Characteristics of VHF Propagation at 50 km. Beyond Radio Horizon

ABSTRACT

Observations for a period of fifteen months on VHF propagation at a distance of 50 km. beyond radio horizon are reported here. The annual cycle of monthly median signal exhibits two peaks in February and August and two valleys in May and October. The monthly median for morning hours is generally higher than for afternoon hours. Correlation of transmission loss with ΔN (1 km.), i.e. difference between refractivity at the surface and at a height of 1 km., has been found to be 0·6. Correlation with surface refractivity N_s is poor and with surface relative humidity, RH, is good if monsoon months are excluded. Monthly median transmission loss has been observed to change by 0·5 db. per unit change of monthly mean ΔN (1 km.). Within-the-hour fading range increases with increase of short-term median value of signal. The ratio of scattered to diffracted component of signal estimated from within-the-hour fading range agrees well with values predicted by means of empirical formulas. The long-term distribution of daily median signal is found to be log-normal. Diffraction is the dominant mechanism of propagation over this path and scattering accounts for the fading of the signal.