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The research work to be presented focuses on the study of multiband antennas for wireless indoor communications and wearable transceivers. An introduction to the essential antenna parameters and the theory behind the electromagnetic simulators used for this research sets the background necessary for understanding the procedure used when designing and measuring antennas. A study of the characteristics of textile materials at microwave frequencies using non-resonant transmission methods is later performed. The basis of small antenna design by top loading and the design of the Planar Inverted F Antenna is also covered. The main research work is divided into two parts. The first part investigates antennas for mobile and wireless indoor communications networks. The antennas are intended for Distributed Antenna Units to be located in buildings. A dual band Planar Inverted F antenna previously developed at the University of Kent is chosen as the prototype to achieve multiband and broadband operations. A parasitic resonator on the ground plane and side resonators are introduced in a study where additional bands are added. The derivation of equivalent function planar antennas from three dimensional multiband PIFAs is also covered. The second part of the work focus on antennas for wearable applications. The antennas are intended for WLAN on-body communication networks. A novel approach is taken by using metallic button structures to create wearable antennas. Dual frequency band operation is achieved on the button structures by adding a metallic plate on the top of the antenna and a cylindrical via connector. Miniaturization techniques are later applied to the button structure to reduce the size of the antenna while maintaining a match at the 2.4GPIz and 5 GHz wireless bands.

The research work carried out and reported in this thesis focuses on the application of additive manufacturing (AM) for the development antennas and novel frequency selective surfaces structures. Various AM techniques such as direct writing (DW), material extrusion, nanoparticle conductive inks are investigated for the fabrication of antennas and FSS based sensors. This research has two parts. The first involves the development of antennas at the microwave and millimetre wave bands using AM techniques. Inkjet printing of nanoparticle silver inks on paper substrate is employed in the fabrication of antennas for an origami robotic bird. This provides an exploration on the practicability of developing foldable antennas which can be integrated on expendable robots using low-cost household inkjet printers. This is followed using Aerosol jet printing in the fabrication of fingernail wearable antennas. The antennas are developed to operate at microwave and millimetre wave bands for potential use in 5G Internet of Things (IoT) or body-centric networks. The second part of the research work involves the development of frequency selective sensors. Trenches have been incorporated on an FSS structure to produce a new concept of liquid sensor. The sensor is fabricated using standard etching techniques and then using FDM method in conjunction with nanoparticle conductive ink. Finally, a new concept displacement sensor using an FSS coupled with a retracting substrate complement is introduced. The displacement sensor is a 3D structure which is conveniently fabricated using AM techniques.

This thesis presents the development and the additive manufacturing of smart antennas and frequency selective surfaces (FSS) for microwave, millimetre wave, and low-Terahertz applications. Numerous fabrications techniques such as etching, low-cost inkjet-printing, Fused Filament Fabrication (FFF) and Aerosol Jet Printing are employed to fabricate the antennas and FSS. Firstly, frequency reconfigurable antennas with close-coupled biasing technique are developed using double-sided etching on a thin mylar substrate for smart current and voltage sensing applications. The frequency tunable antenna also acts as a novel current sensing antenna in a smart sensing system that can sense the alternating current passing through a wire. The design is followed by introducing a low-cost inkjet-printed industry ready solution for frequency reconfigurable antennas where a single antenna aperture solution is utilised to demonstrate frequency reconfigurability in both switching and tuning configurations. Development of Frequency Selective Surfaces using low-cost printing machine is demonstrated in which FSS structures are inkjet-printed as wallpaper posters for Radio Frequency (RF) shielding and signal enhancements for 4G and 5G applications. A novel approach for fully 3D printing an FSS structure is also demonstrated using a low-cost open-source printer, which was modified to print the filament and the conductive inks simultaneously. Widespread investigation of industry-grade Aerosol Jet printing is utilised for additive manufacturing of bandstop and bandpass FSS designs for RF shielding and signal filtering. The bandstop designs are developed for microwave and millimetre-wave applications. The bandpass slot FSS designs are developed for millimetre wave and low-Terahertz applications for futuristic Beyond 5G and 6G systems.

This thesis presents methods to make use of additive manufacturing (AM) or 3D printing (3DP) technology for the fabrication of antenna and electromagnetic (EM) structures. A variety of 3DP techniques based on filament, resin, powder and nano-particle inks are applied for the development and fabrication of antennas. Fully and partially metallised 3D printed EM structures are investigated for operation at mainly microwave frequency bands. First, 3D Sierpinski fractal antennas are fabricated using binder jetting printing technique, which is an AM metal powder bed process. It follows with the introduction of a new concept of sensing liquids using and non-planer electromagnetic band gap (EBG) structure is investigated. Such structure can be fabricated with inexpensive fuse filament fabrication (FFF) in combination with conductive paint. As a third method, inkjet printing technology is used for the fabrication of antennas for origami paper applications. The work investigates the feasibility of fabricating foldable antennas for disposable paper drones using low-cost inkjet printing equipment. It then explores the applicability of inkjet printing on a 3D printing substrate through the fabrication of a circularly polarised patch antenna which combines stereolithography (SLA) and inkjet printing technology, both of which use inexpensive machines. Finally, a variety of AM techniques are applied and compared for the production of a diversity WLAN antenna system for customized wrist-worn application.

In traditional radio frequency (RF) front-end subsystems, the passive components, such as antennas, filters, power dividers and duplexers, are separately designed and cascaded via the 50 Ω interfaces. This traditional approach results in a bulky and heavy RF front-end subsystem, and suffers from compromised efficiency due to the losses in the interconnections and the mismatching problems between different components. The frequency responses of the antennas such as the frequency selectivity and bandwidth are usually degraded, especially for microstrip antennas. To improve the frequency responses and reduce the size of RF front ends, it is important to investigate novel highly integrated antennas which exhibit multiple functions such as radiation, filtering, power dividing and combining or duplexing, simultaneously. In this thesis, several innovative designs of compact, multi-functional integrated an-tennas/arrays are proposed for wireless communication applications. First, new methods of designing integrated filtering antenna elements with broadband or dual-band performance are investigated. These antennas also feature high frequency selectivity and wideband harmonic suppression. Based on these studies, several integrated filtering array antennas with improved gains and frequency responses are developed for the first time. Compared with traditional array antennas, these proposed antennas exhibit improved bandwidths, out-of-band rejection and wideband harmonic suppression. The application of the filtering antennas in millimeter-wave (mm-Wave) frequency band is also investigated as it can potentially reduce the cost of the mm-Wave front-end subsystems significantly while providing the improved impedance bandwidth. The integrated design techniques are further developed to design novel dual-port highly integrated antennas with filtering and duplexing functions integrated. Such a new concept and the prototypes could find poten-tial applications in wireless communication systems and intelligent transportation system (ITS). In this thesis, comprehensive design methodologies and synthesis methods are provid-ed to guide the design of the integrated filtering antennas. The performance is evaluated with the help of full-wave electromagnetics (EM) simulations. All of the prototypes are fabricated and tested for validating the design concepts. Good agreement between the simulation and measurement results is achieved, demonstrating the integrated antennas have the advantages of compact size, flat gain performance, low losses and excellent harmonic suppression performance. These researches are important for modern wireless communication systems.