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In order to obtain accurate radiation characteristics of an antenna, it is required to reduce unbalanced current on the surface of the feeding cable. To accomplish the goal, a balun is often used in-between the antenna under test (AUT) and the feeding cable that connects AUT and the test equipment. Miniaturized baluns of coaxial cavities are presented to show high chocking impedances at two separate bands. An equivalent circuit with inductance and capacitance elements is employed to characterize the circuit behavior of the dual-band balun. In particular, the relation between the physical structure of coaxial cavities and operating frequencies is investigated thoroughly. Both numerical values and experimental results are presented to illustrate the validity of the design method.