Summary form only given. There has been a drive in recent years to produce ultra-high power short pulses for a range of applications. These high power pulses can be produced by microwave pulse compression. Sweep-frequency based microwave pulse compression using smooth bore hollow waveguides is one technique of pulse compression, however at very high powers this method has some limitation due to its operation close to cut-off. In optimum cases, the frequency at the beginning of an input pulse should be only 0.5-1% above the cutoff frequency. If a Cherenkov type TWT is used to drive the compressor then it is inevitable that the low-frequency part of the amplification band is below the cutoff which then reflected from the compressor back to the amplifier resulting in its possible parasitic self-oscillation. If a relativistic BWO is used as a source of frequency-modulated pulses for a smooth-waveguide compressor, then the necessary frequency sweep that would be required can only be produced by using a difficult-to-realize voltage modulation on the BWO. The two combined problems of wave reflection from a compressor and optimum frequency modulation can be solved using a waveguide with a special helical corrugation of its inner surface. A special helical corrugation of a circular waveguide ensures an eigenwave having a strongly frequency-dependent group velocity far from cut-off, which makes the helically corrugated waveguide attractive for using as a compressor of pulses from very high power amplifiers and oscillators. The results of proof-of-principle experiments and calculations of the wave dispersion using a PIC code are presented. In the experiments a 70 ns 1 kW pulse from a conventional TWT was compressed in a 2 metre long helical waveguide. The compressed pulse had a peak power of 10.9 kW and duration of 3 ns. In order to find the optimum pulse compression ratio the waveguide's dispersion characteristics must be well known. The dispersion of the helix was calculated using the PIC code MAGIC and verified using an experimental technique. For a helical compressor the optimum negative frequency sweep can quite naturally be realized at the falling edge of a relativistic BWO pulse. For example, if a BWO generates hundreds of MW during 50 ns over a voltage drop fro- m 700 kV to 400 kV, this frequency modulated radiation can be compressed up to 10-20 times in power with efficiency higher than 50%. Future work detailing plans to produce short, ultra-high power (multi-GW) pulses discussed.