This paper explores the application of the concept of characteristic modes on the chassis of a mobile phone to purposeful design of its radiation characteristics. Focus is on the resonant chassis modes which dominate the overall radiation properties. Modal radiation quality factors are derived from characteristic mode eigenvalues. Antenna -chassis coupling, previously only considered in terms of equivalent circuits, is discussed from a field theoretic point of view. A numerical approach for characteristic mode analysis is presented which is an eigenmode solver extension to the well known NEC2 code. Numerical results are given for wire grid models of a bar-type and a folder-type phone chassis.
Feed point impedance and mutual coupling of monopole array elements on a small ground plane are found to depend critically on the ground plane size when one length dimension comes near to half-wavelength. This is demonstrated to be due to the excitation of the lowest order chassis mode, which acts as an additional radiator element parasitically coupled to the array elements. A model is given in terms of impedance matrix description of the array including the effect of excited chassis mode. Important consequences of the chassis mode excitation and coupling are formulated for the characteristics of phased arrays and for diversity-and MIMO concepts.
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