The paper presents a synthesis of the research performed on the electromagnetic properties and characterization of textile and non-textile materials with applications in shielding and protection from the electromagnetic field. The composite structures of functional textiles intended for protective clothing or general applications for electromagnetic immunity are presented and characterized. There are analyzed composite textiles with amorphous, ferrous or non-ferromagnetic metallic threads manufactured by means of woven and knitting classical technologies as well as materials using non-metallic, electrically conductive powders. The properties of the plain jersey, rib jersey, full and half cardigan fabric, Milano rib, are presented, too. Besides textiles, there are also characterized some composite and non-composite structures using metallic yarns and carbon powder. Another direction of interest relates to the use of textile materials with amorphous metal structure with the scope of achieving a more efficient protection to the electromagnetic fields used in cellular systems and Wi-Fi networks. In addition, a comparative analysis of the methods of characterization of composite structures is made.
By applying a non-standardized methodology and by using electric- and magnetic-field probes of small dimensions (< 1cm), we experimentally characterized the electromagnetic shielding properties of a fabric containing ferromagnetic microwires weaved on a single direction. Electronic microscopy and X-ray spectroscopy revealed the structure, dimensions and chemical elements content of the amorphous magnetic material. Electric shielding factor proved to be very low in the investigated frequency range, but magnetic shielding factor was high, especially when the weaving direction of the magnetic wires corresponded to the polarization direction of the emitting antenna, and showed some resonances. The magnetic shielding efficiency, if checked against an increasing incident magnetic flux density, proved not to change up to 200 nT. The investigated fabric have been previously proved to be very efficient in shielding the field emitted by a mobile phone in its near field, but present results show that near and far field shielding properties are different.
A dual band mobile phone model was used to check the shielding properties of an amorphous ferromagnetic textile against the radiation emitted by the handset. Two frequencies belonging to the 2nd and 3rd generation of mobile emission technologies were used, 897 MHz and 1950 MHz. The specific absorption rate (SAR) of energy deposition in a human head phantom was measured in standardized conditions. The textile contained micrometric-diameter wires of a ferromagnetic mixture embedded in a thin glass coat and weaved in a specific way. A set of fabric orientations and configurations (layering) were provided in the experiment in order to achieve a better shielding to the phone’s radiation. Compared with the non-shielded handset, SAR deposited in the head while using the fabric-covered phone could be decreased up to 30 % of its initial value – in case of 2G technology and up to 24 % – in case of 3G technology. This type of material shows one of the highest shielding efficiencies of the electric-field component in near-field exposure conditions reported until now. A cubic curve of SAR decrease in depth of the head was revealed in both uncovered and covered handset, the effect of shielding being larger at the higher frequency.
An analysis of the shielding effectiveness by the method of dual transversal electromagnetic cell is proposed for a set of conductive materials of the type wire-inserted textiles. Based on the experimental results, some of the materials are further on used to test their capability to effectively reduce the specific absorption rate of energy deposition in the head due to mobile phone, when they are covering the surface of the handset. It is demonstrated that both reducing and increasing the radiation that penetrates the head may appear, and this depends on both the material structure, its position and orientation relative to the device and to the frequency band the phone is emitting.