Multibeam Antenna for Telecommunications

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009 665

Multibeam Antenna for TelecommunicationsNetworks Using Cylindrical EBG Structure

H. Chreim, M. Hajj, E. Arnaud, B. Jecko, C. Dall’omo, and P. Dufrane

Abstract—We present in this letter the study of a multibeam an-tenna for telecommunication networks by using cylindrical electro-magnetic band-gap (EBG) structures. The EBG structure behavesas a partially reflecting surface (PRS) and enhances the directivityof a simple radiating source. We begin by presenting the principleof the multibeam antenna, and then we present a first version, con-ceived to operate in WiMAX band [5.4–5.7] GHz. Simulation re-sults of this antenna will show us that the strong mutual couplingbetween the different excitation sources of the EBG structure badlyinfluences its performance. In order to reduce the mutual coupling,metallic walls are inserted between the excitation sources. More-over, excitation sources have been changed to improve the radia-tion performance of the antenna. A prototype of the final antennais fabricated to validate our simulation results, and the measure-ments results are compared to the simulated ones.

Index Terms—Coupling, electromagnetic band-gap (EBG)structure, multibeam antenna, patch, sectoral radiation pattern.

I. INTRODUCTION

E LECTROMAGNETIC band-gap (EBG) materials are pe-riodic structures that can control the propagation direc-

tion of electromagnetic waves [1]. In the antenna domain, EBGmaterials have been used to conceive high-gain antennas withdifferent radiation patterns, which avoids the use of arrays andtheir complex feeding mechanisms. For example, planar EBGstructures have been used to conceive highly directive [2], [3] orsectoral [4], [5] antennas. Cylindrical EBG structures have alsobeen used to conceive other types of antennas, like omnidirec-tional ones [6], [7]. Generally, this type of antenna is composedof a thin metallic core acting as a ground plane and surroundedby a cylindrical EBG structure. An excitation source is posi-tioned between the EBG and the metallic core. The EBG struc-ture behaves as a partially reflecting surface (PRS) and forms acoaxial resonant cavity with the ground (core). A significant en-hancement of the excitation source directivity is achieved. Theaim of our work is to use the omnidirectional antenna to con-ceive a multibeam antenna for WiMAX applications [5.4–5.7]GHz, by using cylindrical EBG structures. The different simu-lations have been performed using CST 2006B, a finite-integra-tion-technique (FIT)-based software.

Manuscript received January 20, 2009; revised March 03, 2009. First pub-lished April 14, 2009; current version published July 09, 2009.

H. Chreim, M. Hajj, E. Arnaud, and B. Jecko are with the Faculté des Scienceset Techniques, XLIM—CNRS UMR 6172, 87060 Limoges, France (e-mail:[email protected]).

C. Dall’omo and P. Dufrane are with Radiall Systems, 87069 Limoges,France.

Digital Object Identifier 10.1109/LAWP.2009.2020923

Fig. 1. Electromagnetic field distribution and three-dimensional (3D) radiationpattern of the omnidirectional antenna.

Fig. 2. Electromagnetic field distribution and 3D radiation pattern of the sec-toral antenna.

II. PRINCIPLE OF THE MULTIBEAM ANTENNA

In the omnidirectional antenna case, the central metallic corehas a small diameter. Thus, the electromagnetic field is able toresonate all around the core, providing an omnidirectional ra-diation pattern in the azimuth (Fig. 1). By increasing the di-mensions of the central core, we can obtain a sectoral antenna,where the electromagnetic field will be evanescent around thecore, providing a sectoral radiation pattern (Fig. 2).

The multibeam antenna can be obtained by adding multipleexcitation sources between the core and the cylindrical EBGstructure, which feed separately the coaxial resonant cavityusing a background switching system.

III. ANTENNA DESIGN AND SIMULATION

A. Antenna Design

The required antenna is vertically polarized and must produce18 beams in the azimuth with 50 of radiation beamwidth and15 dBi of directivity, which requires 18 excitation sources to beplaced around the central core. As a first step, we will simulatethe antenna with only the excitation source that feeds the reso-nant cavity “one source case” and compare its performance tothose of the one without the EBG structure to notice the direc-tivity enhancement. Then, the antenna will be simulated withthe presence of all the excitation sources “multisource case.”

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666 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009

Fig. 3. Schematic of the antenna with the presence of the source that feeds thecoaxial resonant cavity. (a) 3D view and (b) top view.

Fig. 4. E-plane and H-plane radiation patterns of the antenna without and withthe EBG structure at 5.6 GHz in one-source case.

Fig. 5. Mutual coupling between two adjacent sources.

B. One-Source Case Simulation

The first studied antenna is shown in Fig. 3. It is composed ofa large cylindrical metallic core with a diameter mm.The EBG material composed of 36 cylindrical metallic rodswith a diameter mm is surrounding the core with a ra-dial distance mm. The angular periodicity of the rodsis . The excitation source is a simple patch antenna.The height of the antenna is mm.

This antenna has been simulated without and with themetallic rods in order to emphasize the EBG structure’s effectson the directivity. Fig. 4 shows the radiation patterns of copo-larization with and without the cylindrical EBG structure in theE-plane (xoz) and H-plane (xoy) of the antenna at 5.6 GHz. Wecan see clearly that the E-plane radiation pattern of the antennawithout the rods is rippled due the presence of the central core,while it is less disturbed in the H-plane. After introducingthe cylindrical EBG structure, the directivity is increased by

Fig. 6. E-plane and H-plane radiation patterns of the antenna at 5.6 GHz inmultisource case.

Fig. 7. Schematic of antenna with metallic walls. (a) 3D view and (b) top view.

Fig. 8. Return loss and mutual coupling between two adjacent sectors of theantenna.

7.2 dB. The E-plane radiation pattern becomes directive with aradiation beamwidth of 20 , while it remains sectoral with 55of radiation beamwidth in the H-plane due to the cylindricalshape of the EBG.

C. Multisource Case Simulation

Now, all 18 patches, including the one that feeds the coaxialresonant cavity, will be introduced around the central core every20 to notice their influence on the performance of the antenna.Before presenting the radiation patterns, let us take a look at themutual coupling between two adjacent sources (Fig. 5).

The coupling reaches high values inside our desired fre-quency band, especially at 5.6 GHz, where we find its max-imum of dB. This is due the multiple reflections insidethe coaxial resonant cavity, which allows the field excited

CHREIM et al.: MULTIBEAM ANTENNA FOR TELECOMMUNICATIONS NETWORKS 667

Fig. 9. Antenna with metallic walls: Directivity versus frequency, E-plane and H-plane radiation patterns.

Fig. 10. Schematic of the new antenna with only three unit cells. (a) 3D viewand (B) top view.

by the feeding patch to reach the others close by. This highcoupling between two adjacent sources will clearly influencethe radiation patterns of the antenna (Fig. 6).

In the E-plane, the coupling did not change the pattern’sshape. However, the sources are placed in the H-plane; thismeans that this plane’s radiation pattern will be influenced.Indeed, the radiation beamwidth is increased (55 to 70 ); thus,the directivity is decreased by 2 dB.

IV. MUTUAL COUPLING REDUCTION

As we saw in the previous paragraph, the strong mutual cou-pling influenced the radiation patterns and changed their char-acteristics. That is why we will try to reduce this parameterby reducing the intensity of the electromagnetic field before itreaches the patches close by. A simple solution consists of in-serting metallic walls between the patches.

A. Antenna Design

The new antenna is shown in Fig. 7. In fact, introducing themetallic walls has shifted the operating frequency. That is whywe have changed the radial distance to 33 mm to get back tothe desired frequency. The metallic walls are 3 mm thick with

Fig. 11. Return loss of the new antenna.

the same height of the antenna. Thus, the new antenna isconsidered like a cylindrical array of EBG sectoral antennas,where each one is composed of two metallic walls, three rods,and a part of the core as a ground plane.

B. Antenna Simulation

The antenna, as we said before, is fed by only one patch ata time. The antenna is well matched (return loss lower than

dB) in the frequency band [5.57–5.7] GHz, and the mutualcoupling between two adjacent sectors is lower than dB(Fig. 8).

Since the coupling between two adjacent sectors is lower thandB, we will simulate from now on only three unit cells

(sectors) of the complete antenna, where the fed sector is themiddle one, in order to reduce the simulation time. The fol-lowing presented results are those of one sector. Other sectors’performance will be the same, but with other radiation direc-tions. In the frequency band [5.4–5.7] GHz, the directivity variesbetween 12.9 and 15.8 dBi, while the radiation beamwidths inthe E-plane and H-plane are respectively 18 and 54 (Fig. 9).

In the next paragraph, we will apply some changes to theantenna with the metallic walls, in order to ameliorate the returnloss on the one hand and the directivity on the other hand. Then,the fabricated prototype will be presented, and the measurementresults will be compared to the simulated ones.

668 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009

Fig. 12. New antenna: directivity versus frequency, E-plane and H-plane radiation patterns.

Fig. 13. Photograph of the fabricated prototype.

Fig. 14. Measured return loss of each sector compared to the simulated one.

V. PERFORMANCE IMPROVEMENT AND FABRICATED

PROTOTYPE

A. Performance Improvement

Until now, the antenna is well matched over 130 MHz (2.3%).The directivity presents 3 dB of variations between 5.4 and5.7 GHz. Thus, our objective is to widen the well-matched fre-quency band and decrease the directivity variation throughoutour desired band. In order to achieve this task, we have re-duced the number and the diameter of the used metallic rods(one 3-mm-diameter rod per unit cell instead of three 4-mm-di-ameter rods). By doing this, the cavity becomes less resonant,which will help to ameliorate the matched band. However, thedirectivity will decrease, and its variation inside our frequencyband will be lower. The directivity can be increased again byfeeding each sector with two patches instead of one. Fig. 10shows the new antenna to which we have added two polycar-bonate discs to maintain the components. As we said before, the

Fig. 15. Measured E-Plane radiation patterns compared to the simulated one.

Fig. 16. Measured H-plane radiation patterns compared to the simulated one.

antenna with only three unit cells is simulated. We note that thepatches are glued on a metallic sheet using a thin layer (100 m)of cyanoacrylate adhesive, which is also considered in our sim-ulation.

The well-matched band of this structure, as shown in Fig. 11,is now 220 MHz (4%). The directivity reaches a maximum of15.3 dBi at 5.6 GHz, with lower variation (1.3 dBi) between 5.4and 5.7 GHz. Finally, the radiation beamwidths in the E-planeand H-plane are 17 and 48 (Fig. 12).

B. Fabricated Prototype

The fabricated prototype, shown in Fig. 13, is composed ofnine sectors—three adjacent sectors every 120 . The metalliccomponents are made of aluminum, and they are maintained by

CHREIM et al.: MULTIBEAM ANTENNA FOR TELECOMMUNICATIONS NETWORKS 669

two discs of polycarbonate (Fig. 13). The patches are glued onmetallic sheets using cyanoacrylate adhesive

.The measured return loss of the nine sectors are presented in

Fig. 14 and compared to the simulated one. We notice a fre-quency shift that is due to manufacturing tolerances (in fact,there was an air gap between the patches and the metallic sheeton which the patches are glued).

The maximum measured realized gain of each sector isaround 13.2 and 13.6 dB at 5.64 GHz, while the maximumsimulated directivity is 15.3 dBi at 5.6 GHz. This difference ofvalues is due to different losses (dielectric and metallic losses,losses of the divider and coaxial cables—which have been usedduring the measurement session—and coupling losses).

Figs. 15 and 16 show the measured copolarization radiationpatterns of the nine sectors in the E-plane and H-plane, respec-tively, at 5.64 GHz, where we notice a very good agreementbetween the simulation and measurements. For three adjacentsectors, the beam overlapping is located at dB.

VI. CONCLUSION

We presented in this letter the concept of a multibeam antennausing cylindrical EBG structure. The antenna structure and itscomponents are detailed. Several steps of the design process are

described, and predicted performance values obtained by com-puter simulation are presented. Finally, to validate our simula-tion results, a protototype of the antenna with nine beams isfabricated and characterized. The measured results agreed wellwith the simulated ones.

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