Terahertz source technology has been an active area of research for a number of years. This has helped develop continuous wave solid-state sources that are highly desirable in a wide range of applications spanning from Earth science to medical science. However, even with advances in terahertz technology, the generation of fundamental source power at these frequencies is still challenging. Promising electronic solid-state devices fall short in overcoming source power shortage due to electronic breakdown mechanism and fabrication limits at terahertz frequencies. The fundamental physical limitation of photonic devices, such as low photon energy, force cryogenic operation which at times is impractical. Schottky diode frequency multipliers often offer a very practical solution for generating continuous wave radiation based on solid-state technology. This harmonic source technology is today a most certain candidate for many applications where compactness and room temperature operation is desired. However, despite of all the advances in Schottky diode fabrication and their use in frequency multiplication, output power falls rapidly with increasing frequency. Thermal constrains, fabrication limits, assembly errors and parasitic losses all constitute changes that affect the performance of these devices and make it difficult to reproduce experimental data. To overcome these problems and progress towards the generation of milliwatts of power at terahertz frequencies, the study of existing methods to generate and handle high power is necessary. In the first part of the thesis, the design, fabrication and development of two Schottky diode-based frequency doublers is discussed. The work focuses on the generation of high-power sources that are capable of handling higher input powers while maintaining good thermal efficiencies. A detailed study into the machining tolerances, assembly errors and temperature effects are evaluated for the frequency doublers. High frequency effect such as velocity saturation is also addressed. Depending on the design frequency and power handling, two different circuit configurations are employed for the frequency doublers. While the high-power 80/160 GHz frequency doubler used a discrete flip-chip diode configuration, the 160/320 GHz frequency doubler employed an integrated diode membrane to mitigate sensitivity issues encountered during assembly and enable correlation between simulated and measured data. The second part proposes the use of millimeter-wave Schottky diode-based radiometers for imaging of composites samples. The focus of this experiment is the introduction of an alternate EM inspection method with the use of broadband Schottky diode components. This technique combines two different fields {--} non-destructive testing and radiometry, which presents a potentially new and interesting area for research. Since no single method can qualify to be the most accurate for all inspections, and with the future integration bringing down manufacturing costs of high frequency components, this demonstration presents a new approach to consider for future material imaging and evaluation experiments.

In many engineering applications, including radar, sonar, communications and seismology, the direction of impinging signal wavefronts can be used to discriminate between competing sources. Often these source signals cover a wide bandwidth and conventional narrowband beamforming techniques are ineffective, since spatial resolution varies significantly across the band. In this thesis we consider the problem of beamforming for broadband signals, primarily when the spatial response remains constant as a function of frequency. This is called a frequency invariant beamformer (FIB).¶ Rather than applying the numerical technique of multi-parameter optimisation to solve for the beamformer parameters, we attempt to address the fundamental nature of the FIB problem. The general philosophy is to use a theoretical continuous sensor to derive relationships between a desired FI beampattern and the required signal processing structure. Beamforming using an array of discrete sensors can then be formulated as an approximation problem. This approach reveals a natural structure to the FIB which is otherwise buried in a numerical optimisation procedure.¶ Measured results from a microphone array are presented to verify that the simple FIB structure can be successfully implemented. We then consider imposing broadband pattern nulls in the FI beampattern, and show that (i) it is possible to impose an exact null which is present over all frequencies, and (ii) it is possible to calculate a priori how many constraints are required to achieve a null of a given depth in a FIB. We also show that the FIB can be applied to the problem of broadband direction of arrival (DOA) estimation and provides computational advantages over other broadband DOA estimators.¶ Through the theoretical continuous sensor approach, we show that the FIB theory can be generalised to the problem of designing a general broadband beamformer (GBB) which realizes a broadband angle-versus-frequency beampattern specification. Coupled with a technique for radial beampattern transformation, the GBB can be applied to a wide class of problems covering both nearfield beamforming (in which the shape of the impinging wavefront must be considered and farfield beamforming (which is simplified by the assumption of planar wavefronts) for a broadband beampattern specified over both angle and frequency.

This research study presents a new vibration-based non-destructive global structural damage identification and condition monitoring technique that can be used for detection, localization and quantification of damage. A two-stage damage identification process that combines non-model based and model-based damage identification approaches is proposed to overcome the main difficulties associated with the solution of structural damage identification problems. In the first stage, performance assessment of various response parameters obtained from the time-domain, frequency-domain and spectral-domain analysis is conducted using a non model-based damage detection and localization approach. In addition, vibration response parameters that are sensitive to local and global damage and that possess strong physical relationships with key structural dynamic properties are identified. Moreover, in order to overcome the difficulties associated with damage identification in the presence of structural nonlinearity and response nonstationarity, a wavelet transform based damage-sensitive parameter is presented for detection and localization of damage in the space domain. The level of sensitivity and effectiveness of these parameters for detection and localization of damage are demonstrated using various numerical experimental data determined from one-dimensional and two-dimensional plate-like structures.