Suppression of Mobile Phone Exposure by Using Compact Electromagnetic-Bandgap Array Rupam Das 1 , Sang-Bock Cho 1 , and *Hyoungsuk Yoo 2 1 School of Electrical Engineering, University of Ulsan, Ulsan, Republic of Korea 2 Department of Biomedical Engineering, Hanyang University, Seoul, Republic of Korea, *E-mail: hsyoo@hanyang.ac.kr Abstract - In this paper, a mobile phone case incorporating electromagnetic band gap (EBG) material is studied to intercept electromagnetic waves from the phone and reduce the specific absorption rate (SAR). A loop antenna for the Personal Communications Service (PCS) 1900 MHz band is considered. In addition, three different mobile phone cases associating EBG structures with optimized positions are used to improve SAR reduction. A human head phantom, to replicate the specific anthropomorphic mannequin (SAM) model was used for measurements. From simulation and measurement it is found that placing an EBG structure in a phone case can improve antenna gain as well as a maximum SAR reduction of 24% were found. This study reveals future applications of EBG structures for SAR reduction in mobile phones. Index Terms — Electromagnetic exposure, electromagnetic band gap, gain, phone antenna, specific absorption rate. 1. Introduction Electronic devices are recognized as an essential element of daily life, and interest in mobile phones has significantly increased. Different techniques and ideas are used in mobile phone antennas for 4G applications in order to increase antenna bandwidth and efficiency [1]. In addition, more antennas installed in a phone introduce thermal heating. Moreover, usage of mobile phones close to the body makes it more dangerous than other electromagnetic devices, due to electromagnetic exposure and thermal effects at specific frequencies. However, it is difficult to measure the calorific value of these thermal effects. Therefore, Specific Absorption Rate (SAR), which measures the electromagnetic energy absorbed per mass of tissue, is used in limiting mobile phone radio frequency exposure. An Electromagnetic Band Gap (EBG) structure has high EM surface impedance, which is capable of suppressing the propagation of surface current, and it acts as a perfect magnetic conductor (PMC) in a specified frequency range. The EBG can also reduce the surface waves that generate EM waves that travel toward the human head. Kwak et al. [2], placed a square and interdigitated patch EBG structure in front of the phone antenna beside the head to reduce the SAR. In [3], they furthered the study by incorporating their design in a mobile set. However, modern smartphones contain several antennas (4G/3G, Wi-Fi, Bluetooth) and these antennas can interfere with the EBG structure if placed inside the smartphone’s circuitry. In addition, designing an EBG to obtain the desired characteristics may require a thicker substrate, which can increase the size of the smartphone. Since the vast majority of people nowadays use a mobile phone case to protect or decorate their valuable smartphones, we propose placing an EBG structure within the mobile phone case. 2. Methods and Results The phone antenna [4] considered for this study was proposed by Apple and implemented in IPhone 4 and later versions. As this is a commercially available product, we did not include the elaborate design details. The first EBG unit cell (EBG 1) considered for this study is based on the simplistic hairpin resonator. It consists of two patch arms with a gap between them, as shown in Fig. 1(a). Second EBG (EBG 2) structure is made of concentric rectangular patch ring as depicted in Fig. 1(b). The lattice type EBG in Fig. 1(c) comprises numerous slots in the patch, and length or width of these slots can be regulated to match the resonant frequency. The via (diameter = 0.6 mm) is connected to the ground and p is the lattice constant or periodicity (i.e., gap between the two adjacent unit cells = 0.5 mm). Photographs of the fabricated antenna and EBGs, as well as measurements setup are represented in Fig. 2. Furthermore, three phone cases were designed to check the variation in SAR. Before calculating the SAR value, the radiation patterns of the antenna were simulated at 1900 MHz and measured. The simulated and measured values for radiation patterns were compared by using the E-plane, as depicted in Fig. 3. In the E- plane, the angle range from 90 o to 270 o is the area that faces the human head. Some sharp peaks were observed in the no EBG cases during measurement as opposed to cases with EBGs. It is clear from the E-plane pattern from 40 o to 350 o , that gain value is reduced for different EBG cases, as compared to cases with no EBG. Mathematically SAR is related to the electric field, and this result illustrates that the SAR can be reduced with the proposed EBG phone case. From Semcad simulation, 1g SAR distributions through the cross- Fig. 1. Different unit cell shapes of an EBG: (a) Hairpin. (b) Ring. (c) Lattice. 2018 International Symposium on Antennas and Propagation (ISAP 2018) October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea [ThD3-4] 311