Multipath propagation, and in particular the interference between the ordinary and the extraordinary waves, places a fundamental constraint on the performance of wideband HF skywave radio links. Furthermore, the dispersive nature of ionospheric propagation causes phase nonlinearity and hence distortion of narrow pulses. In the paper an FMCW wideband sounder built for the purposes of characterising the channel is described. Spectral analysis of the audio output of the sounder via the FFT algorithm is shown to permit measurement of the amplitude/frequency function, the polarisation bandwidth, the fade rate, the fade depth and the distortion of a narrow pulse, all for a desired isolated ionospheric propagation mode. The sounder was used to collect data over an oblique path in the UK. The results of applying the FFT processing technique to the experimental data are presented.