In this paper, active frequency selective surfaces (FSS) having a squared aperture with a metal plate loading are described. Active FSS elements using switched PIN diodes are discussed with an equivalent circuit model. A unit cell consists of a square aperture element with metal island loading and one PIN diode placed at the upper gap, considering the vertical polarization. The electromagnetic properties of the active FSS structure are changed by applying dc bias to the substrate, and they can be estimated by the equivalent circuit model of the FSS structure and PIN diode. This active FSS design enables transmission to be switched on or off at 2.3 GHz, providing high transmission when the diodes are in an off state and high isolation when the diodes are on. The equivalent circuit model in the structure is investigated by analyzing transmission and reflection spectra. Measurements on active FSS are compared with numerical calculations. The experimentally observed frequency responses are also scrutinized.

Recent advancements in the ubiquitous sensor network field have brought considerable feasibility to the realization of a ubiquitous society. A ubiquitous sensor network will enable the cooperative gathering of environmental information or the detection of special events through a large number of spatially distributed sensor nodes. Thus far, radio frequency identification (RFID) as an application for realizing the ubiquitous environment has mainly been developed for public and industrial systems. To this end, the most existing applications have demanded low-end antennas. In recent years, interests of ubiquitous sensor network have been broadened to medical body area networks (BAN), wireless personal area networks (WPAN), along with ubiquitous smart worlds. This increasing attention toward in ubiquitous sensor network has great implications for antennas. The design of functional antennas has received much attention because they can provide various kinds of properties and operation modes. These high-end antennas have some functions besides radiation. Furthermore, smart sensor nodes equipped with cooperated high-end antennas would allow them to respond adaptively to environmental events. Therefore, some design approaches of functional antennas with sensing and reconfigurability as high-end solution for smart sensor node, as well as low-end antennas for mobile RFID (mRFID) and SAW transponder are presented in this paper.