Design of a Compact and Low-Cost Fractal-Based UWB PCB Antenna

Transcription

1 SETIT th International Conference: Sciences of Electronic, Technologies of Information and Telecommunications March 22-26, 2009 TUNISIA Design of a Compact and Low-Cost Fractal-Based UWB PCB Antenna Mohammed AL HUSSEINI, Ali RAMADAN, Ali EL HAJJ and Karim KABALAN ECE Department, American University of Beirut, Beirut, Lebanon husseini@ieee.org ahr06@aub.edu.lb elhajj@aub.edu.lb kabalan@aub.edu.lb Abstract: In this paper, two compact PCB antennas operable over the ultra-wide band frequency range are presented. Both designs feature a 1.6-mm-thick substrate based on the low-cost FR4 epoxy material, a microstrip-line feed with a matching section, a patch that efficiently uses the available substrate's surface, and an optimized partial ground plane comprising a rectangular and a semicircular part. The second antenna is a modified version of the first, basically obtained by rounding the corners of a rectangular patch and using fractal geometry in its construction. These modifications lead to better return loss values and an improved UWB bandwidth. FEM and MoM-based EM packages were used to simulate and compare the two designs. Both yielded omnidirectional radiation patterns and good gain and efficiency figures, over the bands of operation. Key words: Fractal geometry, PCB antenna, UWB INTRODUCTION PCB antennas are characterized by their low profile, small size, light weight, low cost and ease of fabrication [BAL 05], which make them very suitable for satellite and communications applications. They are also compatible with wireless communication integrated circuitry due to their simple feed methods, especially microstrip-line and coplanar waveguide feeds. However, they suffer from inherently narrow bandwidths. Techniques to increase the bandwidth of microstrip antennas were discussed in [KUM 03]. Another technique consists of the use of partial ground planes. The PCB antenna presented in [LOW 05] operates over the GHz band. In addition to flushing its ground plane with the feed line, the authors introduced slots into the rectangular patch and tapered the connection between the patch and the feed line for better impedance matching. In [YAN 05], a partial ground plane was also used in addition to top-loading the antenna. The reported impedance bandwidth is 8.2 GHz in the GHz range, but for VSWR < 2.5. The bandwidth is much smaller for VSWR < 2, especially for the design with the small ground plane. Geometries with round corners also lead to bandwidth improvement. In [LEE 02], a broadband round corner rectangular wide slot antenna was proposed. This antenna provided better wideband characteristics compared to a normal rectangular wide slot antenna. A quadrate bowtie antenna with round corners was introduced in [QUS 05]. In addition to its smaller size compared to the conventional triangle bowtie antenna, this antenna resulted in enhanced bandwidth. Multiband, wideband and/or compact antenna designs can be achieved using fractal geometries [WER 03]. A fractal is a self-repetitive curve whose different parts are scaled copies of the whole geometry. Consequently, a radiator based on a fractal or self-similar geometry is expected to operate similarly at multiple wavelengths [KHA 04], and might keep similar radiation parameters through several bands. In [RAH 05a], an antenna that uses a Sierpinski carpet fractal was presented. This ultrawideband design achieved a return loss better than 10 db in the 1 10 GHz frequency range. A multiband operation, in the 1 10 GHz frequency range, was attained in [RAH 05b] through the use of the Sierpinski gasket fractal. In this paper, we exploit the above techniques to design a miniature low-cost microstrip-line-fed PCB antenna for operation in the UWB frequency range. Two designs are introduced, and both are based on a 1.6-mm-thick FR4 epoxy substrate with dimensions - 1 -

2 2 2.5 cm 2. For the two designs, the ground is partial and comprises a rectangular section and a semicircular one, and a matching section of trapezoidal shape connects the patch to the 50-Ω feed line. The patch in both designs occupies a large percentage of the substrate s top area. Beyond these shared characteristics, the second design (Design II) has a patch that is derived from a 2 nd -iterated Sierpinski carpet. Here, the basic geometry of the fractal is a rectangle whose corners were cut round. The patch of Design I has the shape of a whole rectangle. The two designs were simulated in Ansoft HFSS [HFS 07], which is based on the finite element method, and the MoM-based ADS Momentum [ADS 06]. In the following sections of this paper, details of the antennas geometries, the return loss plots, the radiation patterns, the gain and the efficiency will be given. It is shown that both designs are suitable for UWB operation. Moreover, Design II has better return loss properties and a wider bandwidth. For Design II, the feed is 7-mm long. The trapezoidal matching section has a height of 5 mm, and a 1.5-mm top width. The rectangular part of the ground plane is 20mm 5mm. The semi-circular part has a base of 20 mm, and a depth of 6.5 mm. The patch is a modified 2 nd -iterated Sierpinski carpet where the basic geometry is rectangle with round corners. The carpet is designed as follows: a rectangle 20mm 13mm in size is intersected with an ellipse to obtain the round corners. The ellipse is co-centered with the rectangle, and has a major radius of 11 mm (along the x-axis) and a minor radius of 8.8 mm. The resulting geometry is used to generate a 2 nd -iterated Sierpinski carpet. The large slot in the middle of the carpet is one-third the size and has the same shape as the basic geometry. The small slots, also the same shape, are one-ninth the size. Finally, a 0.8-mm-wide strip is used to bridge the large slot, along the y-axis. 1. Antenna geometry and design guidelines The geometries of Design I and Design II are shown in Fig. 1 and Fig. 2, respectively. For both designs, the FR4-based substrate having ε r =4.4 is 1.6 mm in thickness, 2 centimeters along the x-axis and 2.5 centimeters along the y-axis. The feed line is 3- mm wide for a characteristic impedance of 50 ohms. Figure 2. Geometry of Design II Figure 1. Geometry of Design I For Design I, the feed is 5.5 mm in length, and the trapezoidal matching section that connects the feed to the patch has a 3-mm base size, a height of 6 mm, and a 1.4-mm width at the connection with the patch. The patch is a rectangle that is 20 mm along the x-axis and 13.5 mm along the y-axis. The rectangular part of the ground plane is 20mm 3.5mm. Its semi-circular part has a base of 20 mm, and a depth of 7.5 mm. The above specifications guaranteed an ultrawideband operation of the two antennas, especially the inclusion of the following: 1. The partial ground plane, which is made of a rectangular and a semi-circular part. The dimensions of the two parts were optimized for each design. 2. The tapered section, which was used to match the patch to the feed line over an ultra-wide frequency range. Tapered connections between the feed line and the main patch are known to smooth the current s path, thus providing wider impedance bandwidth. 3. The modified 2 nd -iterated Sierpinski carpet, which constituted the patch of Design II. Recursive geometries such as the Sierpinski carpet are able to keep similar radiation properties throughout the operating bands. The large space-utilizing patches of the two designs helped reduce the antennas overall size - 2 -

3 without sacrificing the gain. The use of fractals in Design II led to a wider operational band that starts at lower frequency, as compared to Design I. For the same bandwidth and same lower operation frequency, fractals could yield a smaller antenna size. 2. Results and discussion The HFSS-computed return loss (S 11 ) plots of the two antennas are given in Fig. 3. The bandwidth of Design I for S db is GHz. For Design II, the bandwidth is GHz. Both antennas are ultra-wideband, and the bandwidth improvement obtained achieved by Design II is clear. In Fig. 4, the Momentum-computed return loss plots are displayed. According to ADS Momentum, the bandwidth of Design I is GHz, and that of Design II is GHz. Despite the difference with HFSS, the same property still hold; both designs are UWB, and Design II has better S 11 values and a larger bandwidth. The difference in computed results between HFSS and Momentum is due to the fact that Momentum assumes an infinite substrate, whereas HFSS simulates the finite-size design. This leads to some differences, especially at high frequencies where the two simulators also have different ways of modeling substrate losses. witnessed. For smaller db values of the return loss, the real part of the impedance is closer to 50 Ω and the imaginary part to 0 Ω. For Design II, the range of values is smaller compared to Design I. The same property is deduced from the Momentum-computed impedance. Figure 5. Input impedance of Design I: real part (solid line) and imaginary part (dashed line) Figure 6. Input impedance of Design II: real part (solid line) and imaginary part (dashed line) Figure 3. Return loss of the two designs computed in HFSS Figure 4. Return loss of the two designs computed in Momentum The HFSS-computed input impedance of Design I is depicted in Fig. 5, and that of Design II in Fig. 6. For both, consistency with the return loss plots is Fig. 7 shows the HFSS-computed radiation patterns of Design I in the X Z and Y Z planes, for 4, 6, 8, and 10 GHz. Those of Design II, at 3.75, 5, 7, 9, and 10.6 GHz, are shown in Fig. 8. For both designs, the patterns are consistently omnidirectional, having almost equal radiation in the X Z plane. In the lower region of the operation band, the radiation pattern in the X Y plane has the shape of an 8, corresponding to a 3D donut shape. However, at higher frequencies, sidelobes can appear, as clear in the patterns of Design I at 10 GHz. Beyond 11 GHz, the radiation patterns of Design II lose the omnidirectional property and become directional. At 12 GHz, the X Z plane pattern of this antenna has the shape of an 8. The HFSS-simulated patterns were verified in Momentum. Similar omnidirectional properties were obtained. However, because of Momentum's infinite substrate assumption, extra nulls were shown in the X- Z-plane patterns for θ = 90 o. The Momentumcomputed 2D pattern of Design I at 4 GHz, and that of Design II at 10.6 GHz, are shown in Fig. 9 and Fig. 10, respectively

4 plots. At 3.75 GHz, the gain of both antennas is about 2.5 db, and increases with frequency. The gain is between 4 db and 5.4 db in the range 7 10 GHz. The gain starts to decrease after 8.5 GHz, but increases again starting at 12.5 GHz. This latter increase is a result of the directional patterns that appear at these high frequencies. Figure 7. Normalized radiation patterns of Design I in X-Z plane (solid line), and Y-Z plane (dotted line) Figure 9. Normalized Momentum-computed radiation pattern of Design I in X-Z plane (red line) and Y-Z plane (blue line) at 4 GHz Figure 10. Normalized Momentum-computed radiation pattern of Design II in X-Z plane (red line) and Y-Z plane (blue line) at 10.6 GHz Figure 8. Normalized Radiation patterns of Design II in X-Z plane (solid line), and Y-Z plane (dotted line) The peak gain values of the two antennas, as computed in HFSS-computed, are plotted in Fig. 11. A first remark is the obvious similarity between the two Figure 11. Peak gain of the two designs - 4 -

5 The HFSS-calculated radiation efficiencies are shown in Fig. 12. Again, the two antennas are very similar in terms of efficiency, with a maximum difference of 1% between the two. The efficiency starts at about 97.7%, and monotonically decreases due to more losses in the dielectric material at higher frequencies. At 13.5 GHz, the efficiency is about 81%. Figure 12. Radiation efficiency of the two designs Similar gain and efficiency properties of the two antennas are obtained in Momentum, but with some value difference compared to HFSS. This is again due to Momentum s infinite substrate assumption but also to the different models for substrate losses at high frequencies. 3. Conclusion Two miniaturized low-cost PCB antennas were presented. Both designs use a 1.6-mm-thick FR4- epoxy-based substrate, a microstrip-line feed that includes a tapered section, and a partial ground plane made of a rectangular and a semicircular part. For Design I, the patch is a whole rectangle, whereas for Design II, the patch is based on a modified 2nditerated Sierpinski carpet. The carpet s constituting geometry is a rectangle with round corners. The two antenna designs were simulated in HFSS and Momentum, and their computed parameters presented. Both of them showed ultra-wideband behavior, with Design II achieving better return loss values and an improved bandwidth. Good gain and efficiency figures were also obtained, and the radiation patterns were satisfactorily omnidirectional over the entire UWB band. Fractals, 21 st National Radio Science Conference, pp. 1-25, March 16-18, [KUM 03] G. Kumar and K.P. Ray, Broadband Microstrip Antennas, Artech House, [LEE 02] H.L. Lee, H.J. Lee, J.G. Yook, and H.K. Park, Broadband planar antenna having round corner rectangular wide slot, IEEE Antennas and Propagation Society International Symposium 2002, June [LOW 05] Z.N. Low, J.H. Cheong, and C.L. Law, Low-Cost PCB Antenna for UWB Applications, IEEE Antennas and Wireless Propagation Letters, vol. 4, [QUS 05] S. Qu and C. Ruan, Quadrate bowtie antenna with round corners, IEEE International Conference on Ultra- Wideband 205 (ICU 2005), Sept [RAH 05a] M.K.A. Rahim, A.S. Jaafar, and M.Z.A.A. Aziz, Sierpenski Gasket Monopole Antenna Design, Proceedings of Asia-Pacific Conference on Applied Electromagnetics, pp , December 20-21, 2005, Malaysia. [RAH 05b] M.K.A. Rahim, M.Z.A. Abdul Aziz, N. Abdullah, Wideband Sierpinski Carpet Monopole Antenna, Proceedings of Asia-Pacific Conference on Applied Electromagnetics, pp , December 20-21, 2005, Malaysia. [WER 03] D. H. Werner and S. Ganguly, An Overview of Fractal Antenna Engineering Research, IEEE Antennas and Propagation Magazine, Vol. 45, No. 1, pp , February [YAN 05] T. Yang, W.A. Davis, and W.L. Stutzman, Small, Planar, Ultra-Wideband Antennas with Top-Loading, IEEE Antennas and Propagation Society International Symposium 2005, July REFERENCES [ADS 06] ADS Momentum 2006, Agilent Technologies, Palo, CA 94304, USA. [BAL 05] C.A. Balanis, Antenna Theory, Analysis and Design, Wiley-Interscience, [HFS 07] Ansoft HFSS 11, Pittsburg, PA 15219, USA. [KHA 04] S. E. El-Khamy, New Trends in Wireless Multimedia Communications based on Chaos and - 5 -