Limited Effectiveness of Balancing a Coaxial Feeder with a Balun for Radio Frequencies

A balun is commonly used at radio frequencies to go from an unbalanced line to a balanced line and vice versa. Among other usages, it is commonly used to balance the coaxial cable feeding a symmetrical antenna. However, it occurs to be impossible to fully balance a coaxial cable with a balun. The reason for this is the current flow route through the outer layer of the coaxial cable shield and the coupling of this current with the antenna through the stray capacitances existing between the feeding cable and the antenna elements. This paper analyzes the described problem and shows a method to improve the balance in the case of a shielded cable.