広島および阿蘇におけるスポラディックE層および大気ダクトによるVHF帯異常伝搬の観測

摘要

In non line-of-sight VHF radio wave observation, anomalous propagations due to ionospheric sporadic-E (Es propagation) and tropospheric duct (tropospheric ducting) have been frequently observed. They are known to cause an interference problem in the television, radio broadcasts and wireless communications, so that it is important to understand them. In order to make clear the characteristics of the anomalous propagations, we have observed over-horizon FM radio waves in VHF band for six years in Hiroshima and Aso. For analyses of large amounts of data set observed over years, it is important to classify and extract the radio waves automatically. In this paper, we show a method to classify above two anomalous propagations automatically. Procedures of the method are based on their propagation characteristics and received signal strength. Further the anomalous propagations and broadband noises are separated using dual frequency method. In order to evaluate the method, we examined the occurrence of the Es propagation and tropospheric ducting detected by the above method using the data observed from 2005 to 2010. It was found that the Es propagations were mostly observed in summer season and more frequently observed months in a year were June and July. Further, they were frequently observed from 10 to 12 and from 16 to 18 o'clock in a day. These results had the same tendency as past observations at mid-latitude. On the other hand, the tropospheric ducting was observed from night to morning time in spring and fall at Aso observatory. These results were consistent with past observations of occurrences of inversion layer in the troposphere from fall to spring. In contrast, it was confirmed that there were few tropospheric ducting in summer season while the past observations reported the inversion layer occurred. Since propagation characteristics of the FM radio waves have been clearly shown, it was confirmed that the classification method worked effectively.%1．はじめにrn　　　　これまで短波帯からミリ波帯を利用した通信が多くあり、その伝搬に影響を与える現象についrn　　て多くの研究が行われてきた。特にVH下帯は、地上TV放送のディジタル化に伴う周波数再編でrn　　新たなサービスが期待されている（凡打C血仁和pa打，2011など）。VHF帯近傍の電磁波の伝搬に大きな影響を与える現象として、電離層反射（上ねI元旦1990；C℃甥1992など）やダクト伝搬（」牧聯血rn　et al，1966；Hosoya，1999）などがある。これらの異常伝搬は、テレビ放送やFM放送に混信を生rn　　じさせる等から、本論文ではその発生の特性を理解するために、広島および阿蘇においてFM放rn　　送波の見通し外異常伝搬の観測を行った。