In this paper, it is shown that 2.4 GHz WiFi band is the most suitable source for radio frequency (RF) energy harvesting (EH) indoors. By studying the RF EH work done previously, a trend of increasing the amount of energy harvested by using DC combining antenna array, smaller antenna elements, and shorter elements separation is observed. To increase the area utilization of such arrays, impedance transformers are removed while at the same time, the size of each element is reduced to less than 0.25 λ. With careful review, the maximum element/cell size of 0.1 λ is determined such that the concepts of antenna array, electrically small antenna (ESA), and metamaterials (MM) can be combined for RF EH antenna array.
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