диафрагмированные волноводные фильтры / 5cf18362-0152-4c34-b386-4660103e0478

GAUSSIAN BEAM ANTENNAS FED BY A HORN OR BY A PLANAR SOURCE

R. Sauleau1, Ph. Coquet2,3, J.P. Daniel1, T. Matsui2

2Communications Research Laboratory, MPT, Koganei-shi, Tokyo 184-8795, Japan

1Lab. Antennes et Réseaux, UPRES A CNRS 6075, Université de Rennes 1, 35042 Rennes, France 3 ENS de Cachan Antenne de Bretagne, Campus de Ker Lann, 35170 Bruz, France

E-mail : Ronan.Sauleau@univ-rennes1.fr

1.INTRODUCTION

There has been recently a growing interest in the development of millimeter-wave short range broadband systems, such as high speed wireless LANs. In order to reduce the influence of multipath propagation distortion, radiating structures featuring a low side lobe level are possible candidates for baseor user-station antennas. For that purpose, a quasi-planar Gaussian Beam Antenna (GBA), with a gaussian distribution of the aperture electric field, has been proposed by Matsui et al. [1,2] : a halfwavelength plano-convex Fabry-Pérot (FP) resonator, with partially transparent mirrors, is excited by a guided source (waveguide or horn) or by a printed source.

In this paper, radiation characteristics (radiation patterns, gain, efficiency) of GBAs in the 60 GHz band are investigated theoretically and experimentally by considering the same FP cavity fed either by a pyramidal horn antenna (section 2) or by a microstrip patch antenna (section 3). As a conclusion (section 4), we specify typical achievable radiation performances of GBAs, depending on the kind of feeding (guided or planar) and on the geometry of the FP resonator.

2. GBA FED BY A HORN ANTENNA

2.1/ Geometry : a GBA fed by a horn antenna is depicted on Fig. 1. The FP cavity consists of a plano-convex (thickness 'D', radius of curvature 'R0') spherical (diameter φl ) low loss dielectric lens

(εr ; tanδ) with two highly reflecting mirrors Mi,i=1,2. Each mirror Mi is a two dimensional metallic grid with a square symmetry. The spatial period of each grid is labeled 'ai', and the width of the metallic

strips 'di'. The FP resonator is placed in the aperture plane of the pyramidal horn, which is assumed to have a square aperture 'c' and a length 'Lh'.

The cavity considered throughout this paper is in fused quartz (εr = 3,80 ; tanδ = 3×10-4 at 60 GHz). Its diameter is φl = 30 mm. It has been designed using the FDTD technique combined with periodic boundary conditions [3], in order to provide a fundamental gaussian beam mode TEM000 at 57 GHz. The dimensions of the FP resonator and its intrinsic quasi-optical characteristics (when illuminated by a plane wave under normal incidence) are presented in Table 1. Both mirrors are identical. They were fabricated using a precise lift-off process of sputtered copper film (t = 1,3 µm). Their power

reflectivity 'R' at the measured quasi-optical resonant frequency (fres,qo = 56,82 GHz) is R = 99,3 %, resulting in a 200 MHz half-power bandwidth (BW3dB). The dimensions of the 15 dB horn antenna

exciting the resonator are : c = 9 mm and Lh = 16,7 mm.

Table 1 : Geometry and quasi-optical characteristics of the plano-convex FP cavity

2.2/ Radiation properties : as shown on Fig. 2, the GBA behaves as a bandpass filter with a maximum measured gain of 14 dB at the resonant frequency fres,1 = 57,09 GHz. The measured

reflection coefficient is only -5 dB at 57,09 GHz because of poor coupling between the horn and the resonator. Future work will consist in optimizing the profile of the horn to reduce reflection losses.

The radiation patterns measured in E- and H-planes at 57,09 GHz are given on Fig. 3. The side lobe

level in both planes is lower than -35 dB and the half-power beamwidth is θ3dB,1 = 20°. The focusing effect of the FP cavity is clearly demonstrated in comparison with the radiation patterns of the horn

alone. In particular, the -16 dB side lobes of the horn in E-plane disappear completely. Besides, the cross-polarization level of the GBA remains lower than -28 dB and is of the same order of the crosspolarization of the horn alone. The FP cavity has no depolarization effect. The co-polar components have been computed using the complex source point theory [4]. This method takes into account the aperture size of the horn and the grid parameters of the plane mirror [5]. Agreement with experimental results is very good in the paraxial region. However, the method does not enable to compute the crosspolarization of the antenna. Its level has been arbitrarily set to -40 dB.

Here, their level is relatively high (≈ 4,3 dB) because of the high reflectivity (R = 99,3 %) of the grid mirrors. Efficiencies up to 70 % have been obtained with coarser grids (Cf. section 4).

3. GBA FED BY A PLANAR SOURCE

3.1/ Geometry : the plano-convex FP cavity is excited by a planar antenna situated at a distance L (in free space) from the plane mirror M1, as shown on Fig. 4. Here, the source is an elementary microstrip patch antenna (Fig. 5). It was fabricated on a 150 µm fused quartz substrate using a lift-off process of sputtered copper film. The maximum gain of the patch antenna is 6,4 dB around 56,5 GHz. Therefore, its efficiency, ηsource, is very high (greater than 80 %).

3.2/ Radiation properties - Comparison with a GBA fed by a horn : the maximum measured gain

of this antenna is 15,3 dB at the resonant frequency fres,2 = 56,6 GHz for a distance L equal to 2,50 mm. fres,2 is lower than the quasi-optical resonant frequency of the FP cavity (fres,qo = 56,82 GHz – Cf. Table 1) because the antenna structure can be viewed as the combination of two resonators in cascade (the

first one is the FP cavity, and the second one is constituted by the mirror M1 and by the ground plane of the patch). This also explains physically why the optimum gain is obtained for a distance L slightly shorter than half a wavelength in free space at the operating frequency.

The radiation patterns measured at 56,6 GHz for L = 2,50 mm are represented on Fig. 6. In H-plane, the radiated field is a symmetric gaussian beam, whereas a -20 dB side lobe appears in E-plane around 30°. This side lobe is probably due to the V connector used for RF feeding of the patch antenna. The

half-power beamwidth is θ3dB,2 = 15°. It is smaller than the one obtained with the horn (θ3dB,1 = 20° - Cf. section 2) because it has been proved theoretically that the directivity of the GBA decreases if a

horn antenna is used to excite the cavity [5]. The focusing effect of the FP resonator appears clearly in E-plane, where we can also notice that the co-polar component of the patch antenna is highly distorted by the V connector (shadowing for negative angles, and reflection on the connector for positive angles). The cross-polarization of the antenna is lower than -28 dB ; the same level has been obtained with the horn.

The antenna efficiency (26 % at 56,6 GHz - Cf. Table 3) is lower than the one measured with the horn (Cf. section 2) because of dielectric, metallic and surface wave losses in the printed source.

Fig. 3 : Radiation patterns of the GBA fed by a horn – E- and H-planes

Fig. 6 : Radiation patterns of the GBA fed by a patch antenna – E- and H-planes