  The paper proposes the design of a tri-mode coupled coil which enables three
modes of operation for inducing electromagnetic field, where the focal
adjustment of the E field can be optimized. Methods: The setup consists of two
identical figure-of-eight coils, namely coil 1 and coil 2, coupled to each
other by magnetic resonance coupling. Coil 1 is driven by active source, where
coil 2 is driven by the magnetic field coupled from coil 1. The frequency of
operation would affect the coupling between the coils and hence the current
ratios induced. Results: In the first and the second modes, the current
dominates at the first coil and the second coil, respectively. In the third
mode, both coils conduct similar amount of currents. Conclusion: The concept is
proven by measuring the current ratios of the coils and the voltage induced in
biological tissues. The current ratios in the first and the second modes are
measured as maximum 13.0 and minimal 0.258 at frequencies 487.7 kHz and 453.2
kHz, respectively, while the current ratio measured in the third mode is 1.03.
Significance: The tri-mode coil could potentially be applied for biological
applications such as pulsed electromagnetic energy treatment and thermoacoustic
imaging.
