A new formulation for above-the-MUF loss

When existing HF ionospheric propagation predictions are compared with measurements, the monthly median signal strength is found to be in error by more than 20 dB in an appreciable number of cases. Probably the main reason for such large discrepancies is the unrealism of any existing treatment of above-the-MUF (maximum usable frequency) loss. A programme of measurements of HF signal and noise intensities on path lengths of 120-2300 km, primarily directed to the study of fading statistics, is providing information relevant to the determination of above-the-MUF loss, because constraints on the frequencies which could be transmitted resulted in the operating frequency equalling and exceeding the basic MUF on a significant number of occasions. The experimental data and the theory support the hypothesis that the main physical mechanism involved in above-the-MUF propagation is ground side-scatter, rather than ionospheric forward-scatter