[电] directivity of a directional cou-pler

dān xiàng ǒu hé qì de zé xiàng xìng ㄉㄢ ㄒㄧㄤˋ ㄡˇ ㄏㄜˊ ㄑㄧˋ ㄉㄜ˙ ㄗㄜˊ ㄒㄧㄤˋ ㄒㄧㄥˋ