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We present a new system for measuring ambient RF energy in the 2.4GHz ISM band. This apparatus is intended to establish the feasibility of harvesting ambient RF energy to power emerging ultra-low-power sensors and microcontrollers. We simultaneously acquire RF measurements from a spatial and polarization diversity antenna system, with both a spectrum analyzer (frequency-selective but slow), and a log amp (wideband but fast), explain key tradeoffs in the measurement configuration, and present a post-processing algorithm which provides a reliable characterization of the RF energy available in the 2.4GHz ISM band. Preliminary results suggest enough energy is available to support a low duty cycle wireless sensor node system. An average RF power of 11nW is observed 10m away from a typical Wi-Fi access point in an office environment, suggesting the possibility of low duty cycle, wirelessly powered sensing and communication using a Bluetooth Low Energy (BLE) or another ultra low power uplink.