title: Submillimeter, Millimeter and Microwave Frequency Selective Surfaces, Design and Development
Creators: name: Hussein, MN
Publisher: name: University of Liverpool Repository
Year: 2018

description: This dissertation presents new approaches to design and development of submillimeter, millimeter, and microwave frequency-selective surfaces (FSSs) having extensive applications in wireless communications and radar systems. The theory of the surfaces is introduced in Chapter 3 where a new approach to miniaturise the size of an FSS array element is presented by interconnecting array elements in one direction in a two-layer FSS structure. The top layer acts as an enhanced inductor while the bottom layer acts as a capacitor. The interconnection between adjacent array elements changes the equivalent circuit and produces a strong cross-layer capacitance, which lowers the resonant frequency significantly. The dimensions of the miniaturised FSS element are much smaller than the wavelength at the resonant frequency (periodicity &lt;&lt; λ). Chapter 4 introduces a new methodology to design the FSS by maximizing the value of the capacitance between adjacent layers. The proposed structure offers three distinctive advantages: Firstly, the strong cross-layer capacitance makes the FSS element very compact. Secondly, for the proposed structure, the lower the profile, the stronger the cross-layer capacitance, and the lower the resonant frequency. This is unique to the proposed structure since the resonant frequency is usually higher for a lower profile than for traditional structures. Thirdly and most importantly, any external dielectric material attached to the FSS will not significantly affect the performance of the FSS due to this strong cross-layer capacitance. Chapter 5 introduces novel methodologies to design dual band spatial filters by using FSS periodic arrays composed of a bandpass and a bandstop element. The fabrication of the dual band filters is significantly simplified by using a single metal layer on a dielectric substrate. Chapter 6 introduces a new schematic to design a miniaturised high order bandpass FSSs (N ≥ 1), where N is the order of the FSS filter, with high performance with a flat in-band frequency response and fast roll-off is introduced. Two miniaturised resonant surfaces coupled by a non-resonant inductive layer are used to build the proposed FSSs. An FSS operating at around 3.8 GHz is designed to verify the method. The element size is smaller than 0.076λ×0.076λ for the proposed structure. This is significantly smaller than the element size of second-order FSSs designed using conventional approaches. The overall thickness is less than λ/24. The method could be particularly useful for the design of FSSs at lower frequencies with longer wavelengths. Thus, a novel approach for designing extreme low profile high-order bandpass frequency selective surfaces is introduced in this chapter. The structure is built in such way to obtain bandpass response by the coupling between the third harmonic responses of the resonators instead of the fundamentals. By parametric study of the proposed structure, one can make the coupling between the third harmonics weak with a thinner substrate, and then a flat in-band response can be achieved. The overall thickness can be reduced to λ/75. Chapter 7 demonstrates FSSs with sharp transition edges and almost flat bandpass for submillimetre wave and terahertz applications. The proposed structure exhibits a low insertion loss in the desired band. The structure is realised by combining bandstop and bandpass FSS structures on the same plane. By cascading more than one layer of surfaces, separated by dielectric slabs, the response with the desired flat passband characteristics can be achieved. The structure is polarisation independent and exhibits low insertion loss at the passband around 170 GHz. Finally, Chapter 8 demonstrates an extremely small-size high impedance surface (HIS) array element. A trade-off between a miniaturised element size and a lowered thickness of the grounded substrate is made to design an extremely low profile HIS. Additionally, we propose a way to modify existing classical RFID tag designs to enable them to operate well when they are attached to dielectric materials. Compared with using an HIS, the antenna bandwidth after being loaded with the proposed FSS is increased by approximately 100%.