Far- and near-field focused WPT using (sub) arrays

Due to transmit power limits and spherical spreading, the received RF power levels in radiative Wireless Power Transfer systems are very low. The RF-to-DC Power Conversion Efficiency of the receiver's rectifier therefore is of paramount importance. This efficiency can be increased by applying a multi-sine excitation at the transmitter. Here, we propose to use a pulse-delayed array excitation as a special form of multi-sine excitation. As reported earlier, this excitation will lead to the creation of a focal area with high energy density at the location of the receiver. In this paper, we discuss practical considerations for increasing the peak- and average-power level in the focal area. We find that, relative to an array excited with non-delayed pulses, we may expect at the focal position a peak-power gain up to the number of array elements squared and an average-power gain between zero and the number of array elements. The array size is in the order of the distance to the focal point. The array is best used for focusing in front of and to the side of the array, so that for a complete forward coverage two arrays should be used.