A Study of vhf Signal Variations in the Diffraction Region

Abstract

It is known that beyond the radio line-of-sight or the diffraction region as it is called, a vhf signal shows time variations which depend in a complex manner on different propagation mechanisms; both long term and short term fadings are observed. During a field strength survey of Amritsar TV transmitter (operating on TV band 3, channel 7: 188-195 MHz), it was found that the signal at Jullundur was showing fading characteristics of diffraction region. To study the phenomenon further about 50 hr of recording spread over a week at different hours of the day were made during the winter of 1973-74. The analysis of these recordings indicate that there is a regular diurnal variation of the signal, the signal being high in the early daytime, low at noon and early evening and tending to rise again 'towards the night. The diurnal variation of the signal shows good correlation with meteorological parameters. Apart from this long term variation, considerable short term fluctuations were also observed and the cumulative distribution of the field so obtained agreed well with theoretical one calculated for "temperate climate". Similar observations made at Bulsar on the Bombay TV transmitter (band 1, channel 4: 61-68 MHz) are also described.