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This paper contributes to the solution of the antenna-chassis coupling problem for handheld devices in mobile communication. That problem, up to now addressed in terms of equivalent circuits, is discussed here from a field theoretical point of view. A numerical analysis of characteristic modes on the chassis of the device provides a resonant mode which dominates the overall radiation properties. The modal excitation coefficient which relates the above mentioned chassis dominant wavemode to that of a given antenna element (exciter) should be maximized to achieve an optimal modal coupling. This paper establishes conditions to achieve maximum modal excitation coefficient which lead to an optimal placement of the antenna element on the chassis. A numerical approach for the evaluation of the optimal location of the exciter (electric and magnetic) on the chassis is performed. In this paper a theoretical approach based on some assumptions of the evaluation of the modal excitation coefficient is presented for the cases of an electric and a magnetic exiter. Numerical results are given for a bar-type and a folder-type phone chassis.