SPECTRUM ANALYSIS OF ANOMALOUS PROPAGATION OF FM RADIO BROADCASTING WAVES POSSIBLY RELATED TO EARTHQUAKES

Abstract

. We have observed electromagnetic waves in VHF (Very High Frequency) band at Tateyama in Chiba prefecture to investigate correlation between the anomalous propagation of FM radio broadcasting waves and earthquakes. A tuning meter· of a commercial FM receiver and a spectrum analyzer were used simultaneously to receive the waves in the frequencies at 77.2 ± 0.5 MHz. We investigated two cases of anomalous propagation of FM radio broadcasting waves. One case showed slow fluctuations with about 10-minute period of baseline level of the tuning meter outputs and the other showed rapid fluctuations with less than I-sec period. We calculated correlation between the output of the tuning meter and the power received in the frequency band with the spectrum analyzer. The results of the spectrum analysis of the observed data reveal that the origins of the baseline fluctuations in these two cases are signals at different frequencies, i.e. the origin of the slow fluctuation is radio waves at 77 .1 MHz from Sendai, while that of the rapid fluctuation is waves at 77 .5 MHz from Niijima. This fact could imply that mechanisms of two cases of fluctuations are different from each other.