EP1551078A1 - Omnidirectional antenna with steerable diagram - Google Patents

Abstract

The antenna has an omnidirectional antenna (11) transmitting and receiving electromagnetic radiation beam and having two conical surfaces around z axis. Discrete reflecting units (20) are associated to the antenna (11) and have variable reflectivity based on frequency of the beam. The units are disposed along concentric circles (31-34) centered on the axis, and have bars connected by diodes controlled by continuous voltage.

Description

The present invention relates to a configurable antenna for emit or pick up at least one beam of electromagnetic radiation 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 the field of the diffusion of services broadband type wireless local area network (WLAN), WIFI, LMDS (Local Multi-point Distribution System) and even UWB (Ultra Wide Band).

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

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

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

The solution to the technical problem consists, according to the present in that said configurable antenna also comprises associated with said omnidirectional antenna, reflective elements discrete reflectivity controllable, arranged on at least one centered circle 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 active antennas where radiation is controlled by radio frequency components. In other words, the combination of an omnidirectional type antenna with a system of discrete reflector elements transforms, according to the invention, the omnidirectional coverage of the antenna in a single / multi-beam coverage of varying widths.

We understand 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 cover can be modified without the need to change the antenna or its positioning.

According to a particular embodiment of the invention, said elements Discrete reflectors are linear elements each consisting of discontinuous metal bars interconnected by components of electrical conductivity controllable by a DC voltage. These elements were developed by the Institute of Fundamental Electronics of the University Paris Sud-Orsay ("Numerical and Experimental Demonstration of Electronically Controllable PBG in the Frequency Range 0 to 20 Ghz »A. Lustrac, T. Brillat, F.Gadot and E. Akmansoy, Proceedings of the Congress Antennas and Propagation 2000, 9-14 April 2000, Davos, Switzerland) with the aim of to achieve an electromagnetic band-gap meta-material based on the principle of photonic forbidden bands, the spatial distribution of elements according to 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 allows obtain for certain frequency bands a transmission through this pseudo-crystal which, if it had been perfect, would have reflected all the frequencies.

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

In practice, the invention provides that said components of controllable electrical conductivity are diodes or switches micro-mechanics known by the acronym Anglo-Saxon MEMS for "MicroElectroMechanical System", both 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 properties and characteristics have been described in Chapter 8 "The Biconical Antenna and its Impedance "from J.D. Kraus Antennas, McGraw Hill, Electrical and Electronical Engineering Series, 1950.

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

In order to be able to conform the beam of radiation into elevation, that is to say in the plane passing through the z axis, especially for to obtain a beam centered on another direction than 90 ° with respect to the axis z, it is provided by the invention that said biconical antenna has cones asymmetrical or that said biconical antennas are networked with a variable phase shift.

Finally, in applications more specifically dedicated to mobile telephony, there is advantage that, according to the invention, the elements Discrete reflectors have a variable reflectivity depending on the frequency of electromagnetic radiation. This allows for inclusion of defects in the meta-material constituted by said discrete elements to obtain radiation beams of different coverage according to the frequency band: GSM, UMTS, .... Similarly, the invention envisages also that the antenna of the invention comprises second elements discrete reflectors arranged orthogonally to said reflector elements discreet. This dual structure can be controlled separately in polarization Horizontal and vertical offers the possibility to achieve polarizations at ± 45 °.

The following description with reference to the accompanying drawings, given in As non-limitative examples, it will make clear what constitutes the invention and how it can be realized.

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

FIG. 2 is a sectional view along the z axis of the antenna of FIG. 1.

Figure 3a shows a polarized reflective element.

FIG. 3b represents the reflector element of FIG. polarized.

Figure 4a is a top view of a distribution of elements non-polarized reflectors.

Figure 4b shows the distribution of Figure 4a in a single-beam polarization configuration of the reflector elements.

Figure 4c shows the distribution of Figure 4a in a multi-beam polarization configuration of the reflector elements.

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

In FIGS. 1 and 2 is shown a configurable antenna 10 comprising an omnidirectional antenna 11 broadband or multiband which, in the example illustrated in these figures, is of the biconical type. In accordance with the general principle of biconical antennas set out in the previously cited work of J.D. Kraus, the omnidirectional antenna 11 consists of two substantially conical surfaces 111 and 112 arranged head-to-tail around 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 radiation electromagnetic way omnidirectional, ie isotropic around of 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 on FIGS. 1 and 2, the directivity maximum is obtained for a direction D radiation at an angle  of 90 ° to the z-axis, that is to say in the xy plane.

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

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

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

In FIG. 3a, the diodes 22 are biased by a DC voltage and, because of their very low electrical resistance, realize the equivalent of a only bar of greater length than each individual bar 21. This bar, referenced then 20 ', is a reflector from the point of view electromagnetic. We understand that the spatial distribution of the elements Linear 20 'shorted bars 21 forms a reflector that allows distribute the radiation at will in space.

In FIG. 3b, the diodes are not polarized and therefore have a very high impedance. There is no electrical connection between bars and the equivalent 20 "bar is transparent to the waves electromagnetic. In practical terms, it is advantageous for the length of an elementary bar 21 remains less than one fifth of the most small wavelength to limit the disturbance of these bars 21.

The advantage of using this reflector system ordered is mainly due to the fact that the diodes are polarized with a continuous voltage. So there is no typical RF complex component amplifier or phase shifter. Only the diode 22 must be chosen so as to have the lowest internal resistance at the frequencies considered when is polarized, in order to obtain a better short-circuit.

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

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

As shown in FIG. 4a, the angular distribution of the elements 20 is variable according to the radius of the circle considered in order to obtain a step linear δ constant for all circles 31, 32, 33, 34.

The number of concentric circles of reflective elements 20 is fixed in order to have sufficient attenuation in the short-circuited area since the metal bar 20 'is a localized element and that the superposition of concentric layers can best simulate a metal cylinder reflector. Similarly, the radial spacing between each concentric circle must be low enough that the circle repetition generates a portion cylindrical reflector

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 so to obtain a maximum dimension compatible with the desired application.

Depending on whether the linear elements 20 are polarized or not, one can achieve the desired electromagnetic radiation beam distribution, for example an omnidirectional distribution (Figure 4a), a distribution single-beam (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 space free by playing on the polarization configuration of the linear elements of the first concentric circle 31. For example, to get an angle effective opening of 60 °, it will be necessary to leave the elements 20 "of the first circle 31 in open circuit on a larger angle.

It can be seen in FIG. 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 voltage continuous on the upper cone 111 in order to be able to polarize independently each linear element 20 by a control housing 50 placed either on the upper cone 111, the elements 20 being grounded on the cone lower 112 (FIG. 2), ie under the lower cone 112, the elements 20 being connected directly to the upper cone 111 for connection to the voltage positive.

It is the need to realize the mechanical connection between the elements 20 and the cones of the biconical antenna 11, as well as the passages insulators 40, which explains that surfaces 111 and 112 are not rigorously conical but affect a pseudo-conical form adapted to this requirement.

Vertical networking of antennas 10 according to the invention illustrated in the previous figures offers a great interest to increase the vertical directivity of the radiating structure. In 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 reflector elements 20 in short circuit or open circuit is the even for all of the two biconical antennas 11a and 11b, in order to generate the coverage (s) in azimuth. The feeding 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 feeds to obtain an optimal radiation following  = 90 °, as explained more top, for the specific application to mobile telephony for example, and to adapt as the serialization of the antennas 11a and 11b.

The integration of the linear elements 20 controlled is done the same way only for a simple biconical antenna. The polarization 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 you want to make a beam of radiation in another direction than that defined by  = 90 °, we can apply a phase shift variable between the different biconical antennas networked in a multiple configurable antenna. The same result can be obtained with a Multiple Configurable Antenna Networking Biconical Antennas unsymmetrical.

It should also be noted that with reflective elements 20 to variable reflectivity with 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, we can note the possibility of obtaining a double polarization vertical and horizontal by integrating the system of reflective elements 20 vertical another structure of orthogonal reflective elements to drive horizontal radiation and thus be able to achieve radiation at ± 45 °.

Claims (15)

A configurable antenna for transmitting or sensing at least one electromagnetic radiation beam in an adjustable direction and angular width, said antenna (10; 10 '; 10 ") including an omnidirectional antenna (11; 11'; a given z axis, characterized in that said configurable antenna (10; 10 '; 10 ") also comprises, associated with said omnidirectional antenna (11; 11';11"), discrete reflective elements (20) of controllable reflectivity , arranged in at least one circle (31,32,33,34) centered around the given z axis. Antenna according to claim 1, characterized in that the reflectivity of said discrete reflector elements (20) is controlled by a DC voltage. Antenna according to claim 2, characterized in that said discrete reflector elements are linear elements (20) each constituted by discontinuous metal bars (21) interconnected by components (22) of electrical conductivity controllable by a DC voltage . Antenna according to claim 3, characterized in that said controllable electrical conductivity components are diodes (22). Antenna according to claim 3, characterized in that said controllable electrical conductivity components are micromechanical switches. Antenna according to any one of claims 1 to 5, characterized in that said discrete reflector elements (20) are distributed at constant linear pitch (δ) over a plurality of concentric circles (31,32,33,34) of axis z. Antenna according to claim 6, characterized in that said constant linear pitch (δ) is identical for all said concentric circles (31,32,33,34). Antenna according to any one of claims 1 to 7, characterized in that the length of the reflector elements (20) is less than one fifth of the smallest wavelength of the electromagnetic radiation. Antenna according to any one of claims 1 to 8, characterized in that said omnidirectional antenna is constituted by a biconical antenna (11). Antenna according to claim 9, characterized in that said biconical antenna has asymmetrical cones. Antenna according to any one of claims 1 to 8, characterized in that said omnidirectional antenna (11 '; 11 ") is constituted by a plurality of networked biconical antennas (11a, 11b, 11c). Antenna according to claim 11, characterized in that said biconical antennas (11a, 11b, 11c) are arrayed with a variable phase shift. Antenna according to any one of claims 9 to 12, characterized in that said reflector elements (20) pass through the cones (111, 112) of the omnidirectional antenna (11) without electrical contact. Antenna according to any one of claims 1 to 13, characterized in that the discrete reflector elements have a variable reflectivity as a function of the frequency of the electromagnetic radiation. Antenna according to any one of claims 1 to 14, characterized in that it comprises second discrete reflector elements arranged orthogonally to said discrete reflector elements (20).

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