Polarization Insensitive and Transparent Frequency Selective Surface for Dual Band GSM Shielding

This article proposes a single-layer, dual-band, optically transparent frequency-selective surface (FSS) for GSM shielding. The design evolves from a fractal cross dipole to achieve dual-band response, polarization insensitivity, and wide angle of incidence stability with a miniaturized size. A two-axis symmetric structure helps to provide a stable frequency response for incident waves with different polarization angles up to 60°. Capacitive loading further reduces the unit size and extends the maximum stable incident angle. The working mechanism of the design has been explained through an equivalent lumped element circuit model (ECM), which provides a generic approach to retune or optimize the design for another frequency band. Both the full-wave EM and circuit simulations are in good agreement. A prototype has been realized by screen printing a custom silver nanowire (Ag NW)-based transparent ink on a flexible polymer substrate. A decent reflection performance has been achieved from 0.71 to 1.25 GHz and from 1.73 to 2.16 GHz in measurements, which are consistent with the simulations. Optical measurements reveal a transparency of 81.6%. Printed FSS is completely flexible, and the performance does not deteriorate after bent or rolled up conditions. Field testing on a glass box demonstrates decent shielding from the incoming GSM signals. The highly transparent and flexible nature of the FSS structure makes it suitable for mounting on glass windows of cars or home environments for EM shielding purposes.

I. Introduction

With the growing popularity of wireless communication, radio frequency (RF) devices are covering more and more frequencies to cater for either additional bandwidth requirements or new applications [1]–[10]. Although there are national standards for maximum electromagnetic interference (EMI) and radio power ratings for each device, the large numbers of RF devices working in close proximity suffer from radio interference. Besides, there are increasing concerns about the effects of extensive electromagnetic exposure to humans. This is particularly true for vulnerable patients and newborn babies in hospital environments. Therefore, there is considerable interest in selective RF shielding and researchers are striving to develop new materials and designs to shield the electromagnetic waves. For example, high-performance shielding materials, capable of absorbing the EM wave, have been widely studied in the past few decades [11]–[14]. However, the absorption of these materials is frequency insensitive, which means that both wanted and unwanted frequencies of EM waves are shielded. Moreover, the absorbed EM waves are converted into heat, which is not conducive to many applications. Frequency-selective surface (FSS) reflector, on the contrary, provides selective shielding where unwanted frequency bands are reflected and other bands are allowed to pass through. FSS has been widely used in a radar radome design, special filtering, electromagnetic shielding, and many other fields [15]–[17]. Novel fabrication techniques, such as screen printing with conductive ink, are also providing FSS with flexibility and transparency for new applications [18]–[22]. With flexibility and transparency, FSS can provide EM shielding for unconventional mediums and applications. For example, a newborn incubator in a hospital or a glass window in a car or home can reflect the unwanted EM waves without losing the glass transparency.