Dielectric Resonator Antenna for Wireless LAN Applications Pejman Rezaei*(1,2), Mohammad Hakkak(1,2), Keyvan Forooraghi(1) (1) Tarbiat Modarres University, Department of Electrical Engineering (2) Iran Telecommunication Research Center, Transmission Department P.O. Box: 14155-3961, Tehran 14399, Iran
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Introduction Wireless LAN (WLAN) has attracted a great deal of attention as a technology that enables short-range wireless communications between wide varieties of devices such mobile phones, and notebooks. The dielectric resonator antenna (DRA) is a resonant antenna, fabricated from low-loss dielectric material the resonant frequency of which is predominantly a function of size, shape, and material permittivity. DRAs offer the advantages of small size, lightweight, low profile, low cost, and high radiation efficiency, making them attractive candidates for WLAN applications [1, 2]. DRAs frequently are available in rectangular, cylindrical, and hemispherical geometries. Rectangular DRAs offer more design flexibility since two of the three of its dimensions can be varied independently for a fixed resonant frequency and known dielectric constant of the material [3]. DRA was first proposed in the early 1980’s [4]. After then various investigate offer significant enhancements to parameters such as bandwidth, gain, polarization, or power coupling. Over last decades, various bandwidth enhancement techniques have been developed for DRAs. An overview on these techniques has also reported, where these techniques were classified into three broad categories: Lowering the inherent Q-factor of the resonator; using external matching networks; and combine multiple dielectric resonators [5]. In this paper, we introduce the two-segments rectangular DRA in side by side form. This structure composed of two different rectangular DRA, which separated by a Perfect E plate. Former, the overlap multisegment DRA has been developed to enhance the amount of coupling from the microstrip line [6]. But the proposed arrangement of segments, offers bandwidth broadening of a DRA. Also, in literature demonstrate that the volume of a conventional dielectric resonator antenna can be reduced by approximately half [7-10]. As well, more reductions in the volume of the DRA can be obtained by utilizing sector structure [11]. The technique relies on employing an extra conducting plate in the DRA, which acts as an electric wall. The proposed form of rectangular DRA, fed by microstrip coupling, in this case is shown in Figure 1.
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(a) (b) Figure 1. (a)- Antenna fed by microstrip line, (b) - profile view The use of the metal plate can be likened to the shorting post used in patch antennas to reduce their length from Ȝ/2 to Ȝ/4 [7]. We can still maintain the same field distribution in the driven side of the DRA. Because the field distribution remains the same we can expect the resonant frequency to remain the same and therefore we have effectively reduced the volume of the DRA by half. In the presented structure, two half DRA resonated in two adjacent frequencies. The attentions focus on a type of DRAs that can offer two-resonant frequencies and these frequencies can be merge red into a broad band, which can be used for WLAN application at 5-6GHz band [10]. For this purpose, the effects of various combinations of a1 and a2 dimensions on their impedance characteristic and consequently on matched bandwidth are investigated. Then we report the results of our experimental investigations on this structure on a finite ground plane. Simulations and results Full wave analysis of the dielectric antenna configurations were performed using Ansoft HFSSTM (ver. 9.2) with Finite Element Method. Extensive simulations were carried out using the software in order to obtain optimal design parameters for the antenna. The initial dimensions of the radiating portion of antenna were determined using the equations developed for the dielectric waveguide model (DWM) for a rectangular resonator in free space [3]. The optimum dimensions of antenna parameters were determined with experimental optimization. In general, to achieve strong coupling, the DRA must be fabricated from high permittivity materials. However, to operate over a wide bandwidth, the DRA must have a low dielectric constant [1]. Previous work has shown the critical coupling is possible for DRA’s having a high value (20 or much) of dielectric constant [3]. In this work, for simplicity, material of two segments is similar and the dielectric constant of the designed DR (İr) is 20. The height (b/2) and weight (d) of the DR are 9.6mm, and 6mm respectively. Furthermore, the optimum lengths of two segments (a1 and a2) are 4.8mm, and 5.7 mm respectively. Also, the size of square ground plane is 5cm × 5cm. The microstrip feed line was 1mm wide and on a 0.33mm thick substrate with a relative dielectric constant, İs=2.2, to give a characteristic impedance of 50ȍ. In the direct micro strip coupling scheme of excitation, the far end of a microstrip line is terminated in an open circuit and the coupling can be easily controlled by varying the lateral distance between the DRA and the micro strip line. Former work has shown the maximum
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coupling is achieved when the DRA was placed at a distance of integral multiples of Ȝ/2 from the open end [3]. For this case finest coupling and consequently maximum bandwidth for ǻL=4.1mm was achieved. When operating in a simple structure of a DRA, the bandwidth is typically about 10%. For the optimum two-segments DRA, a 10dB return loss bandwidth 19% is obtained. The related input impedance and return loss curves of designed two-segments DRA are shown in figure 2.
Figure 2. Input impedance and return loss of simulated two-segments DRA The simulated results have shown impedance bandwidth at 4.99-6.04GHz band, with two resonant frequencies at 5.18GHz and 5.86GHz. Moreover, another advantage of the proposed structure is that it offers two optimum locations for antenna on the ground plane by readjusting the ǻL length after rotating the antenna in the feed line direction. In above case, by pivoting the antenna (i.e. exchanging the position of a1 and a2), the optimum return loss curve for ǻL=6.1mm is obtained (figure 3).
Figure 3. Return loss of the optimum pivoted DRA In this condition, the bandwidth has slightly narrowed and has shifted to higher frequencies, and the resonant frequencies have shifted to 5.36GHz and 5.92GHz. Also, the impedance bandwidth has shifted to 5.09-6.07GHz with 17.5% bandwidth. With regard to IEEE 802.11a standard, this plan is still suitable for WLAN application. The multisegment DRAs with more separate sections (over than 2 segment), in comparison to an equal unit DRA, show good performance, which can be a subject of future study. Although because of the additional metallic plate, a decrease in radiation efficiency will occur.
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Conclusion In this work, we have investigated the performance of the proposed two-segment DRA structures using experiments and also numerical software simulations. For the suggested optimum DRA, the impedance bandwidth 19% and 17.5% (for S11>10dB) are obtained in both arrangements on microstrip line feed. Therefore, this plan is easily meeting the WLAN system specifications. Acknowledgment This work was supported by the Iran Telecommunication Research Center. The authors also express their appreciation to Mr. Pirhadi and Mr. Hakkak, for significant advices. References: [1] A. Petosa, A. Ittipiboon, Y.M.M. Antar, and D. Roscoe, “Recent advances in dielectric resonator antenna technology,” IEEE Antennas and Propagation Magazine, vol. 40, no. 3, pp. 35-48, June 1998. [2] R. K. Mongia and P. Bhartia, “Dielectric resonator antennas - A review and general design relations for resonant frequency and bandwidth,” International Journal of Microwave Millimeter-Wave Engineering, vol. 4, pp. 230-247, July 1994. [3] R.K. Mongia, and A. Ittipiboon, “Theoretical and experimental investigations on rectangular dielectric resonator antennas,” IEEE Transactions on Antennas and Propagation, vol. 45, no. 9, pp. 1348-1356, September 1997. [4] S.A. Long, M.W McAllister, and L.C. Shen, “The resonant cylindrical dielectric cavity antenna,” IEEE Transaction on Antennas and Propagation, vol. 31, no. 3, pp. 406-412, May 1983. [5] K.M. Luk and K.W. Leung, Dielectric Resonator Antennas, Research Studies Press LTD., London, 2003. [6] A. Petosa, N. Simons, R. Siushansian, A. Ittipiboon, and M. Cuhaci, "Design and analysis of multisegment dielectric resonator antennas,” IEEE Transaction on Antennas and propagation, vol. 48, no. 5, pp. 738-742, May 2000. [7] M.T.K. Tam, R.D. Murch, “Half volume dielectric resonator antenna designs,” Electronics Letters, vol. 33, no. 23, pp. 1914-1916, November 1997. [8] J. Juntunenl, O. Kiveka, J. Ollikainen, and P. Vainikainen, “FDTD simulation of a wide-band half volume DRA,” 5th International Symposium on Antennas, Propagation and EM Theory, pp. 223-226, August 2000. [9] S.G. O'Keefe, S.P. Kingsley, S. Saario, “FDTD simulation of radiation characteristics of half-volume HEM and TE-mode dielectric resonator antennas,” IEEE Transaction on Antennas and Propagations, vol. 50, no. 2, pp. 175-179, February 2002. [10] K. Lan, S.K. Chaudhuri, and S. Safavi-Naeini, “A compact wide-dual-band antenna for bluetooth and wireless LAN applications,” IEEE Antennas and Propagation Society International Symposium, vol. 2, pp. 926-929, June 2003. [11] M.T.K. Tam and R.D. Murch, “Compact circular sector and annular sector dielectric resonator antennas,” IEEE Transaction on Antennas and Propagations, vol. 47, no. 5, pp. 837-842, May 1999.
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