Highly selective miniaturised-element frequency selective surfaces with tuning capabilities
Creators: name: Afzal, Waseem
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400608
subject: Wireless communication systems and technologies (incl. microwave and millimetrewave)
classificationCode: 400807
subject: Engineering electromagnetics

description: Frequency selective surfaces (FSSs) are periodic structures engineered to work as spatial filters in interaction with the electromagnetic (EM) waves of different frequencies, polarisations, and incident angles in a desired and controlled way. They are usually made of periodic elements with dimensions less than the operational wavelength. For the last few decades, FSSs were a subject of interest among many researchers for designing spatial filters in various applications from low microwave up to optical frequency ranges.

This thesis investigates the applications of frequency selective surfaces to design improved spatial filters with highly selective responses and tuning capabilities. Initially this research focuses on the conception of two novel dual-band FSS structures with reconfigurability and tunability. A mechanically tunable dual-band bandpass FSS structure is proposed with independently tunable passbands. Tunability is achieved through varying the coupling capacitance by laterally translating one layer of the FSS. This technique doesn’t require additional biasing circuitry to tune the frequency response. A switchable FSS has also been designed which offers dual bandpass, single bandpass at two centre frequencies, and total reflection frequency response options. In addition, the structure shows a consistent frequency response under a wide range of oblique incidence angles for both the TE and TM polarizations.

Further, a varactor-tunable dual band bandpass frequency-selective (FSS) with independently tunable passbands is proposed. The FSS is comprised of three stacked layers. A wire grid in each layer in combination with metallic vias provide a bias voltage to the varactor diodes. This arrangement facilitates tunability without the necessity of any extra circuit or network for biasing the individual varactors and thus avoids unwanted responses associated with an additional bias grid. An equivalent circuit model is provided to analyze the operation principle of the FSS. Simulations prove that the passbands can be independently tuned from 2.2GHz to 2:7 GHz to 4:3 GHz respectively. A prototype of the design has been fabricated and measured results are in good agreement with the simulations.

Lastly, the design of a low-prole miniaturised-element frequency selective surface (MEFFS) with narrowband, second-order bandpass response is presented. The miniaturised-element unit cell comprises a capacitively loaded parallel LC unit cell resulting in a narrowband bandpass response. A higher-order filtering response is obtained by implementing five metallic-layers in the FSS, comprising of two layers of parallel LC resonators with an inductive layer in between them. The proposed design offers a very thin prole and an insensitive frequency response to the polarisation of the incident electromagnetic wave. A design procedure is developed for the proposed FSS structure based on an equivalent circuit model analysis. Using this model, a prototype of MEFSS with second-order bandpass response at 3:34 GHz and a fractional bandwidth of 3:7% is designed, realised and characterised experimentally. The measurement results of the FSS verify the effectiveness of the FSS design procedure and stability of its frequency response up to 60 degree oblique incidence angles.