A Compact Multiband Antenna for GSM and WiMAX Applications M. Ali Babar Abbasi, M. Rizwan, Saleem Shahid, Sabaina Rafique, Haroon Tariq Awan, S. Muzahir Abbas Department of Electrical Engineering, COMSATS Institute of Information Technology, Islamabad, Pakistan m.babarabbasi@gmail.com, engr.mrizwan@yahoo.com, saleemshahid@live.com, sabaina.rafique@hotmail.com, haroontariq.awan@gmail.com, muzahir_abbas@comsats.edu.pk Abstract. This paper presents a compact low profile microstrip patch antenna suitable for Global System for Mobile Communications (GSM) and Worldwide Interoperability for Microwave Access (WiMAX) applications. FR4 epoxy is used as substrate with relative permittivity 4.4 and tangent loss 0.02. The prototype contains slots, shorted pin, partial ground, tuning stub and dual feeding techniques. Parametric analysis has been carried out to observe the effects of techniques. Simulation results show that antenna has enough impedance bandwidth to cover the desired frequency bands. The prototype is fed using transmission line. Antenna exhibits omni-directional radiation patterns with reasonable gain on all operating frequency bands. Characteristic analysis and simulation of prototype was done using Ansoft HFSS (High Frequency Structure Simulator). Keywords: Microstrip Patch Antenna, WiMAX, GSM, Partial Ground, Stub, Radiation Pattern. 1 Introduction In 21st century, wireless technologies require compact and low profile components for communication systems. The characteristic parameters of antenna mainly depend upon the size of radiating element which is approximately one-half of a free-space wavelength. This implies a tradeoff between the size of antenna and operating frequency. Microstrip patch antennas are narrow band which applies limitation on the impedance bandwidth. Research has been done to reduce the tradeoffs and limitations by proposing different improvement techniques. Research shows implementation of few techniques like partial grounding [1], shorting pin [1-2], cutting slots and slits in radiating patch and ground plane [1][3-5], and tuning stubs [1][6-7]. The paper presents a multiband antenna which covers bands of GSM 1800MHz and WiMAX bands at 3.5GHz (3.4 3.6GHz), 3.6 3.8GHz and 5.8GHz (5.725 adfa, p. 1, 2011. Springer-Verlag Berlin Heidelberg 2011
5.850GHz) with return loss S11 10dB [8]. The antenna also operates for PCS 190 (1850-1990MHz) [9-10], LTE Band 42 (3.4-3.6GHz), 43 (3.6-3.8GHz) [10] and WLAN 5.8 (5725-5825GHz) [8] with stable, sufficient gain and omni-directional radiation patterns [11]. Section-2 explains the antenna configuration. Modeling and simulation has been discussed in section-3. Simulation results and analysis has been presented in section-4, section-5 presents the comparison between measured and simulated results while section-6 concludes the paper. (a) (b) Fig. 1. Geometry of proposed antenna (a) Front view (b) Side view.
2 Antenna Configuration The geometry of proposed antenna is shown in Fig. 1. Patch has been printed on FR4 epoxy substrate with dimensions 38mm x 38mm, having relative permittivity 4.4 and thickness 1.6mm, is fed using transmission line having impedance of 50Ω and 2.85mm width. Dimensions are tabulated in Table 1. Table 1. Dimensions Of Proposed Antenna Design Parameter Dimension (mm) Parameters Dimension (mm) a 10.4 g 21 b 7.7 h (Stub) 18.525 c 3.85 Slot1 14.25 d 9.5 Slot 2 22.8 e 10.5 Connection 1 40.75 f 3.325 Connection 2 32.3 3 Modeling and Simulation The objective was to design a compact multiband antenna for GSM and WiMAX applications and to implement the characteristic improvement techniques in a single antenna to improve its results as well as to observe the effects of specific techniques. The first step was to design a microstrip patch antenna with rectangular configuration. Stair slots were added along the non-radiating edges to convert them to radiating edges [4]. A stair slot was also added to the upper radiating edge of antenna to disturb the path of current and to shift the resonating frequency to lower bands. For further size reduction, a shortening pin was added to the center of patch. The shorting pin technique [2] shifted the resonant frequency to lower bands, thus for a fixed radiating element size, the antenna operates at a lower frequency, independent of the limit that size of radiating element should be approximately equal to one-half of free-space wavelength. The return loss of antenna before and after shortening pin is shown in Fig. 2.
Fig. 2. Simulated return loss before and after pin shortening. Size reduction at a specific central frequency decreased the impedance bandwidth. Using partial ground, impedance bandwidth was increased [1]. The parameter g (ground) has been varied at different values of g (i.e. g=19mm, 21mm and 25mm). The coupling between ground and radiating element is controlling the GSM frequency band, as the coupling increases return loss also increases. The return loss corresponding to different values of g is shown in fig. 3. Fig. 3. Simulated return loss at various values of g while other parameters were unchanged.
Slots were added to disturb the path of current [1] [3-4]. The slots controlled the return loss and shifted the resonating frequency towards the desired GSM band (1.8GHz). The positions of slots were adjusted according to the current distribution on the patch of antenna. Fig. 4 shows the return loss comparison before and after the addition of slots. Fig. 4. Simulated return loss before and after addition of slots. Connection 1 was established between the transmission line and right side, close to the center of the patch. This results in improved return loss at the GSM band without modifying the geometry of patch. Dual feeding was achieved using this connection. Keep in mind that the connection has provided the current another path to enter the radiating area. Fig. 5 shows the improvement in return loss by establishing the above mentioned connection. Fig. 5. Simulated return loss before and after establishing connection 1.
Adding stub on left side of transmission line with tunable length (h=18.525mm) improved the return loss at the higher frequencies. Stub behaved as monopole resonator producing coupling effect with lower left side of patch. The coupling between stub and patch introduced two new bands at higher frequencies i.e. 5.3-5.7GHz and 7.1-7.8GHz. Without disturbing the structure of radiating patch, an improvement in return loss for higher frequency band of WiMAX was achieved by shortening the farthermost radiating edge of patch, thus a symmetric current path was available for lower frequencies which will not disturb the behavior of radiation at lower frequencies. However, at higher frequencies, it has virtually divided the patch into two parts. First part includes transmission line, connection 1 and area around slot 1 (i.e. lower portion of the patch). Second part includes connection 2 and area around slot 2 (i.e. upper portion of the patch. Fig. 6 shows the return loss after the addition of stub and connection 2. Fig. 6. Simulated return loss before and after adding stub and connection 2. 4 Simulation Results and Analysis 4.1 Return Loss Fig. 7 shows the simulated return loss of the proposed antenna at various frequency bands.
Fig. 7. Simulated return loss of the proposed antenna. 4.2 Radiation Pattern Fig. 8 presents the simulated E-field and H-Field radiation patterns of the proposed antenna at various frequencies. Omni-directional patterns are clearly seen in radiation patterns at 1.8GHz, 3.5GHz and 5.8GHz. As the frequency increases the gain also increases but radiation pattern gets distorted and antenna behavior changes from omni-directional to directional radiation pattern. The pattern is distorted due to radiation of partial ground and mismatch of impedance. (a)
(b) Fig. 8. Radiation pattern of the proposed antenna at 1.8GHz, 3.5GHz and 5.8GHz (a) E-field (b) H-field 4.3 Current distribution Fig. 9 shows the current distribution of antenna at 1.8GHz, 3.5GHz and 5.8GHz. (a)
(b) Fig. 9. Current distribution at (a) 1.8GHz (b) 3.5GHz (c) 5.8GHz (c) 5 Measured Results And Comparison Fig. 10 shows the fabricated antenna. The antenna is fabricated on FR4 epoxy substrate with the proposed dimensions.
(a) (b) Fig. 10. Fabricated antenna (a) Front view (b) Ground view Fig. 11 shows the measured return loss of proposed antenna. The results are measured using Agilent Network Analyzer. Fig. 11. Measured return loss using Network Analyzer. Fig. 12 shows the comparison between measured and simulated return loss of proposed antenna.
Fig. 12. Simulated and measured return loss comparison. 6 Conclusion A multiband tunable antenna with impedance bandwidth to cover applications of GSM 1.8GHz and WiMAX bands 3.5GHz (3.4 3.6GHz), 3.6 3.8GHz and 5.8GHz (5.725 5.850GHz) has been presented. Antenna is also tunable at PCS 1900 (1850 1990MHz), LTE Band 42 (3.4 3.6GHz), 43 (3.6 3.8GHz) and WLAN 5.8 (5725 5825GHz) The antenna is designed using rectangular microstrip configuration with optimization techniques implemented on it to achieve the best possible desired results. The bandwidth of proposed antenna for GSM 1.8GHz is 90MHz (1750 1840MHz), WiMAX 3.5GHz is 1060MHz (3330 4390MHz) and 5.8GHz is 125MHz (5725 5850MHz). The bandwidth of antenna is enough to tune antenna to operate for GSM and WiMAX applications. References 1. Kin-Lu Wong, Compact and Broadband Microstrip Antenna, John Wiley & Sons, Inc. 2002. 2. Shan-Cheng Pan and Kin-Lu Wong, Dual-frequency triangular microstrip antenna with a shorting pin, IEEE Transactions on Antennas and Propagation, December 1997, pp. 1889 1891. 3. C. Kamtongdee and N. Wongkasem, A novel design of compact 2.4 GHz microstrip antennas, 6th International Conference on Electrical Engineering/Electronics, Computer, Telecommunications and Information Technology (ECTI-CON), May 6-9, 2009, BangkadiPathumthani, Thailand,vol. 2, pp. 766 769.
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