X-Band Tunable Frequency Selective Surface Using MEMS Capacitive Loads

A tunable frequency selective surface (FSS) based on slotted ground is presented. Tuning of the resonance frequency is achieved by using a metallic MEMS bridge over the slot. The bridge acts as a capacitive load, increasing the equivalent capacitance, and so decreasing the resonance frequency. Electromagnetic and electromechanical simulations are performed to investigate the designed FSS. S-parameter measurements of the FSS unit cell are performed in a waveguide simulator, showing more than 1.7 GHz frequency shift in the X-band, achieved by using only one MEMS bridge. A measured bandwidth of 400 MHz at the resonance frequency of 9.59 GHz is achieved. The designed MEMS bridge benefits from an unconventional method of using SU-8 as the sacrificial layer, resulting in low loss at high frequencies (3.2 dB loss at the resonance frequency of 9.59 GHz). Devices with different heights of the MEMS bridge were fabricated to study the variation in the resonance frequency. The MEMS bridge was tested at fixed heights. Simulated and measured results show excellent agreement. An FSS array is designed based on the FSS unit cell results. The design procedure to maximize the quality factor and controllable frequency range, and improve the radiation characteristics of the FSS array is discussed. Further simulations are performed to examine the performance of the FSS array with regards to grating lobes, oblique incidence and tunability.