PIERS ONLINE, VOL. 6, NO. 4, 21 35 A Beam Switching Planar Yagi-patch Array for Automotive Applications Shao-En Hsu, Wen-Jiao Liao, Wei-Han Lee, and Shih-Hsiung Chang Department of Electrical Engineering, National Taiwan University of Science and Technology, Taiwan Abstract In this work, a planar beam switching antenna is proposed for wireless communication used in automotive environment. By integrating a Yagi patch antenna array with a one-to-four RF switch, RF power can be delivered to four different driven patch elements and generate directive beams toward different directions. The prototype antenna is designed to operate at the GSM 18/19 MHz band for cellular phone uses. The size and height of the fabricated antenna are 293 mm by 293 mm by 3 mm, respectively. Measurement results indicate the antenna s gain is approximately 9.478 dbi with a 3 db beamwidth of 68. 1. INTRODUCTION Due to the rapid growth of wireless communication applications, interference and multi-path fading phenomena become critical and need to be resolved to ensure communication quality. In this paper, a planar beam switching antenna, which is a combination of logic control circuits, a one-to-four RF switch, and Yagi patch antennas [1 5] are proposed to provide spatial diversity. The antenna configuration is shown in Fig. 1. The Yagi patch comprises four driven patch elements, which are connected via a one-to-four RF switch and five parasitically coupled director patch elements. By selecting a specific through path of the RF switch, RF power is fed to one of the driven patch elements. Because directive radiating elements are used, the antenna can be reconfigured to four different beam directions and therefore reduces potential blind angles. Since the antenna comes with a planar structure, it can be employed in the automotive environment. For instance, the planar antenna can be mounted on top of a vehicle. The switching directive beam enables the moving vehicle to track a specific base station and extends the communication distance. Such a planar beam switching antenna can also be used for mobile devices to ensure connection quality. The design procedure and geometric parameters of the planar one-to-four beam switching antenna are presented in Section 2. The measured antenna performance, including the operational frequency, gain, 3 db beamwidth, and the squint angle, are provided in Section 3. Analysis based on the comparison of simulated and measured results is also given. A brief summary is drawn at the end. Figure 1: The one-to-four beam switching antenna system.
PIERS ONLINE, VOL. 6, NO. 4, 21 351 2. ANTENNA CONFIGURATION AND ANALYSIS Figure 2 shows the geometric configuration of a Yagi patch antenna. This design is aimed to operation around the GSM 18/19 band. It consists of a driven patch element and four parasitically coupled director patch elements. All elements are square patches and are elevated above the grounded by a dielectric slab of 4 mm thick. The sizes of the five patches are 52 mm 52 mm. The substrate used is Teflon. Its dielectric constant ε r is 2.2, while the loss tangent tanδ is.1. According to the microstrip antenna design formula (1) [3], the dimensions of the microstrip antenna can be reduced approximately 1.45 times by using Teflon substrate instead of air spacing. The length L 1 determines the resonant frequency and the position of the feed point (L 2 ) decides the input impedance. The input impedance increases when the feed point approaches toward the edge. Since the characteristic impedance of the cable line is 5 Ω, the goal of antenna matching is 5 Ω also. The width W 2 affects the antenna efficiency. When the width W 2 increases, so does the efficiency performance. The analysis of the antenna is carried out using HFSS, which is a full wavelength numerical EM tool based on the FEM technique. Figure 3 shows simulated reflection coefficients of the basic Yagi patch antenna. The antenna resonates at 1.85 GHz. The 1 db bandwidth is 5.4% and covers the frequency band from 1.8 to 1.9 GHz. 1 2 W = When w 2 f µ r ε ε r 1 h > 1 (1) In order to provide four radiation beams toward different directions, two Yagi patch antennas are arranged as a cross as shown in Fig. 4. It consists of four driven patch elements located at the ends, five parasitically coupled director patches placed in the middle. Each driven element contains one RF feed to produce a directive beam. Figure 5 shows simulated reflection coefficients of the four-port Yagi patch antenna. Multiple nulls are observed around 1.8 GHz band due to the cross-shaped geometry. Although the bandwidth is nominally increased, the directive radiating pattern is also altered due to additional resonant modes as shown in Fig. 6. This is because the horizontal and the vertical Yagi patch antennas are mutually coupled, the side lobe is even bigger than the main lobe. Fig. 7 clearly reveals the strong side lobes at directions perpendicular to the main lobe. Similar results are also observed for the Reflection Coefficient(dB) 1. 1.5 2. 2.5 3. Figure 2: Geometric parameters of the basic Yagi patch antenna. [W 1 = 72 mm, W 2 = 52 mm, L 1 = 52 mm, L 2 = 14 mm, D = 3.5 mm]. Figure 3: Simulated reflection coefficients of the basic Yagi patch antenna. Reflection Coefficient(dB) 1. 1.5 2. 2.5 3. Figure 4: Geometric configuration of the cross Yagi patch antenna. Figure 5: Simulated reflection coefficients of the cross Yagi patch antenna.
PIERS ONLINE, VOL. 6, NO. 4, 21 352 other three Yagi patch antennas. In order to improve the isolation performance of the cross Yagi antenna design, we refer to the crossing element design with fillisters proposed in [7] to reduce coupling. Fig. 8 shows the revised cross Yagi patch antennas. The fillister dimensions are 1 mm 3 mm. According to simulation results, the isolation performance is dramatically improved. Note the shape of patch element is changed to a rectangle. Since the length and width are not equal, resonant frequencies of two orthogonal directions are separated and therefore improve the isolation performance. Because the cross Yagi patch antennas has a symmetric structure, only results of one feed are presented. Fig. 9 shows simulated reflection coefficient of the revised cross Yagi patch antenna. The antenna resonates at 1.84 GHz. The 1 db bandwidth is 7.6% and covers the frequency band from 1.77 to 1.91 GHz. S 31 and S 41, which are equivalent to isolation performance, are below 25 db. Fig. 1 shows simulated radiation patterns of the planar beam switching antenna at 1.85 GHz. Table 1 shows the gain performance and main lobe properties at different frequencies. Note the gain of main beam at 1.9 GHz is smaller than other frequencies. This is because S 21 at 1.9 GHz Figure 6: Simulated 2-D pattern of the cross Yagi patch antenna. (E-plane/1.85 GHz). Figure 8: Geometric parameters of the four ports cross Yagi patch antennas: W 1 = 66 mm, W 2 = 46 mm, W 3 = 1 mm, L 1 = 53 mm, L 2 = 3 mm, L 3 = 14 mm, D = 2 mm. Figure 7: Simulated 3-D pattern of the cross Yagi patch antenna. (1.85 GHz). S-parameter(dB) -25-3 -35-4 -45 5-6 -65 1.21.31.41.51.61.71.81.92.2.12.22.32.4 S11 S21 S31 S 41 Figure 9: Simulated S-parameter spectra of the revised cross, Yagi patch antennas. (a) (b) Figure 1: Simulated 2-D patterns of the revised cross Yagi patch antenna at 1.85 GHz (a) E-plane (b) H-Plane at 47.
PIERS ONLINE, VOL. 6, NO. 4, 21 353 is larger as indicated in Fig. 9, part of the energy is fed to the opposite port. The difference between main beam and side lobe levels are 8.45, 8.369, and 6.568 db at 1.8, 1.85, and 1.9 GHz, respectively. When the operation frequency increases, the squint angle also increases. This is due to the reduced wavelengths at higher frequencies. The constructive interference approaches to the horizontal plane when the horizontal separation distance among patch elements is fixed. On the contrary, the 3 db beamwidth increases when the frequency reduces. The directivity performance also deteriorates accordingly. Table 1: Comparison of simulated performance. Main beam (db) Side lobe level (db) Squint angle 3 db beamwidth 1.8 1.885 2.48 37 68 1.85 1.169 1.8 47 58 1.9 8.93 1.525 57 46 3. MEASURED RESULTS AND ANALYSIS As to the prototype antenna fabrication, instead of using Teflon, an acrylic slab is used due to availability. The dielectric constant ε r of acrylic is 2.2. Fig. 11 shows the fabricated prototype antenna. Measured reflection coefficient of the fabricated cross Yagi patch antenna is provided in Fig. 12. The antenna resonates at 1.865 GHz. The 1-dB bandwidth is 7% and covers the frequency band from 1.8 to 1.93 GHz. The isolation performance, which is represented by S 31 and S 41 spectra are below 3 db. Comparing the simulated and measured S 21, the measured S 21 is smaller than 1 db at 1.9 GHz and therefore ensure its gain performance. Fig. 13 shows measured radiation patterns at 1.85 GHz. Table 2 lists measured gains, side lobe levels, squint angle and 3 db beamwidth. Note the measured gains are smaller than simulated ones by 1 2 db due to the larger loss tangent of acrylic. The difference between the main lobe and side lobe levels are 4.82, 5.916, and 4.819 db at 1.8, 1.85, and 1.9 GHz, respectively. Similar to the simulation results, the main beam squint angle increases while the 3 db beamwidth diminishes as the operation frequency goes up. S-parameter(dB) S11 S21 S31 S 41-25 -3-35 -4-45 5-6 -65 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2. 2.1 2.2 2.3 2.4 Figure 11: Fabricated cross Yagi patch antenna. Figure 12: Measured S-parameter spectra of cross Yagi patch antennas. Table 2: Comparison of measured performance. Main beam (db) Side lobe level (db) Squint angle 3 db beamwidth 1.8 8.455 3.653 36 7 1.85 9.478 3.562 4 68 1.9 8.857 4.38 45 63
PIERS ONLINE, VOL. 6, NO. 4, 21 354 (a) (b) Figure 13: Measured 2-D patterns of the cross Yagi patch antenna at 1.85 GHz (a) E-plane (b) H-Plane at 4. 4. CONCLUSION A Planar one-to-four beam switching antenna with low profile, low cost, and directive beams has been proposed by utilizing rectangular patches with fillisters. At the designated frequency band, which extends from 1.8 to 1.9 GHz, the antenna s main beam is tilted upward with a 45 squint angle. The isolation is below 3 db, and the directive gain is approximately 9 dbi. The antenna gain performance can be further improved by substituting the lossy acrylic slab with less lossy substrates. As to the future work, the feeding path switches can be cascaded to extend the number of Yagi patch antennas accessible from an RF reader, which therefore extends its coverage area and achieves a better spatial diversity performance. REFERENCES 1. Huang, J. and A. C. Densmore, Microstrip Yagi array antenna for mobile satellite vehicle application, IEEE Trans. Antennas Propagation, Vol. 39, 124 13, 1991. 2. Huang, J., Planar microstrip Yagi array antenna, IEEE Antennas Propagat. Soc./Ursi Symposium Dig., 894 897, June 1989. 3. Lee, K. F., et al., Microstrip antenna array with parasitic elements, IEEE AP-S Symposium Digest, 794 797, June 1987. 4. Yang, X.-S., B.-Z. Wang, W. Wu, and S. Xiao, Yagi patch antenna with dual-band and pattern reconfigurable characteristics, IEEE Trans. Antennas and Wireless Propagation, Vol. 6, 168 171, 27. 5. Padhi, S. K. and M. E. Bialkowski, Parametric study of a microstrip Yagi antenna, Proc. 2 Asia-Pacific Microw. Conf., 715 718, Sydney, Australia, December 2. 6. Balanis, C. A., Antenna Theory: Analysis and Design, 3rd Edition, John Wiley & Sons Inc., 25. 7. Lin, Y.-S., A study on RFID microstrip array antennas and associated circuits, Master s Thesis, The Department of Electrical Engineering, National Taiwan University of Science and Technology, 29.