Modeling & Simulation Analysis of Frequencyselective Fabrics (fsf) in Gsm Bands

Abstract— Frequency selective surface (FSS) is a repeated structure is transmitting, reflecting or absorbing based up on the mode of interest by using patches or slots. The patch and slot arrays effectively create band stop and band pass filters. The FSS have potential applications in providing sufficient Shielding in the desired frequency ranges. The work proposed in this paper is to study & analyze the FSS structural requirements to shield against the GSM band frequencies. The persons working near the mobile towers are exposed to strong EM fields, specifically near fields, hence they need to guard themselves against these fields. The concept of FSS is extended to print the structures on the plane fabrics, and the same can be worn by the working personnel. The paper is primarily based on the simulation analysis of the designed fabrics using EM software tools. The simulation results are validated using experimental results.

Keywords— Frequency selective surface, Frequency selective Fabric, Shielding effectiveness

I. Introduction

Cell phone technology has make far researching changes in the telecommunication scenario in India. Cell phone technology has enormous changes in the last decade. Currently, there are more cell phone users as well as cell phone towers to meet the communication demand. The numbers of cell phones and cell towers are increasing day by day without knowing its disadvantages. Cell tower antennas transmit in the frequency range of 869 - 894 MHz (CDMA), 935 - 960 MHz (GSM900) and 1810 – 1880 MHz (GSM1800). Also, 3G which base station antenna transmits in the frequency range of 2110 – 2170 MHz. The cell towers transmits a power of 20-25watts and mobile phone transmits a power of 1-2watts, the radiation from the mobile phone and cell towers affect and cause serious threat to the human health due to Electromagnetic field (EMF) Radiations from mobile towers and mobile handsets. An Antenna is designed in such a way that mobile phone should be able to transmit and receive signal for proper communication up to a few kilometers. Most of the towers are mounted near the residential and office buildings to provide good mobile phone coverage to the users. These cell towers transmit radiation 24x7, so people living or working within 10’s of meters from the tower will receive stronger signal than required for mobile communication. In India, crores of people reside within these high radiation zones. International Commission on Non-Ionizing Radiation Protection (ICNIRP) studies possible adverse effects on human health from exposure to non-ionising radiation[1]-[2]. ICNIRP's principal aim is to disseminate information and advice on the potential health hazards of exposure to non-ionizing radiation. As per the ICNIRP Guidelines the present limits/levels are detailed in below table.

The microwaves from cell phone towers can interfere with our body’s own EMFs causing a altered white blood cells in children; childhood leukemia, impaired motor function, headaches, dizziness, fatigue, weakness, Memory loss, Birth defects, Cancer and DNA damage etc[3]-[5].

Thus, developing a fabric with filtering properties in a specific frequency band of the GSM-1800 is important to human health. GSM-1800 (Global System for Mobile Communications) uses 1810–1880 MHz to receive information from the mobile station to the base transceiver station (downlink). In recent years, various methods have been proposed for manufacturing electromagnetic shielding textiles for protection against electromagnetic interference (EMI) waves [6]. In this field, the customized, flexible, lightweight, and porous conductive fabrics were developed for either EM shielding or functional electronic applications by knife-over-roll coating [7]-[8]. In other study, the conductive co-woven-knitted and woven fabrics were produced with desirable properties of electromagnetic shielding effectiveness. The main purpose of EMI shielding methods is to block the electromagnetic radiation, so that the wave cannot pass through the blocking medium. EMI shielding textiles refer to the manufacture of fabrics from conductive and nonconductive materials using various processing methods. In all of the conductive samples described above, these types of fabrics have the role of metal sheets for EMI shielding [9]. Therefore, the problem with these fabrics is that they are unable to shield EM waves in a specific frequency band. Accordingly, the frequency selective surface (FSS) technique is needed to prepare a fabric with EM filtering properties in the GSM-1800 frequency band.

Frequency Selective Surface (FSS) is a repetitive structure acts as a filter and has wide range of applications such as radomes, lenses, RFID, Protection from electromagnetic interference, medical and military sectors [10]. FSS can also stop unwanted mobile signals based on the traditional FSSs Frequency selective fabrics are made for filtering properties in the GSM-1800 band[11]-[13]. The FSS structure can be printed on plane fabrics to shield against GSM frequency band. This fabric is helpful to humans, to protect themselves from the strong radiation fields coming from the cell towers.

II. S-Matrix and Shielding Effectiveness

S-matrix can play a very important role in filtering problems. S-matrix can be written as the linear relation between reflection wave b and incident wave a, and it can be derived from the following formula (1)

b1=a1S11+a2S12

b2=a1S21+a2S22 (1)

Here, S-matrix is called scattering matrix of the two ports, and each of the four S parameters has definite physical significance. Specially, S21 is transmission coefficient of port1 when port2 connects matched load and it can be calculated from formula (2)

S21=b2/a1|a2=0 (2)

The meaning of S21 is illustrated from the microwave transmission line theory, but in many cases, the energy loss due to the difference between EM impedance and the intrinsic impedance of shielding material is more intuitive for designers to characterize and evaluate the material. According to Schelkunoff Principle, for conductive monolithic materials without holes, the shielding effectiveness can be calculated from formula (3)

SE=SEA+SER+SEM=10log (p1/p2) (3)

Where, SE represents shielding effectiveness, which means the attenuation degree of EM wave caused by shielding materials, and the unit is dB. SEA stands for absorption loss, SER stands for reflection loss, SEM stands for multiple-reflection loss. P1 and P2 are received power without and with the tested material. Actually, S21 means the ratio of transmitted wave (equivalent to P2) to total incident wave (equivalent to P1) when EM wave passes through the medium. Therefore, S21 can be calculated from formula (4)

S21=10log (p2/p1) = -SE (4)

As a medium in EM field, periodic structure, like FSS, can be also evaluated through S21 parameter. In this paper, cross like design frequency selective fabric (FSF) was proposed, which were supposed to shield or make through EM wave with the frequency of 1800MHz. Based on S21, the simulation was accomplished and FSFs were fabricated [14].

III. Design Procedure

The unit cell of the proposed FSFs was composed of two layers. The upper layer was FSS structure and the lower layer was flexible fabrics (i.e., cotton fabric). Taking electrical properties of cotton and simulate the structure in Altair hyperworks EM simulation software. The structure is obtained by rotating the alphabet V one to the other by 900 to get a cross like design (CLD). For the convenience of modeling, the fabric was assumed to be a smooth plate with constant thickness and EM properties, and the conductive patch was assumed to be a perfect conductor, with a thickness of 0.035 mm. The parameters of the unit cell geometry are detailed in Table II.