測(cè)量采用Baluns

　　當(dāng)測(cè)量天線，安裝在小的介質(zhì)的特點(diǎn)，結(jié)果是通過(guò)連接同軸電纜存在的影響。本文探討的原因，描述和量化使用套筒扼流圈（倫）來(lái)減少這種誤差。

　　平衡天線

　　大多數(shù)工程師會(huì)認(rèn)識(shí)到，直接將一個(gè)簡(jiǎn)單的偶極天線（一個(gè)基本平衡和對(duì)稱(chēng)的結(jié)構(gòu)）直接連接到同軸饋電電纜是不好的做法；然而，當(dāng)被問(wèn)及為什么這是不受歡迎的，他們是模糊的原因。

　　最簡(jiǎn)單的解釋見(jiàn)于圖1中。這里我們看到一個(gè)偶極子連接到一根同軸電纜上。對(duì)稱(chēng)的建筑產(chǎn)生相等的電容電抗每個(gè)肢體的偶極子天線和同軸電纜的外導(dǎo)體之間。問(wèn)題產(chǎn)生于這樣一個(gè)事實(shí)，即根據(jù)所示的安排，在兩個(gè)電容之間存在的電壓，因此流過(guò)它們的電流并不相等。在圖1中，左側(cè)的肢體直接連接到外部導(dǎo)體，因此，至少在靠近電纜的區(qū)域中，相對(duì)電位很小，而在右側(cè)，在偶極子肢的內(nèi)側(cè)端與電纜的外導(dǎo)體之間存在完全的驅(qū)動(dòng)點(diǎn)電壓。

　　As the currents inside the cable on the inner and outer conductors must be equal， the difference between the currents in the two capacitances must be supported by a current flowing along the outside of the cable. The dipole now supports two current modes， a push-pull current （the expected mode） and a push-push current， which represents the unwanted mode introduced by the method of connection （Figure 2）。 The external current on the outer conductor of the cable will create radiated fields which degrade the null in the classical dipole pattern and reduce the cross polar discrimination of the dipole; the presence of the current will also modify the input impedance measured at the cable input. If the cable is flexible， then all these effects will vary as the configuration of the cable – especially the part close to the antenna – is varied.