Broadband RF Energy-Harvesting Arrays

For RF energy harvesting, where the frequencies, power density, polarization, and direction of incident waves are unknown and variable, broadband rectenna arrays offer a simple scalable approach. In this article, we first compare the design methodology and scalability of narrowband and broadband arrays. An example 10-GHz narrowband rectifier array is presented to showcase the challenges involved in ultralow-power (down to 1 mW/m(2) = 0.1 mu W/cm(2)) ground-based harvesting of narrowband satellite signals. The circularly polarized array consists of subarrays with RF power combining before rectification. This approach is then contrasted to harvesting over a wide RF bandwidth, through a comparison of several broadband arrays with rectifiers in each element. This includes a 2-5-GHz bow-tie screen-printed wearable rectenna array with an efficiency exceeding 30% above 50-mu W/cm(2) incident power density and a 2-18-GHz spiral array tested with powers down to 0.1 mu W/cm(2). The two types of arrays are analyzed in the context of applications in a ground-based satellite transmitter with narrowband directional power receiver and in a broadband case with multiple widely spaced simultaneous signals.