Design of a Novel Multiband Planar Inverted-F Antenna for Mobile Terminals

#Rashid Ahmad Bhatti1, Young Sin Shin2, Ngoc-Anh Nguyen3, and Seong-Ook Park4

1-4 School of Engineering, Information and Communications University Daejeon, 305-714, Korea. 1rab2138@yahoo.com, 4sopark@icu.ac.kr

1. Introduction Mobile convergence is an emerging trend in the wireless communication industry. Modern multi-standard mobile phones are required to provide a variety of location independent services like voice, data, video, the Internet and multimedia content without compromising on their weight, volume and performance. Therefore, it is important to develop compact internal multiband antennas for these mobile terminals while maintaining good return loss and radiation performance over the desired frequency bands. Because of the compact and low profile nature, planar inverted-F antennas (PIFAs) are promising structures for these applications. Planar inverted-F antennas also exhibit low SAR values and experience less detuning effects in comparison with external or internal printed monopole antennas [1]-[2]. Conventional PIFA has limited bandwidth of 4 % to 12 % for a -10 dB return loss [3]. However, variants of PIFA structure in combination with other broadbanding techniques can be utilized to realize antennas with multiband performance.

Multiband internal antenna design for handset applications is an active research area. A wide variety of PIFA based multiband antennas have been reported in the literature [4]-[8]. Conventional PIFA with a short and a feed strip is used as a basic element to get single or dual band performance. PIFA structure is then modified by using various design procedures to get multiband / wideband performance. Commonly used techniques include the use of slots, multiple quarter-wave strip resonators, and parasitic patches. Slots are created in the main radiating element to excite multiple resonances, quarter-wave length resonators are individually designed and arranged in a compact formation, and parasitic elements are primarily utilized for broad banding purposes. Furthermore, Wireless Local Area Network (WLAN) frequency band around 5.2 GHz is also likely to be included in the future personal communication handsets. Therefore, multiband antennas with seven or more frequency bands are needed for the future mobile phones.

In this paper, we propose a PIFA based multiband internal antenna that can support seven frequency bands. The antenna is designed to operate at GSM (Global System for Mobile Communications, 880-980 MHz), PCS (Personal Communication Services, 1880-1990 MHz), UMTS (Universal Mobile Telecommunications System, 1.9-2.17 GHz), WiBro (2300-2390 MHz), Bluetooth (2.4-2.48 GHz), S-DMB (Satellite-Digital Multimedia Broadcasting, 2.630 ~ 2.655 GHz) and WLAN (Wireless Local Area Network, 5.16 ~ 5.5 GHz) frequency bands. Multiple frequency bands have been realized by using slots and quarter-wave length resonating strips. A matching stub and multiple short circuiting strips are utilized for improving impedance matching across the targeted frequency bands. A prototype antenna is fabricated and characterized by measuring return loss and radiation patterns. Comparison of the measured and simulated results is given in the paper. 2. Antenna Design Geometry of the proposed antenna is shown in Fig. 1. The antenna is mounted at the top of the ground plane measuring 45 mm × 107 mm. Size of the antenna is 21 × 45 × 8 mm3. The antenna design is started with a conventional planar inverted-F antenna element with single feed and a short circuiting strip. An L-shaped long slot (slot # 1) is then created to make the antenna resonant at 0.9

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(a) (b)

Figure 1: Geometry of the Proposed Antenna (a) 3-D view (b) Planar View of Antenna Element

GHz and 1.85 GHz. Another L-shaped slot (slot # 2) is created to get an additional resonance at 3.7 GHz. A third slot (slot # 3) is created at the canter of the patch and joined to the slot # 2 for getting an additional resonance at 2.8 GHz. The new resonances do not detune or adversely affect the resonance at GSM band. The three resonances appear within the frequency band of 1.85 GHz to 3.7 GHz. For impedance matching across this band, an L-shaped tuning stub, located in front of the feed strip, is introduced in the wide slot (slot # 3) along with an additional short circuiting strip (short # 2). Addition of the matching stub and short circuiting strip remarkably improves the impedance matching without adversely affecting the GSM frequency band. A WLAN patch is attached to the PIFA to generate an additional resonance at 5.2 GHz. It gives sufficient bandwidth to cover the WLAN frequency band. Simulated return losses are shown in Figure 2 for various antenna structural configurations. Dependence of the resonance at GSM frequency on the length of slot # 3 is shown in Figure 3. Frequency is decreased by increasing length of the slot. Antenna dimensions are given in Table 1.

In order to further illustrate the multiband performance of the proposed antenna, surface currents on the antenna structure at different frequencies are shown in Figure 4. Longest path of the PIFA along slot # 3 is responsible for operation at GSM frequency band. Surface currents at 2.2 GHz are shown in Figure 1 (b), where most of the surface current is flowing across the edges of slot # 3. Current distribution at 5.2 GHz is shown in Figure 1 (c), demonstrating the role of L-shaped WLAN patch in achieving resonance at the targeted frequency. Antenna is optimized in CST Microwave Studio.

Figure 2: S11 for Various Antenna Configurations Fig. 3: S11 Dependence on Length of Slot # 3

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(a) (b) (c) 3. Prototype Antenna Characterization A prototype antenna was fabricated using a copper sheet of 0.2 mm thickness through an inexpensive metal cutting process. The antenna was bent at the dotted lines shown in Figure 1 (b) to form the three-dimensional structure. A semi-rigid coaxial cable was used to feed the antenna and outer shield of the cable was soldered to the PCB ground plane. Antenna is then characterized by measuring its return loss and radiation performance. Return loss, measured using a Network Analyzer, is better than -6 dB across all the targeted frequency bands. Comparison of the measured and simulated return losses is shown in Figure 5.

Figure 5. Measured and Simulated Return Losses Antenna radiation patterns at different frequencies were measured in an RF anechoic chamber. Radiation patterns measured in the three principal planes are shown in Figure 6. Omni-directional radiation performance is achieved in x-z plane at the GSM band. Quasi omni-directional radiation performance is achieved in the x-z plane at other frequency bands. Measured gain values of the prototype antenna at different frequency bands are given in Table 2.

Table 2: Measured Gain of the Prototype Antenna

Frequency (GHz)

0.90 GHz

1.89 GHz

2.0 GHz

2.32 GHz

2.48 GHz

2.65 GHz

5.20 GHz

GAIN

(dB) 2.227 2.53 3.54 2.91 3.3 2.31 2.54

Figure 4: Surface Current Distributions (a) 900 MHz, (b) 2200 MHz, (c) 5200 MHz

Table 1: Dimensional Details of the Antenna Parameter Value Parameter Value

L 45.0 L8 6.0 w1 5.0 L9 4.0 w2 15.0 L10 4.0 w3 2.0 d1 3.0 w4 1.0 d2 10.0 w5 2.0 d3 1.5 w6 2.0 d4 6.0 w7 1.3 s1 1.0 w8 2.0 s2 3.0 L1 8.0 s3 2.0 L2 13.5 h 8.0 L3 12.0 h1 2.0 L4 43.0 h2 3.0 L5 9.0 h3 1.5 L6 5.0 h4 2.5 L7 6.0 L11 11.0

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Figure 5. Measured Radiation Patterns of the Prototype Antenna Acknowledgments

This work was supported by Post National Research Lab program from Ministry of Science

and Technology (MOST), Korea, under the contract 2007EK0700. References [1] K. H. Chan, K. M. Chow, L. C. Fung and S. W. Leong, “SAR of internal antenna in mobile-

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for GSM900/DCS1800/PCS1900/UMTS bands”, Proceedings IEEE IWAT, 2006. [3] C. R. Rowell and R. D. Murch, “A compact PIFA suitable for dual frequency 900/1800-MHz

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