A new frequency selective absorber surface at the unlicensed 2.4-GHz ISM band

Wireless devices in the unlicensed ISM bands cause mutual interference in indoor environments. This interference does not only degrade system performance but also compromises security. An efficient approach to solve this problem is transforming a building wall to a frequency selective surface (FSS), which filters undesired signals, but allows signals of other devices such as mobile phones, radios and televisions. As a band-stop filter, FSS acts as a reflector that causes additional multipath, delay spread and signal degradation. The aim of this study is proposing a design of FSS absorber (FSAS) at the unlicensed 2.45 GHz ISM band to eliminate the reflected signal while it still acts as a band-stop filter. Absorber characteristic is achieved by placing a second FSS layer comprising lossy periodic FSS elements or placing lumped resistors on the conducting paths of periodic element geometries. HFSS software is used for simulation and design purposes. The minimum attenuation was obtained as 20 dB on the transmission (S₂₁) coefficient, while it was obtained as 10 dB on the reflection (S₁₁) coefficient, respectively. The simulation and measurement results obtained show that the desired frequency response is successfully achieved for both TE and TM polarizations.