International Journal of Electronics Letters
ISSN: 2168-1724 (Print) 2168-1732 (Online) Journal homepage: http://www.tandfonline.com/loi/tetl20
SAR effect reduction using Reject Band Filter arrays for Wi-Fi portable devices Hayder S. Ahmed & Taha A. Elwi To cite this article: Hayder S. Ahmed & Taha A. Elwi (2018): SAR effect reduction using Reject Band Filter arrays for Wi-Fi portable devices, International Journal of Electronics Letters, DOI: 10.1080/21681724.2018.1477190 To link to this article: https://doi.org/10.1080/21681724.2018.1477190
Accepted author version posted online: 14 May 2018. Published online: 23 May 2018. Submit your article to this journal
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INTERNATIONAL JOURNAL OF ELECTRONICS LETTERS https://doi.org/10.1080/21681724.2018.1477190
SAR effect reduction using Reject Band Filter arrays for Wi-Fi portable devices Hayder S. Ahmed and Taha A. Elwi Department of Communication Engineering, Al-Mammon University College, Baghdad, Iraq ARTICLE HISTORY
ABSTRACT
This paper suggests a novel 1D Reject Band Filter (RBF) array to prevent the radiation leakage from portable devices around 2.45 GHz. The proposed RBF array is constructed from eight-unit cell elements mounted on the same side of an FR-4 substrate of 1.5-mm thickness. Each two adjacent unit cells are separated with 2 mm. The individual unit cell design is based on fractal-based open-loop resonators of a modified first iteration Minkowski configuration. Such configuration is coupled with an open-stub transmission line to increase the selectivity of the filter stopband response at the frequency of interest. The unit cell performance is evaluated numerically and analytically, and then compared against measurements. Next, the optimal RBF array is fabricated and simulated to evaluate the Specific Absorption Rate (SAR) at 2.45 GHz. The SAR effect on the human thigh tissue is studied numerically to show an excellent reduction from 0.912 to 0.352 W/kg. Finally, the field strength before and after introducing the proposed array underneath a laptop computer, just below the Wi-Fi antenna, is measured and found to be reduced from 497.1 to 171.8 mV/m.
Received 2 February 2018 Accepted 6 May 2018 KEYWORDS
Filter array; Brillouin zone; absorption rate; Minkowski; Fractal
1. Introduction Over the last three decades, the applications of fractal structures helped engineers to meet the requirements of compact size along with high-performance circuit designs in modern microwave systems (Alqaisy, Chakrabraty, Ali, & Alhawari, 2015). The resulted benefits from applying various fractal structures, among many, wider bandwidths (Alqaisy et al., 2015), size reduction (Ahmed et al., 2017), and better performance can be achieved (Elwi, Hamed, Abbas, & Elwi, 2014). In this respect, the printed fractal circuits were applied to achieve miniaturised RBFs and bandpass filters of high performance. Such filters have been designed by the successful application of Minkowski-like and Peano fractal geometries to the conventional square open-ring resonators (Alqaisy et al., 2015; Ahmed et al., 2017; Elwi et al., 2014). SAR is an industrial standard that measures the amount of absorbed energy by the biological tissues (Mary, Priyadarshini, & Ravichandran, 2010). Therefore, many researchers proposed several techniques based on printed circuitries to reduce the SAR effects on the human body. A frequency selective surface was developed in Kumaran and
CONTACT Taha A. Elwi
[email protected] University College, Baghdad, Iraq
Department of Communication Engineering, Al-Mammon
© 2018 Informa UK Limited, trading as Taylor & Francis Group
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H. S. AHMED AND T. A. ELWI
Arunachalam (2016) to reduce the SAR effects on the human head by a 52%. In Bhattacharjee, Mitra, Ranjan, and Chaudhuri (2016), an artificial magnetic conductor defect was proposed to reduce the SAR when a plane wave is incident from different angles. An EBG layer based on a crossed mushroom structure was placed over a printed circuit antenna to minimise the SAR peak on human head tissues (Hariharan, Maheshwaran, Selvam, & Gunavathi, 2015). Recently, the metamaterial structures were introduced as an attempt to minimise the effects of SAR. For example, a meander PIFA printed antenna on a magneto-dielectric nanocomposite substrate was used to study the SAR effects on the human head for mobile communication systems (Han et al., 2015). Metallic rod reflectors were installed enclose to a transmitting antenna to decrease the antenna field inside the human head (Haridim, 2015). In Kwak, Sim, Kwon, and Yoon (2016), a PIFA antenna with an artificial magnetic conductor for SAR reduction in wireless communication systems was developed. A technique for reducing the SAR for mobile applications was investigated for a PIFA designed to cover the most mobile communication bands (Mageed, Pelleti, & Mittra, 2015). In another aspect, the introduction of the metamaterial structures in the design of printed circuit antennas for portable and wearable devices was investigated in Elwi (2017a) and Elwi Imran, & Alnaiemy (2015) to reduce the effects of the backward radiation from the antenna terminals. Nevertheless, the metamaterial use was extended to reduce the surface radiation leakage between the MIMO antenna elements in their arrays as reported in Elwi (2017b). In Imran and Elwi (2017), metamaterial structures were developed as an in-phase reflector to minimise the back lobes affects at the Wi-Fi bands. In this paper, the proposed filter array based on eight RBF unit cells shows excellent rejection at 2.45 GHz for SAR effects reduction in portable wireless systems. The unit cell performance in terms of the S-parameters is evaluated using CST Microwave Studio (MWS) (CST MWS, 2016). An analytical validation based on a circuit-lumped element technique is invoked to evaluate the S-parameters, and then a further validation is applied using a numerical simulation based on finite-element method of HFSS formulations (HFSS, 2016). The electromagnetic band gap properties of an individual unit cell are evaluated at the First Brillouin Zone (FBZ) in terms of the dispersion diagram. A parametric study is conducted to realise the optimal required number of unit cells to maintain the minimum radiation leakage at 2.45 GHz using numerical simulations. Later on, the proposed filter based on a single-unit cell element is fabricated and measured for validation against the simulated results in terms of the S-parameters. The comparison reveals a good agreement between the simulated and measured results with excellent selectivity at 2.45 GHz. This paper is organised as follows. In Section 2, the RBF unit cell design is developed based on an analytical circuit model. Numerical simulations based on two software packages are invoked to realise the proposed RBF array performance in Section 3. The experimental validations are conducted in Section 4 in terms of S-parameters and electric field strength measurements.
2. RBF array geometrical details The proposed RBF structure is developed to prevent the surface wave motion at a certain direction by conducting eight RBF unit cells in 1D array. The individual RBF unit cell is consistent of open-loop resonators: each one with the same iteration level of Minkowski-like
INTERNATIONAL JOURNAL OF ELECTRONICS LETTERS
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pre-fractal structures. According to a derivation in Ahmed et al. (2017), the perimeter Ln and the centre frequency fn of each open-loop resonator before and after applying the fractal geometry with w1 ¼ 13 1 can be determined using the following equations: Ln ¼ ð4 1 sÞ
f¼
27:4058 þ n 18:35
n forð0 n
