Wave dispersion measurement of helically corrugated waveguides using Vector Network Analyser

Gyrotron travelling wave amplifiers, oscillators and microwave pulse compressors based on helically corrugated waveguides have been developed at the University of Strathclyde at X-band Frequencies (8.2GHz to 12.4GHz). This proposal intends to study the use of helically corrugated waveguides to investigate the scalability of this concept to higher Ka-band (26.5GHz to 40GHz) and W-band (75GHz to 110GHz) frequencies. Two substantial problems immediately present themselves as one looks to increase the frequency, namely the requirements to fabricate an adequately accurate profile with a sufficiently smooth surface to produce the required dispersive properties with a tolerable loss. At the lower Xband frequencies hitherto investigated CNC mills have been used to produce the required 3D surface, but at higher frequencies the proportionately reduced tolerances plus the reduced clearances for tool access will prohibit this technique. The project will therefore investigate the alternative fabrication techniques available, including hydraulic and mechanical forming, electroforming, casting and spark erosion, all combined with electropolishing and thin surface plating. The fabricated waveguide sections will be measured by a Vector Network Analyser (VNA). The VNA will be used to measure the wave dispersion of helical structures by comparing the evolution of the principle value of the phase angle of the radiation as a function of frequency as it propagates through a known length of the waveguide. Measurements of the wave dispersion of amplifier and oscillator interaction regions and helical compressor sections will be benchmarked and compared with the predictions of theory and modelling.

Millimetre-waves are similar to the waves employed in microwave ovens but they are shorter wavelength. This project aims to understand, measure and use millimetre-waves with the eventual aim of producing new devices and sources of millimetre-waves that will have significant impact in several application areas including health sciences, defence and security.

It has been shown that by using a carefully designed helical waveguide two modes of wave propagation can be coupled together to provide a new synthesized mode of propagation.
This result has been clearly demonstrated by theoretically designing such a helical waveguide and then measuring its dispersion using a Vector Network Analyser.

The state-of-the-art Vector Network Analyser that was used to successfully achieve the findings of this project was borrowed from the EPSRC equipment loan pool, resulting in a very cost effective research project, which has resulted in a continuing fundamental research programme and transfer of the acquired knowledge to industry.