**Authors:** Jiang-Feng Lin, Qing-Xin Chu

**Source:** FERMAT, Volume 28, Communication 8, Jul.-Aug., 2018

**Abstract:** The differences between the characteristic modes (CMs) and X modes (XMs) are pointed out. Then the relations between these two kinds of modes are discussed through two examples, namely, a thin strip dipole and a rectangular loop. It is found that CMs and XMs have similar resonant frequencies (RFs) and resonant currents (RCs). Finally, it is concluded that the resonant information of an antenna, which includes RFs and RCs, exists only in the imaginary part of the impedance operator (namely, reactance operator) of the antenna.

**Index Terms:** Characteristic Modes (CMs), X Modes (XMs), relations, impedance operator, resonant frequencies (RFs), resonant currents (RCs), antenna theory

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