Self-Mixing Phase Conjugator for Retrodirective Wireless Power Transfer Applications

When using retrodirective antennas for wireless power transmission it is desirable to have a high-power transmitted signal which is sent back in the same direction as a weak pilot signal to the device being powered. A significant issue arises in the retrodirective WPT transmitter in how to efficiently separate the pilot signal from the transmitted WPT signal, especially when the same antenna is being used for transmit and receive. Traditionally, this is done using either circulators or duplexers, although these components can be bulky and expensive and introduce insertion loss into the transmit path, lowering the transmit power amplifier efficiency. In this article, we propose a new self-mixing PA method using the nonlinear characteristics of a power amplifier to allow a pilot tone signal to be extracted as a lower intermediate frequency (IF) signal with no transmit insertion loss and no requirement for circulators or duplexers. We also present a completely new PLL-based phase conjugating circuit that can extract the low-frequency pilot IF signal and provide a high-power phase conjugated drive signal to the self-mixing PA. The system is experimentally validated as a six-element retrodirective array for wireless power transmission application, where the power amplifier is shown to transmit more than 10 W with efficiencies up to 84%. The self-mixing PA was also confirmed to have very low mixing conversion loss with only 6 dB loss between the received pilot signal and the low frequency IF. We see this work as being essential for applications for retrodirective antennas for space solar satellite applications and other applications where transmission and reception by only a single antenna are required.