"....An incoming electromagnetic wave illuminates both an antenna and its feedline. Current induced on the outer surface of a coax shield may enter the feedline at the feedpoint. Common-mode signals induced on parallel transmission line may become differential at the receiver. Either makes the feedline an unintended part of the antenna structure, which can degrade the directive pattern and forward gain. A current balun can attenuate unwanted coax shield current. A current or voltage balun can attenuate common-mode signals on parallel line...."
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