi Antenna" (Nuangpirom pinit et al) describes a dual-band microstrip fed monopole and quasi-Yagi antenna for use in a wireless local area network (WLAN) access point ) dual band.

The document US 2014/043197 A1 describes an ultra-wideband antenna assembly comprising an electromagnetic reflective structure for reflecting electromagnetic waves, an antenna operatively associated with the electromagnetic reflective structure, a director operatively associated with the antenna.

The document US7023396 B2 relates to a broadband antenna with omnidirectional radiation comprising a first circular or semi-circular monopole perpendicular to a ground plane, and at least one second circular or semi-circular monopole, the monopoles being positioned relative to each other so as to have a common diameter.

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 a high directivity.

Presentation of the invention

The present invention is defined by a Yagi-Uda type antenna according to claim 1.

According to a first variant, the conductive return extends parallel to the disc and is located behind the latter, between the disc and the reflective parasitic element.

According to a second variant, the conductive return extends parallel to the disc and is located 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 director parasitic element is configured as a folded monopole, 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, of the side opposite the ground plane and not being connected at their second ends, on the side of the ground plane.

The conductive plate is 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 will be able to cover more than one octave.

Finally, the invention also relates to an aircraft on which is mounted a Yagi-Uda antenna as described above, 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 longitudinal axis of the aircraft and the ground plane being constituted by the skin of the fuselage.

Brief description of 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 appear on reading a preferred embodiment of the invention, described with reference to the attached figures, including:

• [ Fig. 1 ] schematically represents a monopolar patch antenna in the form of a disc;
• [ Fig. 2 ] represents a diagram giving the reflection coefficient of the antenna of the Fig. 1 , as a function of 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 a Yagi-Uda type antenna, 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 underlying 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 since 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 way by a monopolar plate antenna, advantageously chosen to be circular in shape.

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

The operating bandwidth of the circular patch antenna is substantially greater than that of a monopole of height equal to the diameter of the antenna in question. By way of example, we have shown in Fig. 2 a diagram giving the reflection coefficient (magnitude in decibels of the parameter S ₁₁ ) of the antenna of the Fig. 1 depending on the frequency of the antenna signal, this 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.

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

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 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 constituted 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 the rear of the aircraft. 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.

The radiating element 320 is in the form of a disc.

In the case of a disc, the diameter will be chosen in the order of λ /4 where λ is the wavelength corresponding to the lower limit of the operating band of the antenna.

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 constituted 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 shape of the radiating element 320 is an advantageous feature of the invention. This shape makes it possible to increase the impedance of the radiating element of the monopole known from the prior art. Indeed, if the impedance of a monopole disc is about 37 Ohms, that of this monopole in folded configuration is four times higher.

The conductive return 325 may be located in front of the plate 321 of the monopole in the direction of the longitudinal direction Oz. For example, the conductive return may extend parallel to the plate and is situated in front of the latter, between the plate (for example a disc) and the directing parasitic element. Alternatively and preferably, this conductive return will be located behind the plate, between the plate (for example a disc) and a passive reflective 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 take 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 than 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 reflective 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 whose the axis of revolution coincides with 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 on itself having 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 a second parallel conductive segment of the same shape, located at a low distance from the first. The first and second segments are connected together at a common first end 331 on the side opposite 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 lower or even slightly lower than the respective transverse dimensions of the radiating plate 321. For example, when the plate has a circular 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 reflective element, 310, the radiating element, 320, composed of the patch 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 monopolar Yagi-Uda type antenna.

The relative positions of the elements along the Oz axis and their spacings are chosen so as to optimize the shape of the beam, in particular to reduce the secondary lobes thereof, and to allow impedance matching (generally at 50Ω). The introduction of director elements and a reflective element in 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, of the order of 150 Ω, which makes it possible to use directing elements 330 and a reflecting element 310 while reducing the non-radiated power.

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

There 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 is made up of a metal disc with a diameter of 20mm. The antenna further includes a semi-cylindrical reflector element and a director element. We notice on the Fig. 4 that the width of the operating band taken at 10dB extends over an octave from 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 be used as an overall antenna for several on-board air-ground communication systems, in particular when the aircraft is in the approach phase. This antenna can also serve as a relay antenna in the event of use of cellular telephones by the passengers of the aircraft.

There 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 a good directivity at low and medium elevation, a longitudinal emission in the direction of the Oz axis 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 4 GHz, in an elevation plane 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 to a typical situation where the aircraft flies at an altitude of 3km and the ground station is about 30 km away.

The azimuth angular width of the main lobe is more than 120°, which allows high quality of service communications even when the ground station is not aligned 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 strong rejection, which correspondingly reduces the risks of interference in reception.

Claims (8)

1. Yagi-Uda antenna comprising a reflector parasitic element (310), a radiating element (320) and at least one director parasitic element (330) that are placed in this order along a longitudinal axis of the antenna, the radiating element being formed from a conductive plate (321), which plate is placed substantially orthogonal to the longitudinal axis of the antenna and above a ground plane (310) so as to form a monopole, the plate being provided, on the side of the ground plane, with a feed terminal (322) for applying or receiving an antenna signal, the conductive plate being of circular shape, and being equipped, at an end opposite to the ground plane, with a return conductor (325), the return conductor being electrically connected to the ground plane, so that the assembly consisting of the conductive plate and of the return conductor form a folded monopole, the conductive plate taking the form of a disc of diameter of order λ/4, where λ is a wavelength corresponding to a lower limit of a frequency band of operation of the antenna, and the return conductor taking the form of a rod or strip of length substantially identical to the diameter of the disc.
2. Yagi-Uda antenna according to Claim 1, wherein the return conductor lies parallel to the disc and is located behind the latter, between the disc and the reflector parasitic element.
3. Yagi-Uda antenna according to Claim 1, wherein the return conductor lies parallel to the disc and is located in front of the latter, between the disc and the director parasitic element.
4. Yagi-Uda antenna according to Claim 1, wherein the reflector parasitic element has, in the direction perpendicular to the ground plane, a dimension larger than that of the conductive plate in the same direction.
5. Yagi-Uda antenna according to one of the preceding claims, wherein the director parasitic element is configured as a folded monopole, comprising a first conductive segment and a second conductive segment, which are parallel to each other and to the conductive plate, said first and second conductive segments being connected at a common first end, on the side opposite to the ground plane, and being not connected at their second ends, on the side of the ground plane.
6. Yagi-Uda antenna according to Claim 5, wherein the first and second conductive segments have a length smaller than the diameter of this disc.
7. Yagi-Uda antenna according to one of the preceding claims, wherein a passband of operation covers more than one octave.
8. Aircraft comprising a Yagi-Uda antenna according to one of the preceding claims, said antenna being mounted on a lower portion of the fuselage of the aircraft, the longitudinal axis of the antenna being substantially parallel to a longitudinal axis of the aircraft, and the ground plane consisting of a skin of the fuselage.

Images