Abstract:
The antenna comprises several primary sources ( 6 A- 6 C) fed independently and arranged to have different radiation characteristics. These primary sources are placed in a first medium ( 7 A- 7 C) so as to be spatially decoupled. A second medium ( 8 A- 8 C), of a characteristic impedance substantially lower than the first medium covers the first medium. Each primary source has a direction of focus (A-C) perpendicular to the interface between the first and second media, along which the distance (d 1 ) between said primary source and said interface is λ 1 .(2p 1 −1)/4 and the second medium has a thickness (e 2 ) equal to λ 2 .(2p 1 −1)/4, where λ 1  and λ 2  denote the wavelengths radiated by said primary source in the first and second media, respectively, and p 1  and p 2  are integers.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the antennas used with base stations for radio communication. 
     The take-off in cellular mobile communications has necessitated the installation of a large number of base stations. Cellular operators may encounter difficulties in finding appropriate sites. Apart from the problem of site availability, there is also the nuisance problem as perceived by the public due to the size and unattractive appearance of base station antennas which, of course, have to be positioned high up and clearly visible for the network effeciency. In certain countries, regulations or taxes have been introduced with a view to restricting the number of these antennas. 
     Using multi-sector antennas enables the number of base station sites to be reduced for a given coverage (see EP-A-0 802 579). However, due to their directivity and multiplicity, these multi-sector antennas are considerably larger than omnidirectional antennas. 
     In order to increase the gain in directivity of a base station antenna, an array of radiating elements is used, disposed in a specific manner relative to the wavelength to be transmitted and fed by the same radio signals to which appropriate phase shift and amplitude laws are applied. The greater the gain in directivity sought, the larger the array has to be. The order of magnitude of the size of each radiating element is determined by the wavelength transmitted, i.e. in the decimetric range, and their arrayed arrangement leads to antennas that may be one to several meters in dimension. 
     The difficulties outlined above are further aggravated by the deployment of networks using different wavelength ranges. In Europe, for example, second generation digital systems use a band near 900 MHz (GSM, &lt;&lt;Global System for Mobile communications&gt;&gt;) and a band near 1800 MHz (DCS, &lt;&lt;Digital Cellular System&gt;&gt;), and future third generation systems (UMTS, &lt;&lt;Universal Mobile Telecommunication System&gt;&gt;) will use a frequency band near 2000 MHz. In order put in place an infrastructure for a new type of network, an operator who is already operating a different type of network has to provide new antennas. Either he will have to secure new sites or he will have to install more antennas on existing sites. In either case, there will be more antennas. 
     Furthermore, installing antennas operating in frequency ranges whose ratio is a small integer on the same site causes isolation problems due to the reception by one antenna of harmonics of the frequencies transmitted by another antenna. This situation will arise in the case of the GSM and DCS bands, for which, it is considered that the antennas, already cumbersome, must be spaced at least 50 centimeters apart. 
     A main object of the present invention is to propose an antenna arrangement that will enable radiating elements having different radiation characteristics (in terms of directivity and/or frequency) to be used together in a relatively compact layout in order to limit the difficulties outlined above. 
     Accordingly, the invention proposes an antenna for a radio communication base station, comprising several primary sources fed independently and arranged to have different radiation characteristics, the primary sources being placed in a first medium so as to be spatially decoupled. According to the invention, the antenna further comprises at least one second medium covering the first medium and having a substantially lower characteristic impedance than the first medium. Each primary source has at least one direction of focus perpendicular to the interface between the first and second medium, along which the distance between said primary source and said interface is substantially equal to λ 1 .(2p 1 −1)/4 and the second medium has a thickness substantially equal to λ 2 .(2p 2 −1)/4, where λ 1 and λ 2  denote the wavelengths radiated by said primary source in the first and second media, respectively, and p 1  and p 2  are integers. 
     The media surrounding the primary sources exhibit resonance conditions which procure a gain in directivity, in elevation and optionally in azimuth. The principle of physics underlying this resonance has been described in the case of conformed antennas in the article entitled &lt;&lt;Gain Enhancement Methods for Printed Circuit Antennas&gt;&gt; by D. R. Jackson et al., IEEE Transactions on Antennas and Propagation, Vol. AP-33, No. 9, September 1985, pages 976-987. The gain in amplitude obtained by the first and second media, having characteristic impedances Z c1  and Z c2  respectively, is in the order of 2.Z c1 /Z c2 . 
     The characteristic impedance Z c  of a medium with a relative dielectric constant ∈ r  and a relative magnetic permeability μ r  is given by            Z   c     =       Z   c0     ·         μ   r       ɛ   r             ,                          
     where Z c0 =120π. Consequently, the first and second media may have parameters ∈ r  and μ r  adapted as a function of the desired gain. 
     In a preferred embodiment, adaptation will essentially focus on the dielectric constants ∈ r , in order to use more readily available materials. Generally speaking, a medium with a high ∈ r  will be used for the second medium and ∈ r ≈1 in the first medium so as to maximize the ratio            Z   c1     /     Z   c2       =             ɛ   2     ·     μ   1           ɛ   1     ·     μ   2           ≈         ɛ   2       ɛ   1                                  
     (where ∈ r =∈ 1 , μ r =μ 1  in the first medium and ∈ r =∈ 2 , μ r =μ 2  in the second medium). 
     It is also possible to use composite materials, whereby the values of ∈ r  and/or μ r  can be adjusted to suit requirements. 
     In order to further enhance the gain of the antenna, the first medium may be covered by a superposition of focusing layers, the first focusing layer, adjacent to the first medium, being formed by said second medium, and each focusing layer being formed by a medium of a thickness substantially equal to λ i .(2p i −1)/4 along the direction of focus of each of the primary sources, where λ i  denotes the wavelength radiated by said primary source in the medium forming said focusing layer and p i  is an integer. The i-th focusing layer is formed, for each odd integer i, by a medium having a characteristic impedance substantially lower than the media located on either side of said i-th focusing layer. In particular, the i-th focusing layer may be made up, for each odd integer i, of a medium having a ∈ r  substantially higher than the media located on either side of this i-th focusing layer. 
     Increasing the number of focusing layers increases the gain in amplitude, which will be in the order of 2.        2   ·       ∏     m   =   0     k            Z     c        (       2      m     +   1     )           Z     c        (     2      m     )                                    
     if there are 2k focusing layers over the central high impedance medium, and in the order of        2   ·       ∏     m   =   1     k            Z     c        (       2      m     -   1     )           Z     c        (     2      m     )                                    
     if there are 2k−1 focusing layers, Z ci  denoting for i≧2 the characteristic impedance of the (i-1)-th focusing layer (see H. Y. Yang et al., &lt;&lt;Gain Enhancement Methods for Printed Circuit Antennas through Multiple Superstrates&gt;&gt;, IEEE Transactions on Antennas and Propagation, Vol. AP-35, No. 7, July 1987, pages 860-863). 
     In one embodiment of the antenna according to the invention, the primary sources are fed and arranged to radiate at different wavelengths. The antenna is then adapted to sites where base stations operating in different frequency bands are installed. 
     The dielectric media may be disposed parallel to an ground plane, in which case the antenna may be fitted on a wall. In another advantageous layout, the primary sources are disposed along an axis about which said media has revolution symmetry. This being the case, it will be possible to make omnidirectional and/or multi-sector antennas of a reduced size. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a base station fitted with an antenna proposed by the invention; 
     FIGS. 2 and 4 are perspective diagrams of an omnidirectional antenna and a three-sector antenna according to the invention; and 
     FIGS. 3 and 5 are lateral cross-sectional views of other antennas according to the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an antenna  1  according to the invention, installed at the top of a mast  2  (or any other structure) and connected to a base station  4  by means of cables  3 . 
     In the example illustrated in FIG. 1, the antenna  1 , shown in more detail in FIG. 2, is of the omnidirectional type and enables communication with mobile radio terminals in three separate frequency bands. By way of example, these might be the 900 MHz GSM band, the 1800 MHz DCS band and the 2000 MHz UMTS band. This being the case, the base station  4  in effect groups three base stations corresponding to the three types of network, and three coaxial cables (feeders) link these base stations to respective primary sources  6 A,  6 B,  6 C of the antenna  1 . 
     In the example illustrated in FIG. 2, each of the primary sources  6 A- 6 C is a dipole tuned to a central frequency of the frequency band associated with said source. Each dipole is connected to its feeder (not shown in FIG. 2) in a conventional manner, by which it is fed independently of the other dipoles. 
     The three dipoles  6 A- 6 C of the antenna of FIG. 2 are aligned on an axis X and surrounded by a focusing structure which is symmetrical in revolution about the axis X. 
     This focusing structure comprises a central medium having a relatively high characteristic impedance Z c1  with regard to radio waves. If no magnetic materials are used (μ 1 =1), this central medium will simply be selected so as to exhibit a dielectric constant ∈ 1  close to 1 so that Z c1 ≈Z c0 =120π. 
     This high-impedance medium occupies a cylindrical region  7 A,  7 B,  7 C around each dipole  6 A,  6 B,  6 C, aligned and centered on this dipole. The axial height of each of these regions  7 A- 7 C is in the order of the wavelength radiated by the corresponding dipoles  6 A- 6 C. Its radius d 1  (indicated for region  7 A only in FIG. 2) is of the form λ 1 .(2p 1 −1)/4, where p 1  is a positive integer preferably equal to 1, and λ 1  denotes the wavelength radiated by the dipole  6 A,  6 B,  6 C in the medium having impedance Z c1 . The wavelength λ 1  is given by λ 1 =λ 0 .{square root over (∈ 1 +L .μ 1 +L )}, the wavelength λ 0  being that radiated in vacuum by the source  6 A,  6 B,  6 C. 
     The high-impedance central medium  7 A,  7 B,  7 C is surrounded by a focusing layer  8 A,  8 B,  8 C formed by a medium having a relatively low characteristic impedance Z c2 . If no magnetic materials are used (μ 2 =1), a dielectric material with ∈ 2 &gt;&gt;1 is chosen for the focusing layer  8 A,  8 B,  8 C. 
     At the level of each source  6 A,  6 B,  6 C, the thickness e 2  of the focusing layer  8 A,  8 B,  8 C is taken as being equal to λ 2 .(2p 2 −1)/4, where p2 is a positive integer preferably equal to 1, and λ 2 =λ 0 .{square root over (∈ 2 +L .μ 2 +L )} is the wavelength radiated by the corresponding source  6 A,  6 B,  6 C in the low-impedance medium. 
     The high-impedance medium Z c1  used in the antenna  1  may be air. 
     It may also be formed by means of a honeycomb or foam material, whose dielectric constant decreases with density (see &lt;&lt;Radome Engineering Handbook, Design and Principles&gt;&gt;, J. D. WALTON Jr., Editions Marcel Dekker Inc., New York, 1970). Such a material may be made from resins or polymers, for example of the polyester, epoxy, phenolic polyimide or polyurethane type. 
     For the focusing layers of low impedance Z c2 , organic materials may be used in particular, such as a polyester (∈ r  of 4 to 5), an epoxy (∈ r ≈4) or a polyimide (∈ r =3.5). 
     If the cost of the antenna is not the most critical factor, materials with a very high permittivity may be used as an alternative, in particular inorganic compounds such as used in high-speed and high-temperature radomes, for example Al 2 O 3  (∈ r ≈9) or TiO 2  (∈ r ≈100). Such materials may be diffused in a ceramic base matrix, for example in silica, enabling the value of ∈ r  to be adjusted. 
     For reasons pertaining to cost and/or ease of manufacture, it may be practical to use composite dielectrics instead of natural dielectrics in order to obtain the desired values for the parameters ∈ r  and μ r . 
     By &lt;&lt;natural dielectric&gt;&gt; is meant a pure dielectric compound or a mixture of pure dielectric compounds on a microscopic scale. For example, polystyrene (∈ r =2.5) or lead glass (∈ r =7). 
     A composite dielectric is a macroscopic assembly of discrete metal or dielectric particles, disposed regularly in three spatial dimensions and in various forms: spheres, discs, strips, rods or wires. The assembly is held together by a base: for example, the particles are coated in a homogeneous dielectric medium or disposed on dielectric plates. In each case, the index of the base is not much different from 1. If the dimensions of the particles and the distance between particles are small compared with the wavelength, the behavior of these assemblies will be identical to that of a natural dielectric. The weight, on the other hand, may be very much reduced and the dielectric constant can be quite finely adjusted. 
     The value of ∈ r  for such an artificial dielectric is determined on a sample or by approximation formulae. For example, an arrangement made up of N metal spheres of a radius a per unit volume will give a dielectric constant of a value: ∈ r =1+4πNa 3 . It is thus possible to obtain an ∈ r  ranging from 1 to 9. 
     For the media with high impedance Z c1 , it is possible to adjust the parameter μ r  in a similar way and obtain inexpensive, lightly magnetic, low loss composite materials with an appropriate concentration of iron particles in a base of plastics or resin material. 
     The focusing structure is assembled by a molding process, for example, once the sources  6 A- 6 C and their feeders have been placed in position. If the mechanical strength of one or other of the dielectric media so requires, it may be reinforced, for example with glass fibers. It is also possible to use base, coating or protective elements provided they do not interfere with the electromagnetic behavior of the unit. 
     The focusing structure may also be made on a modular basis. 
     The largest dimension of the antenna  1  of FIG. 2 is its axial height which, in the example illustrated here, may remain in the order of 50 cm. The multi-frequency antenna therefore meets the requirement of a very compact system. 
     Each of the dipoles  6 A,  6 B,  6 C has an omnidirectional radiation pattern, with a set of focusing directions A, B, C contained within the equatorial plane of the dipole. The above-mentioned resonance phenomenon enhances focusing of the waves transmitted by the dipoles  6 A- 6 C in these directions A-C (elevation focusing). The gain in amplitude secured by the composite focusing structure is given by 2.Z c1 /Z c2 . The power gain g, expressed in dB, is given by g=20.log 10 (2.Z c1 /Zc 2 ). As may be seen, focusing gains of several decibels are readily obtained. 
     This gain may be increased by adding focusing layers of alternating high and low impedance. 
     The antenna  11  illustrated in FIG. 3 is of a generally flat design. The high-impedance medium  17 A,  17 B,  17 C containing the dipoles (or other primary sources)  16 A,  16 B,  16 C is deposited on a conductive ground plane  15 . On a level with each source  16 A,  16 B,  16 C, this medium  17 A,  17 B,  17 C forms a layer of thickness λ 1 .(2q−1)/2, λ 1  being the wavelength radiated in the medium by the relevant source and q a positive integer advantageously equal to 1. The distance d 1  between the source  16 A,  16 B,  16 C and the interface with the first low-impedance focusing layer  18 A,  18 B,  18 C is of the form λ 1 .(2p 1 −1)/4. The thickness e i  of the (i−1)-th focusing layer (i≧2) is of the form λ i .(2p i −1)/4. The successive focusing layers ( 18 A,  19 A,  20 A), ( 18 B,  19 B,  20 B), ( 18 C,  19 C,  20 C) are of alternating low impedance and high impedance, i.e. for each odd integer i, the i-th focusing layer is formed by a medium whose characteristic impedance Z c2  is lower than that Z c1  of the media located on either side of this i-th layer. 
     The antenna  11  illustrated in FIG. 3 may be installed on a wall, for example, so as to radiate directively (directions A-C) towards a zone to be covered by the base station. 
     FIG. 4 is a schematic illustration of a multi-sector antenna made in accordance with the invention. The geometry of the focusing structure is symmetrical in revolution about the axis X along which three primary sources  26 A,  26 B,  26 C are aligned. Each of these primary sources is for example provided in the form of a square conducting patch formed on a dielectric substrate (microstrip technology). This type of source has directivity in both azimuth and elevation, in a direction A, B, C perpendicular to the substrate. The focusing structure, of a cylindrical geometry, enables elevation focusing, and hence the gain of the antenna  21 , to be enhanced. In order to limit the size of the sources  26 A- 26 C at the core of the focusing structure, they may be formed on a substrate with a high ∈ r . 
     In the example illustrated in FIG. 4, the three directive primary sources  26 A- 26 C are tuned to the same frequency and are disposed along the axis X so that their focusing directions A-C are radial directions oriented at 120° relative to one another. The antenna therefore covers three sectors. 
     The high-impedance central medium  27  and the focusing layer  28  (and optionally subsequent layers not illustrated) have dimensions determined as mentioned above, taking account of the wavelength radiated by the sources  26 A- 26 C. 
     It should be pointed out that it is possible to add an omnidirectional antenna such as a dipole to the primary sources  26 A- 26 C forming a multi-sector antenna of the type illustrated in FIG. 4, thereby obtaining a combined antenna. 
     The antenna  31  illustrated in FIG. 5 is of a design generally similar to that of FIG. 3, with a single low-impedance focusing layer  38 A,  38 B,  38 C over the high-impedance media  37 A,  37 B,  37 C containing the dipoles  36 A,  36 B,  36 C. The different media  37 A-C,  38 A-C fulfil to the above-discussed spatial resonance conditions. The interface between the successive media is inclined relative to the ground plane  35  and the primary sources  36 A-C so that the refraction of the waves inclines the focusing directions A-C downwards in the example illustrated here. This enables the radiation pattern of the antenna to be adapted to suit requirements. 
     In another embodiment based on the same principle, the interfaces between dielectric layers are parallel with the earth plane and it is the dipoles which are inclined. 
     Naturally, the focusing directions could be inclined in a similar manner in the case of an antenna designed to be symmetrical in revolution of the type illustrated in FIG. 2 or  4 , which will then be conical in shape rather than cylindrical. 
     An antenna according to the invention may be made using various types of primary sources (simple or crossed dipoles, slots, microstrip patterns), each disposed outside the transmission lobes of the others in order to ensure that they are electromagnetically decoupled from one another. 
     In the case of a multi-sector antenna, the primary sources may be placed on or conformed to a non-planar metal surface, for example a cylindrical or conical surface, which improves the forward-backward ratio of the antenna. The cylinder or cone bounded by this surface is symmetrical relative to the axis of the antenna. For example, it has a circular, triangular or polygonal section.