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This letter proposes a new approach for estimating resonant characteristics of near-field coupled small antennas. It is based on the equivalent circuit representation of the interaction of two antennas using the addition theorem of spherical modes. Using the proposed method, the splitting of the resonant frequency and the resonant impedance of the near-field coupled small antennas can be obtained by the impedance characteristics of the isolated antenna. The results are shown to be in good agreement with the full electromagnetic (EM) simulation.