Hexa-Band Multi-Standard Planar Antenna Design for Wireless Mobile Terminal

Electronic devices such as mobile phones and laptop computers are parts of modern life. Users of portable wireless devices always desire such devices to be of small volume, light weight, and low cost. Thanks to the rapid advances in very large scale integration (VLSI) technology, this dream has become a reality in the past two decades. As technology grows rapidly, a mobile is not just a phone recently. The highly integration of circuits makes the mobile phone and the PDA (personal digital assistant) been combined into a single handset, which is called a smart phone. Also, the Internet carries various information resources and services, such as electronic mail, online chat, file transfer and file sharing, these attractive proprieties make wireless internet service becomes an important function that should be integrated into mobile devices. There are many ways for the user to connect to the internet. The traditional wireless local area network (WLAN) is a popular communication system for accessing the Internet. However, the reach of WiFi is very limited. WLAN connectivity is primarily constrained to hotspots, users need to find the access points and can only use it in certain rooms or areas. As the user get out of range of the hotspot, the signal will become very weak and the user may lose the connection. This disadvantage limits the mobility of wireless communication. Except for the widely used wireless local area network, third generation (3G) mobile telephony based on the High Speed Downlink Packet Access (HSDPA), which is part of the UMTS standards in 3G communications protocol, is another high speed wireless internet access service. It has become popular nowadays that people can get to the internet via cellular communication system. This technology gives the users the ability to access to the Internet wherever the signal is available from the cellular base station. However, the quality sometimes depends on the number of users simultaneously connected per cellular site. In addition to utilizing WLAN/3G dual-mode terminals to enhance efficiency of mobile number portability service, WiMAX (the Worldwide Interoperability for Microwave Access) is an emerging telecommunications technology that provides wireless data transmission in a variety of ways, ranging from point-to-point links to full mobile cellular-type access. WiMAX is similar to Wi-Fi but it can also permit usage at much greater distances. The bandwidth and range of WiMAX make it suitable for the applications like VoIP (Voice over Internet Protocol) or IPTV (Internet Protocol Television). Many people expect WiMAX to emerge as another technology that may be adopted for handset devices in the near future.

The rapid progress in mobile communication requires that many functions and wireless communication systems be integrated into a mobile phone.When portability is taken into account, antenna that can be built in the phone device is desirable.This has led to a great demand for designing multiband antennas for handset devices.Among existing built-in or internal type scheme, the inverted-F (IFA) or planar inverted-F antenna (PIFA) are the most promising candidates.The linear inverted-F antenna, which is the original version of the PIFA, has been described by R. King in 1960 as a shunt-driven inverted-L antennatransmission line with open-end (king et al., 1960).The PIFA, which is constructed by replacing the linear radiator element of IFA with a planar radiator element, can also be evolved from a microstrip antenna.Taga first investigated PIFA's performance for 800MHz band portable unit radio in 1987 (Taga & Tsunekawa, 1987).He also wrote a chapter in his textbook to teach how to design a single band PIFA (Hirasawa & Haneishi, 1922).The PIFA or IFA are not only small in size but also have a broadband bandwidth.Since it is cheap and easy to fabricate, it has become very popular with mobile phone manufacturers.Many references concerning PIFA and its relatives were published in the decade.
In order to integrate all the wireless services into a mobile terminal and have an effective usage of the precious board space in the mobile device, multiband antenna that is designed to operate on several bands is necessary.However, designing a multiband antenna in a narrow space is a great challenge; a method that decrease the complexity of the antenna structure is also necessary to be investigated.Guo et. al. have recently designed quad-band antennas for mobile phones (Chiu & Lin, 2002;Nashaat et al., 2005;Karkkainen, 2005) and dual-band antennas for WLAN operations (Su & Chou, 2008).However, few of these antennas simultaneously cover the following communication standards: GSM (880-960 MHz), DCS (1710-1880 MHz), PCS (1850-1990 MHz), UMTS2100 (1920( -2170 MHz) MHz), WLAN + Bluetooth (2400-2480 MHz), WiMAX (2500-2690 MHz), HiperLAN/2 in Europe (5150-5350 / 5470-5725 MHz) and IEEE 802.11a in the U.S. (5150-5350 / 5725-5825 MHz) (Liu & Gaucher, 2004, 2007;Wang et al., 2007;Rao & Geyi, 2009;Nguyen et al., 2009;Anguera et al., 2010;Kumar et al., 2010;Liu et al., 2010;Hsieh et al., 2009;Yu & Tarng, 2009;Hong et at., 2008;Guo et al., 2004;Li et al., 2010).This chapter proposes a planar multiband antenna that comprises a dual-band inverted-F resonator and two parasitic elements to cover all the communication standards mentioned above.bandwidth.This hepta-band antenna is designed for a mobile device and the parasitic element broadens the impedance bandwidth to about 45.5%.This antenna is extended to simultaneously operate in WLAN, WiMAX, and WWAN systems.It covers all cellular bands world-wide and all wireless network bands, such as the following communication standards: GSM/DCS/PCS/UMTS/WLAN/WiMAX/HIPERLAN2/IEEE 802.11.The antenna structure that measures only 50 mm x 12 mm x 0.5 mm can be easily fabricated by stamping from a metal plate.The following describes the details of the proposed antenna as well as the experimental results.

Design of a dual-band antenna
Modern mobile terminals require small and thin design, therefore, planar inverted-F antenna, which requires a spacing of about 7 mm ~ 12 mm between the antenna and the substrate to achieve the sufficient operating bandwidth, is not suitable to be integrated with the present thin mobile terminals although it is popular and widely used.Fig. 1(a) shows a three dimensional view of the proposed design.The antenna, which is mounted on the top edge of the printed circuit board (PCB), is fed by a 50 Ω coaxial cable.The antenna is coplanar with the system ground of the PCB.The dielectric constant of the PCB used here is 4.4 and the thickness is 1.58 mm.As shown in Fig. 1(b), this radiating structure measures 50 mm × 12 mm × 1.5 mm and can be extended to a single metallic plate.It is basically an inverted-F antenna in which the quarter-wavelength characteristic is obtained thanks to a short-circuited metallic strip.As indicated in Fig. 1(b), this design comprises a direct-feed dual band main resonator with two branches (A) and (B), and two parasitic elements (C) and (D) excited by electromagnetic coupling, to achieve multiband operation.Shown in Fig. 2 is a typical configuration of an inverted-F antenna.It can be fed by a minicoaxial cable which is connected to the RF module.Here, H is the height of the radiator above the ground plane, L F is the horizontal length from the feed point to the open end of the antenna, and L B is the horizontal length from the feed point to the closed end of the antenna.This antenna is a quarter-wavelength radiator with one short end and one open end.The resonant frequency can be easily calculated by the formula:

4( )
the where c is the speed of light.The resonant frequency can be adjusted by changing the value L F , and the distance L B between the feed point and shorting strip can be used to adjust the input impedance.The height H of the antenna is closely related to the impedance bandwidth where the Q factor can be reduced by increasing the antenna height to broaden the bandwidth and vice versa.Variations of IFA Antenna height cause some effects on bandwidth.Fig. 3 shows the simulation results with different antenna height H.It is found that increasing the height will increase the impedance bandwidth.

Bandwidth enhanced by a parasitic element
Creating multiple resonant paths of the inverted-F antenna is helpful to generate multiple resonances.However, the coupling between each resonant path makes it difficult to match the antenna at each frequency band.To cover the wide bandwidth from 1900 MHz to 2700 MHz, this work introduces a parasitic resonator C near the main driven resonator.This parasitic element is excited by electromagnetic coupling from the main dual band resonator.Thus, a dipole-like antenna that resonates at 2250 MHz is formed by both the introduced resonator C, and the main resonators A and B. Fig. 7 shows the surface current distributions on the resonators and the ground plane.Finding show that part of the dual band resonator and the parasitic element form a dipole antenna.From point a, through point b, c, and d, then to point f in Fig. 7, the total length (39 mm + 3 mm + 9 mm + 19 mm = 70 mm) is closed to 0.5 wavelength at 2250MHz (67 mm).This allows the antenna to generate an additional 0.5-wavelength resonant mode at 2250 MHz to cover the desired operation bands.

Advanced Transmission Techniques in WiMAX 10
To demonstrate the effect of the parasitic element covering from 1900 MHz to 2700 MHz, Fig. 8 shows the parameter study of the proposed antennas with different length of the parasitic element.By Investigating the Smith chart shown in Fig. 9, it is evident that the input impedance is closer to 50 Ω as length L increases, because the longer the parasitic element, the more the loaded capacitance (Chi, 2009).The narrow gap between the main resonator and the parasitic element C introduces a proper capacitance to compensate for possible inductance contributed from the dual-band main resonator.capacitance neutralizes the effect due to inductance of the strip.Therefore, the capacitive coupled parasitic element creates a new resonant mode but does not change the original two resonant modes at 900 MHz and 1800 MHz.The length of the parasitic element is selected to be 19 mm to have the return loss better than 6 dB in the band of operation.The achieve bandwidth of the parasitic element is about 34.78 %, covering from 1900 MHz to 2700 MHz, which is enough for WLAN, WMAN, and WWAN operations.Finally, this work applies another technique to tune the higher order resonances for the U-NII band.The quarter wavelength resonating at 6.0 GHz is only about 12.5 mm.A short resonator D with a length of 10.5 mm, as Fig. 1(b) shows, is introduced to the short-circuited pin of the main resonator to form an inverted L-shape parasitic element.The capacitive coupling between the strip and the chassis increases its electrical length since the radiating strip is only 1 mm above the ground plane.Adding this parasitic element improves resonance performance at the U-NII band.

Results and discussion
This study constructs and tests the proposed antenna based on the design dimensions shown in Fig. 1(b).The test structure was shown in Fig. 12 and the measurement of scattering parameters was performed by an Agilent E5071B network analyzer.Fig. 13 shows the measured and simulated return loss where the solid red line is the measured result and the dotted blue line is the simulated one.Findings show good agreement between the measured data and simulated results.The antenna covers all cellular bands used worldwide is evident.The achieved bandwidths with return loss better than 6 dB are 80 MHz (880-960 MHz) in the GSM band, 1000 MHz (1700-2700 MHz) in the DCS/PCS/UMTS/WiFi /WiMAX band and 1270 MHz (4820-6090 MHz) in the 5 GHz U-NII band.When ground plane length varies from 80 mm to 120 mm, frequency shifting is slight (Chi, 2009).

│S 11 │ (dB)
This study performed radiation-pattern and gain measurement in the anechoic chambers of SGS Ltd.Taiwan, as shown in Fig. 14.Fig. 15 shows the measured and simulated radiation patterns at the xy-cut, xz-cut, and yz-cut.The measured radiation patterns show a good match to the simulation results except at 925MHz.In the small antenna measurement, the patterns are easily affected by the feeding RF cable in the GSM band (Chen et al., 2005).This work finds that the dual-polarization radiation-patterns have very suitable characteristics for portable devices.For the radiation shown in Fig. 14(a), more energy for E θ is radiated in the lower band as compared to Eφ.The Eφ field has some dips at 900 MHz on the xz-plane or 1800 MHz on the xy-plane.This is probably due to current cancellation on the strips and the ground plane.By using the commercial electromagnetic simulation software HFSS, this research carries out simulations for the theoretical gains to investigate antenna performance and compare it with the measured results (Chi, 2009).Good agreement confirms that the measured data are accurate.The two-dimensional average gain is determined from pattern measurements made in the horizontal (azimuth) plane for both polarizations of the electric field.The results are then averaged over azimuth angles and normalized with respect to an ideal isotropic radiator (Chen, 2007).Finally, Table 1 lists the measured peak gain, two-

Measured E-theta Measured E-phi Simulated E-theta Simulated E-phi
dimensional average gain and radiation efficiency for all the operation bands, showing that all radiation efficiencies are over 50 percent, meeting the specification requirement.

Summary
This chapter reported a down-sized multiband inverted-F antenna to integrate the 3.5G and WLAN/WiMAX antenna systems.It is comprised of a dual-band antenna with one feed point and two parasitic elements to cover many mobile communication systems including GSM900 /DCS /PCS /UMTS /WLAN/ WiMAX /HiperLAN2 /IEEE802.11a.Measured parameters including return loss, radiation patterns, three-dimensional peak gain and average gain as well as radiation efficiency were presented to validate the proposed design.
Since this antenna can be formed by a single plate, it is both low cost and easy to fabricate, making it suitable for any palm-sized mobile device applications.
One element is devoted to generating a dipole m o d e a n d a no t h e r i s h e l p f u l t o e x c i t e a l o o p m o d e s o a s t o b r o a d e n t h e i m p e da n c e www.intechopen.com

Fig. 1 .
Fig. 1.The proposed antenna (a) Three-dimensional configuration of the proposed antenna (b) Plane view of the antenna structure.

Fig. 4 .
Fig. 4. A variation of typical inverted-F antenna.Fig.4showsanother kind of inverted-F antenna while the shorting pin is moved to the bottom for size reduction.The mechanism of this alternative is the same as the previous one, but the input impedance is matched by adjusting the length of the shorting strip L S .The dual band inverted-F antenna can be simply accomplished by creating two resonant paths of the antenna element.As shown in Fig.5, the dual-band main resonator consists of two branches (A and B).The length of the longer branch (B) is about 83 mm (9 + 44.5 + 6 + 23.5 mm) which is one-quarter of the wavelength at 900MHz.The lower resonant mode for GSM operation can be excited on this resonator.On the other hand, branch (A) in the middle creates a shorter path of 42 mm, which is about a quarter of wavelength at 1800 MHz.As a result, the resonant mode for DCS operation can be excited.Simulation result of the dual band antenna is shown in Fig.6.The input impedance can be adjusted by changing the

Fig. 14 .Fig. 15 .
Fig. 14.Radiation Pattern measurement in a 3D anechoic chamber.Findings also show a dipole-like pattern at the frequency 2170 MHz.Radiation patterns shown in Fig. 15(b) confirm this deduction.The radiation pattern of this mode is similar to a small dipole oriented in the y-axis leading to a directional pattern in the E-plane (xy-plane, blue line) and omni-directional pattern in the H-plane (xz-plane, blue line), as Fig. 15(b), shows respectively.The resonators C and B at 2170 MHz have strong current distributions along the z-direction which also contribute to radiation fields.The radiation pattern of this current distribution is due to a small dipole oriented in the z-axis leading to a bidirectional pattern in the E-plane (xz-plane, red line) and omni-directional pattern in the H-plane (xyplane, red line), as Fig. 15(b), shows respectively.Findings also show an asymmetric radiation pattern at the U-NII band (5-6 GHz) and some variation and nulls, since different modes are excited in this U-NII band.

Table 1 .
Measured three-dimensional peak gain, average gain, and radiation efficiency.