The relationship between signal intensity and maximum observed frequency in long-distance propagation of VHF radio waves based on the reflection from the ionosphere

摘要

Observation of VHF radio waves with high frequency resolution has been carried out at Tateyama, Japan. 48.25 MHz radio waves transmitted from Bangkok, Thailand, located in the distance of 4500 km from Tateyama were frequently received in 2001. These radio waves have a carrier of TV broadcast used in Thailand and vestigial side band whose bandwidth is 6 MHz. It is found that, simultaneously with the reception of the radio wave, maximum observed frequency (MOF) increased in spring, autumn, and winter. Since the electron density in the ionosphere was large in these seasons in 2001, the reception of the radio wave is obviously due to one-hop F_2 reflection. Simultaneous observations of both the reception of the radio wave and the enhancement of MOF have never been reported in the previous studies. Using ray tracing calculation, it is confirmed that this reception is due to the reflection from the ionosphere. The increase of the signal intensity corresponds to the crossing of the skip distance at Tateyama.