A Miniaturized Printed Integrated Inverted-F Antenna for Bluetooth or Wireless LAN Application M. Z. Azad1*, M. Ali1, Senior Member, IEEE, and H.S. Hwang2, Member, IEEE 1. Department of Electrical Engineering, University of South Carolina, Swearingen Building, Columbia, SC 29208. Tel: (803) 777 1488. Email:
[email protected] 2. Kyocera Wireless, San Diego, California
Abstract A miniaturized printed integrated inverted-F antenna (PIIFA) is proposed which operates in the Bluetooth frequency band (2.4-2.485 GHz). The antenna is derived from a modified Hilbert geometry and can be easily fed using a 50Ω printed transmission line. The antenna being an integral part of the printed circuit board (PCB) reduces the cost and complexity of manufacturing the whole unit. The antenna on standard FR4 substrate (εr = 4.5) measures 9.25 mm by 4.125 mm and has a bandwidth of 3.5 % within 2.5:1 VSWR. The peak gain is 1.2 dBi. An overall size reduction of 67% can be achieved compared to a conventional inverted-F antenna.
Introduction With the recent rapid growth in wireless communications, many consumer electronics devices have emerged with either Bluetooth or WLAN functionalities operating in the 2.4-2.485 GHz band [1]. For portable terminals, such as mobile phones, PDAs, and laptops either surface mount PIFA, integrated board mounted IFA, or some sort of printed monopole geometries are used [2]. The first board mounted integrated inverted-F antenna was reported by Ali et al. [3, 4]. Later many variants of this configuration have been introduced both for 2.4 and 5 GHz WLAN applications. Some authors have proposed meandering the antenna geometry to achieve miniaturization [5]. Recently researchers have focused on the Hilbert geometry to achieve antenna miniaturization [6]. We have reported a miniaturized modified Hilbert PIFA for mobile phone applications [7]. In this paper the prospects of extreme miniaturization is explored for a printed integrated inverted-F antenna. We investigate modified Hilbert antennas of various different sizes and observe their bandwidth, pattern and gain characteristics.
Antenna Design The antenna geometry conforming to the Hilbert profile effectively increases the current flow path making it possible to develop a small antenna. We
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consider FR4 (εr = 4.5) substrate and a ground plane measuring L = 110 mm and w = 50 mm. The substrate x thickness and hence the L antenna height is fixed at 1.5 mm. First an antenna Bottom PCB consisting of a 3rd order ground Hilbert geometry is w considered. The position of the antenna is shown in Fig. 1(a). The antenna is y directly printed on the Antenna Dielectric substrate substrate. In this figure, Fig. 1.(a). Geometrical location of antenna. part of the dielectric material has been shown removed to create a window to visualize the antenna location. In reality, the dielectric will be present. Figs. 1(b) and 1(c) show the different antenna configurations with feed and shorting pin positions. These antennas will be placed in the location indicated in Fig. 1(a). x
x
s
Shorting pin Feed
s p m
t d1
Shorting pin
Feed p
t
m
Fig. 1(c) Antenna B y
n
d1
d2
n
y d2
Fig. 1(c) Antenna B
Fig. 1(b) Antenna A
Table 1: Antenna parameters (mm). Antenna A Antenna B
m
n
s
t
p
9.45 3.75
9.45 5.75
2.73 1.25
0.5 1.5
0.9 0.375
Along the window area, all metal will be removed except the antenna trace, its feed line and shorting pin. The trace width of Antenna-A in Fig. 1(b) is 0.63 mm in both x and y directions while for Antenna-B in Fig 1(c) it is 0.25 mm. By reducing the trace width and utilizing higher and higher order Hilbert or modified Hilbert curves one can miniaturize the antenna. The effects of such miniaturization, such as bandwidth and gain must be carefully studied. One must also be mindful about fabrication resolution. The gap between the edge of the window and antenna is fixed at d1 = d2 =1 mm. The values of other antenna parameters are given in Table 1. Design and analysis of the antenna was conducted using Ansoft HFSS 9.0.
Computed results Computed VSWR data for both antennas are shown in Figure 2(a). The third order Hilbert Antenna-A resonates at 2.45 GHz. The band width for Antenna A is 4.5% within VSWR 2.5:1. Antenna A has an area of 9.45 x 11.95 mm2 (or 113 mm2). We further tried to minimize the antenna dimension which resulted in the design of Antenna B. This antenna has an area of 4.125 x 9.25 mm2 (or 38 mm2) which is one third in terms of size compared to Antenna-A and resonates at 2.43 GHz with bandwidth of 3.5 % within VSWR of 2.5:1. Due to its wide trace width Antenna-A has a wider bandwidth than Antenna-B. Antenna A Antenna B
Antenna A Antenna B
(b) (a) Fig. 2. Computed (a) VSWR and (a) input impedance of both antennas.
Eθ Eφ (b) (a) Fig. 3 Computed radiation patterns for antenna B (a) φ = 0o and (b) φ = 90o at 2.43 GHz Computed input impedance is plotted in Fig. 2(b). It is apparent that the locus of Antenna-B is very close to the centre of the smith chart indicating a good impedance match. Computed normalized radiation patterns for Antenna-B is shown in Fig. 3. Realized gain patterns are computed at 2.43 GHz. These gain patterns are studied in the context of the antenna and PCB geometries shown in the Figs. 1(a) and (c). It is observed from Fig 3(a) that the Eφ component is fairly uniform and dominant.
The Eθ component is strong in those angular regions where the strength of the Eφ component is less than -4 dB. The field pattern in this plane is uniform and good angular coverage is achieved. The yz-plane pattern [Fig. 3(b)] shows that although the Eθ component is not the dominant component it reinforces the Eφ component. The peak gain of Antennas A and B are 3.5 and 1.2 dBi respectively at f =2.45 GHz.
Conclusions A small printed integrated inverted-F antenna (PIIFA) conforming to a modified Hilbert curve is proposed. Significant miniaturization is achieved for 2.45 GHz WLAN application. An antenna with 38 mm2 area has a 3.5% bandwidth within 2.5:1 VSWR. The peak gain of the antenna is 1.2 dBi. Further miniaturization can be achieved by narrowing the Hilbert trace width at the expense of narrower bandwidth and reduced antenna gain. Acknowledgement: This work was supported in part by the National Science Foundation (NSF) Career Award ECS-0237783.
References 1. M. Ali, R.A. Sadler, and G.J. Hayes, “A Uniquely Packaged Internal Inverted-F Antenna for Bluetooth or Wireless LAN Application,” IEEE Antennas and Wireless Propagation Letters, vol. 1, no. 1, pp. 5-7, 2002. 2. M. Ali, “Miniaturized Packaged (Embedded) Antennas for Portable Wireless Devices,’’ Wiley Encyclopedia of RF and Microwave Engineering,’’ (in press). 3. M. Ali, and G.J. Hayes, “Analysis of Integrated Inverted-F Antennas for Bluetooth Applications,” IEEE Antennas and Propagation Conference for Wireless Communication Digest, Waltham, MA, Nov. 2000, pp. 21-24. 4. M. Ali and G.J. Hayes, “A small printed integrated inverted-F antenna for Bluetooth application,” Microwave Opt. Technol. Lett., vol. 33, no. 5, pp. 347-349, June 5, 2002. 5. C.-C. Lin, S.-W. Kuo, and H.-R. Chuang, “2.4-GHz Printed Meander-Line Antenna for WLAN Applications,” in proc. IEEE Antennas and Propagation Society Symposium, vol. 3, Monterey, USA, June 2004 pp. 2767 - 2770 6. M.Z. Azad and M. Ali, “A Miniature Hilbert Planar Inverted-F Antenna (PIFA) for Dual-Band Mobile Phone Applications,’’ IEEE Antennas and Propagation Society International Symposium and URSI/USNC Meeting, Monterey, CA June 2004, Vol. 3, pp. 3127 - 3130. 7. M. Z. Azad and M. Ali, “A Miniaturized Hilbert PIFA for Dual Band Mobile Wireless Applications,’’ IEEE Antennas and Wireless Propagation Letters (in press).
