In this paper, a method for calculating the received power in a radio-frequency wireless power transfer system with array antennas is proposed. The received power is derived based on the superposition of the electric fields that radiate from individual transmitter (Tx) elements and are captured by each receiver (Rx) element. That is expressed in a finite series form with the radiation patterns of an element considering mutual coupling in the array and corresponding distances between the elements of Tx and Rx. Unlike conventional methods (such as the Friis and Goubau formulas), this approach is able to calculate the received power precisely in both Fresnel and far-field regions. It is also efficiently applicable to various cases, such as those involving beamforming and with varying positions of Rx. The calculated results using this method are applied to a 5.2 GHz WPT system with array antennas and verified through comparisons with both simulation and experimental results.

A new approach to estimate the direction-of-arrival (DOA) in a Time-Modulated Array (TMA) system with a bipolar squared periodic time-modulating sequence is proposed in this letter. Using such a sequence can increase the power of harmonic components of the received signal. Based on this fact, we formulated a new parameter pertaining to the ratio of the upper and lower sideband first harmonic components of the received signal, to improve the accuracy of the DOA estimation in a low signal-to-noise ratio (SNR) environment. A numerical analysis was performed to verify the effectiveness of the proposed DOA estimation method.

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.

A triple-stacked microstrip patch antenna is presented for a multiband system comprising cellular phone, global positioning system and personal communication services. It consists of three microstrip patches, and there is a slot on the middle patch to couple the first patch to the third patch. The measured results are in agreement with the simulated results at the frequencies of interest.

In this study, a rectenna is adopted in a microwave-based wireless power transfer system to rectify the received radio frequency (RF) to DC power. Each rectenna comprises a DC-DC converter at the DC output port to ensure a stable power supply and a Schottky diode for the DC-combining network. The results indicate that the RF-DC conversion efficiencies of the rectifier circuit and 4×1 rectenna were 71% and 71.2% when the unit-cell RF input levels were 19 dBm and 19.6 dBm, respectively. Furthermore, the DC output levels of a 4×8 rectenna with and without the DC-DC converter were 28 V and 30 V, respectively, when the unit-cell RF input level was 20.2 dBm. Additionally, the RF-DC conversion efficiencies were 52.6% and 59.9% with and without the DC-DC converter, respectively. Thus, we experimentally verified that the Schottky diode combined with the DC-DC converter at the DC output port can increase the DC power combining level, making it suitable as an extendable array rectenna system.