Measurement of Specific Absorption Rate of Monopole Patch Antenna on Human Arm

Transcription

1 190 Measurement of Specific Absorption Rate of Monopole Patch Antenna on Human Arm Sakshi Kumari* and Vibha Rani Gupta Department of Electronics and Communication Engineering Birla Institute of Technology, Mesra, Ranchi, Jharkhand , India. Abstract- This paper presents the measurement of electromagnetic energy absorption in the human biological tissue for designed monopole patch antenna in body area network (BAN). The two factors that affect the design of antenna for body area network are; first, the impact of human body s electromagnetic characteristics on the performance characteristics of an antenna and second, electromagnetic energies absorbed in the biological tissue. The performance parameters of antenna in terms of reflection coefficient and bandwidth are measured in free space and close to arm. A novel and simplified method is adopted to calculate specific absorption rate (SAR), from experimental data to check the compliance with the FCC safety guidelines. Index Terms- BAN, Monopole patch antenna, measurement of SAR. I. INTRODUCTION Advancement in communication and electronic technologies have enabled the development of compact and intelligent devices that can be placed on the human body or in close proximity to it, thus facilitating the introduction of Body Area networks (BANs). The development of antenna for the body area network presents several challenges. The presence of human body in close proximity to the antenna affects its performance characteristics such as radiation pattern, efficiency, resonant frequency, input impedance etc. Apart from these, amount of electromagnetic energy absorbed by the human body is also one of the major concerns [1]. The EM energy of different power levels and different frequencies penetrate into the human body, which cause potential health risks [2-3]. The rate at which EM energy is absorbed by the tissues of the human body is quantified by the specific absorption rate or the SAR value [4]. SAR is usually averaged either over the whole body, or over a small sample volume typically 1 g or 10 gram of tissue. If the value of SAR is computed in the cell of 1 gram of tissue then it is specified as local SAR and average SAR if it is computed in a cell of 10 gram of tissue.sar can be related to the electric field at any point by SAR = σ[e] ρ Watt/kg (1) Where E is the electric field in V/m, is conductivity of tissue in S /m and is the density of tissue in g/cm 3. Thus it depends on the electrical properties of the tissue mass and the strength of the EM waves inside the body. The interaction between human head model and mobile handset has been explored in the past [5-9] for dipole, monopole, patch and planar inverted F antennas. SAR measurement based on the different methods like numerical simulation [8,10-11], experimental evaluation based on electric field[12], thermographic[13], and thermal sensor using optical fiber [14] have been reported in recent years. In this paper a new method based on power measurement has been presented for the calculation of SAR. In case of body area networks, devices can be placed anywhere on human body but arm and forearm are the most suitable and convenient location on the body to place any device. So the performance of antenna with respect to these

2 191 particular parts of the body needs to be studied. In the present paper the interaction of antenna with arm has been considered. Patch antennas being inexpensive, having greater flexibility in design and shape, lighter in weight and low profile [15-16] have been considered for the design and are then its specific absorption rate is measured. First a monopole patch antenna is designed and then its performance characteristics are measured in free space and in close proximity to arm. II. ANTENNA DESIGN The impedance bandwidth of a simple microstrip patch antenna is only 2%-3% [16]. When placed on the human body the performance characteristics get modified and would result in the change of resonant frequency and bandwidth. Thus an antenna designed for ISM, HIPERLAN and WLAN bands in free space may resonate at some other frequency in the body proximity. Hence to simplify the study process, monopole patch configuration[17] is preferred for the design, which offers wider bandwidth. A simple rectangular monopole antenna is designed on FR4 substrate having relative permittivity 4.4, loss tangent and the thickness of 1.6 mm. The radiating patch of dimension (L patch x W patch ) is printed on the substrate whose size is L sub x W sub. A simple rectangular conducting ground plane of width W g and length L g is placed on the other side of the substrate. The radiating patch is fed by 50 microstrip line of width W f and length L f. The gap (g) between ground plane and patch has been optimized by iterative simulations on High Frequency Structure Simulator (HFSS). The geometry (with detailed design dimensions) and photograph of the fabricated antenna is shown in Fig.1 and Fig. 2 respectively. Fig.1. The Detailed geometry of Monopole Patch Antenna Fig.2. Fabricated antenna III. MODELLING OF HUMAN ARM For BAN our aim is to place the antenna on the arm. The interaction of antenna with these body parts will depend on its electrical property. Hence it becomes necessary to model these body parts in terms of dielectric and conductive property. The Human arm consists of four layers skin, fat,

3 192 muscles and bone. The arm consists of only one dense bone called humorous [18]. The human arm is modelled in HFSS [19] considering the electromagnetic properties of biological tissues [20], which is depicted in Fig.3. Finally, the SAR is calculated using the formula: SAR = (3) Where V R is the volume of human body tissue illuminated by electromagnetic energy, also known is SAR averaging volume. V = patch area (4) is the average density of skin, fat, muscle and bone. is the depth of penetration of electromagnetic field in the human body tissue, also known as skin depth. = (5) Fig.3. Modelling of human arm IV. MEASUREMENT The block diagram of the measurement setup is shown in the Fig.4. The antenna under test is connected as a receiver and the transmitting antenna is connected to VNA (PNA N5230A, Agilent Technologies). The total power absorbed by the body has been calculated in two steps. First the antenna is placed in the space and the power (P m (free space)) received by the antenna at the desired frequency is measured with the help of the spectrum analyzer [Rohde and Schwarz (9MHz- 30 GHz)]. Next the antenna is placed on the arm and again the power P m (arm) ) received by the antenna is measured. The difference in the power is the power absorbed (P m ) by the arm. Fig.4. Block diagram for SAR measurement set- up P m = P m(free space) - P m(arm) (2) where f is the desired operating frequency of operation, and are permeability and electrical conductivity of the tissue respectively. V. RESULT AND DISCUSSIONS The simulated and the measured S 11 parameter of the designed antenna in the free space are shown in the Fig. 5. It is evident from the figure that simulated and measured results are in good agreement. The bandwidth of the antenna is 0.69 GHz and 4.23 GHz at 2.45 and 7.5 GHz respectively which covers the ISM ( GHz), HIPERLAN ( GHz) and WLAN (5.725GHz to 5.850GHz) bands in free space. The measured and simulated S parameter of the antenna, when placed on the arm has been compared in the Fig.6. It is obvious from the Fig.6 that the performance characteristics of the antenna gets affected when placed on the arm. This time the impedance band width covered is 1.23 GHz and GHz, which includes the GSM, WLAN and UWB bands. In case of simulation, average value of thickness for various human tissues of arm have been considered

4 193 while in measurement, the thickness of human arm tissues may not be same due to different bodily structure or physique which has resulted in slight discrepancy in simulated and experimental results. S11(dB) Fig.5. Simulated and measured S 11 parameter of Monopole Patch Antenna in free space S11(dB) simulated [S(11)] db measured [S(11)] db Frequency(GHz) and are depicted in the Table 1 which comply with the safety guidelines provided by FCC [21]. As with the increase in the frequency the skin depth decreases hence power absorbed by the arm also reduces. Thus for a particular antenna, increase in frequency results in the reduction of the SAR. Table 1: SAR calculation at different frequencies Application ISM (2.45 GHz) HYPERLAN (5.2GHz) WLAN (5.75GHz) Penetration depth (mm) Received Power(dBm) free space P m( free space) Arm P m(arm) Total SAR (W/kg) VI.CONCLUSION We have examined the effect of arm on the performance of antenna in terms of return loss and the impedance bandwidth. It has been observed that the bandwidth of antenna increases by 45% at lower frequency and 66.45% at higher frequency due to the electrical properties of the human body. SAR is also calculated for the frequencies 2.45 GHz, 5.2 GHz and 5.75 GHz. From this observation it can be concluded that SAR decreases with the increase in frequency. ACKNOWLEDGMENT measured S11 (db) simulated S11(dB) Frequency in GHz The authors acknowledge the financial support granted under the Self-Assistance Program (SAP) of University Grants Commission, Government of India, New Delhi. Fig.6. Simulated and measured S 11 parameter of Monopole Patch Antenna when placed on human arm Using the formula (3), SAR's are calculated for the frequencies 2.45 GHz, 5.2 GHz and 5.75 GHz REFERENCES [1] Arthur W. Guy, Analyses of electromagnetic fields induced in biological tissues by thermographic studies on equivalent phantom models, IEEE Trans. Microwave Theory and Techniques, Vol.19, No. 2, pp , Feb.1971.

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