Wideband measurements of coherence over an HF skywave link and implication for spread‐spectrum communication

Abstract

A wideband frequency‐modulated, continuous‐wave (FMCW) oblique incidence sounder was used in an experiment to measure the coherent bandwidth of a one‐hop F 2 layer propagated ionospheric path. The experimental data were collected over a 234‐km link in the United Kingdom by using frequency sweeps of up to 5 MHz repeated at 10‐s intervals. The path had the characteristics of quasi‐longitudinal propagation. The results indicated that the limiting factors to the coherent bandwidth are mainly polarization interference and frequency dispersion. Data on the coherent bandwidth and fade depth as a function of frequency and the time variation of the amplitude frequency response are presented.