A Wide Band Pattern and Frequency Reconfigurable Microstrip Patch Antenna using Varactors for WLAN Applications

Transcription

1 A Wide Band Pattern and Frequency Reconfigurable Microstrip Patch Antenna using Varacrs for WLAN Applications Ros Marie C Cleetus 1 and Dr.T.Sudha 2 Department of Electronics and Communication Engineering N.S.S. College of Engineering, Palakkad Kerala, India ros_bina@yahoo.in 1, sudhat@nssce.ac.in 2 Abstract Wireless communication systems are being used for a number of applications nowadays and for that, a number of antennas or a single antenna with multiple functional capabilities has become inevitable. This paper attempts design a frequency as well as pattern reconfigurable microstrip patch antenna using electrical reconfiguration method. Three cases are being taken in account. The first case results in an operation at 5.2GHz and the remaining two cases offer operations at 5.2GHz and also at 2.4GHz. In the 5.2GHz band a figure of 8 E-plane pattern and an equal gain H-plane pattern are obtained in all the cases, whereas in the 2.4GHz band an equal gain E-plane pattern and 180º switchable H-plane patterns will be resulted according the switching status. This antenna is an attractive candidate for Wireless Local Area Network (WLAN). Index Terms Reconfigurable antenna, microstrip patch antenna, electrical reconfiguration, return loss, gain pattern I. INTRODUCTION Reconfigurable antennas have attained an unavoidable position in the field of communication as the need of wireless communication devices is increasing day by day. The characteristics of antenna, such as resonant frequency, radiation pattern, polarization, etc. can be reconfigured and be used in a more efficient and effective manner. Reconfigurable antennas have found their extensive applications at cellular radio systems, radar systems, satellite communication systems, smart weapon protection, etc. A large number of standards can be supported by reconfigurable antennas, in mobile and satellite communication. The most significant advantage of reconfigurable antennas is that, they can replace a number of single function oriented antennas. Reconfiguration of the antenna can be done by different techniques. The first one, that uses radio frequency micro electromechanical systems (RF -MEMS) [1], PIN diodes [2] or varacrs [3] as switching devices are called Electrically Reconfigurable Antennas. Optical switches are used achieve reconfiguration in the second technique, and such antennas are called optically reconfigurable Antennas [4]. In the third technique, antenna radiating parts are physically altered and these are called Physical Reconfigurable Antennas [5]. And also, the antennas can be reconfigured by introducing changes in the substrate characteristics by using materials such as ferrites, liquid crystals, etc []. All these switches, especially varacrs add the scalability of reconfigurable antennas [7]. So that, out of these four techniques here adopted the electrical reconfiguration method that uses varacrs for switching. In this paper, a microstrip line-fed rectangular patch with a partial ground plane base equipped with two varacr diodes is proposed get both frequency and pattern Page 721

2 reconfigurability. Two varacr switches are mounted over the slots in the ground plane. Three switching cases are considered. In the first, the antenna is operable over the 5.2 GHz band whereas, at 2.4 GHz and 5.2 GHz in the other two cases. A figure of 8 E-plane pattern and an equal gain H plane pattern are obtained in all cases in the 5.2 GHz band. The two cases where operation at 2.4 GHz is possible, equal gain E-plane patterns and 180 switchable H- plane patterns are obtained. The Ansoft High Frequency Structure Simular (HFSS) software is used as the ol for simulation. Section II is a briefing on microstrip antenna design, while Section III presents antenna configuration, Section IV shows the results and discussion. Finally, conclusion is given in Section V. II. MICROSTRIP ANTENNA DESIGN Nowadays, antennas with smaller dimensions that influence device portability are preferred for wireless applications. Low-profile antennas will be required, in applications like high-performance aircraft, spacecraft, satellite, etc. Microstrip antennas can be considered as the solution. There are different shapes of microstrip patch antenna such as rectangular, square, circular etc. The rectangular patch is considered as the most widely used configuration. This paper also deals with a rectangular patch. The value of parameters like the dielectric constant of the substrate (ε r ), the resonant frequency (f r ), and the height of the substrate (h) should be known when designs the patch. The designing procedure is as follows [8]: First the width (W) is determined from, W = (1) C 0 is the free-space velocity of light. Then the effective dielectric constant of the microstrip antenna can be determined using the following equation: ε Afterwards the extension of the length L can be determined using the equation: L =. (. ). (. ). (3) The actual length of the patch can now be determined by using this equation: = 2 (4) From these equations that mentioned above, we can see that the resonant frequencies are inversely proportional the width and length of the patch. III. ANTENNA CONFIGURATION The geometrical structure of the antenna, including dimensions, is shown in Fig. 1. The antenna is based on a Rogers RT/Duroid 5880 substrate with a dimension of 50mm X 32mm with the dielectric constant, ε r of 2.2 and a thickness of 1mm. The patch which is rectangular is of 22.8mm X 18.92mm dimension, and is fed using a 2.8mm-wide microstrip line. The ground plane is constructed in such a way that, a rectangle of dimension 39mm X 22mm is subtracted from the full ground plane at (5.5,10,-1). Later, two symmetrical 1.4mm-wide rectangular slots are created on the ground plane. Both the slots are at 8.mm from the antenna's symmetry axis. Two 1.4mm X 2.5mm varacr diodes are mounted across the slots, as indicated in Fig. 1. The diodes that can be used are Skyworks SMV silicon abrupt-junction common-cathode pairs [9] that are connected in parallel achieve a capacitance range of pf from 0 30V with an equivalent series resistance of almost 0.55 Ohm. ε = (2). Page 722

3 Good gain, directivity and return loss characteristics are exhibited by the antenna. A Gain higher than 2dB is recorded at 2.4GHz, whereas a gain higher than 5.dB is obtained in all cases at 5.2GHz. The return loss is less than -15dB at 5.2GHz and less than -21dB at 2.4GHz. The percent bandwidths are 30.19% for the first case, 25.42% and 31.35% at 2.4GHz and 5.2GHz for the second and third case. Fig. 1 Antenna configuration IV. RESULTS AND D DISCUSSION The antenna is designed and simulated using Ansoft HFSS [10], an EM simular based on the Finite Element Method (FEM).T he three cases, considered achieve the desired frequency and pattern reconfigurability are given in Table I. It is considered in this antenna system that the varacrs operate with the capacitance value of 1.2pF according zero bias voltage. Among the three cases considered, Case 1 deals with the condition that both the varacrs are in reverse bias state, the first varacr alone in reverse erse bias in Case 2, and the second alone in reverse bias in Case 3. The HFSS-computed operating frequency ( f r ), return loss (S 11 ), peak gain (G 0 ), operable band of frequencies, bandwidth and peak directivity are also listed in Table I. Fig. 2 Simulated return loss of the antenna for Case 1 Fig. 3 Simulated return loss of the antenna for Case 2 TABLE I. OPERATING FREQUENCY, RETURN LOSS,PEAK GAIN, OPERABLE BAND, B BANDWIDTH AND PEAK DIRECTIVITY OF THE ANTENNA A FOR EACH BIASING CONDITIONN f r (GHz) Return Loss G Band (GHz) Peak Directivity Fig. 4 Simulated return loss of the antenna for Case 3 Figures 2-4 depict the simulated return loss plots for the 3 biasing cases. The first case results in a resonant frequency of 5.2GHz band, and bandwidth centered at this frequency is extended from GHz. For the cases 2 and 3, the antenna resonated at 2.4GHz and Page 723

4 5.2GHz. The bandwidth at 2.4GHz extends from GHz and that at 5.2GHz from 4.37 GHz, so that these can cover 5.2/5.8GHz ( GHz/ GHz) and 2.4GHz ( GHz) WLAN standards. The good return loss characteristics and high bandwidth exhibited at these resonant frequencies make this antenna suitable for WLAN applications. 2.4GHz (c) 5.2GHz 2.4GHz 5.2GHz (a) (b) 5.2GHz Fig. 5 Normalized Gain Pattern of the antenna in the X Z (Red line) and Y Z (Black line) planes for (a) Case 1, (b) Case 2 and (c) Case 3 The simulated gain patterns in the X Z (H) plane and Y Z (E) plane are shown in Fig.5, for each case. At 5.2GHz, all three cases result in almost similar radiation patterns in the H-plane and a figure of 8 pattern in the E plane. At 2.4GHz band, the antenna exhibits patterns with nulls directing wards the user. These types of patterns are very much suitable for reducing the biological effects of electromagnetic radiations from the antenna wards the user side. The H- plane pattern in both cases show similar characteristics as at 5.2 GHz but with a null in one of the ±90 direction. The H-plane pattern is 180º switchable in the two cases 2 and 3. The E-plane pattern exhibited omnidirectional patterns in both cases which makes it suitable for WLAN application. A. Parameter study The varacr used here can achieve a capacitance range of pf from 0 30 V with an equivalent series resistance of almost 0.55 Ohm. So that, a parameter study is performed by varying the capacitance value with which the particular varacr can be operated, keeping all other parameters same. Here also among the three cases considered, Page 724

5 Case 1 deals with the condition that both the varacrs are in reverse bias state, the first varacr alone in reverse bias in Case 2, and the second alone in reverse bias in Case 3. Each modified structure is simulated. The results of the simulations are presented in Table II-IV. TABLE II. OPERATING FREQUENCY, RETURN LOSS, PEAK GAIN, OPERABLE BAND, BANDWIDTH AND PEAK DIRECTIVITY OF THE ANTENNA FOR EACH BIASING CONDITIONS WITH THE MODIFICATION 2. Modification 2: When the Capacitance value is fixed at 3.pF f r (GHz) Return Loss G Band (GHz) Peak Directivity The simulated parameters of the antenna structure for a different capacitance value, named as Modification 2 are given in Table II. The percent bandwidth is much better when it compares with that of the reference antenna structure. At 5.23GHz, it is greater than 31% and at 2.73GHz, it is greater than 30%. TABLE III. OPERATING FREQUENCY, RETURN LOSS, PEAK GAIN, OPERABLE BAND, BANDWIDTH AND PEAK DIRECTIVITY OF THE ANTENNA FOR EACH BIASING CONDITIONS WITH THE MODIFICATION 3. Modification 3: When the Capacitance value is fixed at 4.pF f r (GHz) Return Loss G Band (GHz) Peak Directivity The resulting parameters of the antenna for modification 3 are given in Table III. The percent bandwidth is better here also, when it compares with that of the reference antenna structure. At 5.22GHz, it is greater than 30% and at 2.72GHz, it is greater than 32%. TABLE IV. OPERATING FREQUENCY, RETURN LOSS, PEAK GAIN, OPERABLE BAND, BANDWIDTH AND PEAK DIRECTIVITY OF THE ANTENNA FOR EACH BIASING CONDITIONS WITH THE MODIFICATION 4. Modification 4: When the Capacitance value is fixed at 5.4pF f r (GHz) Return Loss G Band (GHz) Peak Directivity The resulting parameters of the antenna for modification 4 are given in Table IV. The percent bandwidth is better than that of reference antenna structure. At 5.25GHz, it is greater than 30% and at 2.74GHz, it is greater than 32%. At 5.25GHz, peak directivity and peak gain are better when compares with that of the reference antenna structure. The results of the parameter study show that reconfigurable antennas with better gain, directivity and bandwidth from the reference antenna can be obtained by slight modifications in the capacitance value from the existing structure. V. CONCLUSION This antenna structure uses two varacr diodes, mounted over two slots in the ground plane so as obtain both frequency and pattern reconfigurability. In the first switching scenario, the antenna is operable over the 5.2GHz band, whereas a dual-band operation at 2.4GHz and 5.2GHz is obtained in the other two scenarios. An equal gain pattern in the H- plane and a figure 8 pattern in the E-plane are Page 725

6 obtained in all cases in the 5.2GHz band. And when it becomes operable at 2.4GHz, the antenna has equal gain E-plane patterns and 180º-switchable H-plane patterns. Also, a parameter study is performed by modifying the capacitance values and could find that these structures can have improved gain, directivity and bandwidth values. This antenna finds its application in WLAN. REFERENCES [1] C. W. Jung, M. Lee, G. P. Li, and F. De Flavis, Reconfigurable scan-beam singlearm spiral antenna integrated with RF- MEMS switches, IEEE Trans. Antennas Propag., vol. 54, no. 2, pp , Feb [2] S. Shelley, J. Costantine, C. G. Chrisdoulou, D. E. Anagnosu, and J. C. Lyke, FPGA-controlled switchreconfigured antenna, IEEE Antennas Wireless Propag. Lett., vol. 9, pp , [3] E. Annino-Daviu, M. Cabedo-Fabres, M. Ferrando-Bataller, and A. Vila-Jimenez, Active UWB antenna with tunable bandnotched behavior, IEEE Electron. Lett., vol. 43, no. 18, pp , Aug [4] C. J. Panagamuwa, A. Chauraya, and J. C. Vardaxoglou, Frequency and Beam Reconfigurable Antenna using Phoconductive Switches, IEEE Trans. Antennas Propag., vol. 54, no. 2, pp , Feb [5] S. Jalali Mazlouman, M. Soleimani, A. Mahanfar, C. Menon, and R. G. Vaughan, Pattern Reconfigurable Square Ring Patch Antenna actuated by Hemispherical Dielectric Elasmer, Electron. Lett., vol. 47, no. 3,pp , Feb [] W. Hu, M. Y. Ismail, R. Cahill, J. A. Encinar,V. Fusco, H. S. Gamble, D. Linn, R. Dickie,N. Grant, and S. P. Rea, Liquid-crystal-based reflectarray antenna with electronically switchable monopulse patterns, Electron. Lett., vol. 43, no. 14, Jul [7] I. Gutierrez, E. Hernandez, and E. Melendez, Design and Characterization of Integrated Varacrs for RF Applications. New York: Wiley, 200. [8] Constantine A. Balanis, Antenna Theory: Analysis and Design, Wiley- Interscience, [9] Skyworks Solutions Inc. Woburn, MA. [10] Ansoft HFSS, Pittsburg, PA 15219, USA. Page 72