Azimuthal Variation of Measured HF Noise

HF noise (2-8 MHz) as a function of azimuth, frequency, and time of day was measured in England during the winter of 1966-1967. For a given frequency interval eight unequal azimuthal sectors (with five sectors between 45°-180°) were sampled each four hours in a standardized fashion. Power levels for the tenth, fiftieth, and ninetieth percentiles were derived and plotted. The results provide the integrated total external noise (both local and propagated). In addition to the well-known distinctive diurnal and frequency changes, a marked variation with azimuth is present. A separate examination of noise intensities measured simultaneously from several azimuths reveals strong differences with azimuth. The investigation indicates that noise levels as a function of azimuth are needed and are the ones to be applied to practical HF systems.