Evaluation of SAR Reduction for Dipole Antenna Using RF Shield

Lakshmappa K. Ragha Electronics and Telecommunication Engineering,

SIES Graduate School of Technology, Nerul, Navi Mumbai 400706, India.

e-mail: laxman.ragha@rediffmail.com

Manmohan Singh Bhatia Laser and Plasma Technology Division,

Bhabha Atomic Research Centre Mumbai 400085, India.

e-mail: monibhatia@rediffmail.com

Abstract—In this paper we propose to use radio frequency

(RF) shield on dipole antenna to reduce Specific Absorption Rate (SAR) in the spherical head model. RF Shields made of Ferrimagnetic material are used to suppress surface current on antenna. Many kinds of simulation are performed to investigate the effect of various parameters like thickness, size, location and type of the shield on the SAR using CST-Microwave studio, the field simulation software. Paper includes numerical evaluation of the SAR reduction and also analyzes the SAR data for shielding effectiveness. Drastic reduction in SAR was observed in case of Ferrite3 material with increase in shielding size and thickness. However there was no appreciable change in SAR for change in shielding location. Simulation results will be useful for compliance testing of wireless communication devices.

Keywords-SAR; SAR reduction factor (SRF); Dipole

antenna; Ferrimagnetic material and RF shield.

I. INTRODUCTION The widespread use of wireless communication devices

with close proximity to the human body remains a topic of growing public concern. Hence there is a need to develop techniques to reduce radiations from wireless devices for compliance testing [1]. The SAR is normally used to evaluate the degree of the hazard to the humans. Since live human heads can not be safely experimented for SAR measurements, computational RF dosimetry provides the best estimate of SAR in actual human heads [2]. The SAR in the human head is defined by “(1)” [3].

2

/E

SAR w kgσ

ρ= (1)

Where σ the electrical conductivity of tissue, E is the rms magnitude of the electric field strength vector at a given point in the head model, and ρ is the mass density of tissue. As per the FCC, ANSI/IEEE and Safety code 6 standards the recommended SAR values are 0.08 W/Kg for a Whole body and 1.6W/Kg for over any 1gram(g) of tissue.

In this paper we propose to use RF shield on dipole antenna to reduce SAR in the spherical head model.

While various absorbing materials can be used for making RF shields, ferrite absorbers are preferred because they are light in weight and do not cause degradation of the radiation pattern in the maximum direction of the radiation in

both the planes. The effectiveness of the RF shielding can be specified in terms of an SAR reduction factor (SRF) and is defined as follows [3][4]:

(%) 100abs sTotal

abs

P PSRFP

−= × (2)

1 1 ,1

1

(%) 100g g sg

g

SAR SARSRF

SAR−

= × (3)

10 10 ,10

10

(%) 100g g sg

g

SAR SARSRF

SAR−

= × (4)

Where TotalSRF is the total SRF, absp is the power

absorbed in the head model , sp is the power dissipated in

the RF shield, 1gSRF is SRF for lg peak SAR, 10gSRF , is

SRF for l0g peak SAR, 1gSAR is 1g peak SAR(without RF

shield), 1 ,g sSAR is 1g peak SAR (with RF shield), 10gSAR

is l0g peak SAR(without RF shield), and 10 ,g sSAR is l0g peak SAR(with RF shield), respectively. Larger SRF value implies greater shielding effectiveness.

II. MATERIALS In our study, two tissue human head model is used. The radius chosen for bone tissue is 10cm and that of brain tissue is 9cm. A λ/2 dipole antenna at 900MHz is used as the radiating source. Radius of cylindrical dipole shown in figure 1 was set to 0.045cm. Material used in modeling the antenna was a perfect electric conductor (PEC) and it is excited by Gaussian pulse using discrete port.

Figure 1. Model of human head and Dipole antenna with RF shield.

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TABLE I. SUMMARY OF DIELECTRIC PARAMETERS

TABLE II. SAR AND SRF FOR DIFFERENT TYPES OF RF-SHIELDS

A. RF Shielding Material Ferrimagnetic materials are used as RF absorbers. The

most important group of ferrimagnetic materials are the ferrites. In ferrites the conductivity is low which results in much smaller induced currents in the material when electromagnetic waves are applied [5].

The dielectric parameter values ( rε , σ , rμ , ρ ) for modeling and simulation of dipole antenna and spherical head model at 900 MHz are as listed in Table I. rε and

rμ values at 900 MHz for various types of shielding materials are shown in Table II.

III. METHOD The commercial field simulation tool, CST-Microwave

studio [6]... [9] was used to model RF shield, the dipole antenna and the spherical head for evaluating the SAR reduction. The basic parameters used for the simulation are as mentioned below:

The Perfect boundary approximation is used for spatial discretization. The mesh is produced by an automatic mesh generator, which ensures a good compromise between accuracy and simulation time. An ‘‘add space’’ boundary condition representing the electromagnetic wave propagating towards the outer space is set. Mesh type : Hexahedral; Minimum mesh step : 4.9; Solver Type : Transient solver; Excited discrete port : 1; Reference impedance : 50.0 ohms; Steady state accuracy limit : -30dB;

Number of processors used : 01; The antenna output power : 0.5 watt (rms).

The antenna output power is defined as;

out abs s radp p p p= + + where absp the power absorbed in

the head model, sp is the power dissipated in the RF shield

and radp is the power radiated to the far-field.

A. SAR reduction mechanism Before deriving the SAR reducing effects using RF

shield, the mechanism of SAR reduction is discussed [10]. The mechanism behind reduction of SAR is due to the suppression of surface currents on the antenna. When an electromagnetic wave (EMW) traveling through free space encounters a different medium, the wave will be reflected, transmitted and/or absorbed. EMW absorption materials absorb the energy in electromagnetic waves as magnetic loss, and convert that energy, in the end, to heat. Current density on the dipole antenna is suppressed by putting RF shield on to it which in turn reduces the corresponding surface SAR in human head.

IV. NUMERICAL RESULTS AND ANALYSIS The results discussed in this section are the effect of

various parameters like location, size, thickness and type of RF shield on the SAR, SRF, SRFtotal and Average Power. Analysis is based on the results from Table II and Figure 2 to Figure 8.

Table II contains Maximum SAR, SRF, Total SAR, and Average Power values simulated using various RF shielding material. Ferrite3 material gives highest SRF (i.e.

Material Relative permittivity

rε

Relative permeability

rμ

Conductivity σ

(s/m)

Density ρ

(g/ cm^3)

Thermal Conductivity

(w/mm/k)

Air 1 1 0 1 0 Simulating Brain at 900

MHz

55 1

1.23

1.03

0

Simulating Bone at 900 MHz

8

1

0.105

1.85

0

PEC Perfect electric conductor(PEC)

RF Shielding Material

Type

rε rμ Maximum SAR(w/kg) 1gm 10gm

(rms)

Total SAR(w/kg)

(rms)

Average Power (rms) [W/mm^3]

SRF (%) (1gm) (10gm)

(rms) Re Im Re Im

Ferrite1 (F-1)

7 0.6 2.8 3.3 1.87621 1.4559 0.02877 3.5517e-008 14.21 14.47

Ferrite2 (F-2)

20 10 9 1 1.15471 0.91392 0.05201 6.4215e-008 47.20 46.31

Ferrite3 (F-3)

18 15 15 17 0.39316 0.30221 0.06545 8.0805e-008 82.02 82.25

Ferrite4 (F-4)

38 28 12 3 0.46068 0.36113 0.11651 1.4384e-007 78.94 78.78

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82.02% reduction for 1g sample) followed by Ferrite4 and Ferrite2 (78.94% and 47.20% reduction for 1g sample respectively). Ferrite1 gives lowest SRF (i.e. 14.21% reduction). Higher SRF indicates greater SAR reduction effect.

Figure 2 shows comparison of SAR (W/Kg) values for without ferrite (WF) and with ferrites (that is ferrite1 (F-1) to ferrite4 (F-4)). Simulations ware carried out at 5mm, 10mm and 15mm gap between head model and dipole. SAR values decreased with increase in gap for all types .Lowest SAR was observed for F-3 type followed by F-4, F-2 and F-1 as compare to WF.F-3 type suppresses more surface currents than other types resulting in low energy absorption (that is low SAR).

Figure 3 shows comparison of Total SAR (W/Kg) values for WF and F-1 to F-4 type. Total SAR values decreased with increase in gap for all types. Lowest Total SAR was observed for F-1 type followed by F-2, F-3 and F-4 type.

Figure 4 shows comparison of average power (rms) for WF and F-1 to F-4 type. Average power decreased with increase in gap for all types. Lowest average power was observed for F-1 type followed by F-2, F-3 and F-4 type.

Figure 5 shows SRF (%) as a function of ferrite location for 1g and 10g samples. The antenna portion between ±1.11 to ±22.22mm is taken as location-1, the portion between ±22.22 to ±44.44mm is taken as location-2, and the portion between ±44.44 to ±66mm is taken as location-3.Location-2 gives highest SRF value for 10g sample compare to other locations.

Figure 6 shows SAR (W/Kg) as a function of Ferrite1 size for 1g and 10g samples. Size of cylindrical shield cover of 1mm thick is varied simply by varying its height.15mm gap was maintained between head model and dipole. Size-0: WF; size-1: ±1mm to ±11.11mm; size-2: ±1mm to ±22.22mm; size-3: ±1mm to ±33.33mm; size-4: ±1mm to ±44.44mm; size-5: ±1mm to ±55.55mm; size-6: ±1mm to ±66.66mm. Decrease in SAR is observed with increase in Ferrite1 size.

Figure 7 shows SRF (%) as a function of ferrite thickness. 15mm gap was maintained between head model and dipole. Thickness of Ferrite1 was changed from 1mm to 2.5 mm in steps of 0.5mm.Increase in SRF was observed with increase in shield thickness.

Figure 8 shows TotalSRF [%] as a function of ferrite thickness. 15mm gap was maintained between head model and dipole. Thickness of Ferrite1 was changed from 1mm to 2.5 mm in steps of 0.5mm.Absorbed power (Pabs) in the head model decreased with increase in shield thickness.

Figure 2. SAR (W/Kg) Vs Gap between head model and dipole

Figure 3. Total SAR (W/Kg) Vs Gap between head model and dipole

Figure 4. Average Power Vs gap between head model and dipole

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Figure 5. SRF (%) as a function of ferrite-1 location

Figure 6. SAR (W/Kg) as a function of Ferrite-1 Size

Figure7. SRF (%) as a function of ferrite-1 thickness

Figure 8. TotalSRF [%] as a function of ferrite thickness

V. CONCLUSIONS In the present study simulations are carried out using

CST-Microwave studio. Results indicate that Ferrite3 material gives highest SRF (i.e. 82.02% reduction for 1g sample) compare to other types. Reduction in SAR, total SAR and average power was observed with increase in gap between head model and antenna for all types of shielding. However reduction observed in SAR was maximum for Ferrite3 type. Comparatively SRF was maximum for location2 in case of 10g sample for Ferrite1. Decrease in SAR was observed with increase in Ferrite1 size. SRF increased as the Ferrite1 thickness changed from 1mm to 2.5mm in steps of 0.5mm. TotalSRF Decreased as the Ferrite1 thickness increased.

ACKNOWLEDGMENT The authors would like to thank Mr.Handu V.K, Head,

Vacuum Physics and Instrumentation Division, BARC, Mumbai and his colleagues for technical support.

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SAR distributions from two anatomically based models of the human head using CAD files of commercial telephones and the parallelized FDTD code”, IEEE Trans. Antennas Propagat., Vol.46,pp.829-833, June 1998.

[2] Lin JC: Cellular Mobile Telephones and Children. IEEE Antennas and Propagation Magazine.2002, 44(5):142-145.

[3] Jung Minseok & Lee Bomson, ‘Evaluation of SAR Reduction for Mobile Communication Handsets’, IEEE Antennas and Propagation Society International Symposium, Vol. 1, pp. 444-447, June 2002.

[4] J.Wang and 0. Fujiwara, “Reduction of electromagnetic absorption in the human head for portable telephones by a ferrite sheet attachment”, IEICE Trans. Commun., vol.E80-B, no.12, pp.1810-1815, Dec. 1997.

[5] Johann Pretorius, “Design and Manufacture of a Ferrimagnetic Wave Absorber for Cellular Phone Radiations”, IEEE Electron Devices for Microwave and Optoelectronic Applications, 12th International Symposium, pp. 119- 123, Nov. 2004.

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[6] CST user’s manual, HP Design and analysis; CST MICROWAVE STUDIO ® 4—Getting started.

[7] CST user’s manual, CST MICROWAVE STUDIO ® 5—Advanced Topics.

[8] CST user’s manual, CST MICROWAVE STUDIO ® Tutorials. [9] CST STUDIO SUITETm2006 User’s Manual;http://www.cst.com [10] J. Wang and O. Fujiwara, “Reduction of SAR in human head by

suppression of surface currents due to a portable telephone”, Proc. 13th Int. Zurich Symp.on Electromagn. Compat., Switzerland, Feb., 1999, pp. 59-62.

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