WO2001052356A1

WO2001052356A1 - Resonant cavity antenna having a beam conformed according to a predetermined radiation diagram

Info

Publication number: WO2001052356A1
Application number: PCT/FR2001/000109
Authority: WO
Inventors: Ronan Sauleau; Philippe Coquet; Toshiaki Matsui; Jean-Pierre Daniel
Original assignee: Universite De Rennes 1
Priority date: 2000-01-12
Filing date: 2001-01-12
Publication date: 2001-07-19
Also published as: AU2001231893A1; FR2803694A1

Abstract

The invention concerns a resonant cavity antenna having a beam conformed according to a predetermined radiation diagram and comprising: a resonant cavity (21; 31) especially having a lower face and an upper face and consisting of a dielectric substrate (22; 32) covered respectively on the bottom and top faces thereof with a network of semi-reflective patterns (24, 23; 34, 33); excitation means (25; 35) of said resonant cavity in said bottom face for a radiating source; optionally, at least one layer consisting of a dielectric buffer substrate (26) located between the bottom face of said resonant cavity and the aforementioned excitation means. According to the invention, said resonant cavity is a flat plane cavity, the top and bottom faces of the dielectric substrate of said resonant cavity being flat and substantially parallel relative to one another.

Description

Resonant cavity antenna having a beam shaped according to a predetermined radiation pattern.

The field of the invention is that of focusing devices for electromagnetic waves, mainly (but not exclusively) at millimeter and submillimeter wavelengths.

More specifically, the invention relates to an antenna of the type comprising a resonant cavity and means for exciting the latter, so as to present a beam shaped according to a predetermined radiation pattern. The predetermined radiation pattern can in particular, but not exclusively, be Gaussian. The invention has numerous applications, such as for example point-to-point communications applications, which are currently developing in millimeter bands (intra-building wireless communications, high-speed communications (60 GHz), anti-collision radars automotive (77 GHz), ...).

More generally, it can be applied in all cases where more or less directive and compact radiation devices ("general public" applications) are necessary.

In this type of application, the main techniques known from the prior art are the following: planar reflectors, lens antennas, substrate lenses, antenna arrays and Gaussian beam antennas. However, it appears that none of these techniques is satisfactory. In fact, planar reflectors and antenna arrays sometimes have insufficient efficiency. Furthermore, the lens antennas and the substrate lenses do not have sufficient compactness. Finally, the Gaussian beam antennas also have faults which are discussed in detail below. A classic Gaussian beam antenna (or GBA, for "Gaussian Beam Antenna" in English) is illustrated in FIGS. 1A (overall simplified diagram of the antenna), 1B (top view of the upper grid) and 1C (diagram of Gaussian radiation). The resonant cavity 1, of the Perot-Fabry type, comprises a dielectric substrate 2 (permittivity ε _r ) of "convex piano" shape: the lower face is planar while the upper face is spherical (radius of curvature R ₀ ). This dielectric substrate 2 has a thickness D (≈ λJ2) in the center. On each of these faces, a grid is formed two-dimensional (2D) semi-reflective and uniform 3, 4 (cf. fig.lB), the patterns of which are identical and repeat periodically (spatial period a, and track width d „with i equal to 1 (for the lower grid ) or 2 (for the upper grid)). The lower 3 and upper 4 grids can be identical or different. The radiated field (cf. fig. LC) has the form of an undepointed Gaussian beam, more or less directional and having no secondary lobe. The resonant cavity 1 is excited from the side of the underside thereof, by a radiating source 5 (source of horn type, as in the example illustrated).

With such a conventional GBA antenna, it is the spherical geometry of the cavity which is responsible for establishing a transverse distribution of the Gaussian field.

The curved face of the "convex piano" substrate, on which the upper uniform 2D grid is placed, forms a spherical mirror and plays the role of an equiphase surface for the generated Gaussian beam. Its radius of curvature R ₀ determines in particular the directivity of the antenna, that is to say its focusing power. Unfortunately, the convex piano substrates are expensive and do not lend themselves easily to the usual process of making grids by photolithography, especially for "low" values of R ₀ (UV diffraction due to the curvature (arrow) of the piano-convex substrate ).

In addition, the spherical surface is generally obtained by polishing and the thickness of the substrate (which forms a lens) can only be guaranteed to within a few tens of microns. As the resonant frequency of the cavity strongly depends on the thickness D of this lens, it is essential to measure this thickness precisely beforehand, then to choose the grid parameters of the two faces accordingly, in order to respond to a specification of loads imposing resonant frequency and bandwidth. Otherwise, the frequency shift, which can reach several bandwidths at half power, penalizes the power budget (hence a degradation in the quality of a wireless link for example).

The invention particularly aims to overcome these various drawbacks of the state of the art. More specifically, one of the objectives of the present invention is to provide an antenna of the type comprising a resonant cavity and means for exciting the latter, this antenna not requiring a convex piano substrate.

The ^invention also aims to provide an antenna that is simpler to manufacture and less expensive than conventional antenna GBA discussed above.

These various objectives, as well as others which will appear subsequently, are achieved according to the invention using a beam antenna conformed according to a predetermined radiation pattern, of the type comprising: a resonant cavity having in particular a face lower and an upper face, and consisting of a dielectric substrate covered on each of its upper and lower faces with a network of semi-reflecting patterns, upper and lower respectively, means for exciting said resonant cavity, on the side of said lower face, by a radiating source, - optionally, at least one layer of a buffer dielectric substrate, situated between the lower face of said resonant cavity and said excitation means.

According to the present invention, said resonant cavity is a cavity with planar faces, said upper and lower faces of the dielectric substrate of said resonant cavity being plane and substantially parallel to one another. In addition, said arrays of upper and lower semi-reflecting patterns and / or said excitation means and / or said at least one layer of buffer dielectric substrate are chosen with characteristics making it possible to simulate the effect of at least one curvature. on at least one of said flat faces of said cavity, and thus allow the obtaining of said predetermined radiation pattern.

The general principle of the invention therefore consists in using a blade with parallel faces (instead of a convex piano lens) and playing on certain specific parameter (s) of at least one of the elements included. in the antenna to simulate the effect of at least one curvature of the cavity, and therefore adjust in particular the directivity of the antenna. By specific parameters, we mean in particular, but not exclusively, the dimensions, the geometry, the permittivity, the uniformity, the thickness, etc. Thus, we play for example on the dimensions and / or the geometry of each semi-reflective pattern taken individually, or on the homogeneity and / or the thickness of the layer (s) of dielectric substrate (s) buffer (s) , or also on the geometry of the excitation means. It is important to note that the antenna may possibly not include a layer of buffer dielectric substrate. In this case, one can of course only play on the networks of semi-reflecting patterns (or semi-reflecting mirrors) and / or the excitation means.

Such a blade with parallel faces is inexpensive and easily lends itself to the usual method of manufacturing semi-reflective patterns by photolithography. It is also understood that the antenna according to the present invention makes it possible to easily respond to specifications imposing resonant frequency and bandwidth, since these are parameters of constituent elements of the antenna that should be chosen appropriately. In a first preferred embodiment of the invention, said network of lower semi-reflective patterns is uniform, and said network of upper semi-reflective patterns is non-uniform and has characteristics such that the planar upper face of said resonant cavity simulates the effect of at least one curvature.

In this first preferred embodiment, we therefore play on the non-uniformity (or non-periodicity) of the patterns of the upper network to simulate at least one curvature on the upper flat face of the cavity. The characteristics (or parameters) of this upper network, which it is appropriate to choose according to the present invention, are in particular the geometry and / or the dimensions of each pattern. For example, if the patterns of this network form a grid, the spatial (pseudo-) period and the width of the metal tracks constituting this grid are chosen. This choice is for example such that the equiphase surface of the wave reflected on the internal face of the non-uniform upper network is spherical, with a radius of curvature R _Q. It is the optical path which compensates for the phase difference existing between the reflection coefficients at the different reflection points. Advantageously, said excitation means belong to the group comprising: planar antennas and radiating openings having no conductive collar and / or not creating phase discontinuity on the underside of the cavity.

These different excitation means do not create phase discontinuity on the underside of the cavity. According to an advantageous variant, said excitation means and / or said at least one layer of buffer dielectric substrate create at least one phase discontinuity on said underside of said resonant cavity, so that the planar underside of said resonant cavity simulates at least one curved face.

In this particular case of the first preferred embodiment, it is therefore also created at least one phase discontinuity, to simulate at least one curvature on the flat face of the cavity which carries the lower network. In other words, a total of at least two curvatures is simulated (at least one on each of the plane faces of the cavity). It should be noted that by combining several phase discontinuities, it is also possible to create several curvatures on each face of the resonant cavity. A non-exhaustive list of such excitation means creating at least one phase discontinuity is given below, in relation to a second embodiment of the invention.

In a second preferred embodiment of the invention, said arrays of upper and lower semi-reflecting patterns are uniform, and said excitation means and / or said at least one layer of a dielectric buffer substrate create at least one discontinuity phase on said underside of said resonant cavity, so that the planar underside of said resonant cavity simulates at least one curved face.

In this second preferred embodiment, one therefore does not play on the networks of semi-reflecting patterns but on the excitation means and / or on the layer (s) of buffer dielectric substrate. This involves simulating (by creating at least one phase discontinuity) at least one curvature on the lower flat face of the cavity (which carries the network of lower semi-reflective patterns).

Advantageously, said excitation means belong to the group comprising: radiating openings with conductive flange; slots or networks of slots; planar antennas each associated with at least one layer of a partially metallized buffer dielectric substrate on its upper face. Preferably, said networks of semi-reflecting patterns belong to the group comprising: networks of uniform semi-reflecting patterns and networks of non-uniform semi-reflecting patterns.

It is clear that this list is not exhaustive. Furthermore, whether they are uniform or not (that is to say periodic or not), the patterns can be of different types, and even possibly associated with solid surfaces.

Advantageously, said antenna operates at millimeter and sub-millimeter wavelengths.

Other characteristics and advantages of the invention will appear on reading the following description of a preferred embodiment of the invention, given by way of non-limiting example, and the accompanying drawings, in which: FIG. 1A presents a simplified diagram of a Gaussian beam antenna according to the prior art; Figure 1B shows a top view of the upper grid appearing in Figure 1 A; - Figure 1C shows the radiation pattern of the antenna according to the prior art shown in Figure 1 A; FIG. 2A shows a first preferred embodiment of an antenna according to the present invention; Figure 2B shows the radiation pattern of the antenna shown in Figure 2A; Figure 3 shows a second preferred embodiment of an antenna according to the present invention; and FIGS. 4A and 4B illustrate the operation of the antenna of FIG. 3, with in particular a phase discontinuity (fig.4A) and a simulated curvature (fig.4B). The invention therefore relates to an antenna of the type comprising a resonant cavity and means for exciting the latter, so as to present a beam shaped according to a predetermined radiation pattern.

In the following description, we only discuss the case of a Gaussian single-beam radiation diagram. It is clear that the present invention applies more generally to all types of radiation diagrams, substantially Gaussian or not, mono or multi-beam (x), of revolution or not, defocused or not, electronically controllable, ...

The antenna according to the invention operates for example in linear or circular polarization.

We now present, in relation to FIGS. 2A and 2B, a first preferred embodiment of an antenna according to the present invention.

The resonant cavity 21 is a dielectric strip 22 with parallel faces, of permittivity ε _r , of thickness D and of diameter φ ^ The dielectric constituting the strip 22 may be air or any other dielectric material. In the case of air, at least one layer of dielectric material 26, 27 should be provided, on either side of the cavity, in order to form a support for the cavity made up of air.

On the lower face (excitation side) of this dielectric strip 22 is produced a uniform semi-reflective 2D grid 23. On the upper face (radiation side) is produced a non-uniform semi-reflective 2D grid 24.

The excitation is done by a planar antenna 25, via a layer of a dielectric buffer substrate 26 (air for example, or any other dielectric material).

The parameters (spatial period and width of the metal tracks in particular) of the non-uniform grid 24 are for example chosen so that the equiphase surface of the wave reflected on the internal face of the non-uniform grid 24 is spherical. In this case, as illustrated in FIG. 2B, the radiation diagram is of revolution of the Gaussian type. The equiphase wave surface has a radius R ₀ . It is the optical path which compensates for the phase difference existing between the reflection coefficients at the different reflection points. If it is assumed that the spatial period of the non-uniform grid 24 is constant, the problem therefore consists in defining the width d (p) of the metal tracks. Let T _{0 be} the reflection coefficient at the center of the non-uniform grid 24 (at p = 0). The non-uniform grid 24 behaves like a spherical equiphase surface (of radius of curvature Ro) at the resonance frequency f _res = c ₀ / λ ₀ , if the argument of T (p) is defined by the following relation:

To determine the width of the metal track at the distance p from the center, it suffices to refer to charts giving the evolution of the phase of the reflection coefficient of a plane wave at normal incidence on a grid located at an interface. dielectric (ε _r ) / dielectric (s) buffer (s) 26. The approach therefore amounts to performing a linear approximation by pieces of the spherical wavefront defined by the relation (1) above.

Thus, from a resonant cavity with planar faces, the effect of a curvature on the upper face (radiation side) is simulated. The directivity of the antenna can be adjusted by checking the non-uniformity of the upper grid 24.

By correctly choosing the type of grids and their parameters, this technique can of course extend to the formation of other types of radiation patterns, not necessarily Gaussian. In addition, this technique also makes it possible to generate single or multi-beam (x) radiation patterns, possibly depointed, electronically controllable, etc.

We now present, in relation to FIGS. 3, 4A and 4B, a second preferred embodiment of an antenna according to the present invention.

As in the first embodiment, the resonant cavity 31 is a dielectric strip 32 with parallel faces, of permittivity ε _r , of thickness D and of diameter φ _j . On each of the two faces (lower, excitation side, and upper (radiation side)) of this dielectric strip 32, a uniform semi-reflective 2D grid 33, 34 is produced.

The excitation is done by a horn 35 (for example pyramidal) with conductive flange creating a phase discontinuity (cf. fig. 4A) on the underside of the resonant cavity. This phase discontinuity can be represented by an equivalent convex mirror with a radius of curvature R ₀ ', causing the beam to be focused inside the resonant cavity. In other words, the lower (flat) face of the resonant cavity simulates a curved face with a radius of curvature R ₀ '(cf. fιg.4B), and therefore allows the formation of a beam, for example of the Gaussian type. The directivity and the opening angle at - 3 dB depend essentially on the geometry of the grid printed on the lower flat face and on the dimensions of the horn.

In general, all types of excitation means allowing the creation of such a phase discontinuity can be used, such as in particular waveguides with conductive flange, slots, networks of slots, etc.

It is of course possible to combine the two embodiments described above. In this case, at least one curvature is simulated on each of the two flat faces of the resonant cavity, thanks to the use of a non-uniform upper grid (first embodiment) and of excitation means creating a phase discontinuity. on the underside (second embodiment).

Optionally, one can also play on the thickness and / or the homogeneity of the dielectric buffer substrate layer 26, so as to create at least one additional phase discontinuity on the underside of the resonant cavity, and therefore simulate at minus an additional curvature on this underside. We now present an embodiment of such a combination. This example has been successfully tested in the 60 GHz frequency range. The radiation pattern (not shown) of this antenna is indeed directional and Gaussian in appearance with secondary lobes less than - 30 dB.

The resonant cavity consists of a dielectric plate (molten quartz substrate, with permittivity ε _r = 3.80, diameter φj = 20 mm and thickness D = 1.259 mm) with parallel faces on which grids are made ( lower and upper). This resonant cavity is excited by a pyramidal horn with square opening and side c = 9 mm. The assembly comprising the cavity and the horn constitutes the antenna. On the exciter side, the lower grid is uniform and on the outside, the upper grid is non-uniform and simulates a curved face. The geometric characteristics of the resonant cavity in this example are as follows ("a," denotes the spatial period of the grids and "d," the width of the metal tracks):

- lower face (i = 1): a _! = 1.3 mm d ^ a, = 50% reflectivity at resonance: 97.7%

- upper face (i = 2): d ^ = 0.6 mm

30% <d ₂ / a ₂ <51.6% reflectivity R at resonance: 96.2% <R <99.7% simulated radius of curvature: 400 mm

It is clear that many other embodiments of the invention can be envisaged.

We can in particular replace the two-dimensional inductive grids described above by other types of networks of semi-reflecting patterns: - grids, capacitive or inductive, mono (1D) or two-dimensional (2D); grids with Jerusalem cross; concentric rings of possibly variable lengths and / or radii; patterns networks forming openings in a predetermined shape (pseudo-circular, pseudo-square, pseudo-rectangular, ...); dipole arrays; etc. A one-dimensional grid (1D) is for example made up of a plurality of tracks or conductive strips parallel to each other. A two-dimensional (2D) grid is for example made up of two sets of tracks or conductive strips parallel to each other within the same set, each set being associated with a separate direction (the directions of the two sets can be orthogonal or not ).

Claims

1. Beam antenna shaped according to a predetermined radiation pattern, of the type comprising: a resonant cavity (21; 31) having in particular a lower face and an upper face, and consisting of a dielectric substrate (22; 32) covered on each from its upper and lower faces of a network of semi-reflecting patterns, upper and lower respectively (24, 23; 34, 33), means (25; 35) of excitation of said resonant cavity, on the side of said face lower, by a radiating source, possibly, at least one layer of a buffer dielectric substrate

(26), located between the underside of said resonant cavity and said excitation means, characterized in that said resonant cavity is a cavity with planar faces, said upper and lower faces of the dielectric substrate of said resonant cavity being planar and substantially parallel to each other, and in that said arrays of upper and lower semi-reflecting patterns and / or said excitation means and / or said at least one layer of buffer dielectric substrate are chosen with characteristics making it possible to simulate the effect of 'at least one curvature on at least one of said flat faces of said cavity, and thus allow obtaining said predetermined radiation pattern.

2. Antenna according to claim 1, characterized in that said array of lower semi-reflective patterns (23) is uniform, and in that said array of upper semi-reflective patterns (24) is non-uniform and has such characteristics that the planar upper face of said resonant cavity simulates the effect of at least one curvature.

3. Antenna according to claim 2, characterized in that said excitation means belong to the group comprising: planar antennas (25); radiant openings having no conductive collar and / or not creating phase discontinuity on the underside of the cavity.

4. Antenna according to claim 2, characterized in that said excitation means (35) and / or said at least one layer of buffer dielectric substrate create at least one phase discontinuity on said underside of said resonant cavity, so that the flat underside of said resonant cavity simulates at least one curved face.

5. Antenna according to claim 1, characterized in that said arrays of upper and lower semi-reflecting patterns (34, 33) are uniform, and in that said excitation means (35) and / or said at least one layer of a buffer dielectric substrate create at least one phase discontinuity on said underside of said resonant cavity, so that the planar underside of said resonant cavity simulates at least one curved face.

6. An antenna according to any one of claims 4 and 5, characterized in that said excitation means belong to the group comprising: radiating openings with conductive flange (35); slots or networks of slots; planar antennas each associated with at least one layer of a partially metallized buffer dielectric substrate on its upper face.

7. An antenna according to any one of claims 1 to 6, characterized in that said arrays of semi-reflective patterns belong to the group comprising: arrays of uniform semi-reflective patterns; - networks of non-uniform semi-reflective patterns.

8. An antenna according to any one of claims 1 to 7, characterized in that it operates at millimeter and sub-millimeter wavelengths.