EBG STRUCTURE FOR INDOOR WIRELESS LAN WITH BEAM SHAPING. A. H. Alomainy, Y. Hao, C. G. Parini

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1 EBG STRUCTURE FOR INDOOR WIRELESS LAN WITH BEAM SHAPING A. H. Alomainy, Y. Hao, C. G. Parini Department of Electronic Engineering Queen Mary, University of London Mile End Road, London E1 4NS, U.K. ABSTRACT We present a new application of Electromagnetic BandGap, specifically Uniplanar Compact structures (UC-EBG) in antenna beam shaping for indoor Wireless Local Area Network (WLAN) systems. Such UC-EBGs are used to construct Fabry-Perot type cavities in which the enhanced directivity can be achieved from conventional patch antennas. In this paper, it is demonstrated that antenna beam can be shaped by shifting the defects (patch antennas) from the centre of UC-PBG cavities. Theoretical predictions are verified by numerical simulations and measurement results, and it appears that antenna beam shaping in UC-EBG cavities can achieve the same radiation patterns as complex planar antenna arrays used for improving the quality and strength of sent/received signals in WLAN systems. INTRODUCTION Electromagnetic Band Gap (EBG) structures (also called Photonic Band Gap (PBG) structures [1], [2]) offer the opportunity to control and manipulate electromagnetic wave propagation as a result of their being formed from smallscale periodic geometric structures. This detailed microscopic configuration can be designed to cause the structures (macroscopically) to exhibit either effective permittivity or permeability that is negative over a finite frequency range. Such EBG structures offer pass and stop bands to electromagnetic waves in the same way semiconductors offer these properties to electrons. Its application to antennas and RF passive components includes the suppression of surface waves [3], [4] the construction of Perfect Magnetic Conducting (PMC) planes [5], antenna gain improvement [6], isolation enhancement in the diplexer [7] and self-diplexing patch antennas [8] etc. It is also noted in [9] and [1] that high directivity can be obtained from conventional patch antennas using a defect mode of Fabry-Perot type EBG cavities in the same way as photonic devices in PBGs [12], [13]. Almost all directive photonic antennas are constructed from either layered dielectric materials with different permittivity (commonly called as Bragg mirrors) or alumina rods layers and thin cylindrical metal wires [9]-[13]. Under this circumstance, Bragg reflections are achieved with a onedimensional EBG/PBG, and hence the enhanced directivity is limited. By extending the periodicity from one to two dimensions, it is possible to control the electromagnetic wave propagation in a plane and therefore achieve with pencillike radiation beam from single radiators such as a monopole or patch antenna. It is verified recently by Sauleau [14] using plane-parallel non-uniform metallic strip gratings as reflecting mirrors to achieve Gaussian beam like directive antennas. In this paper, a novel directive antenna design using Uniplanar Compact (UC) EBG structures as Fabry-Perot cavities is presented. UC-EBG [3], [15] has the advantage of ease of fabrication, and most UC-EBGs have been made by etching a class of periodic patterns on the metallic ground plane. Furthermore, it is demonstrated from our studies that one can obtain shaped beam antenna by simply shifting radiators from the centre in the proposed UC-EBG cavities. In applications like WLANs, the limited available power from access points must be evenly distributed over wide areas, without creating over or under-illuminated zones. This implies that the antenna radiation pattern must compensate for path loss, which corresponds to the well-known cosec 2 characteristic in the elevation plane. The antennas are also required to discriminate multipath losses by diversity, and hence sector coverage in the azimuth plane is required. In this paper, properties of the proposed Fabry-Perot cavities respectively made from a single-layer and a double-layer UC-EBGs are examined. Specifically, radiation characteristics from the offset defects in the cavity are investigated compared with those from which the defect is in the centre to verify the beam shaping property. Experimental verification has been made using a conventional microstrip patch antenna as the defect in the cavity. Such antennas demonstrate the improved radiation properties with high directivity, and the evidence on beam shaping is also

2 demonstrated. For demonstration, all designs are made at GHz for High Performance Radio LAN (HiperLAN) applications and simulation results are presented with experimental verification. THEORY AND DESIGN Understanding EBG structures behaviour in directive antenna design involves the study of the Bragg Mirror Reflection, Diffraction and Fabry-Perot Cavity concepts. Naturally occurring electromagnetic fields can be represented by the superposition of either a discrete set or a continuum of plane wave travelling in different directions. Such a set of plane waves is known as an angular spectrum [16]. For many microwave antennas, the angular spectrum is of negligible amplitude outside a narrow range of angles. When such antennas are placed inside Fabry Perot type EBG cavities, the latter acts as spatial filters which only let a certain amount of angular spectral components pass and thus focus the incident fields radiating by the antennas [14]. In some wireless applications, such as WLANs, the limited available power from access points must be evenly distributed over wide areas, without creating over or under-illuminated zones. This implies that the antenna radiation pattern must compensate for path loss, which corresponds to the well-known cosec 2 characteristic in the elevation plane. The antennas are also required to discriminate multipath losses by diversity, and hence sector coverage in the azimuth plane is required. The idea using EBG cavity to shape antenna beam is very straightforward. Once the defect, namely the patch antenna in the cavity is offset from the centre, its angular spectrum of radiation field will be alternated and becomes asymmetrical due to uneven attenuation from the EBG. Geometry of a layered UC-EBG cavity used for antenna beam shaping campaign is shown in Fig. 1, and the structure consists of UC-EBGs as angular filters with a conventional patch antenna as radiating source. UC-EBGs and the metallic ground plane form a Fabry Perot type cavity and the patch antenna is seen as a defect in the cavity. Reference [9] indicates that the distance from the patch antenna to UC-EBG is set to be half wavelength at the resonating frequency to maximize power radiating from the source. However considered the perturbation from thin dielectric substrates, according to our experience, the distance h 2 should be optimized as 3mm at 5.2GHz. Both UC-EBG and the patch antenna are fabricated on RT/Duroid boards with substrate thickness mm and dielectric constant 3. A dielectric mirror, also known as a Bragg reflector, consists of identical alternating layers of high and low refractive indices. The optical thickness values are typically chosen to be quarter-wavelength long, that is λ o /4 at some operating wavelength λ o. The standard arrangement is to have an odd number of layers, with the high index layer being the first and last layer. A quarter-wave phase-shifted multilayer structure is obtained by doubling the layers and then inserting a quarter-wave layer between the two groups of high and low layers; such a structure is referred to as a Fabry-Perot resonator (FPR). The quarter-wave layers present, acts as a narrow transmission filter, with the transmission band-width becoming narrower as the number of layers increases. Lp UC-EBG layer UC-EBG layer H1 Dielectric Substrate More angular attenuation region Less angular attenuation region H2 Ground plane Patch antenna Fig. 1. A side view of the layered UC-EBG cavity proposed

3 Transmission Power (db) E+9 4.5E+9 5.E E+9 6.E+9 6.5E+9 Frequency (Hz) E B G T ransm issio n pattern at 9 angle E B G T ransm issio n pattern at 45 angle E B G T ransm issio n pattern at angle Fig. 2. Simulated UC-EBG frequency responses at different wave incidence angles As mentioned earlier, to obtain the full picture of the antenna radiation and band gap existence problem for different elevation angles the periodicity of the UC-EBG structure was extended to cover more than one dimension with the period given as, g c l p λ 2 = (1) 2 ε f where λ g is the guided wavelength at the operating frequency. This produces and approximate period of 2 mm, Fig. 1. NUMERICAL ANANLYSIS AND MEASUREMENT VERIFICATIONS eff As a prior step to simulating and verifying the performance of the structure shown in Figure 1, the behaviour of the proposed UC-EBG structure was analysed to examine the band gap property at different planar angles, Fig. 2. The figure shows distinct band gaps of the UC-EBGs at different wave incidence angles. Most of the simulation results presented were obtained using Agilent Momentum. Highly Directive Antenna Using UC-EBG Cavities Fig. 3. shows a photograph of the constructed UC-EBG Cavity Antenna. Both UC-EBG and antenna structures were fabricated on an RT/Duroid boards with substrate thickness of mm and dielectric constant of 3. The dimensions of the rectangular patch are 16.3mm 12.74mm. The 5Ω line width is 3.9mm. The width and length of the impedance transformers are.4mm and 8.3mm respectively. The measured resonant frequency was found in fig. 4 at 5.9 GHz and good agreement with simulation results is shown. The microstrip patch E- and H-plane radiation patterns are shown in Fig. 5. To investigate the performance enhancement expected from the UC-EBGs the patch antenna was first placed in a one layer UC-EBG cavity. The antenna beam narrowing is evident from the measurement results presented in Fig. 5, with the simulated directivity increasing from 6 db to 2 db and beam-width narrowed by 6% in the main direction of radiation. res Fig. 3: Photographs of the microstrip antenna embedded within the cavity bounded by the ground plane and the EBG structure

4 Refelection Coefficient (db) Frequency (GHz) 2 UC-EBG Layers (Measured) 2 UC-EBG Layers (Simulation) Patch Only (Simulation) Patch Only (Measured) Fig. 4: Comparison of simulated and measured return loss responses for both cases with/without UC-EBG Cavity The E and H field cross polar patterns imply low level of backward radiation due to their low radiation level, Fig. 5. The H-field pattern of the one-layer UC-EBG cavity shows some asymmetric characteristics and this can be avoided by using different feeding circuitry. When the two UC-EBG layers were used an additional improvement on both directivity and directionality was achieved, with directivity of 22 db and beam-width of less than 25, Fig. 6. The differences in the side lobes level between the measured and simulated patterns are due to the assumption of infinite ground plane considered in the Agilent Momentum simulation package. The return loss response (Measured/Simulated) is shown in Fig. 4. Beam-Shaping Analysis and Results As the main objective of the work presented here is to introduce the property of beam shaping of the radiation pattern by applying EBG structures to a single element patch antenna, instead of an array system, numerical and experimental analysis were performed in order to validate the possibility of obtaining such characteristic. The cavity structure was numerically simulated with different positions and degrees of offsetting of the patch antenna in regards to the UC-EBG Patch Only (Co-Pol) Patch Only (Cross-Pol) Patch Only (Co-Pol) Patch Only (Cross-Pol) a) E Field Patterns b) H Field Patterns Fig. 5: Comparison between conventional patch and antenna with 1 UC-EBG layer on measured radiation pattern. A) E field. B) H field

5 Patch with 1 UC-EB G (Sim) Patch with 1 UC-EB G (M eas.) Patch with 2 UC-EBG (Sim) Patch with 2 UC-EBG (M eas.) Fig. 6: Comparison between the simulated and measured E-plane patterns of one layer and two layer UC-EBG cavities layers. The produced pattern showed asymmetric pattern with attenuation at certain directions more than others, as expected. Many measurement experiments have been performed and they produced various patterns with different characteristics but with the main beam direction not changing notably. Fig. 7 shows the radiation pattern resulted from one of those measurements with an apparent variation in the attenuation level at both sides of the main beam. This was expected since the periodicity of the structure changes with the change of the structure dimensioning, which in order affects the propagation in each direction with different characteristic. In addition, the propagating wave, in this case, will be under different cavity properties since the mirrors (UC-EBG layers and ground plane) have changed their dimensions. The cavity structure studied here can be applied to different application in the Wireless Communication sector, such as Wireless LAN and Mobile Communication Systems. Thus, it was interesting to study the behaviour of the antenna at different frequencies around the resonance and to obtain the bandwidth of the antenna. Fig. 8 shows that the directivity keeps good values when the frequency is slightly shifted and the bandwidth is approximated to be around 2%, which illustrates the possibility of using such structure in the newly emerging WLAN application, HiperLAN (Frequency range 5.15 to 5.35 GHz) E pattern patch only E pattern with 1 UC-EBG E pattern shifted 1 UC-EBG Fig. 7: Comparison between measured patch antenna, antenna with 1 UC-EBG layer and with shifted UC-EBG structure patterns

6 25 2 Directivity (db) Frequency (GHz) Fig. 8: Simulated frequency response of the UC-EBG cavity directivity with beam shaping In order to validate the practicality of the investigation and structure presented and produced in this literature the measured radiation patterns of the shifted EBG structures for both one and two layers were compared to that produced be a 9-element antenna array, providing patterns desired in some mobile service coverage applications. Fig. 9 shows good agreement with slight differences in the existence of null points in the results obtained in this study. However, future work including the design and study of various EBG structures and modelling techniques would allow the introduction of a fully configurable EBG structures permitting the forming of different patterns according to the applications in use. CONCLUSION A new and novel application of the UC-EBG concept has been introduced not only to improve directivity and gain of conventional patch antenna but also to provide the feature of beam forming required currently in many systems, such as element array pattern Patch w ith 1 EBG layer Patch w ith 2 EBG layers Fig. 9: Comparing Beam Shaping patterns with that of a typical desirable mobile service coverage pattern produced by a 9-element array

7 Wireless LANs. In this paper, it is demonstrated that antenna beam can be shaped by shifting the defects (patch antennas) from the centre of UC-PBG cavities. The experimental results show good agreement with the theoretical and numerical results. The presented concept can be used as the basis for future work in developing low-cost and simple antenna systems to be implemented in many communication applications without the return to complicated array systems. ACKNOWLEDGEMENT The authors would like to thank Mr. J. Dupuy, Antenna Lab Manager, at the Department of Electronic Engineering, Queen Mary University of London, U. K., for his assistant on antenna fabrication and measurement. REFERENCES [1] E. Yablonovitch, Inhibited spontaneous emission in solid state physics and electronics, Phys. Rev. Lett., vol. 58, pp , [2] Y Rahmat-Samii and H Mosallaei, Electromagnetic Bandgap Structures: Classification, Characterisation and Applications, Antenna and Propagation, 21. Eleventh International Conference on (IEE Conf. Publ.), Volume: 2, 21. PP [3]. Y. Hao, C. G. Parini, Microstrip Antennas on Various UC-PBG Substrates, IEICE Trans. on Electronics, special issue on microwave and millimeter-wave technology, Aug., 23. [4] R. Gonzalo, P. De Maagt, and M. Sorolla, Enhanced patch-antenna performance by suppressing surface waves using photonic-bandgap substrates, IEEE Trans. Microwave Theory Tech., vol. 47, pp , Nov [5] L. Zhan and Y. Rahmat Samii, PBG, PMC and PEC ground planes: A case study of dipole antennas, in Proc. IEEE Int. Symp. Antennas and Propagation Society 2, vol. 2, Salt Lake City, UT, July 2, pp [6] K. M. Shum, Q. Xue, C. H. Chan, and K. M. Luk, Gain enhancement of microstrip reflectarray incorporating a PBG structure, in Proc. IEEE Int. Symp. Antennas and Propagation Society 2, vol. 1, Salt Lake City, UT, July 2, pp [7] T.-Y. Yun and K. Chang, A new PBG diplexer for a multi-band transceiver antenna system, in Proc. IEEE Int. Symp. Antennas and Propagation Society 21, vol. 2, Boston, MA, July 21, pp [8] Y. Hao, C. G. Parini, Isolation Enhancement of Anisotropic UC-PBG Microstrip Diplexer Patch Antenna, IEEE Antennas and Wireless Propagation Letters, Vol [9] M. Thévenot, C. Cheype, A. Reineix and B. Jecko, Directive Photonic-Bandgap Antennas, Microwave Opt. Technology Letter, Vol. 47, No. 11, pp November [1] T. Akalin, J. Danglot, O. Vanbesien and D. Lippens, A Highly Directive Dipole Antenna Embedded in a Fabry- Perot Type Cavity, IEEE Microwave and Wireless Components Letters, Vol. 12, No. 2, February 22 [11] B. Temelkuran, E. Ozbay, J. P. Kavanaugh, G. Tuttle, and K. M. Ho, Resonant cavity enhanced detectors embedded in photonic crystals, Appl. Phys. Lett., vol. 72, no. 19, pp , May [12] G. Guida, D. Maystre, G. Tayeb, P. Vincent, Electromagnetic Modelling for Three-Dimensional Metallic Photonic Crystals, Journal of Electromagnetic Waves and Applications, vol. 12, pp , [13] I. Abram, I. Robert, and R. Kuszelewicz "Spontaneous Emission Control in Semiconductor Microcavities with Metallic or Bragg Mirrors", IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 34, NO. 1, pp , JANUARY [14] R. Sauleau, P. Coquet, T. Matsui and Jean-Pierre Daniel, A New Concept of Focusing Antennas Using Plane- Parallel Fabry-Perot Cavities With Nonuniform Mirrors, IEEE Trans on Antennas and Propagation, Vol. 51, No. 11, pp Nov. 23. [15] C. Caloz, C. C. Chang and T. Itoh, 'A Novel Anisotropic Uniplanar Compact Photonic Band-Gap (UC-PBG) ground Plane', the 31st European Microwave Conference, London 21. Page(s): vol. 2. [16] R. H. Clarke & John Brown, Diffraction Theory and Antennas, Ellis Horwood Limited.