The document describes a compact planar inverted-F antenna (PIFA) designed for Bluetooth headset applications. Due to space constraints from the headset design, the PIFA is mounted on a small circuit board ground plane that is less than a quarter wavelength in size. Input matching for Bluetooth operation is achieved by connecting a thin conductive wire from the bottom corner of the ground plane. Measurements show the antenna achieves over 95 MHz bandwidth for Bluetooth and maintains good radiation efficiency when integrated into a headset and attached to a head phantom. A simple matching wire enables the PIFA to be used on small circuit boards.

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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. 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. 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. 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
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of GSM850/900/1800/1900/UMTS, for wireless wide area net-
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942–946. the mobile phone as the internal antennas often require the use
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Opt Technol Lett 50 (2008), 180–183. result, the occupied volume of the multiband PIFAs is usually
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increasing volume makes such conventional multiband PIFAs
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less attractive for practical applications in the modern mobile
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7. M.C. Huynh and W. Stutzman, Ground plane effects on planar In this article, we present a small-size PIFA capable of gen-
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DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 12, December 2009 2805