SAR reduction in the modelled human head for the mobile phone using different material shields
© Dutta et al. 2016
Received: 13 June 2015
Accepted: 6 January 2016
Published: 11 April 2016
Every mobile phone emits radio frequency electromagnetic energy. The amount of this energy absorbed by the human head is measured by the specific absorption rate (SAR). There are standard limits, according to which phones sold should be below certain SAR level. To maintain these limits, shields can be used for the mobile phones. In this paper using ANSYS HFSS, modelling of the human head is done. The modern mobile phone design is used for simulation. SAR distribution is measured for different layers of the head due to exposure to radiation from the mobile phone operating at GSM-1800 frequency band. The performance of the mobile phone antenna due to different shields is observed. Proper shielding material properties are found.
Over the years, a lot of attention have been paid to the analysis of SAR in the human head due to the complexity and large scale involved in this kind of problems. Recently, research efforts have been devoted to the reduction of peak SAR in the human head for handset applications. Mobile phone antenna performance gets affected due to the human head has been investigated in many published papers [10–14]. The impact of mobile phones on the human head can be measured using SAR [13–18]. The radiation from the mobile phones can be reduced using a shield that results in the decrease of SAR. However, this shield affects the performance of the mobile phone antenna. Proper shield material is to be selected such that it reduces the radiation while maintaining a good quality of signal transfer and reception from the mobile phone.
Global system for mobile communications (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe protocols for second-generation (2G) digital cellular networks used by mobile phones. In Africa, Europe, Middle East and Asia, most of the providers use 900 and 1800 MHz bands. The GSM-900 system uses the frequency band (890–915) MHz for uplink and the band (935–960) MHz for downlink. While, the GSM-1800 system uses the frequency band (1710–1785) MHz for uplink and the band (1805–1880) MHz for downlink. In GSM systems, up to eight users share the same frequency channel, and each phone transmits only one-eighth of the time as the time division multiple access (TDMA) is used, so the average power is one-eighth of the peak power. GSM-900 phones have a peak power of 2 W, and as the maximum average power is one-eighth of the peak power. Hence, the GSM-900 mobile phones radiate an average power of 250 mW. GSM-1800 phones have a peak power of 1 W; hence the GSM-1800 mobile phones radiate an average power of 125 mW . In this paper, the mobile phone is designed to operate in GSM-1800 frequency band.
Some research was done in this area, where the old models of mobile phones with dipole and helix antennas were used [20–22]. The mobile phone models discussed in those papers are not used in this modern time. In a paper , aluminium was used as the shield, but using conductive materials as the shield will degrade the mobile phone antenna performance.
In this paper, modern type of mobile phone with the shield is tested. With the help of HFSS, 3D head model is designed. SAR in the human head is measured. In a modern mobile phone, Planar Inverted-F Antenna (PIFA) is used. The advantage of PIFA is that they are compact in size and have small back lobes, which make them ideal for mobile phone antennas. A mobile phone equipped with PIFA is analyzed.
Properties of the layers of the head at 1.8 GHz frequency
Mass density (kg/m3)
Properties of materials used
Mass density (kg/m3)
Properties of unknown materials at 1.8 GHz frequency
Mass density (kg/m3)
Results and discussion
Shields made up of conductors
Aluminium and copper shields are tested with and without slit. The thickness of the shields is taken as 0.1 cm.
Shields made up of insulators
Teflon and glass shields are tested with and without slit. The thickness of the shields is taken as 0.1 cm.
Shields made up of unknown materials
Five unknown materials defined in “Simulation model” section are tested with and without slit. The thicknesses of the shields are taken as 0.1 and 0.2 cm.
From Fig. 16, it is observed that for the shield made up of materials 1, 4, and 5 there is no change in the radiation pattern. For the shield made up of material 2 large loss in radiated power is observed. For the shield made up of material 3 change in radiation pattern is noticed. Same results are noticed with 0.2 cm thick shields made up of materials 1, 2, 3, 4, and 5, only the radiated power gets reduced in this case when compared with 0.1 cm thick shields.
Maximum local SAR for skin and brain without shield
Local SAR in W/kg
Maximum local SAR for skin and brain with shields having slit by varying shield material and shield thickness
Local SAR in W/kg
0.1 cm thick shield
0.2 cm thick shield
From Tables 4 and 5, it is observed that a good amount of SAR reduction is achieved using shields made up of materials 1, 2, 3, 4, and 5. Shields made up of materials 1, 4 and 5 will be better mobile phone radiation shield as they have a negligible effect on mobile phone antenna performance. It is noticed that shield made up of material 1 absorbs more radiation compared to materials 4 and 5 due to less mass density; but due to the high mass density of materials 4 and 5, less SAR is observed in skin and brain because of less radiated power through them.
Shields made up of leather and germanium
After all the above analysis, it is observed that shields made up of conductors and insulators cannot be used for mobile phone shielding. Shields made up of materials 1, 4 and 5 can be used for mobile phone shielding. Few more tests are performed to find the exact properties of the materials that will give the best result in terms of reducing the radiation and maintaining good antenna performance of the mobile phone. There are three important properties of a shield material i.e. conductivity (σ), relative permittivity and mass density (ρ). From Eq. 1, it can be noticed that relative permittivity does not have any effect on SAR.
Coming to mass density (ρ), on earth the metal with highest mass density is osmium, having a mass density of 22,570 kg/m3. Materials 1, 4 and 5 in Table 3, have a mass density of 1000, 10,000 and 20,000 kg/m3, respectively. From Table 5, it can be observed that by increasing the ρ from 1000 to 10,000 to 20,000 kg/m3, very small change in SAR is observed in skin and brain. Hence, if there are two materials with same conductivity, same relative permittivity, but different mass density then that material will be a better shield which has the lowest mass density.
Coming to conductivity (σ), which is the main parameter in deciding whether a material can be used as a shield for the mobile phone or not and which material will be a better shield. Comparing materials 1 and 2 having σ = 1 and σ = 10 S/m, respectively, it is noticed that material 1 gives good radiation absorption with good antenna performance whereas material 2 gives better radiation absorption but a poor antenna performance. So few more tests are performed by taking conductivity 0.1 S/m and incrementing it by 1 up to 9 S/m, keeping relative permittivity = 1, mass density = 1000 kg/m3 and thickness of the shield as 0.1 cm. After these tests, it is found that materials having conductivity between 0.1 and 3 S/m are suitable for mobile phone shield, as they have a negligible effect on mobile phone antenna performance. Compared to the mobile phone antenna without a shield, with a shield having σ = 0.1 S/m 0.24 % loss in radiated power is observed; with a shield having σ = 1 S/m 1.11 % loss in radiated power is observed; with a shield having σ = 2 S/m 1.74 % loss in radiated power is observed, with a shield having σ = 3 S/m 2.15 % loss in radiated power is observed. If conductivity is taken less than or equal to 0 S/m, it behaves like an insulator and do not absorb any radiation. If conductivity is increased above 3 S/m, increase in loss of radiated power and degradation of antenna performance is observed. If shield having a conductivity of 0.1 S/m is used, better antenna performance and less radiation absorption is achieved compared to shield having a conductivity of 3 S/m. So a trade-off is to be made between these two properties as per the requirement in the mobile phone.
Now, the properties of the materials are known; that will be a good shield for the mobile phone with good radiation absorption and negligible effect on the mobile phone antenna performance. Based on these properties, materials are searched that exist in the real world having conductivity between 0.1 and 3 S/m. Two materials those have these properties are skin (leather) having σ = 1.1847 S/m and germanium having σ = 2.17 S/m. The simulations are performed with the shields having slit and made up of skin (leather) and germanium having thicknesses of 0.1 and 0.2 cm. Results are shown below.
Maximum local SAR for skin and brain due to shields having slit, made up of skin (leather) and germanium having thicknesses of 0.1 and 0.2 cm
Local SAR in W/kg
0.1 cm thick shield
0.2 cm thick shield
Shield with slit having a thickness of 0.1 cm and made up of skin (leather) provides 20.49 % SAR reduction in the skin and 13.64 % SAR reduction in the brain with a change in return loss from −22.9 to −19.4 dB. Shield with slit having a thickness of 0.2 cm and made up of skin (leather) provides 36.07 % SAR reduction in the skin and 31.82 % SAR reduction in the brain with a change in return loss from −22.9 to −15.7 dB.
Shield with slit having a thickness of 0.1 cm and made up of germanium provides 21.14 % SAR reduction in the skin and 15.45 % SAR reduction in the brain with a change in return loss from −22.9 to −17.2 dB. Shield with slit having a thickness of 0.2 cm and made up of germanium provides 36.55 % SAR reduction in the skin and 33.63 % SAR reduction in the brain with a change in return loss from −22.9 to −16.4 dB.
Author PKD drafted this manuscript, performed simulation using the data sets and analysed the results. Authors Dr. PVYJ and Dr. VSSNSB suggested the methods used in this study and provided technical support throughout the project. All authors read and approved the final manuscript.
Prabir Kumar Dutta was born in West Bengal, India in 1992. He received the B.Tech degree in ECE from SIEM, Siliguri, West Bengal, India in 2013. He received the M.Tech degree in Radio-frequency and Microwave engineering from GITAM University, Visakhapatnam, Andhra Pradesh, India in 2015. P. V. Y. Jayasree was born in Andhra Pradesh, India in 1968. She received the B.E. degree in ECE from GITAM University, Visakhapatnam, Andhra Pradesh, India in 1989. She received the M.E. degree in Electronic Instrumentation from Andhra University, Visakhapatnam, Andhra Pradesh, India in 1999. She received the Ph.D. in EM shielding techniques from JNTU University, Kakinada, Andhra Pradesh, India in 2010. She is now Associate Professor of ECE department in GITAM University, Visakhapatnam, Andhra Pradesh, India. V. S. S. N. Srinivasa Baba was born in Andhra Pradesh, India in 1967. He received the B.E. degree in ECE from Andhra University, Visakhapatnam, Andhra Pradesh, India in 1988. He received the M.S. degree in ECE from JNTU University, Kakinada, Andhra Pradesh, India in 1993. He received the Ph.D. in Radio-frequency and Microwave Communication engineering from Andhra University, Visakhapatnam, Andhra Pradesh, India in 1999. He was a professor at GITAM University (for 16 years 11 months) and Andhra University (for 3 years 3 months). He is now working at Prithvi Information Solutions as General Manager, RF from 2008.
We would like to acknowledge GITAM University for providing us with full access to IEEE journals.
The authors declare that they have no competing interests.
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