Small Film Antenna for Wireless LAN Hirotaka Furuya, Hiromasa Futamata, Tomoko Miyashita, Hirotaka Sawada and Kazuya Akashi Previously we have developed a film antenna for wireless LAN available for notebook personal computer (PC). The previous size of the film antenna was 55 mm × 5 mm × 0.1 mm. We recently developed a small film antenna that has a reduced area of 70% this time. This antenna is available for 2GHz and 5GHz bands, and the size of this antenna is 25 mm × 2.5 mm × 0.1 mm. This antenna has omni-directional radiation pattern and high gain, and it is flexible. So, this antenna is suitable for mounting in small mobile devices such as personal digital assistants (PDA). 1. Introduction In recent years, the number of a place called 'hot spot' where we can use radio communications systems, such as wireless local-area network (LAN), at a public place is increasing. In Japan, it rose as a new telecom infrastructure in about 2001, and is increasing rapidly since 2002, and the number of hot spot is expected to be about 60,000 in 2006. Moreover, the use of a mobile device in which an antenna is installed is increasing1). Based on this trend, we previously developed a film antenna that can be installed in notebook PC2). This antenna has enough bandwidth, high gain and omni-directional radiation pattern, but the size is too wide to install in small mobile devices. So, we developed a small film antenna for wireless LAN that can be installed in PDA. required. Based on these demands, Table 1 shows the target performance. 3.Miniaturization Figure 1 shows the appearance of the antenna that we have already developed for notebook PC, and Fig. Fig. 1. Antenna (Previously-developed). 2. Target Performance The requirements for this antenna are voltage standing wave ratio (VSWR) characteristics, radiation pattern and size. VSWR of this antenna must have the capability of using 2 GHz and 5 GHz bands because there are two wireless LAN standards using 2 GHz (IEEE802.11b/g, Bluetooth) and 5 GHz bands (IEEE802.11a). The requirement of antenna is omnidirectional because the environment for use, such as movement of the mobile device and the situation of place of communication changes. Moreover, as the gain is high, the communication is fast and stable. Also, the installing space of PDA is narrower than that of notebook PC, so the miniaturization of size is Strong Cable Cable B Weak A Table 1. Target Performance 22 Size 30mm × 3mm Band width (VSWR 2) 2GHz band 100MHz 5GHz band 200MHz Gain(average) −8dBi (main polarization) 2GHz band 5GHz band Fig. 2. Current Distribution of Previously Developed Antenna. Ymm Inner conductor connecting position Outer conductor connecting position C Inner conductor Outer conductor B 25mm A (a) (b) (c) (d) Fig. 3. Miniaturization Study. Fig. 4. Simulation Model. 5 5 Y=2.5mm Y=2.0mm Y=1.5mm 3 2 1 Y=2.5mm Y=2.0mm Y=1.5mm 4 VSWR (−) VSWR (−) 4 3 2 2 2.5 Freq. (GHz) 3 1 5 5.5 Freq. (GHz) 6 Fig. 5. VSWR by Simulation. Strong Weak 2GHz band 5GHz band Fig. 6. Current Distribution. 2 shows the simulated result of current distribution on 2 GHz and 5 GHz bands. In 2 GHz band, the current is strongly distributed on the element A, and in 5 GHz band on the element B. As the current path is half of the wavelength in each resonance frequency, we think that this antenna is resonating 50%. Hence Fujikura Technical Review, 2005 on this basis, we considered a smaller antenna. The outline is shown in Fig. 3. Longitudinal direction was shortened by bending while keeping the length of element A from (a) to (b) as shown in Fig. 3. As for the width, current does not flow on the non-feeding element C, and it has not contributed to the characteristics of the antenna .So, we eliminated the nonfeeding element C from (b) to (c) and the width was narrowed. Next we simulated whether the miniaturization was possible in the form shown in (d). Figure 4 shows the simulation model. When the longitudinal direction was fixed at 25 mm and the width Y varied to 1.5 mm, 2 mm and 2.5 mm, we investigated for the VSWR characteristic and the current distribution of resonance frequency. Figure 5 shows the result. Resonance (VSWR is two or less) has occurred with both 2 GHz band and 5 GHz band at Y = 2.5 mm. We decided to perform trial production examination with this size. Moreover, Fig. 6 shows the current distribution in 2 GHz and 5 GHz bands by simulation. By comparing the wavelength of resonance frequency and path length (arrow in Fig. 6) where there is a strong current for both the frequencies, we can see that the path length was 1/4 of the wavelength in 2 23 5 5 Measurement Calculation 3 2 1 Measurement Calculation 4 VSWR (−) VSWR (−) 4 3 2 2 2.5 Freq. (GHz) 3 1 5 5.5 Freq. (GHz) 6 Fig. 8. VSWR of Single Piece. Table 2. Antenna Gain (unit:dBi) Item Vertical polarization 2GHz 5GHz Max −0.2 0.9 Min −8.9 −8.8 Ave −4.7 −3.3 Horizontal polarization Max Fig. 7. Appearance of Antenna (top :Newly bottom :Previously-developed). 270° 2GHz band 5GHz band 0° 10 −0 0 −10 −20 −30 90° −40 270° 2GHz band 5GHz band 0° 10 −0 0 −10 −20 −30 −40 180° 90° 180° Vertical polarization 270° 180° Horizontal polarization 90° 0° Fig. 9. Radiation Pattern of Single Piece (unit : dBi). GHz band, while the path length was approximately 1/2 of the wavelength in 5 GHz band. Therefore, we concluded that resonance in 2 GHz band is occurring at λ/4 and resonance in 5 GHz band occur at λ/2. 4. Antenna Trial Production and Evaluation of Characteristics 4.1 Production of Small Film Antenna To manufacture the film antenna, we applied the manufacturing technique of our flexible printed circuit (FPC). The overall thickness was kept low, at 24 2GHz 5GHz −4.2 −8.0 Min −24.7 −33.9 Ave −10.8 −16.2 approximately 0.1 mm, through the use of a singlesided copper clad laminate, while the circuit was formed through etching to ensure mass productivity. The antenna surface was coated with an insulating film through screen printing to protect the antenna element. A coaxial cable 0.8 mm in diameter was used for antenna-feeding purposes. The appearance of the film antenna is shown in Fig. 7. 4.2 Independent Characteristics of Small Film Antenna Figure 8 shows VSWR characteristics of small film antenna. Resonance peaks are seen in 2 GHz and 5 GHz bands and thus we can understand that this antenna is operating by both the frequency bands, and this result is corresponding to simulation. Bandwidths of 100 MHz or more are seen in 2 GHz band and that of 500 MHz or more are seen in 5 GHz band. Radiation pattern has omni-directional pattern in vertical polarization of both the bands shown in Fig.9 and the gain is over −5dBi as shown in Table 2. 5. Antenna Characteristics Installed in PDA Considering the actual situation, the film antenna was installed in PDA and we evaluated characteristics. The chassis of PDA is made of plastic, and the size is 75 mm × 140 mm × 25 mm. PDA consists of LCD, a mounting board, and a protection cover. Moreover, we used the 100 mm coaxial cable (φ 0.80 Operating frequency Before adjustment After adjustment VSWR (−) 5 4 Table 3. Antenna Gain in PDA (unit:dBi) Item Vertical polarization 2GHz 5GHz Max −3.6 −3.3 Min −9.3 −32.9 Ave −6.3 −10.7 Horizontal polarization 3 2GHz 5GHz −3.1 −4.4 Min −23.7 −19.3 Ave −14.1 −9.7 Max 2 1 2 3 4 Freq. (GHz) 5 6 Fig.10. VSWR in Installation on PDA. 2GHz band 5GHz band 270° 0° 0 −10 −20 −30 −40 2GHz band 5GHz band 90° 270° 0° 0 −10 −20 −30 −40 90° GHz band. The bandwidth required for wireless LAN is satisfied by adjusting. Next, radiation pattern is shown in Fig. 11 and the gain of all directions is shown in Table 3. There are some parts with low gain points. But in our opinion, this is because radiation is interfered with the mounting parts inside. As the wavelength is especially short in 5 GHz band, the tendency is remarkable. However, we consider that this gain lowering can be improved by examining the place of an antenna. 6. Conclusion 180° 180° Vertical polarization Horizontal polarization 180° Antenna PDA 140 270° 90° 25 0° 75 Loading situation We developed the multi-band small film antenna for wireless LAN. Compared with the antenna for notebook PC, it was miniaturized about 70% and this antenna can be installed in small mobile devices such as PDA. VSWR characteristic has enough bandwidth in 2 GHz and 5 GHz bands, and radiation pattern of this antenna has an omni-directional pattern and high gain in both bands. Moreover, when this antenna is installed in PDA, it has enough bandwidth. We expect that the gain can be improved by examining the loading place of an antenna. Finally we wish to express our deepest appreciation to Professor Itoh of Chiba University and Mr. Nakagishi, a second-year student of the Master's Course in Chiba University. Fig.11. Radiation Pattern in PDA. References mm) for electric supply. Figure 10 shows the VSWR characteristic installing in PDA. Since the resonance peak shifted when the above-mentioned trial production antenna was installed directly, we adjusted the element length. The bandwidth after adjustment is 100 MHz or more in 2 GHz and 500 MHz or more in 5 1) Hosoe, et al.: A Study of the Characteristics of Reverse-F Plate Antenna for Wireless LAN for Installation in Note PC, Fujikura Technical Review, 2005 Technology Journal of the Institute of Image Information and Television Engineers, Vol. 26, No. 67, pp. 21-24, 2002 2) Futamata, et al.: Film Antenna for Use with Wireless LAN, Fujikura Giho, No. 104, pp. 28-31, 2003 25