HF radio signal fading and atmospheric radio noise measurements at low latitudes



[1] Two HF broadcast transmissions, one originating from Sri Lanka (7.1°N, 79.9°E) at 11.800 MHz around the geomagnetic equator and the other from BBC, Masirah (20.6°N, 58.9°E), at 15.310 MHz several degrees north of the geomagnetic equator were monitored at Tirupati (13.5°N, 79.5°E) in southern India to compare the fading characteristics of the links. It is found that the link over the geomagnetic equator exhibits much higher fade rates as compared to the other link. The fade ranges also are, in general, higher for Sri Lanka. Fade allowances derived from these data do indicate that margins higher than those recommended by the International Telecommunication Union Radiocommunication Sector (ITU-R) need to be provided at these latitudes. The diurnal variation in the atmospheric radio noise (ARN), which sets a lower limit to the satisfactory reception of signal, over the Indian subcontinent is in total disagreement with the variations obtained at higher latitudes. This rather puzzling phenomenon has been interpreted as being due to path attenuation differentials between the LOS mode and the ionospheric mode from the lightning source because of significantly greater cloud height in the Indian region.