7. B.K. Kormanyos, W. Harokopus, L. Katehi, and G. Rebeiz, CPW-
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space wavelength at the center operating frequency of the PIFA
[6, 7]. Bandwidth deteriorates when the ground plane length...
Figure 4 Measured 2D radiation patterns at 2440 MHz for the antenna
                                                      ...
8. S.W. Su, C.H. Wu, W.S. Chen, and K.L. Wong, Broadband printed
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Matching a Bluetooth Headset Antenna on a Small System Ground by Using a Conductive Wire

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A simple yet effective method for matching a compact planar inverted- F antenna (PIFA) on a small system circuit board for Bluetooth-headset applications is presented. The antenna is perpendicular to and extends along the top and right sides of the system ground, making it possible for the antenna to occupy almost no limited board space. Results have shown that by introducing a thin conductive wire soldered to the bottom corner of the system ground, good input matching over the 2400-2484 MHz band can easily be achieved for the PIFA mounted on a small ground of length less than a quarter wavelength at 2440 MHz. Radiation measurements of the proposed design in a real headset attached to a standard head phantom are also taken.

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Matching a Bluetooth Headset Antenna on a Small System Ground by Using a Conductive Wire

  1. 1. 7. B.K. Kormanyos, W. Harokopus, L. Katehi, and G. Rebeiz, CPW- fed active slot antennas, IEEE Trans Microwave Theory Tech 4 (1994), 541–545. 8. S.T. Fang and J.W. Sheen, A planar triple-band antenna for GSM/ DCS/GPS operations, IEEE AP-S Int Symp Dig 2 (2001), 136–139. 9. J.H. Lu and Y.D. Wang, A planar triple-band meander line antenna for mobile handsets, IEEE AP-S Int Symp Dig 4 (2003), 146–149. 10. C.T.P. Song, P.S. Hall, H. Ghafouri-Shiraz, and D. Wake, Triple- band planar inverted-F antennas for handheld devices, Electron Lett 36 (2000), 112–114. V 2009 Wiley Periodicals, Inc. C MATCHING A BLUETOOTH HEADSET ANTENNA ON A SMALL SYSTEM GROUND BY USING A CONDUCTIVE WIRE Jui-Hung Chou and Saou-Wen Su Network Access Strategic Business Unit, Lite-On Technology Corporation, Taipei County 23585, Taiwan; Corresponding author: susw@ms96.url.com.tw Received 6 March 2009 ABSTRACT: In this article, a simple, yet effective method for matching a compact planar inverted-F antenna (PIFA) on a small system circuit board for Bluetooth headset applications is presented. The antenna is perpendicular to and extends along the top and right sides of the system ground, making it possible for the antenna to occupy almost no limited board space. Results have shown that by introducing a thin conductive wire soldered to the bottom corner of the system ground, good input matching over the 2400–2484 MHz band can easily be achieved for the PIFA mounted on a small ground of length less than a quarter wavelength at 2440 MHz. Radiation measurements of the proposed design in a real headset attached to a standard head phantom are also taken. Details of the obtained experimental and simulation results are Figure 7 Measured antenna gain for different operating frequencies: given and discussed. V 2009 Wiley Periodicals, Inc. Microwave Opt C (a) 2.4 GHz band; (b) 5 GHz band Technol Lett 51: 2802–2805, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24755 bandwidths for WLAN operation in the 2.4- and 5-GHz bands. The proposed antenna’s radiation characteristics were also Key words: antennas; planar inverted-F antennas; Bluetooth headset observed. antennas ACKNOWLEDGMENTS 1. INTRODUCTION This work is supported by an Inha University Research Grant. For several years, Bluetooth has overwhelmingly existed in many consumer electronic products such as mobile phones, REFERENCES Bluetooth headsets, wireless headphones, and USB dongles. The technology is very useful when transferring data between two or 1. K.L. Wong, Planar antennas for wireless communications, Wiley, more devices near each other without complicated setup of serv- Hoboken, NJ, 2003. 2. T.H. Kim and D.C. Park, Compact dual-band antenna with double ices. Because of a great variety of Bluetooth products, many L-slits for WLAN operations, IEEE Antennas Wireless Propag Lett antenna designs with different form factors have been reported 4 (2005), 239–252. in the literature [1–5]. Despite the fact that the types of these 3. Y. Jan and L.C. Tseng, Small planar monopole antenna with a studied antennas are not the same, the lengths of the system shorted parasitic inverted-L wire for wireless communications in the ground (or antenna ground) planes are all larger than a quarter 2.4-, 5.2-, and 5.8-GHz bands, IEEE Trans Antennas Propag 52 wavelength at the minimum resonance frequency of the antenna. (2004), 1903–1905. It is because for quarter wavelength resonant structures, the 4. Y.L. Kuo and K.L. Wong, Printed double-T monopole antenna for antenna ground is part of the radiator and provides a quarter 2.4/5.2 GHz dual-band WLAN operations, IEEE Trans Antennas wavelength path for the imaged surface currents of the antenna Propag 51 (2003), 2187–2192. (like two radiating portions of a dipole). The antenna is usually 5. H.D. Chen, J.S. Chen, and Y.T. Cheng, Modified inverted-L monop- ole antenna for 2.4/5 GHz dual-band operations, Electron Lett 39 quite difficult to match when the ground is shorter than the (2003), 1567–1568. aforementioned length from empirical experience. Moreover, 6. C. Yoon, S.H. Choi, H.C. Lee, and H.D. Park, Small microstrip previous studies of the ground effects on the planar inverted-F patch antennas with short-pin using a dual-band operation, Micro- antenna (PIFA) impedance bandwidth disclose that for a rectan- wave Opt Technol Lett 50 (2008), 367–371. gular ground, the optimal ground plane length is about 0.45-free 2802 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 12, December 2009 DOI 10.1002/mop
  2. 2. space wavelength at the center operating frequency of the PIFA [6, 7]. Bandwidth deteriorates when the ground plane length is shorter than the criteria. In this letter, we demonstrate that even when mounted on an electrically short system ground plane (less than a quarter wave- length at 2440 MHz), the 2.4-GHz Bluetooth PIFA can easily be well matched with voltage standing wave ratio (VSWR) below 2. The design is mainly aiming for compact Bluetooth headset applications, in which little space is left for the system circuit board and battery with no board space reserved for the antenna. The industrial-design (ID) appearance confines the housing thickness of the headset, and thus, the battery cannot be put above the circuit board simply because the printed circuit board assembly (PCBA) also occupies a certain thickness. In this case, the PCB and battery are placed on the same level in close prox- imity (Figs. 1 and 2). The proposed PIFA is then perpendicu- Figure 2 Photo of a mass-production headset antenna mounted on a larly mounted on the corner of the PCB to avoid occupying small PCB with a matching wire and a typical headset battery in close board space. It is found that with a simple conductive wire sol- proximity. [Color figure can be viewed in the online issue, which is dered to the PCB bottom corner on the same side of antenna available at www.interscience.wiley.com] feed, much better antenna input matching can be realized for Bluetooth operation in the 2.4-GHz (2400–2484 MHz) band. Details of the proposed design are described in this article, and constrained internal space for the PCBA and battery size, the the experimental results thereof are elaborated and discussed. battery in this case cannot be stacked up above the PCBA (over- all height does not fit into the housing). This is the reason why 2. ANTENNA DESIGN the system circuit board is forced to be electrically short (much Figure 1(a) shows the configuration of a Bluetooth headset PIFA less than 0.45-wavelength at the PIFA center operating fre- with a matching wire and a headset battery in the close vicinity quency). Note that to test the design prototype in these experi- (gap of 1 mm). The PIFA is of a metallic strip [2, 8, 9], ments, a 50-X mini-coaxial cable with an I-PEX connector is obtained from a 0.3-mm thick, copper-nickel-zinc alloy, and used. The inner conductor of the cable is connected to feed perpendicularly mounted on the top left corner of a small system point A, and the outer braided shielding is connected to ground ground with dimensions 10 mm  20 mm. The main radiat- point B. ing strip of the antenna extends along and above the top and The matching wire used in this study is a simple conductive right sides of the system circuit board to avoid occupying lim- wire that has an insulating sleeve outside [see Fig. 1(b)]. With ited board space. More detailed size of the PIFA is shown in no matching wire, the input impedance of the antenna are sub- Figure 1(b). Because the ID and appearance of the headset have stantially controlled by the distance between the feed point and shorting portion with a predetermined antenna height above the ground (that is 2 mm here). However, the optimal achievable bandwidth of the PIFA in this case is too narrow to satisfy the Bluetooth operation band, and thus, an extra matching circuit is needed to attain the required impedance bandwidth, which is highly not desired with concerns for BOM cost and board space. However, in cooperation with the matching wire presented, the distribution of the imaged surface currents on the system ground (antenna ground) is expected to be extended, which in turn lengthens the ground plane to be favorable to antenna achieva- ble bandwidth. 3. EXPERIMENTAL RESULTS AND DISCUSSION Figure 3(a) shows the measured and simulated return loss of a constructed prototype. The experimental data compare favorably with the simulation results, which are based on the finite ele- ment method. The achievable impedance bandwidth reaches about 95 MHz (2395–2490 MHz), defined by 10 dB return loss, and satisfies the bandwidth specification for 2.4 GHz Bluetooth operation. Simulation study of the effects of the matching con- ductive wire on the PIFA achievable bandwidth was conducted. Figure 3(b) plots the simulated input impedance curves on the Smith chart in the frequency range of 2–3 GHz. Notice that between two adjacent marks along the impedance curve, a step of frequency 25 MHz is seen. It is seen that, when there is no Figure 1 (a) Configuration of the proposed headset antenna with a wire (L ¼ 0), the desired operating frequencies are all of the matching conductive wire. (b) Detailed dimensions of the planar 2:1-VSWR circle. With an increase in the wire length (from 0 inverted-F antenna in planar structure. [Color figure can be viewed in to 24 mm), the impedance curve moves clockwise along the 50- the online issue, which is available at www.interscience.wiley.com] X resistance locus, and at the same time, its diameter becomes DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 12, December 2009 2803
  3. 3. Figure 4 Measured 2D radiation patterns at 2440 MHz for the antenna studied in Figure 3(a). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com] ment environment, in a CTIA authorized test laboratory [10] in Taiwan. Test results showed that the radiation efficiency dropped to 19.6% (about À7.1 dB) due to a lossy loading caused by the head phantom on the antenna [2]. Figure 6(b) presents the 3D patterns of effective isotropic radiation power (EIRP) of the DUT set in test mode in Channel 39 (that is 2440 MHz; channel spacing of 1 MHz from 2402 to 2480 MHz in Bluetooth system). The antenna input power of 3.1 dBm is cho- sen for Channel 39 here; the total radiated power (TRP) is À3.9 dBm accordingly. 4. CONCLUSIONS A compact PIFA ideally suited to internal antenna applications in Bluetooth headsets has been presented and tested. Since mounted on a comparatively small system circuit board in elec- trically short length, the antenna is difficult to match. By using a simple matching wire connected to the bottom corner on the same side of antenna feed, much improved input matching with VSWR below 2 in the band of interest can be achieved to meet the required bandwidth for Bluetooth operation. In addition, omnidirectional radiation patterns have been obtained with good Figure 3 (a) Measured and simulated return loss of a constructed pro- radiation efficiency above 85% (À0.7 dB) for the antenna in totype; L ¼ 24 mm. (b) Simulated input impedance on the Smith chart free space. For the proposed design integrated in a functioning (frequency range: 2–3 GHz) for the antenna with a matching wire of various lengths. [Color figure can be viewed in the online issue, which headset, the radiation efficiency drops to about À7.1 dB, result- is available at www.interscience.wiley.com] ing in the TRP of À3.9 dBm when giving antenna input power of 3.1 dBm at 2440 MHz. The design concept can be applied to smaller (wider bandwidth). A near optimal value of the match- ing-wire length (L) selected in this study is 24 mm. Figure 4 gives the far-field, 2D radiation patterns at 2440 MHz in Ey and Eu fields for the prototype in free space. The measurement was taken at 3 Â 3 Â 7 m3 fully anechoic cham- ber, with the great-circle method, at Lite-On Technology Corp. Similar radiation patterns measured at other frequencies in the 2.4-GHz band were observed. Good omnidirectional radiation patterns are seen in the x–y plane, which can be treated as the horizontal plane when the Bluetooth headset is in use in talk position [Fig. 6(a)]. The measured peak antenna gain and radia- tion efficiency are plotted in Figure 5. The gain is stable, and at about 2.2 dBi, the efficiency is above 85% or about À0.7 dB. Finally, the proposed design was successfully implemented in a mass-produced, fully functioning, Bluetooth headset avail- able on the market. The headset is regarded as a device under Figure 5 Measured peak antenna gain and simulated radiation effi- test (DUT) [see Fig. 6(a)] and attached to a head phantom, ciency for the antenna studied in Figure 3(a). [Color figure can be viewed which is commonly used for specific absorption rate measure- in the online issue, which is available at www.interscience.wiley.com] 2804 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 12, December 2009 DOI 10.1002/mop
  4. 4. 8. S.W. Su, C.H. Wu, W.S. Chen, and K.L. Wong, Broadband printed p-shaped monopole antenna for WLAN operation, Microwave Opt Technol Lett 41 (2004), 269–279. 9. K.L. Wong, J.H. Chou, and S.W. Su, Isolation between GSM/DCS and WLAN antennas in a PDA phone, Microwave Opt Technol Lett 45 (2005), 347–352. 10. CTIA Authorized Test Laboratory, CTIA, The wireless association. Available at: http://www.ctia.org/business_resources/certification/ test_labs/. V 2009 Wiley Periodicals, Inc. C SIMPLE SMALL-SIZE COUPLED-FED UNIPLANAR PIFA FOR MULTIBAND CLAMSHELL MOBILE PHONE APPLICATION Ting-Wei Kang and Kin-Lu Wong Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan; Corresponding author: kangtw@ema.ee.nsysu.edu.tw Received 6 March 2009 ABSTRACT: In this study, a simple uniplanar printed PIFA occupying a small area of 10 Â 40 mm2 for achieving multiband operation in the clamshell mobile phone is presented. The proposed PIFA is formed by a simple shorted radiating strip coupled-fed by a simple feeding strip and is mounted at the hinge of the clamshell mobile phone; further, the upper ground plane is connected to the main ground plane using an extended connecting strip. With the coupling feed and the connection arrangement between the main and upper ground planes, the proposed PIFA itself is not only an efficient radiator, it can also excite the two ground planes of the clamshell mobile phone as an efficient radiator (dipole-like resonant modes are excited). Thus, with a small occupying area and a simple structure for the proposed PIFA, two wide operating bands at lower and higher frequencies can be provided to cover Figure 6 Measured peak EIRP for the design integrated in a function- GSM850/900/1800/1900/UMTS bands for WWAN operation. The ing Bluetooth headset attached to a head phantom: (a) test setup; (b) 3D antenna also meets the 1-g SAR specification of 1.6 W/kg required for EIRP patterns in Channel 39. [Color figure can be viewed in the online practical applications. V 2009 Wiley Periodicals, Inc. Microwave Opt C issue, which is available at www.interscience.wiley.com] Technol Lett 51: 2805–2810, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24756 a compact wireless device where the system circuit board is Key words: antennas; mobile antennas; handset antennas; PIFA; small and short, and the layout therein does not tolerate further multiband antennas extra matching circuit. REFERENCES 1. INTRODUCTION 1. L. Lu and J.C. Coetzee, Reduced-size microstrip patch antenna for Because of the rapid development in the mobile communication Bluetooth applications, Electron Lett 41 (2005), 944–945. systems, multiband operation, especially the pentaband operation 2. K.L. Wong, M.R. Hsu, W.Y. Li, and S.W. Su, Study of the Blue- of GSM850/900/1800/1900/UMTS, for wireless wide area net- tooth headset antenna with the user’s head, Microwave Opt Technol Lett 49 (2007), 19–23. work (WWAN) communications has been demanded for many 3. C.H. Wu, K.L. Wu, Y.C. Lin, and S.W. Su, Internal shorted monop- modern mobile phones. To meet the multiband operation ole antenna for the watch-type wireless communication device for requirement, the conventional planar inverted-F antennas Bluetooth operation, Microwave Opt Technol Lett 49 (2007), (PIFAs), such as in [1–6], that are promising to be applied in 942–946. the mobile phone as the internal antennas often require the use 4. D.H. Seo, S.G. Jeon, N.K. Kang, J.I. Ryu, and J.H. Choi, Design of of at least two resonant strips or patches for obtaining more res- a novel compact antenna for a Bluetooth LTCC module, Microwave onant modes to cover the desired multiband operation. As a Opt Technol Lett 50 (2008), 180–183. result, the occupied volume of the multiband PIFAs is usually 5. J.H. Yoon, Design of a compact antenna for Bluetooth application, increased as additional resonant strips or patches are added. The Microwave Opt Technol Lett 50 (2008), 2568–2572. increasing volume makes such conventional multiband PIFAs 6. T.Y. Wu and K.L. Wong, On the impedance bandwidth of a planar inverted-F antenna for mobile handsets, Microwave Opt Technol less attractive for practical applications in the modern mobile Lett 32 (2002), 249–251. phones. 7. M.C. Huynh and W. Stutzman, Ground plane effects on planar In this article, we present a small-size PIFA capable of gen- inverted-F antenna (PIFA) performance, IEE Proc Microwave erating two wide operating bands at lower and higher freq- Antennas Propag 150 (2003), 209–213. uencies to cover GSM850/900 (824–894/880–960 MHz) and DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 12, December 2009 2805

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