EP1551078B1 - Omnidirectional antenna with steerable diagram - Google Patents

Description

The present invention relates to a configurable antenna for transmitting or sensing at least one electromagnetic radiation beam in an adjustable direction and angular width.

The invention finds a particularly advantageous application in the field of mobile telephony (GSM bands (Global System for Mobile Communication), DCS (Digital Cellular System), UMTS (Universal Mobile Communication System)), as well as in that of the broadcasting of broadband services such as Wireless Local Area Network (WLAN), WIFI, Local Multi-point Distribution System (LMDS) and even Ultra Wide Band (UWB).

The development of telecommunication systems responding to the problems of communication in a situation of mobility has led operators and industrialists to develop and use more and more complex base stations. At present, due to the constraints related to the number of sites in operation, it is more and more difficult to install new antennas indefinitely. It therefore becomes necessary to use broadband antennas that can replace several single-band antennas or to use the same antenna to cover several different areas.

In mobile telephony, cell coverage can be obtained from mono / multi-beam antennas whose radiation zones are made adjustable in direction and angular width by the use of active elements that control the antenna. feeding planar array antennas or focal array reflector antennas for angles of sight of ± 30 to 40 °, or placed on a cylindrical surface to have the ability to point one or more beams through 360 °. The complexity of the power grid is directly related to the possibilities and agility of the antenna. This complexity is growing even faster with the formation of multiple independent beams. The management of all the beams must be done through radio frequency active elements of the amplifier type, phase shifter, delay line which work in the frequency bands of the antenna. The use of such elements drastically increases the cost of the antenna or limits the possibilities thereof if one wants to obtain a reasonable price (use in narrow band, ...). In addition, the power losses of these active printed network antennas are not negligible and may limit intrinsic performance.

Also, the technical problem to be solved by the object of the present invention is to propose a configurable antenna intended to emit or pick up at least one electromagnetic radiation beam in an adjustable direction and angular width, which would make it possible to eliminate the limitations known antenna systems mentioned above, especially avoiding the use of radio-frequency components.

The document DE 32 37 136 discloses an antenna having an upper metal plate and a lower metal plate, partially conical and reflective elements in the form of bars extending between said plates.

The document AT 520 107 describes an antenna whose structure is that of a conical monopole.

The document US 4,700,197 describes an antenna whose structure is a monopole comprising a plurality of re-connectors mounted on a ground plane.

The document US 3,375,519 discloses an antenna comprising an upper plate and a lower plate and gas-laden tubes extending between the two plates.

. The document GB 1,021,727 discloses a system comprising an antenna formed of a central pole and a plurality of reflectors disposed around said pole.

The solution to the technical problem posed consists, according to the present invention, in that said configurable antenna also comprises, associated with said omnidirectional antenna, discrete reflective reflectivity elements controllable, arranged on at least one circle centered around the given z axis .

Advantageously, the reflectivity of said discrete reflector elements is controlled by a DC voltage.

Thus, the configurable antenna according to the invention uses the modification of the electromagnetic radiation of an omnidirectional antenna, broadband or multi-band, by a system of deflectors controlled by simple direct voltage, unlike conventional active antennas where one pilot the radiation by radio-frequency components. In other words, the combination of an omnidirectional type antenna to a system of discrete reflector elements transforms, according to the invention, the omnidirectional coverage of the antenna into a mono / multibeam coverage of variable widths.

It is understood that depending on the desired coverage, the antenna of the invention can be configured to obtain a beam of radiation in a cell of larger or smaller size or to illuminate several cells in different angular sectors. The coverage can therefore be changed without the need to change the antenna or its positioning.

According to a particular embodiment of the invention, said discrete reflector elements are linear elements each consisting of discontinuous metal bars interconnected by electrical conductivity components controllable by a DC voltage. These elements were developed by the Institut d'Electronique Fondamentale of the University of Paris Sud-Orsay ( "Numerical and Experimental Demonstration of an Electronically Controllable PBG in the Frequency Range 0 to 20 Ghz" A. de Lustrac, T. Brillat, F.Gadot and E. Akmansoy, Proceedings Antennas and Propagation 2000, 9-14 april 2000, Davos, Switzerl and) for the purpose of producing an electromagnetic band gap meta-material based on the principle of photonic band gaps, the spatial distribution of the elements in a bi-periodic network creating the equivalent of a "crystal". The effect of this pseudo-crystal on the propagation of electromagnetic waves is modified by the presence of faults placed inside, which makes it possible to obtain for certain frequency bands a transmission through this pseudo-crystal which, he had been perfect, would have reflected all the frequencies.

For application to the invention, the working frequencies are below the forbidden bands and the meta-material is used as a simple metal reflector controlled.

In practice, the invention provides that said controllable electrical conductivity components are diodes or micro-mechanical switches known by the acronym MEMS for "MicroElectroMechanical System", these two types of components being controllable by a DC voltage .

According to the invention, said omnidirectional antenna is constituted by a biconical antenna.

Biconical antennas are omnidirectional antennas whose properties and characteristics have been described in chapter 8 "The Biconical Antenna and its lmpedance" from JD Kraus Antennas, McGraw-Hill, Electrical and Electronical Engineering Series, 1950 .

As will be seen in detail below, it is possible to increase the directivity of the antenna object of the invention, because said omnidirectional antenna is constituted by a plurality of networked biconical antennas.

So as to be able to shape the beam of radiation in elevation, that is to say in the plane passing through the axis z, in particular to obtain a beam centered on a direction other than 90 ° with respect to the axis z, it is provided by the invention that said biconical antenna has asymmetrical cones or that said biconical antennas are networked with a variable phase shift.

Finally, in applications more specifically dedicated to mobile telephony, it is advantageous that, according to the invention, the discrete reflector elements have a variable reflectivity as a function of the frequency of the electromagnetic radiation. This makes it possible, by inclusion of defects in the meta-material constituted by said discrete elements, to obtain beams of radiation of different coverage according to the frequency band: GSM, UMTS, .... Similarly, the invention also contemplates that the antenna of the invention comprises second discrete reflector elements arranged orthogonally to said discrete reflector elements. This double structure, which can be controlled separately in horizontal and vertical polarization, offers the possibility of making polarizations at ± 45 °.

The following description with reference to the accompanying drawings, given as non-limiting examples, will make it clear what the invention consists of and how it can be achieved.

The figure 1 is a perspective view of a configurable antenna according to the invention.

The figure 2 is a sectional view along the z-axis of the antenna of the figure 1 .

The figure 3a represents a polarized reflective element.

The figure 3b represents the reflective element of the figure 3a unpolarized.

The figure 4a is a top view of a distribution of non-polarized reflector elements.

The figure 4b represents the distribution of the figure 4a in a single-beam polarization configuration of the reflector elements.

The figure 4c represents the distribution of the figure 4a in a multi-beam polarization configuration of the reflector elements.

The figure 5 is a sectional view along the z-axis of two antennas according to the invention mounted in a network.

On the Figures 1 and 2 is shown a configurable antenna 10 comprising a omnidirectional antenna 11 broadband or multi-band which, in the example illustrated in these figures, is of the biconical type. In accordance with the general principle of the biconical antennas set out in the aforementioned work by JD Kraus, the omnidirectional antenna 11 consists of two substantially conical surfaces 111 and 112 arranged head to tail about a common axis z which is also that of the antenna 10. The antenna 11 is able to emit or pick up a beam of electromagnetic radiation omnidirectionally, ie isotropically about the z axis, which constitutes an axis of revolution for the antenna 11. In the where the two cones 111 and 112 are symmetrical, as shown in Figures 1 and 2 the directivity maximum is obtained for a direction D of radiation making an angle θ of 90 ° with the axis z, that is to say in the plane xy.

In order to configure the antenna 10 into a single / multi-beam antenna with adjustable direction and angular beam width (x), the omnidirectional antenna 11 is associated with a system of discrete reflective elements 20 of controllable reflectivity arranged according to at least one circle centered around the z axis. As indicated by figure 1 and more precisely the Figures 4a to 4c said reflective elements 20 are distributed in four concentric circles 31, 32, 33, 34.

In the proposed embodiment on the Figures 3a and 3b , the reflector elements 20 are linear elements each constituted by discontinuous metal bars 21 interconnected by components 22 of controllable electrical conductivity. As can be seen on the Figures 3a and 3b said components 22 are diodes controlled by a DC voltage. The system formed by a regular set of discrete linear elements 20 of this type produces a meta-material, called electromagnetic bandgap, whose properties have been recalled above with reference to the publication of A. de Lustrac et al.

The Figures 3a and 3b illustrate how the linear elements 20 operate when applied to the configurable antenna 10.

On the figure 3a , the diodes 22 are biased by a DC voltage and, because of their very low electrical resistance, realize the equivalent of a single bar of greater length than each individual bar 21. This bar, then referenced 20 ', is a reflector from the electromagnetic point of view. It will be understood that the spatial distribution of the linear elements 20 'of shorted bars 21 forms a reflector which makes it possible to distribute the radiation at will in the space.

On the figure 3b , the diodes are not polarized and therefore have a very high impedance. There is no electrical connection between the bars and the equivalent bar 20 "is transparent to the electromagnetic waves.Practically, it is advantageous for the length of an elementary bar 21 to remain less than one-fifth of the smaller wavelength to limit the disturbance of these bars 21.

The advantage of using this controlled reflective element system is mainly due to the fact that the diodes are biased with a DC voltage. There is therefore no complex RF component type amplifier or phase shifter. Only the diode 22 must be chosen so as to have the lowest internal resistance to the frequencies envisaged when it is polarized, this in order to obtain a better short-circuit.

Of course, other components 22 of electrical conductivity controllable by a DC voltage can be envisaged such as micromechanical switches MEMS mentioned above.

For application to the invention, the configurable reflector system associated with the omnidirectional antenna 11 is constituted by a plurality of concentric circles of axis z on which the reflector elements 20 are regularly distributed in a linear step δ constant.

As the figure 4a , the angular distribution of the elements 20 is variable as a function of the radius of the circle considered in order to obtain a linear pitch δ constant for all the circles 31, 32, 33, 34.

The number of concentric circles of reflective elements 20 is set so as to have a sufficient attenuation in the short-circuit zone since the metal bar 20 'is a localized element and the superposition of concentric layers makes it possible to best simulate a cylinder reflective metal. Likewise, the radial spacing between each concentric circle must be small enough that the repetition of the circles generates a cylindrical reflective portion

It is thus necessary to make a compromise between the number of circles, the spacing between circles and the total size of the antenna 10 in order to obtain a maximum dimension compatible with the desired application.

Depending on whether the linear elements 20 are polarized or not, the desired electromagnetic radiation beam distribution may be achieved, for example an omnidirectional distribution ( figure 4a ), a single beam distribution ( figure 4b ) of variable width or multi-beam distribution ( figure 4c ) with a variable width for each beam.

Note that it is possible to better adapt the antenna 10 to the free space by changing the polarization configuration of the linear elements of the first concentric circle 31. For example, to obtain an effective opening angle of 60 ° it will be necessary to leave the elements 20 "of the first circle 31 open circuit at a larger angle.

We can see on the figure 2 that each linear element 20 passes through the upper metal cones 111 and lower 112 without electrical contact, passing through insulating passages 40. It is then very easy thanks to the coaxial feed of the biconical antenna 11 to bring a DC voltage on the upper cone 111 in order to be able to independently bias each linear element 20 by a control box 50 placed on the upper cone 111, the elements 20 being grounded on the lower cone 112 ( figure 2 ), or under the lower cone 112, the elements 20 being connected directly to the upper cone 111 for connection to the positive voltage.

It is the necessity of making the mechanical connection between the linear elements 20 and the cones of the biconical antenna 11, as well as the insulating passages 40, which explains that the surfaces 111 and 112 are not strictly conical but affect a pseudo-shape. -conical adapted to this requirement.

The vertical networking of antennas 10 according to the invention illustrated in the previous figures offers great interest in increasing the vertical directivity of the radiating structure. On the figure 5 is shown a configurable antenna 10 'constituted by an omnidirectional antenna 11' comprising two biconical antennas 11a and 11b. The configuration of linear reflective elements 20 in short-circuit or open circuit is the same for all of the two biconical antennas 11a and 11b, in order to generate the cover (s) in azimuth. The power supply of the two half-antennas is of the series type, and the spacing between the two biconical antennas 11a and 11b serves to re-phase their respective power supply to obtain an optimal radiation along θ = 90 °, as explained above, for the specific application for mobile telephony for example, and to adapt as and when the series of antennas 11a and 11b.

The integration of the linear elements 20 controlled is done in the same way as for a simple biconical antenna. The bias voltage of the diodes is applied to the central core of the coaxial cable 200 and is recovered on the last cone 111b of the network. The elements 20 pass through the cones without electrical contact and are connected to ground on the lower cone 112a.

If one wants to achieve a radiation beam in another direction than that defined by θ = 90 °, one can apply a variable phase difference between the different biconical antennas networked in a multiple configurable antenna. The same result can be obtained with a configurable multiple antenna networking asymmetrical biconic antennas.

It should also be pointed out that with reflective elements 20 with variable reflectivity with the frequency it is possible to generate beams in certain directions of space for a given frequency band and in other directions for other frequency bands.

Finally, it is possible to note the possibility of obtaining a vertical and horizontal double polarization by integrating into the system of vertical reflective elements another structure of orthogonal reflective elements for controlling the horizontal radiation and thus be able to achieve ± 45 ° radiation. .

Claims (11)

1. Configurable omnidirectional antenna (10, 10', 10") intended to transmit or pick up at least one beam of electromagnetic radiation in a direction and over an angular width that may both be adjusted about a given z-axis, comprising:

   an upper metal cone (111) and a lower metal cone (112) arranged head-to-toe about the shared z-axis;

   - a system of discrete reflective elements (20) of controllable reflectivity extending vertically between said metal cones (111, 112) arranged in a circle (31, 32, 33, 34) centred on the z-axis of said antenna;

   characterized in that:

   - each reflective element (20) is composed of a plurality of discontinuous metal bars (21) mutually connected by components (22) of controllable electrical conductivity, said reflective elements passing through the two metal cones (111, 112), forming a biconical antenna, without electrical contact with said metal cones.
2. Antenna according to Claim 1, characterized in that the reflectivity of said discrete reflective elements (20) is controlled by a DC voltage.
3. Antenna according to Claim 2, characterized in that said components of controllable conductivity are diodes.
4. Antenna according to Claim 2, characterized in that said components of controllable conductivity are micromechanical switches.
5. Antenna according to any one of Claims 1 to 4, characterized in that said discrete reflective elements (20) are distributed with a constant linear pitch (δ) over the plurality of concentric circles (31, 32, 33, 34) of axis z.
6. Antenna according to Claim 5, characterized in that said constant linear pitch (δ) is identical for all said concentric circles.
7. Antenna according to any one of Claims 1 to 6, characterized in that the length of the reflective elements is less than one-fifth of the shortest wavelength of the electromagnetic radiation.
8. Antenna according to one of the preceding claims, characterized in that the lower (112) and upper (111) cones are asymmetrical.
9. Antenna according to any one of Claims 1 to 8, characterized in that said antenna comprises a plurality of networked biconical antennas (11a, 11b, 11c).
10. Antenna according to Claim 9, characterized in that said biconical antennas (11a, 11b, 11c) are networked with a variable phase shift.
11. Antenna according to any one of Claims 1 to 10, characterized in that it contains a structure of discrete reflective elements arranged orthogonally to the system of vertical reflective elements.

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