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978-1-4799-6013-2/14/$31.00 ©2014 IEEE A Compact Multi-Band Planar Inverted-F Antenna With Reduced SAR for Mobile Terminals Hrudya.B.Kurup 1,3,5 ,Vijesh.K.R 1,3 , P.Mohanan 2 , V.P.N.Nampoori 1,3 , Bindu.G 3,4 , Jubaira 5 1.International School of Photonics, 2.Center for Research in Electromagnetics and Antennas (CREMA) Department of Electronics, Cochin University of Science and Technology (CUSAT), 3.Swadesi Science Movement, 4. Nansen Environmental Research Center, Cochin 5.KMEA Engineering College, Department of Electronics Cochin- 22, Kerala, India. Abstract—This paper proposes a novel low SAR multi-band PIFA structure with reduce exposure of the human head to mobile-set antenna radiation. Triple band PIFA structure operating at GSM 900, GSM 1900 and Wi-Fi bands are designed and fabricated .Analysis of SAR is SAR is decreased by reducing the radiation towards human head ,while maintain significant radiation in all other directions. This SAR reduction is accomplished by inserting suitable slots in the top radiating patch and ground plane of PIFA at appropriate positions so as to modify the current distribution and radiation pattern. Keywords - PIFA, internal antenna, FR4 dielectric, planar element, return loss, VSWR I. INTRODUCTION Recent years has witnessed a very rapid expansion of wireless communications. As a result of which personnel communication devices, especially mobile phones have become as integral part of our day to day life. A wide verity of mobile phones is available in market which can support various applications and provide different types of services. Antennas are the crucial elements which support these hand- held transceivers for personal communications with its various capabilities. For antennas to satisfy the requirements of the current market, they must be compact with a low congestion in the telecommunications devices. More and more communication standards are introduced every day and they are required to be supported by handsets. This recent growth and rapid development of mobile communication and introduction of new frequency bands and services has lead to the requirement of antennas with multi- band operation. All these services data voice internet and multimedia has to be delivered simultaneously without compromising their weight, volume and performance. Therefore the antenna in hand held devices are generally required to be compact along with multi band capability. Therefore mobile phone manufacturers are now increasingly focusing on low profile, compact, multi-band capabilities for antennas. Recently, there has been a great demand for mobile devices and antennas that have small size with multiband operation because of widespread use of Bluetooth, GSM, and Wi-Fi which can be easily fabricated with low manufacturing cost. Due to the rapid growth in the use of mobile phones and other wireless communication systems there’s a huge public health crisis looming from this one particular threat: electromagnetic radiation from cellular phones. As the number of services and applications supported by mobile phones increased, there is an increased usage and dependency for these devices, in our day to day life. As the duration of usage increased considerably, there is a great concern about the harmful radiation from these devices. The human head is one of the most sensitive parts of the human entire body when exposed to electromagnetic radiation. This electromagnetic radiation interacts with the human head and may lead to detrimental effects on human health. [1] Therefore, it is necessary to decrease the interaction of electromagnetic energy towards human head when mobile handset is in operation. For this reason, various public organizations in the world have established safety guidelines for electromagnetic wave absorption. [2, 3] For electromagnetic wave exposures, these guidelines are based on peak specific absorption rate (SAR). The power absorption of electromagnetic energy in human tissues induces temperature increases inside tissue. Apart from these thermal effects, there are non- thermal effects which are even more harmfull. In this paper, the design of a simple , compact, multi-band planar inverted F-antenna (PIFA) working in GSM 900 , GSM 1900 WI-Fi and Bluetooth bands is presented. Slots are inserted in the radiating element and ground plane, with a relatively simple structure. Suitable dimensions are selected for the antenna to achieve the required multi-band operation. Next section explains the basic structure of simple PIFA and discusses the relationship between various parameters. Section III discusses the design of the proposed antenna and its properties using CST microwave studio simulation software. The fabrication and device testing results are presented in section IV. Section V provides conclusion and section VI is acknowledgement.

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978-1-4799-6013-2/14/$31.00 ©2014 IEEE

A Compact Multi-Band Planar Inverted-F Antenna With Reduced SAR for Mobile Terminals

Hrudya.B.Kurup1,3,5,Vijesh.K.R1,3, P.Mohanan2, V.P.N.Nampoori1,3, Bindu.G3,4 , Jubaira5

1.International School of Photonics, 2.Center for Research in Electromagnetics and Antennas (CREMA) Department of Electronics, Cochin University of Science and Technology (CUSAT), 3.Swadesi Science Movement,

4. Nansen Environmental Research Center, Cochin 5.KMEA Engineering College, Department of Electronics Cochin- 22, Kerala, India.

Abstract—This paper proposes a novel low SAR multi-band PIFA structure with reduce exposure of the human head to mobile-set antenna radiation. Triple band PIFA structure operating at GSM 900, GSM 1900 and Wi-Fi bands are designed and fabricated .Analysis of SAR is SAR is decreased by reducing the radiation towards human head ,while maintain significant radiation in all other directions. This SAR reduction is accomplished by inserting suitable slots in the top radiating patch and ground plane of PIFA at appropriate positions so as to modify the current distribution and radiation pattern.

Keywords - PIFA, internal antenna, FR4 dielectric, planar element, return loss, VSWR

I. INTRODUCTION Recent years has witnessed a very rapid expansion of wireless communications. As a result of which personnel communication devices, especially mobile phones have become as integral part of our day to day life. A wide verity of mobile phones is available in market which can support various applications and provide different types of services. Antennas are the crucial elements which support these hand-held transceivers for personal communications with its various capabilities. For antennas to satisfy the requirements of the current market, they must be compact with a low congestion in the telecommunications devices. More and more communication standards are introduced every day and they are required to be supported by handsets. This recent growth and rapid development of mobile communication and introduction of new frequency bands and services has lead to the requirement of antennas with multi-band operation. All these services data voice internet and multimedia has to be delivered simultaneously without compromising their weight, volume and performance. Therefore the antenna in hand held devices are generally required to be compact along with multi band capability. Therefore mobile phone manufacturers are now increasingly focusing on low profile, compact, multi-band capabilities for antennas. Recently, there has been a great demand for mobile devices and antennas that have small size with multiband operation because of widespread use of Bluetooth, GSM, and

Wi-Fi which can be easily fabricated with low manufacturing cost. Due to the rapid growth in the use of mobile phones and other wireless communication systems there’s a huge public health crisis looming from this one particular threat: electromagnetic radiation from cellular phones. As the number of services and applications supported by mobile phones increased, there is an increased usage and dependency for these devices, in our day to day life. As the duration of usage increased considerably, there is a great concern about the harmful radiation from these devices. The human head is one of the most sensitive parts of the human entire body when exposed to electromagnetic radiation. This electromagnetic radiation interacts with the human head and may lead to detrimental effects on human health. [1] Therefore, it is necessary to decrease the interaction of electromagnetic energy towards human head when mobile handset is in operation. For this reason, various public organizations in the world have established safety guidelines for electromagnetic wave absorption. [2, 3] For electromagnetic wave exposures, these guidelines are based on peak specific absorption rate (SAR). The power absorption of electromagnetic energy in human tissues induces temperature increases inside tissue. Apart from these thermal effects, there are non- thermal effects which are even more harmfull. In this paper, the design of a simple , compact, multi-band planar inverted F-antenna (PIFA) working in GSM 900 , GSM 1900 WI-Fi and Bluetooth bands is presented. Slots are inserted in the radiating element and ground plane, with a relatively simple structure. Suitable dimensions are selected for the antenna to achieve the required multi-band operation. Next section explains the basic structure of simple PIFA and discusses the relationship between various parameters. Section III discusses the design of the proposed antenna and its properties using CST microwave studio simulation software. The fabrication and device testing results are presented in section IV. Section V provides conclusion and section VI is acknowledgement.

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II. PIFA THEORY

A. PIFA Structure PIFA antenna structure has emerged as one of the most promising candidate in the category of antennas used in handheld devices and most built-in antennas currently used in mobile phones include planar inverted F- Antenna (PIFA). Broad range of applications employs PIFA as their basic antenna. PIFA structure consists of a ground plane, a radiating element i.e. a patch, a feed wire or strip & one or more shorting pins or plates to connect the top patch and the ground plane. The antenna is fed through feeding pin which connects to the ground plane. This type of feeding technique allows designer to place it at any desired location in the patch.

The shorting pin and shorting plate allows good impedance matching achieved with the patch above ground plane of size less than λ/4. Resulting PIFA structure is of compact size than conventional λ/2 patch antennas.This structure resembles a short-circuit micro strip antenna .Therefore PIFA can be thought of as a shorted micro strip patch antenna with air as dielectric.[4] Fig. 1 shows a basic PIFA structure which is fed at the base by a feed wire.

Fig.1: Basic PIFA Structure

In a PIFA structure there are several design variables which can be varied and the performance of the desired antenna is achieved [5]-[6]. Some of the design variables are width, length and height of the top radiating patch, width and position of shorting pin or plate, location of the feed point, dimensions of the ground plane.

B. Basic Design Equation The frequency at which PIFA resonates can be calculated by using a basic formula as given below

………………………………………. (1)

Where c is the speed of light, L1 and L2 are the width and length of the top patch, f is the resonant frequency The substrate electrical properties as well as thickness affect the performance of PIFAs in terms of gain and bandwidth. Substrates with high loss tangent are very lossy and result in low gain. Substrates of high permittivity or of narrow thickness lead to poor radiators of narrow bandwidth. Hence antennas are often designed with thick substrates of low permittivity. However, the thickness of the substrate should be limited so that surface waves, which deteriorate the radiation efficiency, are not generated [7]. Therefore a modified approximation of the above equation including dielectric permittivity and shorting plate width is given as

L1 + L2-W = c /4f√ ……………………………………… (2)

Above equation represents that the sum of the width and length of the top plate should be λ/4. The performance of the antenna can be enhanced by varying ground plane length. Optimum length of the ground plane is 0.4λ at the operating frequency [8]. This approximation is very rough and does not cover all the parameters that significantly affect the resonant frequency of the antenna [9]. In several designs, position of the antenna on the dielectric substrate is important as enhancement in the operating bandwidth can be achieved to few more percentage. Location of feed point and the type of feed used and position of shorting pin or plate etc are some other significantly influencing parameters

III. PROPOSED ANTENNA The proposed antenna is shown in Fig.2 (a) and 3D structure of the proposed PIFA antenna is shown in Fig.2 (b). The proposed PIFA antenna consists of main radiating patch, a rectangular ground plane, a shorting plate, coaxial feed and a ground plane. The antenna is designed using a dielectric material as FR-4 which has loss tangent, δ=0.02, dielectric constant, εr = 4.4 and substrate height, h = 1.6 mm. Slots of optimum dimensions are inserted at appropriate positions in the ground plane and top radiating patch so as to obtain multi-band operation.Those are used: to create new resonances, to lengthen the electric lengths and to create new resonators. The slits disturb the current flowing on the surface, forcing them to meander and thus the electrical length of the

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patch antenna changes. Accordingly, the operating frequency and radiation pattern changes whereas the physical size of the patch is unaffected. Here lowest frequency means the biggest wavelength in comparison to other higher frequencies and hence the respective current path should be the longest. Introducing slot or slit in radiating patch is simple and efficient method for obtaining the desired compactness, multiband and broadband properties since these shapes radiate electromagnetic energy efficiently. .

Fig.2: a)Proposed antenna b) 3-D view in CST

Feeding point source is used to excite the structure. Total dimensions of the radiating parts of the antenna are 46 x 14 mm2. And that of ground plane are 120 x 50 mm2. It can be observed that radiating parts covers small portion of the total size of the antenna leaving more space available for other electronic components [11]. Fig 3 and 4 shows the detailed dimensions of the top radiating patch and the ground plane

Fig.3: Dimensions of Top Radiating Patch

Fig.4: Dimensions of Ground Plane

The simulation and analysis of the proposed antenna is done using High Frequency Structure Simulator (HFSS). The simulated reflection coefficient (S11) also known as return loss is presented in Fig. 5. It can be observed from S11 plot that the antenna covers the GSM900, GSM 1900 , Bluetooth and Wi-Fi bands.

The space between ground plane and top plate is air filled; here air is used as dielectric material [10]. Using a dielectric material between ground plane and top plate has effect on gain and bandwidth of PIFA antenna. To get good return loss and gain, the height of top plate selected is 10 mm. The ground plane, shorting plate and top plate are made perfect electrical conductor (pec) [11].

Fig. 5: The Simulated S11 (dB) of proposed PIFA

Return loss of -27.87 dB obtained at resonant frequency of 867 MHz, -23.36 dB at 1899MHz and -22.30 dB at 2.47 MHz. Also, it is observed from results that at resonant frequency the Voltage Standing Wave Ratio (VSWR) is well below 2dB [11] i.e. at 867 MHz value of VSWR is 1.09 , at 1899 MHz VSWR is 1.17 and at 2.47 GHz VSWR is 1.18. The upper and lower frequency at which return loss of -10 dB is obtained is 0.770 GHz and 1.032 GHz, respectively.

Fig. 6: The Simulated VSWR of proposed PIFA

The bandwidth here can be specified as impedance bandwidth for which return loss S11 is -6 dB as this value is good enough for mobile handset applications. Also frequency bandwidth can be specified for voltage standing wave ratio (VSWR) less than 2:1 which is equivalent to 10 dB level . At this level 10% of the incident power is reflected back at the source. Therefore, the impedance bandwidth of the proposed PIFA design is the difference between upper and lower frequency which is 0.07 GHz, 0.05GHz and 0.06GHz.

The simulated 3-D radiation pattern of the proposed PIFA is shown below. The simulated 3D radiation pattern at 867 MHz, 1899MHz, and 2.47GHz in xyz and xy-z planes is obtained from the simulation results using CST microwave studio is shown in Fig 7(a),(b),(c). It can be seen from the plot that the

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antenna is having good electromagnetic radiation intensity in the Z plane where as radiation is considerably reduced in the –Z direction. [13]

(a)

(b)

(c)

Fig.7 Simulated radiation pattern in XYZ plane and XY-Z plane at a)867MHz b)1899 MHz c) 2.42GHz

The red, orange,yellow,green and blue regions represents radiation intensity in the decreasing order. Therefore the antenna radiation pattern itself clearly shows a decrease in intensity of electromagnetic radiation in the –Z direction (ie towards user head, when the phone is in use )

The cellular bands covered by the simulated antenna are GSM900,GSM1900 ,Bluetooth and Wi-Fi bands. The antenna has good radiation characteristics in all directions except to the user head side thus reducing electromagnetic radiation interaction with user head, which is suitable for mobile handsets.

IV. SAR CALCULATION WITH PHANTOM HEAD MODEL The simulation model of SAM phantom head model provided by CST Microwave Studio®(CST MWS) along with the designed antennas is used for calculation of SAR . The designed antenna is kept at a distance of 1mm from head model and the peak SAR value is studied. This is shown in

figure 8 (a) and (b) respectively. The SAR [14] in human head is defined as follows,

Where

= conductivity of the tissue (S/m)

= mass density of the tissue (kg/m3)

= rms electric field strength (V/m)

(a)

(b)

Fig.8 Proposed PIFA structure at a distance of 1mm from the head model when antenna is placed a)Right side of head model b) Left side of head model

Various public organizations in the world have established safety guidelines for electromagnetic wave absorption. For electromagnetic wave exposures, these guidelines are based on peak specific absorption rate (SAR). SAR value is an important tool in judging the maximum possible exposure to RF energy from a source. FCC and DOT has regulated SAR value as 1.6 W/Kg averaged over 1 gm of tissue. The 1-g SAR results for the respective resonant frequencies are listed in table I, it is clearly seen that 1-g SAR results at all frequencies are well below the SAR limits of 1.6 W/kg standards.

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TABLE I

SIMULATED SAR VALUE OF THE ANTENNA WITH DIFFERENT RESONANT FREQUENCY

Frequency under consideration

SARw/kg(1gm)

Antenna on Left side of head model

SAR w/kg(1gm)

Antenna on Right side of head model

867MHz 0.000127 w/kg 0.000127 w/kg

1899MHz 0.000957 w/kg 0.00104 w/kg

2.42GHz 0.00237 w/kg 0.00240 w/kg

Here, the minimum SAR value obtains at lowest resonant frequency 0.9 GHz and this could be attributed due to the largest wavelength having minimum ability of penetration in the human head. The SAR value can be reduced even more, if casing of the antenna is also considered and if it is placed even more away from the body.

V. HARDWARE IMPLEMENTATION AND RESULTS Selection of appropriate substrate material is critical in the fabrication process of an antenna, so as to get the required characteristics. The selection of dielectric constant of the substrate depends on the application of the antenna and the radiation characteristics specifications. Fabricated antenna and is shown in fig.5.

Fig.9 Fabricated Antenna

High Dielectric constant substrates causes surface wave excitation and low bandwidth performance. Also increasing the thickness of the substrate increases the band width of the antennas at the expense of efficiency owing to increase in surface waves. The antenna studied in this paper is fabricated using Fr4 substrate. FR4 with εr=4.4 tan, δ=.02, h=1.6 mm is used for the study.To raduce losses the top radiating patch is made of metallic plate of 0.2mm thickness.

A. Test Results

The measured return loss of the proposed antenna is shown in fig.7. The center frequencies obtained are 872MHz , 1922MHz and 2.42 GHz .

Fig.10 Measured return loss of proposed PIFA

Measured 2D radiation patterns of the antenna in XY and YZ plane at the resonance frequency is shown in Fig 11 and Fig 12 respectively. The measured pattern is suitable for mobile handset with good radiation in three space quadrants with reduced radiation in one quadrant. .[14] This property can be conveniently employed to reduce the EM interaction towards the head of a mobile phone user

Fig. 11: Measured radiation patern in xy plane

Fig. 12: Measured radiation patern in yz plane

The designed antenna, built on PIFA structure, is very sensitive to any changes to the dimensions of the structure including the ground plane.

VI. CONCLUSION

The design of a compact multi-band PIFA having simple structure with reduced SAR which can work in the GSM 900 , GSM 1900 bluetooth and WI-Fi bands have been presented and proposed. The SAR results for the respective resonant frequencies are also computed placing the antenna in both left and right side of the phantom head model. Simulation results have shown good performance characteristics in terms of return loss, gain, VSWR. There is a good agreement between measured and simulated results .The design details of the antenna can be used as base for increasing the number of bands covering several communication standards.

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VII. ACKNOWLEDGEMENT Authors are thankful to KSCSTE for financial assistance

through emeritus scientist scheme. They are also thankful to Director ISP and Head of the Department ISP CUSAT for providing facilities to carry out the project . Authors also acknowledge interest in the project by Swadesi science movement Kerala.

REFERENCES [1] Neha Kumar1 and Prof. Girish Kumar,” Biological Effects of Cell Tower Radiation on Human Body”, Electrical Engineering Department, IIT Bombay Powai, Mumbai. ISMOT /09/C/318. [2] International Commission on Non-Ionizing Radiation Protection (ICNIRP),1998, “Guidelines for Limiting Exposure to Time-Varying Electric, Magneticand Electromagnetic Fields (up to 300 GHz),” Health Phys., 74, pp. 494–522. [3] IEEE Standard for Safety Levels with Respect to Human Exposure toRadio Frequency Electromagnetic Fields, 3 kHz to 300 GHz, IEEE Std.C.95.1-2005, pp. 1-205, Apr. 2006 [4] Kin-Lu Wong, “Compact and broad-band micro strip antennas”, Published by John Wiley & Sons, Inc., Chapter: 2, Page(s): 46-78, 2003. [5] Ray J.A, Chaudhuri S.R.B., “A review of PIFA technology”, IEEE Indian Antenna week (IAW), Page(s): 1 – 4, Dec. 2011. [6] Belhadef, Y.; Boukli Hacene, N., “PIFAS antennas design for mobile communications”, 7th IEEE International Workshop on Systems, Signal Processing and their Applications (WOSSPA), Page(s): 119 – 122, May 2011.

[7] A. Skikiewicz, “Systematization of the Terminals of Mobile Communication Systems taking into Account their Functionality and Radiation Hazard”, MSc. Thesis, Wroclaw University of Technology, Wroclaw, Poland, 2007 [8] C. Picher, J. Angueral, A. Andújar, C. Puente1, and S. Kahng, “Analysis of the Human Head Interaction in Handset Antennas with Slotted Ground Planes”, IEEE Antennas and Propagation Magazine, Vol. 54, No. 2, Page(s): 36 – 56, April 2012. [9] W. Geyi, Q. Rao, S. Ali, and D. Wang, “Handset Antenna Design: Practice And Theory”, Progress In Electromagnetic Research Journal (PIER) , Vol. 80, Page(s) : 123–160, 2008. [10] Tefiku, F. , “A Mobile Phone PCS PIFA with Low SAR”, IEEE Antennas and Propagation Society International Symposium, Page(s): 4685 – 4688, 2007. [11] Krzysztofik, W.J.; Skikiewicz, A., “Tapered PIFA Antenna for Handsets Terminals”, 17th IEEE International Conference on Microwaves, Radar and Wireless Communications (MIKON), Page(s): 1 – 4, May 2008. [12] Tefiku, F. , “A Mobile Phone PCS PIFA with Low SAR”, IEEE Antennas and Propagation Society International Symposium, Page(s): 4685 – 4688, 2007. [13] D. Laila, R. Sujith, C. M. Nijas, C. K. Aanandan, K. Vasudevan, P. Mohanan “Modified CPW Fed Monopole Antenna with Suitable Radiation Pattern for Mobile Handset,” Microwave Review, Sept, 2011. [14] Lin J C “Cellular Mobile Telephones and Children,” IEEE Antennas and Propagation Magazine, vol. 44 (5), pp.142-145, 2002.