Measurement of narrow pulse distortion over a short HF skywave link: Es and F2 summer results

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Abstract
A high resolution frequency modulated, continuous wave (FMCW) oblique incidence sounder was used in an experiment to measure the minimum pulse width that could be transmitted via a one‐hop F2 layer and Es propagated ionospheric paths. Measurements were made over a 234‐km link in the United Kingdom by using 3 MHz frequency sweeps at 1 MHz/s sweep rate and repeated at 10‐s intervals for 10 min. The collected data were weighted and spectrum analyzed via the fast Fourier transform (FFT) algorithm to a 8–13 μs time resolution. This permitted the resolution of the different multipath components including the magnetoionic waves. Narrow pulse distortion, and consequently the instantaneous bandwidth, was estimated by evaluating the 6‐dB width of the compressed pulses and the slope of the group time delay versus frequency of the Es and one‐hop F2 ordinary and extraordinary waves. The time history of the different modes was obtained by analyzing repetitive sweeps.The results indicated that the sporadic E layer introduced less distortion and was more stable with time than the F2 layer. Moreover, the slope of the group time delay with frequency was often found to give a more accurate estimate of the dispersion suffered by narrow pulses than the 6‐dB width determined by Fourier transforming the FMCW data.

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