The retroreflective method has been used as a radio—frequency (RF) beamforming method for massive antenna array, for wireless communication systems, radars, and microwave power transfer (MPT) systems. In particular, the retroreflective method is suitable for optimum beamforming for both near—and far—field zones. Each transmitter (Tx) and receiver (Rx) circuit in the Tx array has its own phase offset, which should be calibrated for optimum retroreflective operation. This paper presents a simple and effective phase offset calibration method for the retroreflective beamforming arrays using a one—by—one basis optimum phase searching method with a receiver at a reference point. To verify the proposed method for a massive retroreflective array, a 5.2 GHz short— and mid—range (~ 4 m) MPT system, including a Tx array based on 8x8 antennas and an Rx using 6 antennas (5 rectennas for power reception, and 1 additional for pilot transmission) for charging mobile handsets was implemented. Optimal retroreflective beamforming from the Tx array, which was calibrated using the proposed method, was experimentally verified for angles of within ±45° degrees for both xz— and yz—planes. The implemented Tx and Rx circuits achieved a received dc power of about 100 mW at a distance of 4 m.

This article presents a broadband two-stage cascode power amplifier integrated circuit (PAIC) using a 2-μm InGaP/GaAs heterojunction bipolar transistor process. Since higher supply voltage of cascode power amplifier (PA) results in lower impedance transformation ratio of the matching network due to lower current, cascode PAs generally have broader bandwidth than common-emitter (CE) PAs. However, bandwidth analysis for handset PAs with a single-section load matching network, including transistors output capacitance, revealed that cascode PAs had bandwidth similar to CE PAs. Through bandwidth analysis based on power and efficiency contours at the internal plane of the transistor, an optimized shunt inductor with a single-section load matching network was proposed in this article to increase the bandwidth of cascode PA. Performances of both cascode and CE PAs with and without the optimized shunt inductor were compared. A broadband two-stage cascode PAIC with a compensating shunt inductor and an L-section matching network was designed and implemented. The implemented PAIC exhibited an output power of at least 30.1 dBm at frequency band ranging from 1.55 to 2.65 GHz for continuous-wave excitation. Using a long-term evolution signal with a peak-to-average power ratio of 7.5 dB and a signal bandwidth of 10 MHz, a power gain of more than 23.7 dB, power-added efficiency of 30.9% to 38.4%, and average output power of 26.7 to 27.7 dBm were obtained at a given adjacent channel leakage power ratio of -30 dBc. A fractional bandwidth was calculated to be 54.3% based on measured results.

This paper presents a 5.8 GHz RF–DC converter for high conversion efficiency and high output voltage based on a common-ground and multiple–stack structure. An RF isolation network (RFIN) for the multiple-stack RF–DC converter is proposed to combine the DC output voltage of each stack without separating its RF ground from the DC ground. The RFIN is designed using micro-strip transmission lines on a single-layer printed circuit board (PCB) with a common ground for the bottom plate. A 4-stack RF–DC converter based on a class-F voltage doubler for each stack was implemented to verify the proposed RFIN for the multiple-stack and common-ground structure. The performances of the implemented 4-stack RF–DC converter were evaluated in comparison to the single-stack converter that was also implemented. The size of the implemented 4-stack RF–DC converter using bare-chip Schottky diodes is 24 mm × 123 mm on a single-layer PCB. For an input power of 21 dBm for each stack of the RF–DC converter with a load resistance of 4 kΩ, a high efficiency of 73.1% and a high DC output voltage of 34.2 V were obtained.

This paper presents a look-up table (LUT)-based focal beamforming system that can effectively transmit RF power up to mid-range distances (≤ 3 m) including when the Rx is in the near-field zones. The Tx elements control and radiate signals for the Rx even at the near-field zone ensuring the received signals are in-phase. Since the proposed system uses a LUT for storing the phase sets of the signals for the Tx elements, it requires only very simple hardware and a very simple adaptive control algorithm compared to conventional retroreflective method. In order to track the moving Rx, a 2-D adaptive sequential searching algorithm is proposed. The system can find the optimum phase set by sequentially searching the phase sets for a pre-determined 2-D area. The LUT of the phase sets are generated using geometric analysis over the entire 2-D area where the Rx could be located. To verify the proposed method, a 5.2 GHz mid-range (≤ 3 m) MPT system composed of a $4\times 8$ Tx array and a $2\times 3$ Rx array was designed and implemented. Using the proposed 2-D adaptive sequential searching algorithm, the optimum phase set for focal beamforming can be quickly found for a given position of the Rx. In our experiments, the results showed an RF power level of 177.8 mW was received at the Rx with a distance of 1 m with a total radiated RF power of 16W. Since the measured received power levels for various Rx positions agree well with the simulation results, the proposed system was proved to be an excellent candidate for the practical application.

This paper presents a high-efficiency wireless power transfer (WPT) system that can charge multiple receivers (Rx's) of various types. For system analysis, the transmitted power and received power levels are derived using the parameters of the resonators based on electro-magnetic (EM) resonance. Using the analysis results, the power customized resonator (PCR) not only for deriving sufficient output power from the transmitter (Tx) but also for appropriately distributing the received power to the various and multiple Rx's is proposed. In addition to the PCR, the Tx unit is designed to work as a load-dependent voltage source using a differential Class-E power amplifier (PA) through a wide range of load impedances. Therefore, the Tx can naturally adapt to the required Tx power levels with a high efficiency corresponding to various Rx configurations without additional tunable circuits or adaptive control schemes. The WPT system, including the load-dependent voltage source for the Tx, full-bridge rectifiers for the Rx's, and the proposed PCR, was designed for the 6.78 MHz frequency band. The proposed system is validated for use with multiple mobile devices by conducting experiments with nine charging cases using three types of Rx's. For all cases, high system efficiencies of 70.7-85.5% were maintained over received power levels of 8.6-45.7 W at a charging distance of 30 mm, and each of the three types of Rx's were experimentally verified to receive sufficient power.