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A broadband and miniaturized balun is proposed using the artificial coupled line with the imaginary even-mode impedance. The artificial coupled line is a metamaterial transmission line with zero electrical length for the even-mode excitation, whereas it behaves like a TEM transmission line with approximately constant characteristic impedance for the odd-mode excitation. Based on the artificial coupled line, a new concept to design a broadband balun is presented. The broadband characteristic of the balun is explained by the odd- and even-mode analysis. A balun is realized using low-temperature co-fired ceramic fabrication technology, simulated by a full-wave tool, and measured in the three-port vector network analyzer system. It exhibits good amplitude imbalance of 0.3 dB and phase imbalance of 179°±3° with the return loss greater than 10 dB from 3.3 to 7.1 GHz with 73% of fractional bandwidth. Its electrical size is only 0.063×0.44λ g2 and its physical size is 1.6×11.2 mm2. Compared with previous works, the proposed balun possessing good balance has a more compact size with broadband operation.