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X-band passive microwave pulse compression is an interesting area of research in vacuum electronics. Applications include radar technology, plasma diagnostics etc. The scientific study of producing high-power nanosecond microwave pulses, using passive sweep-frequency compression, was undertaken. Two novel helically corrugated waveguides were investigated; 3-fold and 5-fold, respectively. The 3-fold structure couples a TE1.1 travelling wave with a near cut-off TE2.1 wave producing a region far from cut-off with a large change in group velocity as a function of frequency. A 2.08 metre long copper helical waveguide was used to compress a 80 ns, 5.5 kW frequency-swept pulse from a high power TWT (TMD PTC6321), driven by an arbitrary waveform generator and vector signal generator, to a 1.5 ns, 135 kW pulse containing -75% of the energy of the input pulse. To enhance the power capabilities of the microwave pulse compressor a more overmoded larger diameter 5-fold helical waveguide structure which adiabatically couples a TE3.1 traveling wave and a near cut-off TE2.2 wave was studied. Analysis of the dispersion characteristics carried out using CST MWS will be presented.