描述
在無線通訊的實際應(yīng)用中,為有效提高通訊效果,減少天線輸入功率,天線會做成各種帶有輻射方向性的結(jié)構(gòu)以集中輻射功率,由此就引申出“天線增益”的概念。簡單說,天線增益就是指一個天線把輸入的射頻功率集中輻射的程度,顯然,天線的增益與其方向圖的關(guān)系很大,主瓣越窄、副瓣越小的天線其增益就越高,而不同結(jié)構(gòu)的天線,其方向圖的差別是很大的。
在通訊技術(shù)領(lǐng)域,與其它考量功率、電平等參數(shù)的量值同樣,天線增益也采用相對比較并取對數(shù)的簡化法來表示,具體計算方法為:在某一方向向某一位置產(chǎn)生相同輻射場強的時,對無損耗理想基準(zhǔn)天線的輸入功率與待考量天線的輸入功率的比值取對數(shù)后乘以10(G=10lg(基準(zhǔn)Pin/考量Pin)),即稱為該天線在該點方向的增益。常用衡量天線增益的單位是dBi和dBd。對于dBi,其基準(zhǔn)為理想的點源天線,即一個真正意義上的“點”來作天線增益的對比基準(zhǔn)。理想點源天線的輻射是全向的,其方向圖是個理想的球,同一球面上所有點的電磁波輻射強度均相同;對于dBd,其基準(zhǔn)則為理想的偶極子天線。因偶極子天線是帶有方向性的,故二者有個固定的恒差2.15即0dBd=”2″.15dBi。
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