Development of an Active Integrated Antenna for 24-GHz Wireless Power Transfer System

A 24-GHz band active integrated antenna for battery-less sensor networks has been developed. The control unit for power transmission comprises four stacked front-end modules and a 24-element array antenna. The RF output power of the modules is 28 dBm, and the antenna gain is 18 dBi. Since the demonstration system generates heat of 40 W, a heat dissipation structure was investigated to enable continuous operation. The module’s thermal equivalent circuit was derived. The temperature rise estimated from the thermal equivalent circuit and the measured temperature change were in good agreement, and the channel temperature of the final amplifier could be maintained below 145 °C. Beamforming of the phased array was achieved by controlling the IF phase input to the four front-end modules. A transmission experiment was conducted, and LEDs at 1 m away were successfully lit. In addition, the amplitude and phase of each antenna element were measured by over-the-air (OTA) to obtain an ideal beam with equal excitation amplitude and equally spaced phase difference between adjacent elements. A measurement system, including a frequency conversion device, was constructed using a near-field measurement device because the front-end module performs frequency conversion internally. Furthermore, the simulation based on the measured beam patterns reproduced the phase and amplitude errors, and calibration was conducted for fine-tuning. The calibration was verified using the rotating element electric field vector (REV) method principle. As a result, an ideal beam pattern was obtained, showing that beam calibration of active integrated antennas can be easily performed.