Antennas for Portable Communications in the UWB and GPS Spectra.

The thesis describes a study of antennas for solutions in emerging systems. An account of the fundamental principles of these antennas is given together with an outline of the basic electromagnetic theory on which the designs are based. While the main focus is on antennas for Ultra Wideband (UWB) systems, some additional work was carried out on enhancing performance of circularly polarised antennas for satellite navigation systems. The study focussed on the design of highly efficient broadband antennas with particular requirements for size compactness, gain stability, phase-linearity, and the use of low-cost materials. These requirements are particularly important for UWB system antennas that operate in the Federal Communications Commission (FCC) allocated frequency range 3.1-10.6GHz. Very compact solutions were produced in the form of small semi-planar monopole antennas with broad bandwidth impedance and stable radiation properties that showed independence from groundplane dimensions. The investigation of the broadband properties of a rolled planar monopole led to a unique mechanically-reconfigurable antenna, operating in the UWB spectrum, for indoor or mobile applications where tuneable band-notching and omni-directional radiation are required. The impedance matching of this rolled monopole antenna can be adjusted by regulation of the degree of spiral tightness. This innovative solution overcomes several problems which are typical for traditional reconfigurable antennas, such as non-linearity, low gain/efficiency and limited power handling. The requirement for pulse-based UWB communication antennas that exhibit linear-phase led to the investigation of the frequency and time-domain performances of tapered feedlines for non-planar monopoles. By configuring an optimised taper between an antenna’s feed-point and radiating structure, the phase distortions can be significantly reduced without degrading the radiation characteristics. This principle has been proven to be successful for both two and three dimensional monopole antennas. Electromagnetic bandgap (EBG) solutions were proposed for proposing circularly polarised (CP) antennas for Global Positioning Systems (GPS) applications. Miniaturized EBG solutions based on fractal geometries were designed and integrated with stacked CP patch antennas. A radical EBG cell distribution around a CP patch antenna was also proposed. Compared to legacy Cartesian configuration designs, the radical EBG arrangement enhances the axial-ratio performance, reduces the antenna area footprint and simplifies the design of the guard ring between radiator and EBG cells

State-of-the-Art of Metamaterials: Characterization, Realization and Applications

Metamaterials is a large family of microwave structures that produces interesting ε and μ conditions with huge implications for numerous electromagnetic applications. Following a description of modern techniques to realize epsilon-negative, mu-negative and double-negative metamaterials, this paper explores recent literature on the use of metamaterials in hot research areas such as metamaterial-inspired microwave components, antenna applications and imaging. This contribution is meant to provide an updated overview of complex microwave engineering for the generation of different types of metamaterials and their application in topical electromagnetic scenarios

State-of-the-art of Metamaterials: Characterization, Realization and Applications

Metamaterials is a large family of microwave structures that produces interesting ε and μ conditions with huge implications for numerous electromagnetic applications. Following a description of modern techniques to realize epsilon-negative, mu-negative and double-negative metamaterials, this paper explores recent literature on the use of metamaterials in hot research areas such as metamaterial-inspired microwave components, antenna applications and imaging. This contribution is meant to provide an updated overview of complex microwave engineering for the generation of different types of metamaterials and their application in topical electromagnetic scenarios

Directional Dual-Band Slot Antenna with Dual-Bandgap High-Impedance-Surface Reflector

A compact dual-band high-impedance-surface EBG structure is employed as a reflector for a dual-band annular-slot antenna. The reflector comprises an array of miniaturized EBG cells which utilizes square patches augmented by four S-shaped corrugated arms to reduce the resonant frequency of the proposed EBG structure. In order to broaden the bandwidth and adjust the frequency ratio for the dual-band EBG structure, a log-periodic spacing between the S-shaped strips is introduced. The combination of microstrip-fed annular slot and EBG reflector provides directional properties for both frequency bands with reduced size and low-profile

Passive Self-Interference Suppression for Single Channel Full-Duplex Operation

Full duplex radios have become a topic of increased interest in the wireless communications community. As part of this development, many efforts were directed to passively decrease the self-interference level at the antenna outputs. However, in many proposed solutions transmission and reception occur through different propagation channels. This paper demonstrates and quantifies the negative impact of channel differentiation on pivotal applications of full-duplex radio, such as cognitive radio. Antenna designs used for self-interference suppression in full-duplex radio architectures are analyzed. In order to ensure that transmission and reception occur within the same propagation channel, the use of the envelope correlation coefficient is proposed. The paper aims to firstly define the problem and proposes a metric to quantitatively assess full-duplex antenna designs. This is followed by analysis and discussion of representative full-duplex solutions (i.e. their antenna and passive RF feed components) proposed in the literature. Finally, it is demonstrated for the first time, that the passive non-magnetic self-interference suppression comes at the cost of increased losses in the structure. A theoretical upper limit for the antenna efficiency is proposed. Its consistency is verified for three representative full-duplex antenna designs which are highly documented in the literature

Ultra-wideband antennas

The focus of UWB antenna research activity has matured in recent years and currently mainly concentrates on applications such as biomedicine and security. Early UWB antenna designs were driven by the FCC allocation of spectrum in 2002 and focussed on obtaining wide impedance bandwidths with reasonable group delay characteristics. Many of these were simple planar monopoles antennas with canonical geometries. The emergence of new applications channelled the emphasis towards miniaturisation and integration into devices. This required optimisation of the antenna geometries to ensure that good system performance is achieved from the integrated antenna. Many optimisation techniques are available including the spline technique to generate the outline of the antenna element and ground plane. Simple methods based on genetic algorithms are employed and evolutionary algorithms which are capable of optimising for multiple goals are beneficial when multiple antenna parameters are simultaneously investigated. These techniques have proven advantageous especially when time-domain performance is critical and provide solutions for both single-ended and differential feed arrangements. The main applications using UWB channels in the 3.1 GHz −10.6 GHz spectrum are localization and tracking applications, mainly employing impulse radio UWB imaging, and generally using linear polarization. However circularly-polarized UWB antennas have been developed, both directional and omnidirectional and are being investigated across various systems

Analysis and Characterization of Finite-size Curved Frequency Selective Surfaces

Frequency Selective Surfaces (FSSs) are spatial filters that are largely employed in radomes for radars and antennas where high performance is necessary. FSS can be categorised in terms of their frequency behaviour. Band-pass FSS structures are able to let the electromagnetic wave pass in certain frequency ranges and shield others, which are especially suitable for out-of-band stealth of radomes. The properties of frequency selectiveness of these screens are used at microwave and infrared wavelengths. By tuning the electrical size and geometry of the unit-cell (patch or aperture) different frequency behaviours of the structure are generated. Although their analysis and simulation is significantly simplified with the approximation of infinite and planar array, in some real applications FSS are required to be conformal to limited non-planar structures. The literature on the problem of characterizing finite-size and curved FSS has been sparse and more focused on the single application rather than on a unified approach. In the aim of synthesizing valuable techniques, this paper reviews significant research results on truncated and curved FSS which were presented in the dedicated literature

Radial EBG Cell Layout for GPS Patch Antennas

A novel radial layout for mushroom-like electromagnetic-bandgap (EBG) cells surrounding a printed circularly-polarised patch antenna is proposed. Two radial EBG configurations surrounding a circular patch are compared to a reference patch on a conventional ground plane of the same dimension. The radial shape and displacement of the EBG cells around the patch offers improvements in terms of gain and axial-ratio compared to the reference antenna and is more suitable for circular geometries compared to conventional Cartesian layouts. In particular, the distance between the patch and the surrounding EBG cells is independent of the cell period, which can be arbitrarily chosen, and the overall layout offers footprint reduction

Analysis and Characterization of Finite-size Curved Frequency Selective Surfaces

Frequency Selective Surfaces (FSSs) are spatial filters that are largely employed in radomes for radars and antennas where high performance is necessary. FSS can be categorised in terms of their frequency behaviour. Band-pass FSS structures are able to let the electromagnetic wave pass in certain frequency ranges and shield others, which are especially suitable for out-of-band stealth of radomes. The properties of frequency selectiveness of these screens are used at microwave and infrared wavelengths. By tuning the electrical size and geometry of the unit-cell (patch or aperture) different frequency behaviours of the structure are generated. Although their analysis and simulation is significantly simplified with the approximation of infinite and planar array, in some real applications FSS are required to be conformal to limited non-planar structures. The literature on the problem of characterizing finite-size and curved FSS has been sparse and more focused on the single application rather than on a unified approach. In the aim of synthesizing valuable techniques, this paper reviews significant research results on truncated and curved FSS which were presented in the dedicated literature