Improved bandpass FSS for wireless communications

Summary form only given: Mobile radio or cellular phones operating in the 824 to 894 GHz band and the personal communication services (PCS) operating in the 1.8 to 2 GHz band are the first generation wireless communication systems (T. Rappaport, Wireless Comm. Principles and Practice, 2^nd ed., PH PTR, 2002). Wireless local area networks (WLAN) or Wi-Fi, operating in the 2.4 to 2.45 GHz band, are next implemented with wireless networks to replace the cumbersome wires or optical fibers connecting multiple points inside a home, or a building, or an office. It is desirable to contain the WLAN/Wi-Fi signals inside the building, while allowing cellular and PCS signals to pass thru the building´s glass windows. Also, in very hot or cold regions, these windows are usually made of energy saving glass with very thin metallic or metallic-oxide coatings, which pass visible light frequencies but block Infrared frequencies and the wireless signals at all frequencies (G. Kiani, et. al., 2008 IEEE Int. AP Symposium, San Diego, CA.) Hence various bandpass frequency selective surfaces (FSSs) with aperture or slot elements have been proposed to improve the transmission of cellular and PCS signals thru these windows. For example, the hexagonal slot element FSS was shown to provide about 10 dB improvement for the transmission thru these glass windows in the 900 MHz to 2 GHz band (M. Gustafsson, et. at., IEEE Trans., AP-54, 1897-1900, 2006.) Recently a dual layer glass window with circular ring slot FSS was presented by S. Sohail, et. al. (2012 IEEE Int. AP Symposium, Chicago, IL.) Although this ring slot FSS provides about 30 dB attenuation (or protection) for the WLAN/Wi-Fi signals inside the building, the cellular and PCS signals still suffer 11 dB transmission loss thru this FSS window. The major difficulty of designing this bandpass FSS is the rejection band (2.4-2.45 GHz) being very close to the pass band (824 MHz to 2 GHz).In this paper, a dual layer FSS printed with - iniature fractal elements is shown to provide less than 3 dB transmission loss for cellular and PCS signals and more than 38 dB attenuation for the WLAN/Wi-Fi signals in the 2.4 to 2.45 GHz band. The fractal FSS has been previously implemented for a dual band FSS with a reflection to transmission band ratio of 3, i.e. f_r/f_t = 3 (TK Wu, Microwave Optical Tech. Letters, 54, 833-835, 2012). Now this bandpass FSS is designed for even smaller ratio, i.e. f_r/f_t = 2.4/2 = 1.2. It should be pointed out that nontransparent windows are usually required for privacy and securing the WLAN/Wi-Fi signals inside the building or office. In other words, various obscure dielectric substrates, such as the Duroid 6006 or TMM-6 laminates, will also be used here for the design and fabrication of the bandpass FSS.