Optimal antenna placement for mobile terminals using characteristic mode analysis

When an electrically small antenna element is placed on a small finite ground plane, as in mobile terminals, the ground plane resonances strongly affect the impedance and radiation characteristics of the antenna system. In this paper, we show how characteristic mode analysis can predict the optimal locations of antennas on finite ground planes. As patch-type antennas couple to the ground plane resonances mainly through the electric field, it is natural that the strongest coupling and thus the maximum impedance bandwidth is obtained when the antenna element is placed at the maximum of the electric field of a ground plane mode. This is verified by moving a small probe antenna at selected locations over a rectangular 100 mm by 40 mm ground plane and by computing the obtainable bandwidth at each point.