Figure 3.

(a) The fabricated sensor with the coil and the circuit components. It contains a loop antenna, matching network, rectifier, storage capacitor, and power transistor for shifting impedance (top) and photodiode and LED (bottom). (b) The planar model consists of a source surface current density $\mathbf{J}$ and a receiver coil placed within a tissue. The surface current source and the air–tissue interfaces are at the plane of $z=0$ and $z=-z_1$, respectively. The receiver is embedded at the depth of $(z=-z_f)$. (c) The optimal current distribution at 1.5 GHz. (d) A coil structure for the transmitter that realizes the optimal current distribution.