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試解釋天波電離層傳播的多徑效應和衰落現象,通信靜區(qū)和盲區(qū)。

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多徑效應:由于電波從發(fā)射點到接收點的傳播并非只經過一條路徑,往往是既有天波又有地波,而且天波也因反射的電離層不同和反射的次數不同,傳播的路徑也不同。因此到達接收地的電波場強是各條路徑電波的矢量和。衰落現象:因漫射波的傳播路經不同,而隨電離層的反射高度而變動,故各條路徑來的信號不能保持固定的相位差是疊加后的信號幅度發(fā)生變化。通信靜區(qū)(盲區(qū)):在短波波段,地面波隨距離的增加衰減很快,地波能到達最遠距離為D1,當一定頻率發(fā)射的電波以一定的入射角向電離層投射式隨入射角的減小,反射波到達地面處也愈來愈靠近發(fā)射點。進一步減小入射角,電波將穿透電離層,天波傳播反射到地面站有一個最小距離D2。通信靜區(qū)就是由半徑為D1,D2組成的環(huán)形區(qū)域

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