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Abstract-To assess the feasibility of ambient RF energy scavenging, a survey of expected power density levels distant from GSM-900 and GSM-1800 base stations has been conducted and power density measurements have been performed in a WLAN environment. It appears that for distances ranging from 25m to 100m from a GSM base station, power density levels ranging from 0.1mW/m 2 to 3.0mW/m 2 may be expected. First measurements in a WLAN environment indicate even lower power density values, making GSM and WLAN unlikely to produce enough ambient RF energy for wirelessly powering miniature sensors. A single GSM telephone however has proven to deliver enough energy for wirelessly powering small applications on moderate distances.