SAR Reduction in Slotted PIFA for Mobile Handsets using RF Shield

I J W C N, 4(1) June 2012 41

Sar Reduction in Slotted Pifa for Mobile Handsets Using RF Shield

T. Anita Jones Mary1 and C. S. Ravichandran2

1Department of Electronics and Communication, Karunya University, Coimbatore 641114, E-mail: anitajones@karunya.edu2SSK College of Engineering and Technology, Coimbatore, Tamil Nadu, India, E-mail: eniyanravi@gmail.com

Abstract: This paper proposes a multiband handset antennacombining PIFA and novel slots on radiating patch of PIFAfor mobile applications. Two slots were introduced in orderto embody 900 and 1800 MHz band. Radiation patterns arecomputed in free space as well as in the presence of head.Specific Absorption Rate (SAR) of slotted PIFA antenna iscalculated and is compared with the SAR limits in the safetystandards. SAR is investigated, by discussing the effect of RFshield which is introduced between slotted PIFA antenna andhead. Simulation results show that with the introduction ofRF shield of thickness of 1mm, SAR reduces by 12%. .Allsimulations are done in FEKO software.

Keywords: SAR, Mobile phone, RF shield, Relative Permittivityand Permeability.

I. INTRODUCTION

Mobile phones have been used extensively in ourdaily basis. Human interaction with mobile hasbecome pioneering area in research for welfare ofhuman health preventing from hazardousabsorption of radiations.

The purpose of this paper is for the investigationof interactions between Slotted PIFA handsetantennas and human head where SAR is definedas figure of merit to evaluate the power absorbedby biological tissue. For mobile phone compliance,the SAR value must not exceed the exposure limits[2]. For example, the SAR limits specified¯ in IEEEC95.1: 1999 is 1.6 W/kg in a 1 G averaging mass.

Traditionally used external antennas such asmonopoles and helical antennas increase the sizeof the handset. They are easily broken or there is apossibility of bending. These antennas also havesignificant effect on the human body because theyare not easy to shield. PIFA antenna is a recentantenna used in mobile technology. So for all

experiments are done based on monopole anddipole antenna which has so many drawbacks suchas lack of shielding, backward radiation andincreasing the size of the mobile antenna. So in thispaper Slotted PIFA antenna for supporting dualband 900 MHz and 1800 MHz is designed and theinteraction of the antenna with the designedhomogeneous head model with tissue propertiesare determined by using the metric called SARwhich is discussed below.

2. SPECIFIC ABSORPTION RATE (SAR)

Specific absorption rate (SAR) is a measure of therate at which energy is absorbed by the body whenexposed to a radio frequency (RF) electromagneticfield. It is defined as the power absorbed per massof tissue and has units of watts per kilogram (W/kg).The SAR (W/kg) at any point in the model canbe determined from the calculated electric field E(V/m) and it is given by the following equation

��

�

2

( / )E

SAR W Kg

Where E is the internal electric field (V/m), SARis the Specific Absorption Rate (W/kg), � is theconductivity (S/m) of the tissue and � is the massdensity (kg/m3). [6]

3. PLANAR INVERTED F ANTENNA (PIFA)

The PIFA is resonant at a quarter-wavelength dueto the shorting pin at the end as shown in figure1.The feed is placed between the open and shortedend, and the position controls the input impedance.

T. Anita Jones Mary and C. S. Ravichandran

42 I J W C N, 4(1) June 2012

The resonant length of PIFA is given by

L1+L2-W = �/4

Where L1 is the length of PIFA

L2 is the width of PIFA

W is the width of shorting pin

h is the height of shorting pin

D is the distance of the feed from theshorting pin

4. SLOTTED PIFA

In slotted PIFA antenna proposed in this work, themodified dual-band PIFA having 900 MHz and 1800MHz was designed. Two resonances were acquiredby the use of L and L shaped slots in theradiating patch of slotted PIFA antenna as shownin the figure 2.

5. METHOD OF MOMENTS(MOM)

The MoM or boundary element method (BEM) isone of the first full-wave numerical methods usedfor electromagnetic compatibility (EMC) andantenna applications. During the last few years,where computer performance has increaseddramatically, the importance of MoM has decreasedwhile the use of other techniques like FDTD or FEMhas increased. Nevertheless, the MoM still providesadvantages for certain structures, especially whenmetal objects and wires are of interest. The MoM isapplicable to problems for which Green’s functionscan be calculated. These usually involve fields inlinear homogeneous media. This placesconsiderable restrictions on the range andgenerality of problems to which boundaryelements can usefully be applied. Nonlinearitiescan be included in the formulation, although theywill generally introduce volume integrals whichthen require the volume to be discredited before asolution can be attempted, thus removing one ofthe most often cited advantages ofbioelectromagnetics

6. NUMERICAL MODELLING

(A) Modelling Background

The specific absorption rate experiments with realhuman bodies were strictly limited. They lead topoor approximation of real human bodies when thereal time experiments were performed with theanimal bodies. So it is decided to investigate thepossible impacts with numerical field calculationsusing several numerical models of the human body[1].

(B) Planar Inverted F Antenna Geometry andDimensions

The Planar Inverted F antenna is increasingly usedin mobile phone market.The antenna is resonant atQuarter wavelength thus reduces the size of mobilephones.PIFA is popular because of its low profileand omnidirectional pattern and less backwardradiation. The design parameters are as shown intable 1.

Figure 1: PIFA ANTENNA

Figure 2: Slotted PIFA Antenna

SAR Reduction in Slotted PIFA for Mobile Handsets using RF Shield

I J W C N, 4(1) June 2012 43

Table 1Design Parameters of Pifa

PIFA Parameters Values (900 MHZ) in meters

L1 .09078L2 .02000H .03D .03378W .0045

(C) Slotted PIFA Antenna

The design parameters for the dual band slottedPIFA antenna (figure 2) is shown in table 2.

Table 2Slot Specification of Slotted PIFA

Design Parameters Values (in mm)

L1 21.51H1 5.5W1 21.5L2 15H2 18.5W2 5L3 1.5W3 2

(D)Human Head Model and Dielectric Propertiesof Human Head Tissues

The human head is simulated with brain equivalentmaterial (muscle and skin) with a relativepermittivity, conductivity and mass density for 900and 1800 MHz as in table 3.

Table 3Properties of Head Tissue Used in the Simulation

at 900 Mhz [5]

Properties of Tissue 900 MHz

Dielectric permittivity (�,) 50.5Conductivity(�) in S/m 1.5Mass density(�)in kg/ m3 1020

(E) Metal Sheet

The purpose of this paper is to reduce SpecificAbsorption Rate (SAR) by introducing RF shield ofthickness 1mm between mobile loaded with slottedPIFA antenna and human head. RF shield attachedhas electrical conductivity .02670 and relativepermeability of 2.07 [3].The distance between RFshield and head is 10mm.

7. FEKO SIMULATIONS ANDVALIDATION

(A) Free Space Condition

Slotted PIFA is simulated under free spacecondition with the design parameters as specifiedin table 2 (9) is shown in figure 3.

Figure 3: Slotted PIFA Antenna

Figure 4: Simulated S11 of the Complete Model IncludingHandset and Head and RF Shield

The antenna was designed in such a way thatthe return loss in two bands (900 and 1800Mhz) isless than -10 dB (s11< -10 dB)

(B) Slotted Pifa with Human Head Small

Slotted PIFA [9] with human head model [7] asshown in tables 2 and shown in figure 5.

T. Anita Jones Mary and C. S. Ravichandran

44 I J W C N, 4(1) June 2012

(C) In the Presence of RF Shield

For reducing SAR in human using mobile phones,RF shield of thickness 1mm is introduced betweenhuman head and mobile handset with slotted PIFAantenna. From the far field simulated graph ofslotted PIFA antenna shown in fig 6, it can be seenthat backward radiation is less for the desiredfrequency (800MHz) which is one of the importantcharacteristics of PIFA antenna.

Method of Moments (MoM) based platform. [8].Specific Absorption Rate (SAR) for 1G and 10 G byintroducing RF shield slotted PIFA antenna andhuman head is calculated for frequency range from0.7GHz to 2.16GHZ and the simulated results aretabulated below.

SAR comparison table shows that l G and 10GSAR for dual band slotted PIFA with theintroduction of RF Shield of thickness 1 mm isreduced considerably in the frequency range of0.7GHz and 2.16GHz compared to dual band slottedPIFA without RF shield.

Table 41G SAR Distribution (with and without

RF Shield)

FREQUENCY(GHz) 1 G SAR 1 G SAR(RF Shield)

0.7 0.0006358 0.000635

0.862222 0.01509 0.013506

1.02444 0.003855 0.0037885

1.18667 0.0020585 0.0019813

1.34889 0.002174 0.0021088

1.511111 0.00241 0.0026698

1.67333 0.0077578 0.011856

1.83556 0.01658 0.014547

1.99778 0.0025695 0.0025242

2.16 0.0015004 0.001455

Figure 5: Slotted PIFA with Human Head

Figure 6: Far Field

8. RESULT ANALYSIS AND VALIDATION

The FEKO simulation model consists ofhomogeneous human head and Dual band slottedplanar inverted-F antenna mounted on a mobilehandset. By considering the problem size andcomplexity, the FEKO program is simulated using

Figure 7: Graph for 1G SAR Distribution (with and withoutRF Shield)

SAR Reduction in Slotted PIFA for Mobile Handsets using RF Shield

I J W C N, 4(1) June 2012 45

Table 510G SAR Distribution (with and without RF Shield)

FREQUENCY(GHz) 10 G SAR 10 G SAR(FERRITE)

0.7 0.000362 0.0003260.862222 0.013133 0.011561.02444 0.001653 0.0015941.18667 0.000744 0.000731.34889 0.000589 0.0005841.511111 0.000897 0.0007981.67333 0.003586 0.0031451.83556 0.004428 0.0040831.99778 0.00089 0.0008842.16 0.000468 0.00047

amount of reduction in the SAR at centerfrequencies is 10.497% & 12.261% for 1G. The samefor 10 G is calculated as 7.79% and 11.977%respectively.

References[1] O. S. Dautov, Russiaad, Elz Etne. M., Egypt “Application

of FEKO Program to the analysis of SAR on Human Headmodeling at 900 and 1800 MHz from a Handset Antenna”IEEE August 2005.

[2] IEEE Standard-1528, “IEEE Recommended Practice forDetermining the Peak Spatial-Average SpecificAbsorption Rate (SAR) in the Human Head from WirelessCommunications Devices: Measurement Techniques,”December 2003.

[3] M.T Islam, M.R.I Faruque, N. Misram, “Design Analysisof Ferrite Sheet Attachment for SAR Reduction in HumanHead” Progress in Electro Magnetic Research, PIER 98,191-195, 2009.

[4] “Product Standard to Demonstrate the Compliance ofMobile Phones with the Basic Restrictions Related toHuman Exposure to Electromagnetic Fields (300 MHz–3GHz),” European Committee for ElectricalStandardization (CENELEC), EN 50360, Brussels, 2001.

[5] “Basic Standard for the Measurement of SpecificAbsorption Rate Related to Exposure to ElectromagneticFields from Mobile Phones (300 MHz–3GHz),” EuropeanCommittee for Electrical Standardization (CENELEC),EN-50361, 2001.

[6] “Specific Absorption Rate (SAR) Estimation for CellularPhone,” Association of Radio Industries and businesses,ARIBSTD-T56, 2002.M. Young, The Technical Writer’sHandbook. Mill Valley, CA: University Science, 1989.

[7] M. A. Jensen and Y. Rahmat-Samii, “EM Interaction ofHandset Antennas and a Human in PersonalCommunications,” Proc. IEEE, 83, 7-17, 1995.

[8] http://www.feko.info/contact.htm

[9] Corbett R Rowell,R D Murch”A compact suitable PIFAsuitable for Dual frequency 900/1800 MHz Operation”IEEE Transactions on Antennas and Probagation as aLetter 1997.

Figure 8: Graph for 10G SAR Distribution (with and withoutRF Shield)

CONCLUSION

Effect of the reduction in peak SAR for handset withdual band slotted PIFA antenna using RF shield isinvestigated experimentally. Simulation resultsshows that with the introduction of RF shield withthickness of 1mm reduces the SAR value in thehuman head for both 1 G and 10 G cases. The