  The retrodirective antenna array is considered as a mechanism to enable
target tracking of a power receiver for long range wireless power transfer
(WPT) due to its simplicity in implementation using only analog circuits. By
installing the retrodirective capability on both the generator and rectenna
arrays, a feedback loop that produces a high efficiency WPT channel is created.
In this paper, we characterize the dynamics of this phenomenon using a
discrete-time state-space model based on S-parameters and show that the system
can naturally achieve maximum theoretical WPT efficiency. We further confirmed
the theoretical analysis through a hardware experiment using a 12-port circuit
board with measurable S-parameters mimicking a static wireless channel. The
results collected from the hardware experiment show agreement with the proposed
theoretical framework by comparing the theoretical efficiency with the measured
efficiency and by showing that the collected data points follow the predicted
condition to achieve maximum efficiency.
