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 nearfield 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 predetermined 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 4×8 Tx array and a 2×3 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 16 W. 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. INDEX TERMS Microwave power transfer, Look-up table, Focal beamforming, 2-D adaptive sequential searching algorithm.
A single-feed wideband circularly polarized Spidron fractal slot antenna with a grooved dielectric resonator is presented. To realize a wideband axial ratio, the circular polarizations of both a Spidron fractal slot and a grooved rectangular dielectric resonator are merged. The proposed antenna is excited by a simple 50-Ω microstrip feed line which is located underneath the Spidron fractal slot. A prototype of the proposed antenna is fabricated and tested. The measured −10 dB reflection coefficient bandwidth and 3 dB axial ratio bandwidth are 40.33% (1.94-2.92 GHz) and 23.75% (2.30-2.92 GHz), respectively. The measured peak gain of the antenna is 4.23 dBic.
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.INDEX TERMS Array antenna, beamforming, Fresnel region, Friis formula, received power, wireless power transfer, power transmission efficiency.
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