Millimetre wave waveguide enclosed grid frequency multipliers and imaging

Doctoral thesis, 2016

The utilisation of the THz spectrum ( 0.3-10 THz ) is hampered by fundamental difficulties in generating power at these frequencies. Applications within such diverse fields as radio astronomy, security imaging, life sciences, high data rate communications and production monitoring could benefit significantly from compact, high power THz signal sources operating at room temperature. This thesis reports on the design and fabrication aspects of varactor diode based waveguide enclosed grid frequency multipliers. The goal of this work has been to design, fabricate and characterise high power varactor grid frequency multipliers, enabling increased power handling capabilities and output power of future THz frequency multipliers. This approach is expected to offer excellent frequency and power scalability for THz signal sources. A tunable 240 - 290 GHz 72 element grid HBV frequency tripler is presented together with measurement data. With an output power of 35 mW at 247 GHz this is the highest frequency of operation reported to date for waveguide enclosed grid frequency multipliers. Furthermore, a 128 Schottky diode frequency doubler grid is presented, with a measured peak output power of 0.25 W at 183 GHz. With a peak conversion efficiency of 23 % it is the most efficient waveguide enclosed grid multiplier reported to date. A modelling approach using parallel sets of cascaded unit cells has been successfully developed as well as a full 3D simulation for grid frequency multipliers. Two application examples that could benefit from future high power THz sources are also presented. The first one is a 346 GHz imaging system using an imaging algorithm based on the Born approximation to produce images with a sub wavelength pixel size of 0.1×0.1 mm2. The second application example is a FMCW radar transceiver operating at 340 GHz, packaged into a compact modular system designed for array integration.