SPATIAL DIVERSITY ANTENNA FOR WLAN APPLICATION

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1 achieved this scaled design at 300 MHz as well; however, we choose to report the smaller ka design at this frequency to allow a direct connection between these results and those reported in Refs. 2 and 3. The differences between Designs 1 and 2 show that the copper losses significantly impact the radiation efficiency of this resonant system. The performance of the scaled limit case at 300 MHz is essentially the same as its higher frequency versions. The overall efficiencies of these electrically-small-limit systems are very high. The complex input impedance behavior and the far-field radiation patterns for the GPS-frequency Design 3 are shown in Figure 2. The resistance and reactance curves in Figure 2(a) exhibit characteristics analogous to the anti-resonant behavior of an electrically-small circular loop (i.e., a magnetic dipole) antenna [5, 6]. In particular, it is clear from Figure 2(a) that the EZ antenna is antiresonant and matched to the feedline at the source frequency. Figure 2(b) also demonstrates that the EZ antenna is acting like a magnetic dipole over a PEC ground plane. Figure 3 shows the E-and H-field vector plots using xy-plane cuts in the stub and just above the semi-circular loop antenna, respectively, and the current vector plots on the metamaterial-inspired structure. From Figs.3(a) and 3(b), one clearly sees that the metamaterialinspired radiating structure is acting like a uniformly extruded CLL element. SPATIAL DIVERSITY ANTENNA FOR WLAN APPLICATION Yong-Sun Shin and Seong-Ook Park School of Engineering, Information and Communications University (ICU), Daejeon, Korea Received 10 November 2006 ABSTRACT: This article presents a novel printed diversity planar monopole antenna integrated on a PCMCIA network card with minimized the mutual coupling for WLAN application at the 5 GHz bands ( and MHz). The proposed diversity antenna is printed on FR-4 substrate and small size enough to be embedded in the laptop computer. Optimizing the separation distance and antenna parameters with use of two slots on a ground, the good isolation performance (less than 20 db) between the two printed monopole antennas is achieved. The proposed diversity antenna has an impedance bandwidth of 2360 MHz ranging from 4520 to 6889 MHz (S db). There is a good agreement between the simulated and measured results Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: , 2007; Published online in Wiley Inter- Science ( DOI /mop Key words: spatial diversity antenna; WLAN application; slots 3. CONCLUSIONS This research work introduced an efficient electrically-small antenna design methodology in which a self-resonant capacitive structure that is driven by an electrically-small semi-circular loop antenna coaxially-fed through a finite ground plane was obtained. These designs realized an inexpensive, easy-to-build, efficient, and electrically-small antenna. The proposed antenna system is linearly scalable to a wide range of frequencies. The overall efficiency of the antenna system depends on the choice of overall electrical size. Highly electrically-small versions exhibit large conductor losses because of their resonant nature. The type of metal used for the designs can be selected to improve this characteristic. Electricallysmall-limit versions were shown to be highly efficient. Preliminary proof-of-concept experiments have confirmed these results and will be reported elsewhere. ACKNOWLEDGMENTS This work was supported in part by DARPA Contract number HR C A Provisional Patent Application (PPA) covering this work was filed by the University of Arizona on 10/31/2006. REFERENCES 1. N. Engheta and R.W. Ziolkowski, A positive future for double negative metamaterials, IEEE Microwave Theory Tech 53 (2005), R.W. Ziolkowski and A. Erentok, Metamaterial-based efficient electrically small antennas, IEEE Trans Antenn Propag 54 (2006), A. Erentok and R.W. Ziolkowski, A hybrid optimization method to analyze metamaterial-based electrically small antennas, IEEE Trans Antennas Propag, to appear. 4. A. Erentok, P. Luljak, and R.W. Ziolkowski, Antenna performance near a volumetric metamaterial realization of an artificial magnetic conductor, IEEE Trans Antennas Propag 53(2005), A.D. Yaghjian and S.R. Best, Impedance, bandwidth, and Q of antennas, IEEE Trans Antenn Propag 53 (2005), C.A. Balanis, Antenna theory, 3rd ed., Wiley, New York, 2005, pp Wiley Periodicals, Inc. Figure 1 Configuration of the proposed spatial diversity antenna (a) The top view (unit : mm) (b) Detail dimensions of the spatial diversity antenna 1290 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 6, June 2007 DOI /mop

2 antennas, the antennas are placed on both side of the substrate which distance is 32 mm corresponding to a half wavelength at 5 GHz and has a common ground with two slots which size is 1 9 mm 2. Therefore the proposed antenna has obtained a broad bandwidth about 2360 MHz (S db) and the enhanced isolation performance (S db) over 5.2 and 5.8 GHz WLAN bands. Details of the antenna designs, the measured and simulated results are investigated. 2. ANTENNA DESIGN The proposed diversity antenna configuration is shown in Figure 1. The proposed antenna consists of two symmetrical elements and a ground with two slots. Two planar monopole antennas fed by coaxial connector and a ground are printed on the front surface of Figure 2 Measured and simulated S-parameters for the proposed diversity antenna 1. INTRODUCTION A technique of spatial diversity is a well-known method for enhancing the antenna performance by reducing the effect of the multi-path fading and improving the signal quality at the receiving end of wireless communication systems. To minimize the effects of mutual coupling in the spatial diversity antennas, there are several researches involving the enough separation distance between the elements [1 6]. Also, the size and shape of ground plane is the another crucial parameter affecting the isolation characteristic between them because the excited ports of antenna induce the current in the ground plane and the adjacent antennas are affected by these antennas. Therefore, it is essential to obtain appropriate ground size because it affects performances of the antenna such as bandwidth, efficiency, gain, etc [7, 8]. This article presents a novel spatial diversity antenna of the printed planar monopole type with minimized the mutual coupling for 5 GHz WLAN applications. The size of the main substrate is mm 3 considered to be a personal computer memory card international association (PCMCIA) network card for the practical laptop application and the end part of the proposed antenna is folded. To obtain broad impedance bandwidth, the proposed antenna has the step shape close to feed part because there are the several current flows at some different resonant frequencies. For enhancing isolation characteristic between two Figure 3 Measured S-parameter with and without slots Figure 4 Simulated current distribution results with and without slots at 5.15 GHz (a) Simulated results of ant 1 with slots (b) Simulated results of ant 1 without slots DOI /mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 6, June

3 Figure 5 Measured and simulated radiation patterns at 5.15 GHz a FR-4 substrate with relative permittivity 4.6 which size is mm 3 considering the practical laptop computer application. As seen in Figure 1(a), the major radiators are folded for occupying small volume and are placed on both side of the substrate. By adjusting the width of the radiator and the step shape of antenna near feed part, the broad bandwidth can be obtained. To achieve good impedance matching, dimensions of w 1 and w 2 are chosen to be 2 and 3 mm, respectively. In a diversity antenna, because performances of the diversity antenna become deteriorated according to increase coupling, the mutual coupling between antennas must be reduced to the minimum. To achieving good isolation performance, two antennas are 1292 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 6, June 2007 DOI /mop

4 Figure 6 Measured and simulated radiation patterns at GHz spaced with 32 mm, corresponding to a half wavelength at 5 GHz and the ground adding two slots which size is 1 9mm 2 is applied. 3. RESULTS AND DISCUSSION The proposed antenna for operation in 5 GHz WLAN bands has fabricated and measured with an Agilent 8510C network analyzer. Figure 2 shows the measured and simulated scattering parameters. Because of their symmetrical structures, the measured and simulated results of two antennas are mostly equal to each other. Accordingly, the results of antenna 1 are only presented. The measured bandwidth is 41.4% at 5700 MHz which is the center frequency covering the 5200 MHz ( MHz) and 5800 MHz ( MHz) bands. The compared results with and DOI /mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 6, June

5 TABLE 1 The Simulated Diversity Performances of the Proposed Spatial Diversity Antenna Frequency(GHz) e MEG1(dBi) MEG2(dBi) MEG1/MEG without two slots in the ground plane are described in Figure 3. When two slots in the ground plane, better isolation characteristic and broader bandwidth are achieved. To check the current flows at 5150 GHz with and without slots, the simulation results in Figure 4 is obtained from CST Microwave Studio. The dominant current distribution appear in the upper direction of antennas and the length of the current path is about 14.5 mm, corresponding to an approximately quarter-wavelength at 5150 MHz in Figure 4(a). As seen in Figure 4(b), in case of without slots, the mutual coupling increases because of more current flows produced in the common ground plane and antennas. Figures 5 and 6 show the measured and simulated radiation patterns of two antennas at 5150 and 5725 MHz for the ant 1 and ant 2 excitations. As seen in Figures 5 and 6, the results are symmetric between ant 1 and ant 2 because of their identical structures. There is the slight discrepancy between the measured and simulated data because when the radiation patterns of the ant 1 were measured, the ant 2 was connected with a terminated load. Table 1 shows the simulated diversity performance results in terms of envelope correlation coefficient, mean effective gain (MEG) ratio. A good diversity gain can be obtained when the envelop correlation coefficient is less than 0.5 and the estimate of the relative signal strengths received by each antenna branch is obtained from the antenna patterns using the concept of MEG. To guarantee the signal strengths from two antennas are approximately equal, the ratio MEG1/MEG2 is calculated and is checked to be close to unity. From the Table 1, when a uniform propagation environment is assumed, the correlation coefficient shows that two antenna branches received highly uncorrelated signals since [GRAPHIC]. Also, the MEG ratio is almost unity, indicating that the mean power delivered from two antenna branch is the same. 4. CONCLUSIONS A novel diversity antenna of planar monopole type suitable for WLAN application has been proposed and investigated. The proposed antenna with step shape near feed part achieved broad bandwidth about 2360 MHz starting from 4520 to 6880 MHz. Also, the isolation characteristic between two antennas is enhanced by separation distance and two slots in the common ground over 5.2 and 5.8 GHz bands. The antenna can provide the spatial diversity capable of overcoming the multi-path fading problem for 5 GHz WLAN operation. ACKNOWLEDGMENT This work was supported by the National Research Laboratory (NRL) of Ministry of Science and Technology, Korea, under contract No. M This work was supported by Intelligent Radio Engineering Center(IREC) of Korea Science and Engineering Foundation, Korea, under contact R REFERENCES 1. J.S. Colburn, Y. Rahmat Samiil, M.A. Jensen, and G.J. Pottie, Diversity performance of dual antenna personal communication handsets, IEEE Antennas Propag Symp 1 (1996), S.C.K. Ko and R.D. Murch, Compact integrated diversity antenna for wireless communications, IEEE Trans Antennas Propag 49 (2001), K.-L. Wong, Y.-Y. Chen, S.-W. Su, and Y.-L. Kuo, Diversity dual-band planar inverted-f antenna for wlan operation, Microwave Opt Technol Lett 38 (2003), M. Karaboikis, C. Soras, G. Tsachtsiris, and V. Makios V, Compact dual-print inverted-f antenna diversity systems for portable wireless devices, IEEE Antennas Wireless Propaga Lett 3 (2004), Y. Ge, K.P. Esselle, and T.S. Bird, Compact diversity antenna for wireless devices, Electron Lett 41 (2005), G.A. Mavridis, J.N. Sahalos, and M.T. Chryssomallis, Spatial diversity two-branch antenna for wireless devices, Electron Lett 42 (2006), G. Tsachtsiris, C. Soras, M. Karabokis, and V. Makios, Ground plane effect on the performance of a printed minkowski monopole antenna, Int Conf Appl Electromagnetics Commun (2003), X.D. Wu and K. Chang, Ground plane effects on planar inverted-f antenna(pifa) performance, IEE Proc Microw Antennas Propagat, 150 (2003), Wiley Periodicals, Inc. NEURAL MODELS FOR THE V-SHAPED CONDUCTOR-BACKED COPLANAR WAVEGUIDES Kerim Guney, Celal Yildiz, Sabri Kaya, and Mustafa Turkmen Department of Electrical and Electronics Engineering, Faculty of Engineering, Erciyes University, Kayseri, Turkey Received 12 October 2006 ABSTRACT: Neural models based on the artificial neural networks (ANNs) for computing the effective permittivities and characteristic impedances of V-shaped conductor-backed coplanar waveguides are presented. The proposed neural models can also be used for calculating the characteristic parameters of conductor-backed coplanar waveguides. Six learning algorithms, Levenberg Marquardt, bayesian regularization, quasi Newton, scaled conjugate gradient, conjugate gradient of Fletcher-Reeves, and resilient propagation, are used to train the ANNs. The neural results are in very good agreement with the results available in the literature. When the performances of neural models are compared with each other, the best result is obtained from the ANNs trained by the LM algorithm Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: , 2007; Published online in Wiley Inter- Science ( DOI /mop Key words: V-shaped conductor-backed coplanar waveguide; effective permittivity; characteristic impedance; artificial neural networks 1. INTRODUCTION Coplanar waveguides (CPWs) have been widely used in microwave and millimeter-wave integrated circuits as an alternative to microstrip lines [1 3]. Although the original CPW structure [1] has a substrate without any metallization on the backside, in most practical applications, the substrate is backed with conducting material. The conductor-backed CPW (CBCPW) loses its modal power into leaky waves. The CBCPW using a backside ground plane has the advantage of mechanical strength, heat sinking ability, and lower characteristic impedance than conventional CPW. Among these advantages, CBCPW also allows easy implementation of mixed coplanar/microstrip circuits. These and several other advantages make conductor-backed coplanar waveguides 1294 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 6, June 2007 DOI /mop