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An investigation of the dispersion and attenuation characteristics of cylindrical structures supporting guided electromagnetic waves with low attenuation is described. The object of the investigation is to understand how the cross-sectional shape and the nature of the boundary conditions affects the propagation characteristics. Attention is directed towards structures supporting the least number of propagating modes under the conditions which yield low attenuation over a reasonable bandwidth. Elliptical waveguides with both smooth-walls and corrugated walls are studied in detail. This reveals errors in previous-theories which are corrected. Some aspects of corrugated rectangular and circular waveguides are considered. Potential low attenuation waveguides such as the dielectric lined and dielectric waveguides are evaluated.

Concealed weapon detection (CWD) has been a hot topic as the concern about pub- lic safety increases. A variety of approaches for the detection of concealed objects on the human body based on earth magnetic ¯eld distortion, inductive magnetic ¯eld, acoustic and ultrasonic, electromagnetic resonance, MMW (millimetre wave), THz, Infrared, x-ray technologies have been suggested and developed. Among all of them, MMW holographic imaging is considered as a promising approach due to the relatively high penetration and high resolution that it can o®er. Typical concealed target detection methods are classi¯ed into 2 categories, the ¯rst one is a resonance based target identi¯cation technique, and the second one is an imaging based system. For the former, the complex natural resonance (CNR) frequencies associated with a certain target are extracted and used for identi¯cation, but this technique has an issue of high false alarm rate. The microwave/millimetre wave imaging systems can be categorized into two types: passive systems and active sys- tems. For the active microwave/millimetre wave imaging systems, the microwave holographic imaging approach was adopted in this thesis. Such a system can oper- ate at either a single frequency or multiple frequencies (wide band). An active, coherent, single frequency operation millimetre wave imaging system based on the theory of microwave holography was developed. Based on literature surveys and ¯rst hand experimental results, this thesis aims to provide system level parame- ter determination to aid the development of a target detection imager. The goal is approached step by step in 7 chapters, with topics and issues addressed rang- ing from reviewing the past work, ¯nding out the best candidate technology, i.e. the MMW holographic imaging combined with the resonance based target recog- i nition technique, the construction of the 94 GHz MMW holographic prototype imager, experimental trade-o® investigation of system parameters, imager per- formance evaluation, low pro¯le components and image enhancement techniques, feasibility investigation of resonance based technique, to system implementation based on the parameters and results achieved. The task set forth in the beginning is completed by coming up with an entire system design in the end. i

The ability to understand and control the propagation of electromagnetic radiation underpins a vast array of modern technologies, including: communication, navigation and information technology. Therefore, there has been much work to understand the interaction between electromagnetic waves and metal surfaces, and in particular to design materials the characteristics of which can be tailored to produce a desired response to microwave radiation. It is the objective of this thesis to demonstrate that patterning metal surfaces with sub-wavelength apertures can afford hitherto unrealised control over the reflection and transmission characteristics of materials which are an order of magnitude thinner than those employed historically. The work presented herein aims to establish ultra thin cavity structures as novel materials for the selective absorption and transmission of microwave radiation. Experimental and theoretical approaches are used to elucidate the mechanism that allows such structures to produce highly efficient absorption via the excitation of standing wave modes in structures that are two orders of magnitude thinner than the operating wavelength. Also considered is how this same mechanism mediates transmission of selected frequencies through similarly thin structures. Later chapters focus on ultra thin cavity structures which, through higher-order rotational symmetry, exhibit resonant absorption which is almost completely independent of incident and azimuthal angle and polarisation state. A detailed studied of the absorption bandwidth of these devices is also presented in the context of fundamental theoretical limitations arising from the thickness and magnetic permeability of the structure.

Experimental investigations of structurally-dictated surface waves supported by periodically textured metallic substrates with different symmetries, are the primary focus of the work presented in this thesis. The electromagnetic response of three near perfectly conducting substrates perforated with arrays of holes with different geometries,together with a low-profile high-impedance structure are characterised. Experimental measurement techniques are employed to record the transmission, and reflection from the structures under investigation, together with phase-resolved measurements to directly obtain the dispersion of the surface waves supported by these structures. From these measurements information about the nature of the surface modes supported by the structures under investigation can be observed. A study of diffractively coupled surface waves supported by a close-packed array of square cross-section, close-ended holes in the limit where the wavelength of incident radiation and periodicity of the hole array are comparable, is presented. An additional grating, which has a periodicity comparable to the hole array is used to control the strength of diffractive coupling to the mode. Using a free-space measurement technique,information about the dispersion of the modes supported by the structure is obtained by recording the azimuthal-dependent reflection from the structure. It is found that the relative positions of the hole array and `coupling-in' grating is significant, a key issue not addressed in the literature when investigating grating-coupling to surface modes. Good agreement with numerical predictions is demonstrated. Structurally-dictated surface waves on a metallic substrate pierced by a close-packed array of deep, rectangular holes is characterised. In this arrangement, the fundamental resonance in the holes in the orthogonal directions is different and the frequency therefore to which the dispersion of the surface waves supported by the structure is limited, varies with sample orientation. The anisotropic dispersion, resulting from an ellipsoid of limiting frequencies, is directly mapped using a phase-resolved measurement technique. Furthermore by exploiting the anisotropy of the unit cell, a family of higher order surface waves associated with the quantisation of the electromagnetic fields within the holes is explored in this chapter. Once again good agreement with numerical predictions is shown.The `enhanced transmission' recorded through a `zigzag' hole array, attributed to the excitation of diffractively coupled surface waves, is explored. Due to the specific symmetry of the unit cell of the zigzag hole array it is shown that coupling to these surface waves can be achieved with both transverse magnetic and transverse electric polarised incident radiation. Further, incident radiation can directly couple to the surface modes supported by the zigzag hole array, via scattering from its inherent in-plane periodicity. The observed polarisation-selective excitation of individual surface wave bands, agrees well with numerical predictions and is shown to be a direct consequence of the reduced symmetry of the system. Finally, the dispersion of the modes supported by an ultra-thin, high-impedance surface in the form of a Sievenpiper `mushroom' structure, with rectangular geometry is directly recorded. The behaviour of the Sievenpiper structure is rather complex and to aid understanding of the electromagnetic response of the structure, the results are compared with the modes supported by a simpler patch array structure. The anisotropy arising from the rectangular geometry is characterised and an in depth discussion of the origin of the modes presented.

Matter can be identified by its interaction with electromagnetic fields. This can be described by its dielectric and magnetic properties, which typically vary with respect to frequency in the microwave region. Microwave-frequency spectroscopy is capable of making non-contact, non-destructive, non-invasive and label-free measurements with respect to time. It can be used to characterise all states of matter and combinations thereof, such as colloids and microparticulate suspensions. Sensors based upon this technology therefore have great potential for (bio)chemical and industrial point-of-sampling applications where existing measurement techniques are insufficiently portable, low-cost or sensitive. Microfluidics is the manipulation of fluids within microscale geometries. This gives rise to phenomena not observed at the macroscale that can be exploited to achieve enhanced control of fluid flow. This means that microfluidic techniques can be used to perform complex chemistry in a completely sealed environment with minimal reagent consumption. Hence, microfluidics offers an ideal sample interfacing method for a microwave-frequency sensor. This work is concerned with developing novel, low-cost and highly sensitive probes that be easily integrated into a microfluidic device for performing on-chip sample preparation and diagnostics for generic (bio)chemical and industrial point-of-sampling applications. To this end, several novel microwave resonant structures were designed, optimised and integrated into microfluidic devices in order to characterise a variety of liquid-phase samples.

Microwave resonant sensors use the spectral characterisation of a resonator to make high sensitivity measurements of material electromagnetic properties at GHz frequencies. They have been applied to a wide range of industrial and scientific measurements, and used to study a diversity of physical phenomena. Recently, a number of challenging dynamic applications have been developed that require very high speed and high performance, such as kinetic inductance detectors and scanning microwave microscopes. Others, such as sensors for miniaturised fluidic systems and non-invasive blood glucose sensors, also require low system cost and small footprint. This thesis investigates new and improved techniques for implementing microwave resonant sensor systems, aiming to enhance their suitability for such demanding tasks. This was achieved through several original contributions: new insights into coupling, dynamics, and statistical properties of sensors; a hardware implementation of a realtime multitone readout system; and the development of efficient signal processing algorithms for the extraction of sensor measurements from resonator response data. The performance of this improved sensor system was verified through a number of novel measurements, achieving a higher sampling rate than the best available technology yet with equivalent accuracy and precision. At the same time, these experiments revealed unforeseen applications in liquid metrology and precision microwave heating of miniature flow systems.

Metamaterials are a class of artificial material, known to produce electromagnetic (EM) responses not found in nature due to their engineered subwavelength structure. In this thesis very thin subwavelength meshes are utilised to form layered metamaterials. The EM characteristics of the transmission and reflection response from these materials, including the polarisation converting behaviour, are explored to further understanding and develop structures to exploit and control the propagation of microwave radiation. Original experimental studies are presented across two sections; the first examines the response of stacks assembled from metallic meshes and dielectric plates; the second explores a rotated layered structure formed of square symmetric elements in a square subwavelength array that demonstrates chirality through evanescent coupling of the near fields. When metallic meshes are excited with EM radiation below the cut off frequency, only evanescently decaying fields are supported in the holes. By combining these subwavelength metallic meshes with dielectric plates in different arrangements, remarkably wide bands of high transmission and low reflection may be observed. The non-interacting resonant modes allow the response to be tuned through a suitable choice of the metallic mesh geometry and the properties of the dielectric. Further the low frequency band edge and the bandwidth are not dependent on the number of unit cells in the stack; but are dependent on the properties of the unit cell. The second section demonstrates ``evanescent handedness'' proposed as a new type of chirality. Two subwavelength square arrays of square elements are rotated with respect to one another. When the rotated arrays are positioned far from one another in the propagation direction, each acts as an effective medium layer. However when placed in close proximity the structure is shown to rotate the plane of polarisation of the incident radiation. All these mesh based structures share the property of producing an EM response that is tunable by design, allowing a structure to be tailored to a specific application.

Helical devices are employed in the area of microwave technology. Applications such as filters and travelling-wave tubes are some of these in which helical devices are used. Ferrite loaded helical devices have been of interest to scientists and engineers throughout the last half century. The structure that is considered in this thesis is of a helix surrounded by a ferrite tube, both of which are enclosed in a cylindrical waveguide. Maxwell's equations for electromagnetism are employed in order to derive the expressions for the electric and magnetic fields. The parameters of the structure are varied in order to observe how certain factors will affect the dispersiveness, loss and phase shift of the structure. The investigation considers the effect of varying the applied magnetic field. The investigation also incorporates an air gap between the ferrite and helix and between the ferrite and metal waveguide. For the first time, the affects of these air gaps are analysed.