Ultra-wideband antenna design for microwave imaging applications. Design, optimisation and development of ultra-wideband antennas for microwave near-field sensing tools, and study the matching and radiation purity of these antennas within near field environment.

Near field imaging using microwave in medical applications has gain much attention recently as various researches show its high ability and accuracy in illuminating object comparing to the well-known screening tools such as Magnetic Resonance Imaging (MRI), digital mammography, ultrasound etc. This has encourage and motivate scientists continue to exploit the potential of microwave imaging so that a better and more powerful sensing tools can be developed. This thesis documents the development of antenna design for microwave imaging application such as breast cancer detection. The application is similar to the concept of Ground Penetrating Radar (GPR) but operating at higher frequency band. In these systems a short pulse is transmitted from an antenna to the medium and the backscattered response is investigated for diagnose. In order to accommodate such a short pulse, a very wideband antenna with a minimal internal reflection is required. Printed monopole and planar metal plate antenna is implemented to achieve the necessary operating wide bandwidth. The development of new compact printed planar metal plate ultra wide bandwidth antenna is presented. A generalized parametric study is carried out using two well-known software packages to achieve optimum antenna performance. The Prototype antennas are tested and analysed experimentally, in which a reasonable agreement was achieved with the simulations. The antennas present an excellent relative wide bandwidth of 67% with acceptable range of power gain between 3.5 to 7 dBi. A new compact size air-dielectric microstrip patch-antenna designs proposed for breast cancer detection are presented. The antennas consist of a radiating patch mounted on two vertical plates, fed by coaxial cable. The antennas show a wide bandwidth that were verified by the simulations and also confirmed experimentally. The prototype antennas show excellent performance in terms the input impedance and radiation performance over the target range bandwidth from 4 GHz to 8 GHz. A mono-static model with a homogeneous dielectric box having similar properties to human tissue is used to study the interaction of the antenna with tissue. The numerical results in terms the matching required of new optimised antennas were promising. An experimental setup of sensor array for early-stage breast-cancer detection is developed. The arrangement of two elements separated by short distance that confined equivalent medium of breast tissues were modelled and implemented. The operation performances due to several orientations of the antennas locations were performed to determine the sensitivity limits with and without small size equivalent cancer cells model. In addition, a resistively loaded bow tie antenna, intended for applications in breast cancer detection, is adaptively modified through modelling and genetic optimisation is presented. The required wideband operating characteristic is achieved through manipulating the resistive loading of the antenna structure, the number of wires, and their angular separation within the equivalent wire assembly. The results show an acceptable impedance bandwidth of 100.75 %, with a VSWR < 2, over the interval from 3.3 GHz to 10.0 GHz. Feasibility studies were made on the antenna sensitivity for operation in a tissue equivalent dielectric medium. The simulated and measured results are all in close agreement

Wearable devices for microwave head diagnostic systems

Although current head imaging technologies such as magnetic resonance imaging (MRI) and computed tomography (CT) are capable of providing accurate diagnosis of brain injuries such as stroke and brain tumour, they have several limitations including high cost, long scanning time, bulky and mostly stationary. On the other hand, radar-based microwave imaging technology can offer a low cost, non-invasive and non-ionisation method to complement these existing imaging techniques. Moreover, a compact and wearable device for microwave head imaging is required to facilitate frequent or real-time monitoring of a patient by providing more comfort to the patient. Therefore, a wearable head imaging device would be a significant advantage compared to the existing wideband microwave head sensing devices which typically utilise rigid antenna structure. Furthermore, the wearable device can be integrated into different microwave imaging setups such as real-time wearable head imaging systems, portable systems and conventional stationary imaging tools for use in hospitals and clinics. This thesis presents the design and development of wearable devices utilising flexible antenna arrays and compact radio frequency (RF) switching circuits for wideband microwave head imaging applications. The design and characterisation of sensing antennas using flexible materials for the wearable head imaging device are presented in the first stage of this study. There are two main variations of monopole antennas that have been developed in this research, namely trapezoidal and elliptical configurations. The antennas have been fabricated using different flexible substrate materials such as flexible FR-4, polyethylene terephthalate (PET) and textile. Wideband performances of the antennas have been achieved by optimising their co-planar waveguide feeding line structures. Importantly, the efficiencies of the fabricated antennas have been tested using a realistic human head phantom by evaluating their impedance matching performances when operating in close proximity to the head phantom. The second stage of the study presents the development of wearable antenna arrays using the proposed flexible antennas. The first prototype has been built using an array of 12 flexible antennas and a conformal absorbing material backed with a conductive sheet to suppress the back lobe radiation of the monopole antennas. Additionally, the absorber also acts as a mounting base to hold the antennas where the wearable device can be comfortably worn like a hat during the measurement and monitoring processes. The effect of mutual coupling between adjacent antennas in the array has been investigated and optimised. However, the use of the absorbing material makes the device slightly rigid where it can only be fitted on a specific head size. Thus, a second prototype has been developed by using a head band to realise a stretchable configuration that can be mounted on different sizes of human heads. Furthermore, due to the stretchable characteristic of the prototype, the antennas can be firmly held in their positions when measurements are made. In addition, fully textile based sensing antennas are employed in this prototype making it perfectly suitable for monitoring purposes. Low cost and compact switching circuits to provide switching mechanism for the wearable antenna array are presented in the third stage of this study. The switching circuit is integrated with the antenna array to form a novel wearable microwave head imaging device eliminating the use of external bulky switching network. The switching circuit has been built using off-the-shelf components where it can be controlled wirelessly over Bluetooth connection. Then, a new integrated switching circuit prototype has been fabricated using 6-layer printed circuit board (PCB) technology. For the purpose of impedance matching for the radio-frequency (RF) routing lines on the circuit, a wideband Microstrip-to-Microstrip transition is utilised. The final stage of this study investigates the efficacy and sensitivity of the proposed wearable devices by performing experiments on developed realistic human head phantoms. Initially, a human head phantom has been fabricated using food-based ingredients such as tap water, sugar, salt, and agar. Subsequently, lamb’s brains have been used to improve the head phantom employed in the experiments to better mimic the heterogeneous human brain. In terms of imaging process, an interpolation technique developed using experimental data has been proposed to assist the localisation of a haemorrhage stroke location using the confocal delay-and-sum algorithm. This new technique is able to provide sensible accuracy of the location of the blood clot inside the brain. The wearable antenna arrays using flexible antennas and their integrations with compact and low cost switching circuits reported in this thesis make valuable contribution to microwave head imaging field. It is expected that a low-cost, compact and wearable radar-based microwave head imaging can be fully realised in the future for wide range of applications including static scanning setup in hospitals, portable equipment in ambulances and as a standalone wearable head monitoring system for remote and real-time monitoring purposes