Observation results of non-line-of sight 77.1 MHz FM radio waves on three different paths for three years

Abstract

After the Hyogoken-nanbu earthquake in 1995, it was reported that the propagation anomalies of the non-line-of sight 77.1 MHz FM radio waves broadcasted from Sendai were detected. It was then hypothesized that the propagation anomalies were due to ionospheric or atmospheric disturbances. Since then, over-horizon FM radio waves have been observed to clarify the hypothesis at many electromagnetic (EM) observatories. Some of them were observed on one propagation path or in a period less than one year. Therefore, it is highly desired to deal with some more propagation paths and more data during several years. In this paper, we describe the observation method and results for the non-line-of-sight FM radio of 77.1 MHz broadcasted from Sendai to Nobeyama, Yokosuka, Hitachi EM observatories. The observation period was for 3 years from January 2006 to December 2008. From the observation results, it was found that the received levels had seasonal variations in their fluctuations. The fluctuation levels in summer season were larger than those in winter. Comparing the received levels of the 77.1 MHz and the aerological atmospheric refractivity, it was also clarified that the received level fluctuations were mainly affected by the change of the aerological atmospheric refractivity. The cross correlation factors among received levels observed at the 3 sites depended on the propagation profiles. There were high correlations more than 0.6 between the received levels at Yokosuka and Hitachi, while there were low correlations less than 0.4 between the received levels at Yokosuka and Nobeyama. It was concluded that the non-line-of-sight 77.1 MHz FM radio waves were characterized by terrestrial propagations with atmospheric refractivity variations.