VHF Propagation at Short Distance Beyond Radio Horizon

ABSTRACT

Observations on VHF propagation at an angular distance of five milliradians are reported here. The field beyond horizon generally consists of a slow varying component and a rapid-varying Rayleigh distributed component caused by atmospheric scattering. Analysis of short-term fading range reveals that scattering is relatively unimportant at this distance.

The monthly median path loss for afternoon hours over a period of six months shows low field strength during summer months increasing with a rise in humidity and also in winter. The monthly median transmission loss shows a correlation coefficient of about 0·85 with monthly average relative humidity, but the correlation with surface refractivity is almost negligible. The cumulative distribution of daily median transmission loss shows that the long-term distribution of field strength is almost log-normal, but the skewness of the curve for about 30 per cent upper fractile group is much higher. This indicates that the mechanism of propagation, for about 30 per cent of the time is somewhat different from that effective under average condition.