Techniques for the measurement and decomposition of the time varying narrow bandwidth transfer function of an HF sky wave transmission

A technique is presented for the measurement and decomposition into component propagation modes of the time varying narrow bandwidth (10 kHz) transfer function of an HF sky wave frequency modulated continuous wave (FMCW) transmission at 15.085 MHz. One event on a 5244-km transmission path which exhibited nonselective fading within the 10-kHz bandwidth is analyzed and found to exhibit severe phase distortion due to multipath. A component transfer function for an individual propagation mode is also obtained by two-dimensional filtering of the signal in the Doppler frequency-time delay domain, resulting in a significant reduction in phase distortion. The filtering technique can be used on FMCW transmissions with a repetitive waveform for the time varying identification and measurement of component multipath modes with unresolvable group time delay differences.