Design and development of frequency selective surfaces for wireless applications

Abstract

Frequency Selective Surfaces (FSSs) with multi-functional capabilities are a current research interest due to their wide range of applications, mainly in wireless communication, sensing, and radar systems. FSS plays an important role in RF systems within wireless communication networks, such as suppressing interference, enhancing transmission selectivity, improving channel quality, and directionally reflecting electromagnetic waves across various ranges. Considering current requirements, their importance becomes even more critical, especially for meeting communication bandwidth and transmission selectivity needs. This creates new opportunities and challenges to the researchers. This challenge is not just to enhance the existing structures but also adding various functionalities to the entire system keeping relatively low cost and maintaining efficiency. Various FSS designs have been extensively investigated with desirable properties like sharp roll offs, miniaturization, cost-effectiveness and reconfigurability. This thesis primarily focuses on the design and development of Frequency Selective Surfaces (FSSs) for wireless applications. The initial part of the thesis is dedicated in developing various passive FSSs for enhancing the desirable characteristics of the structure. Various techniques have been developed to achieve additional degrees of freedom in design parameters, cost efficiency, manufacturing feasibility and reliability. The advantages of 3-D printing and other low-cost substrate material have been utilized in prototyping different types of Frequency Selective Surfaces and investigating various desirable parameters, primarily to produce multiple bandwidth channels with intervals and sharp roll-off edges, which are highly anticipated in the development of reconfigurable FSS. The term "reconfigurable" refers to a wide range of parametric selectivity in FSS without physical changes to the structure. The reconfigurable FSS(RFSS) achieves a wider operating frequency range either tuning electrically or mechanically. In thesis work, RFSS incorporates active circuit elements, such as variable capacitors, to achieve real-time tuning of the resonating unit cells. Consequently, the development of reconfigurable FSS using active components is more challenging compared to conventional FSS. As a result, the work has been carried out in multiple stages. The desirable features of FSSs are explored and prototyping FSS using 3-D printing is a critical step, providing sufficient knowledge and data resources to finalize the design and viii implementation of reconfigurable FSS. Furthermore, the developed 3-D printed FSS and other low cost FSSs may have a wide range of applications, such as RF shields, reflectors, filters, etc. The key milestones of the work presented in this thesis are briefly discussed below: • The first part deals with two FSS designs to achieve higher selectivity and miniaturization characteristics. In first work, FSS is designed to exhibit filter like characteristics with flat passbands and fast roll-off edges, resulting in better frequency selectivity. Next design deals with miniaturization that led more and more unit cells to be integrated to smaller space thus saving size and space. The miniaturized FSS has been investigated using metallic vias to resonate at frequency bands of 1.24 GHz and 2.65 GHz. These works focus on achieving precise frequency control while maintaining lightweight, small and cost-effective designs. • The operating frequency of 3-D printed FSS has been altered by incorporating the designed elevated pattern on the surface of substrate. The work exploited the unit cell design by varying substrate height and metallization patterns and leads to significant variation in operating bands. The 3-D printed FSSs have also been explored for harmonic radar applications. The harmonic radar transmits at a harmonic frequency and detects the second harmonic frequency of reflected signal. The presented works reject the frequency at 2.5 GHz while passing the second harmonic frequency at 5 GHz frequency band. Another work is presented for RF shielding applications to suppress the various signals for security reasons and prevent cross-coupling between nearby wireless channels. The work also investigated the fabrication tolerances of 3-D printed technique. • The reconfigurable FSSs are explored for wideband tuning characteristics and beam steering applications. First work deals with dual bandstop tuning that can be individually as well as simultaneously tuned for achieving wideband characteristics. The wideband tuning with sharp roll off rejection at upper edge of frequency band is achieved by simultaneous varying the capacitance of varactor diode inserted at the top and bottom side of substrate. Effectiveness of the FSS design is tested by the fabricated prototype mounted with capacitors in order to achieve cost-effectiveness of proposed structure. Another reconfigurable bandpass FSS has been investigated to achieve desirable transmission phase for beam steering applications. The work mainly focuses on demonstrating the steering capability with extensive control over the phase distribution.