This paper aims to provide a general review of the fundamental ideas, varieties, methods, and experimental research of the most advanced frequency selective surfaces available today. Frequency-selective surfaces are periodic structures engineered to work as spatial filters in interaction with electromagnetic (EM) waves with different frequencies, polarization, and incident angles in a desired and controlled way. They are usually made of periodic elements with dimensions less than the operational wavelength. The primary issue examined is the need for more efficient, compact, and adaptable electromagnetic filtering solutions. The research method involved a comprehensive review of recent advancements in FSS design, focusing on structural diversity, miniaturization, multiband operations, and the integration of active components for tunability and reconfigurability. Key findings include the development of highly selective miniaturized FSSs, innovative applications on flexible and textile substrates, and the exploration of FSSs for liquid and strain sensing. The conclusions emphasize the significant potential of FSS technology to enhance wireless communication, environmental monitoring, and defense applications. This study provides valuable insights into the design and application of FSSs, aiming to guide future research and development in this dynamic field.

We propose a periodically modulated metasurface (MTS) capable of producing two-sided electromagnetic (EM) beams with frequency controlled beam reconfiguration. The beam forming functionality is realized through the leaky wave conditions of the periodic structure, and the beam reconfiguration at a quadrature azimuth angles is triggered by switching between the two operating frequencies. The MTS is formed using a periodic homogeneous array of split ring resonators (SRR) with anisotropic characteristics. The electromagnetic bandgap (EBG) properties and leaky wave conditions of the individual SRRs are combined to produce both beam-forming and beam-reconfiguration functionality. The subwavelength SRR unit cells are designed such that they satisfy leaky-wave theory at 1.8 GHz to form a two-sided beam along azimuth angles 00 and 1800, and create a complete electromagnetic bandgap (EBG) at 2.6 GHz to form a two-sided beam along angles 900 and 2700 at 2.6 GHz. Both the TE and TM mode response of the individual SRRs is taken into consideration during the design process. The MTS structure is synthesized and evaluated using full-wave EM simulator, and is validated with experimental results.

The effect of using a superstrate dielectric layer on an on-head matched antenna for biomedical diagnosis applications is investigated. Two on-head matched antennas are considered with different length meandered lines ensuring operation around 0.9 GHz frequency. The first antenna’s conductive radiating structure is in direct contact with the head phantom, whereas the second one utilises a 0.5 mm thick superstrate layer on top of the conducting layer as a buffer. The lateral dimensions of both antennas are held constant at 30 × 30 mm². The electric and magnetic field distribution is analysed and the power penetration, 50 mm inside the head phantom, is derived from the electromagnetic field surrounding the antennas. Both homogeneous and inhomogeneous head phantoms are considered while evaluating the antennas in terms of their reflection coefficient, current distribution, electric field, magnetic field, specific absorption rate (SAR) and power penetration inside the head. The antennas are fabricated and measured utilizing an inhomogeneous phantom to validate the proposed performance improvement using a superstrate. It is shown that the superstrate antenna achieves a ~8 dB increase in power penetration inside the head phantom along with a 0.0731 W/kg decrease in SAR compared to the antenna without a superstrate.