High frequency, multiple beam antenna system

The high frequency, multiple beam antenna system comprises a focusing device having a profile of revolution created by the cross section of a dielectric lens rotating about an axis located in its plane and radiating elements with form of directional-printed antennas with longitudinal radiation.

This application claims the benefit, under 35 U.S.C. 119, of France patent application No. 0350765 filed Oct. 31, 2003.

The present invention relates to a high-frequency, multiple beam antenna system. More specifically, the invention relates to a high gain millimetric antenna with multiple radiating elements (or primary sources) that illuminate a focusing device to radiate 360° in azimuth.

BACKGROUND OF THE INVENTION

The invention is intended more specifically for a high bit rate wireless communication network using the LMDS (Local Multipoint Distribution Service) system, which is based on a cellular architecture. In this architecture, a sending/receiving station equipped with antennas to be able to communicate with the other stations of the cell, can serve as a node of the cell. In this case, the architecture is called “P-MP” (Point-MultiPoint). Another possible architecture for this system is the “MP—MP” (MultiPoint-MultiPoint) architecture, in which each station can be a relay in a call between two other stations of the wireless network.

The millimetric frequencies (30 to 3000 GHz) or EHF (Extra High Frequencies) are used with a view to increasing the information transfer rates in the wireless networks. At such frequencies, the available bandwidths are wide (greater than 1 GHz) but the attenuation as a function of distance is high.

The coverage rate is therefore limited by the short range of the millimetric frequency transmit stations that make up such a wireless network, and by the need to have an “LOS” (Line Of Sight) between a sending station and a receiving station of the network. Despite the low cost and the performance of the LMDS systems at millimetric frequencies, their coverage limitations mean that they cannot be deployed intensively.

In an MP—MP architecture, or mesh network architecture, in which each station of the network can be a relay station, the obstacles can be circumvented. Thus, the coverage and the capacity of the high bit rate wireless network are improved.

The attenuation as a function of distance limiting the transmission range between two stations of the high bit rate wireless network is offset by a high antenna gain. Increasing the gain of an antenna involves improving its directivity and therefore concentrating its radiation pattern in a precise direction. Consequently, the alignment of the antenna must also be accurate.

Furthermore, changing the configuration of the network must involve a reliable realignment of the antenna system of the stations of the network with a 360° coverage in azimuth for each station.

A solution proposed by the Radiant Networks company is an antenna system made up of four high gain millimetric antennas. The system uses an access technique known as “TDMA/TDD” (Time Division Multiple Access, Time Division Duplex). In this technique, the time is divided into frames of a fixed duration, which are in turn subdivided into “slots”. The slots are used individually for sending/receiving between two antennas aligned for a call between their respective stations. The antennas are aligned mechanically through the intermediary of a motor. This solution is complex, expensive and bulky. Furthermore, the mechanical alignment is neither reliable nor instantaneous.

Another solution is described in patent application GB2238174A. This document describes a high-frequency antenna made up of a set of dielectric lenses adjacent to one another and arranged to obtain 360° coverage in azimuth. The rear surface of each lens is itself made up of several radiating elements for sending and receiving. These elements are precisely arranged to send or receive beams according to different, evenly spaced angular directions, the periodicity of which is maintained from one lens to the next. The lenses are delimited on each side by a flat surface, the direction of which passes through the central axis of symmetry of the optical system. This antenna system is complicated to implement. In this system, a number of radiating elements are used for the same lens. The result is, necessarily, that certain of these radiating elements are out of focus. The antenna system does not present the same radiation pattern, and in particular the same directivity, in all the directions corresponding to the feeds.

BRIEF SUMMARY OF THE INVENTION

The invention proposes a simpler millimetric antenna system, which satisfies the requirements of a network using a mesh network architecture and which rectifies the drawbacks described above.

In particular, the invention proposes a millimetric antenna system having a 360° coverage in azimuth and a high gain and which is inexpensive.

To this end, the invention relates to a high-frequency antenna system as described above, this antenna system comprises a focusing device having a profile of revolution created by the cross section of a dielectric lens rotating about an axis located in its plane and radiating elements in the form of directional printed antennas with longitudinal radiation.

The dielectric lens can be axisymmetric, for example with a crescent-shaped cross section or with a circular, monofocal, bifocal, multifocal cross section, with perfect or imperfect focusing, etc.

An antenna system according to the invention can offer the following particular characteristics:the radiating elements are printed on a common substrate.each radiating element is a “Vivaldi” type printed slot antenna which means that the illumination of the antenna system can be adjusted with high design flexibility by adjusting the length and width at the end of the slot forming the “Vivaldi” type radiating element.it is equipped with transmit and/or receive and/or switching circuits arranged on said common substrate.the focusing device has an annular profile of revolution, the substrate is disc-shaped and the radiating elements are arranged along the periphery of the substrate to obtain a 360° coverage in azimuth.the radiating elements are arranged around transmit and/or receive and/or switching circuits which helps to reduce the bulk of the antenna system.the focusing device is made of synthetic foam.

The invention is extended to a sending and/or receiving station with an antenna system as defined above, and to a communication network with sending/receiving stations equipped with an antenna system according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Generally, a focusing device for a millimetric antenna system according to the invention takes the form of a kind of “buoy” with annular profile of revolution and constant radial section.

FIG. 1represents a first exemplary embodiment of a focusing device having a profile of revolution created by the crescent-shaped cross section of a dielectric lens2rotating about an axis1located in its plane. In this example, the focusing area comprising all the focal points is circumscribed on a circle3. The focus is therefore perfect.

FIG. 2shows another example of a focusing device according to the invention. This focusing device has a profile of revolution created by the circular-shaped cross section of a dielectric lens5rotating about an axis4located in its plane. In this example, the focusing area comprising all the focal points is circumscribed in a ring6. The focus is therefore imperfect.

Naturally, the invention is extended to a focusing device with a different profile of revolution, which can be obtained from a cross section of a lens, that is neither circular nor “crescent”-shaped.

FIG. 3very schematically illustrates a printed circuit substrate10on which are printed Vivaldi antenna type radiating elements11and switching and transmit/receive circuits13. This disc-shaped substrate is placed at the centre and in the horizontal plane of symmetry of a focusing device such as, for example, that illustrated inFIG. 1or2.

As can be seen inFIG. 3, the Vivaldi antennas11are distributed in a circle around the periphery of the substrate to provide a 360° coverage in azimuth. The phase centre of each Vivaldi antenna should coincide with a focal point of the focusing area3or6.

Furthermore, the Vivaldi antennas are directional slot antennas with longitudinal radiation. In the case of the invention, the main direction of their radiation corresponds to the plane of the substrate10. This type of antenna provides for relatively easy control of the focusing device (in this case, the buoy), by an adjustment of the length, the profile and the width at the “mouth” of the “Vivaldi” antenna. The illumination control of the focusing system is used to control the radiation pattern and in particular the directivity of the antenna system.

As described above, the reference13designates transmit/receive circuits and a switching device, the latter selecting the radiating element corresponding to the given azimuth direction. As can be seen inFIG. 3, the antennas11are arranged around the circuits13which are thus concentrated at the centre of the substrate10. At the centre of the substrate, it is also possible to print signal processing circuits.

Combining all these elements11and13on a same common substrate simplifies the antenna system and makes it less bulky.

FIG. 4illustrates the radiation pattern of an antenna system according to the invention in the vertical plane20and in the horizontal plane21.

The radiation pattern is obtained by illuminating a portion of the buoy-shaped focusing device via a radiating element11.

It can be seen that in the vertical plane20, the directivity of the radiation pattern22obtained is the same as that obtained from a axisymmetric lens. InFIG. 4, θe designates the aperture angle of the antenna in elevation at −3 dB.

Conversely, in the horizontal plane21, the directivity of the radiation pattern23obtained is less than that obtained from a lens of revolution in the case of identical illumination in azimuth by a radiating element. It is known that, in the case of a lens of revolution, the illumination by a radiating element having a pattern of revolution can be used to obtain an equivalent radiating aperture virtually uniform in phase and in amplitude. In the case of the antenna system according to the invention, the focusing device, by its tubular shape, introduces phase and amplitude distortions resulting in a loss of directivity. InFIG. 4, θa designates the azimuth aperture at −3 dB.

The use of a Vivaldi type slot antenna according to the invention provides for a control of the length, of the profile and of the aperture of the slot at the “mouth”11. A narrower aperture provides illumination of a greater portion in azimuth of the focusing device (greater angle ThetaV). The gain and therefore the directivity of the antenna in azimuth are increased, since the illuminated area is greater. However, illuminating a wider portion in azimuth of the focusing device also causes greater phase distortions. A maximum directivity in azimuth is obtained by optimization, by adjusting the radius24of the focusing device and the directivity of the Vivaldi antenna in the horizontal plane.

The antenna system according to the invention is configured as follows:the focal distance25(F) is determined by the shape of the cross section of the focusing device, the permittivity of the supposedly homogeneous material, and the height26(D) of the radial section of the focusing device according to the axis of rotation such as 1 or 4.the radius24of the focusing device must be greater than the focal distance25. It can be increased to have a greater space available at the centre of the focusing device so that the substrate10can contain not only the Vivaldi antennas but also the excitation system including the transmit/receive circuits and the switching device13.the parameters of the radiating element: θv vertical aperture angle at −3 dB and θh horizontal aperture angle at −3 dB.

An estimation of the gain G of the antenna is given by the relation:

G (in dB)=10 log (K/θe θa) (1) in which K is a constant with a value of between approximately 26000 and 35000 inclusive according to the illumination efficiency of the antenna.

The antenna gain must be sufficient to offset the attenuation as a function of distance and thus be compatible with the requirements of a high bit rate wireless network.

An approximate value of the aperture angle in elevation at −3 dB is given by the following relation:
Θe=kλ/D(2)

in which

λ designates the wavelength of the working frequency

D designates the height of the radial section of the focusing device

k is a constant typically varying between 60 and 80 according to the illumination efficiency of the antenna

In a first approximation, θa can be taken to be equal to θh.

It is always possible to increase the value of the antenna gain by increasing the height26(D). The number N of radiating elements needed to obtain a 360° azimuth coverage and for a gain value greater than Gmin=G−3 dB is given by the following relation:
N=360°/θa(3)

An antenna system according to the invention has been produced and the focusing device presents the following characteristics:homogeneous lens,circular inner profile, elliptical outer profile,the synthetic foam used for the focusing device is typically polystyrene prefilled with dielectric material, the material has a permittivity εr<2, preferably a permittivity equal to 1.56,height D of 11.5 cm,frequency 42 GHz,

An aperture angle in elevation is obtained which is derived from the relation (2) Θe=4° (for k approximately equal to 65).

The radiating element11has an aperture angle of 28° at −3 dB to the horizontal (θh) and to the vertical (θv). If in a first approximation, it is assumed that θa equals θh, the number N of radiating elements needed to have a 360° azimuth coverage which is given by the relation (3) is equal to
N=360/28=13.

With this antenna system construction, an antenna gain is obtained which is:1. G=23.6 dB for K=260002. G=24.9 dB for K=35000

taking into account the 3 dB of losses at the edge of the beam, the minimum gain for the antenna system is between 20.6 and 21.9 dB inclusive.

In the example described above of an antenna system according to the invention, Vivaldi slot antenna dimensions have been calculated to provide, for the antenna system, a minimum gain of between 20.6 and 21.9 dB inclusive, where the length of the profile of the slot must be 26 mm and the aperture 9 mm.

The thirteen Vivaldi antennas are distributed in a circle along the focusing area of the disc-shaped substrate10which has a diameter of approximately 8 cm with a 25 mm diameter space in the centre containing the switching circuits and the transmit/receive circuits13. If necessary, the diameter of the disc10can be increased to provide more space in the centre to contain the rest of the antenna circuits.

The focusing device according to the invention can also have a profile obtained from a cross section of a non-homogeneous dielectric lens, with graded index for example.

The invention can also be applied to interior domestic communication networks in particular at 60 GHz with a mesh network architecture.

In an antenna system according to the invention, the radiating elements have a horizontal polarization as in the case of the Vivaldi antennas. As a general rule, these radiating elements are planar coplanar radiating elements arranged on a substrate extending in the horizontal plane of symmetry of the buoy-shaped focusing device. In the case of a double polarization or a vertical polarization, horns can be used as the radiating elements.