PIFA Bandwidth Enhancement by Changing the Widths ... - IEEE Xplore

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009

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PIFA Bandwidth Enhancement by Changing the Widths of Feed and Shorting Plates Hassan Tariq Chattha, Yi Huang, and Yang Lu

Abstract—The ultrawideband (UWB) systems require antennas having a very broad bandwidth. The planar inverted-F antenna (PIFA) is widely used in mobile systems due to its excellent performance. However, its bandwidth is very limited. This letter is aimed at addressing this problem by investigating the effects of changing the widths of the feed plate and shorting plate on the impedance bandwidth. It is shown that a PIFA with a much wider bandwidth (up to 65%) than previously reported can be achieved by optimizing the widths of the feed and shorting plates. Simulated and measured results are provided to verify the conclusion. Index Terms—Antennas, input impedance bandwidth, planar antennas, planar inverted-F antenna (PIFA). Fig. 1. Geometry of PIFA.

I. INTRODUCTION

T

HE inverted-F antenna is evolved from a quarter-wavelength monopole antenna and is now widely used in mobile and portable applications due to its simple design, lightweight, low cost, conformal nature, attractive radiation pattern, and reliable performance [1]–[4]. The planar inverted-F antenna (PIFA) is an extension of the wire inverted-F antenna in which the wire is replaced with a plate in order to increase the bandwidth. However, PIFA is still generally considered a narrowband antenna, and a significant amount of effort has been made to broaden its bandwidth. It was shown that the height [5], shorting plate width [6], and meandered shorting strip [7] can be used to increase the bandwidth, but in these papers either the impedance bandwidth is not very broad or they are not practically implemental. Feik et al. have shown that diversely shaped feed plates can be used to increase the impedance bandwidth (up to about 25% fractional bandwidth), but they did not investigate the size of feed plate and have used a very large shorting plate [8]. This study, as part of a comprehensive study of the PIFA, investigates the effect of changing both the rectangular feed width and shorting plate width on the impedance bandwidth of the antenna. It is found that a much wider bandwidth than previously reported can be achieved, and the new PIFA may be regarded as a wideband [or even ultrawideband (UWB)] antenna if the correct widths of the feed and shorting plate are chosen. The proposed approach is a Manuscript received January 11, 2009; revised February 27, 2009. First published May 19, 2009; current version published July 09, 2009. The authors are with the Department of Electrical Engineering & Electronics, University of Liverpool, Liverpool L69 3GJ, U.K. (e-mail: [email protected], [email protected], [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2009.2023251

Fig. 2. PIFA with SMA connector.

simple and more practical way of getting the maximum impedance bandwidth than the previously reported methods. II. ANTENNA CONFIGURATION The configuration of a typical PIFA shown in Fig. 1 is chosen , for this study. The radiating top plate has dimensions of . There is a FR4 suband ground plane dimensions are mm and a relative dielectric constant strate of thickness of 4.4 between the rectangular ground plane and feed plate. The antenna height is , and the space between the top plate and the substrate is filled with air (free-space). The model in Fig. 1 is made purely for the ease of construction in our lab. In practice, a substrate is generally just underneath the top plate, but this will make the top plate too heavy to be supported by the shorting and feeding plates. For production, the antenna size can be reduced by loading a dielectric between the ground and top radiating plate, which will make the antenna more robust. The shorting

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Fig. 3. Values of feed plate width versus fractional bandwidth.

Fig. 4. Values of shorting plate width versus fractional bandwidth for mm.

5

W =

XY

XZ 8 = 0) for f = 2:5 GHz. = 90 f = 2:5 GHz.

Fig. 6. (a) Radiation pattern for elevation plane ( (b) Radiation pattern for azimuth plane ( ) for

III. NUMERICAL AND EXPERIMENTAL INVESTIGATION

S11) in dB versus frequency in GHz.

Fig. 5. Return loss (

plate has dimensions of , and the feed plate has di. The shorting plate is placed under the top mensions of corner of the top plate. The horizontal distance between shorting and feed plates is . The PIFA is fed by coaxial cable through a SMA connector as shown in Fig. 2. The software package used for simulation is Ansoft’s High Frequency Structure Simulator (HFSS), which is based on the finite element method.

There are many variables that may affect the PIFA bandwidth. Numerical and experimental approaches are adopted. The effect of changing the width of feed plate on the fractional bandwidth is shown in Fig. 3. It is evident that increasing the width of the feed plate increases the fractional bandwidth up to a certain value, and then further increase in the feed plate width actually decreases the fractional bandwidth. Therefore, an optimum value for the width of feed plate should be selected for achieving the maximum bandwidth. Similarly the effect of changing the width of the shorting plate on the fractional bandwidth is also mm. investigated, and the results are shown in Fig. 4 for

CHATTHA et al.: PIFA BANDWIDTH ENHANCEMENT

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Fig. 7. Return loss [S11] for different values of the length of ground plane.

Fig. 9. Input Impedance Z as a function of the feed plate.

Fig. 8. Simulated Smith chart for impedance bandwidth of PIFA.

It is observed that increasing the width of the shorting plate increases the fractional bandwidth to about 25%, and further increase of the shorting plate width results in the decrease of the fractional bandwidth for a given feed plate. Thus, there is also an optimum width for the shorting plate. It is also observed that increasing the widths of shorting and feed plates results in the increase of the resonant frequency. In addition, it is found that the global maximum bandwidth is achieved when the shorting plate width is relatively small, which could explain why we have managed to achieve much wider bandwidth than the researchers of [8]. All the parameters of PIFA are optimized in order to get the maximum impedance bandwidth, and feed is provided at the edge of ground plane. For example, if the center frequency is around 2.2 GHz, the optimized values of all the parameters aremm, mm, mm, found as follows: mm, mm, mm, mm, mm. The simulated and experimental results of the

return loss are shown in Fig. 5. It is evident that the bandwidth dB is extremely achieved by this technique for broad with a fractional bandwidth of 65% from about 1.6 to 3 GHz, which covers the frequencies for most of the current mobile and portable applications (GSM, PCS, DCS, UMTS, WLAN, WiMax, and Bluetooth). The simulated and experimental results are little different. The major courses for these are: 1) the cables, which are not included in the simulation but presented in the measurements, and 2) the connect, which is also not considered in the simulation. Another important factor is that the patterns are presented in linear scale, which may have made the differences look larger than that shown in a dB plotted graph. The simulated and measured radiation patterns of this antenna as shown in Fig. 6 are similar to that of a conventional PIFA, as changing the widths of feed and shorting plates do not have significant effects on the gain and radiation pattern of the antenna. It is evident that the resultant PIFA has a directional radiation pattern. It can be concluded that a properly configured PIFA can be made as a broadband antenna. The average simulated radiation efficiency of this PIFA is about 85% with the peak efficiency of 95%. The overall antenna size is larger than the radiating plate. This is the penalty of achieving broad bandwidth. The effect of ground plane dimensions is very significant in achieving the maximum bandwidth. Fig. 7 shows the effect of changing the

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length of the ground plane on the return loss of the antenna. Therefore, there exists an optimal ground plane dimension for maximum impedance bandwidth. However, it is found, by extensive simulations with different ground plane dimensions, that the 50% fractional impedance bandwidth can be achieved for any given ground plane dimensions with the given technique by changing and optimizing the other parameters. Now the question is why the bandwidth of the PIFA can be significantly increased using the proposed technique. As we know, the impedance bandwidth of a half-wavelength dipole can be significantly increased by increasing its diameter. We believe for the same reason that the impedance bandwidth of PIFA can also be improved by increasing the size of feed plate. Fig. 8 shows the Smith chart for impedance bandwidth, and Fig. 9 shows the real and imaginary components of impedance of PIFA versus frequency. It is evident that by increasing the width of feed plate, the impedance changes slowly around the resonant frequency, which means an increase in the impedance bandwidth of the antenna. IV. CONCLUSION It has been shown that a very broad band (up to 65% fractional bandwidth) can be achieved for a PIFA by selecting the right values for the feed and shorting plates.

REFERENCES [1] K. Hirasawa and M. Haneishi, Analysis, Design, and Measurement of Small and Low-Profile Antennas. Norwood, MA: Artech House, 1992. [2] K. L. Virga and Y. R. Samii, “Low-profile enhanced-bandwidth PIFA antennas for wireless communication packaging,” IEEE Trans. Microw. Theory Tech., vol. 45, no. 10, pt. 2, pp. 1879–1888, Oct. 1997. [3] P. S. Hall, E. Lee, and C. T. P. Song, “Planar inverted-F antennas,” in Printed Antennas for Wireless Communications, R. Waterhouse, Ed. Hoboken, NJ: Wiley, 2007, ch. 7. [4] Y. Huang and K. Boyle, Antennas: From Theory to Practice. Hoboken, NJ: Wiley, 2008. [5] D. Liu and B. Gaucher, The Inverted-F Antenna Height Effects on Bandwidth. Yorktown Heights, NY: IBM T. J. Watson Research Center, IEEE, 2005. [6] P. S. Hall, C. T. P. Song, H. H. Lin, H. M. Chen, Y. F. Lin, and P. S. Cheng, “Parametric study on the characteristics of planar inverted-F antenna,” Proc. Microw., Antennas Propag., vol. 152, no. 6, pp. 534–538, Dec. 2005. [7] P. W. Chan, H. Wong, and E. K. N. Yung, “Wideband planar inverted-F antenna with meandering shorting strip,” Electron. Lett., vol. 44, no. 6, pp. 395–396, Mar. 2008. [8] R. Feick, H. Carrasco, M. Olmos, and H. D. Hristov, “PIFA input bandwidth enhancement by changing feed plate silhouette,” Electron. Lett., vol. 40, no. 15, pp. 921–922, Jul. 2007.