Radiofrequency (RF) energy harvesting has been in production for years in the form of RFID tags. RFID enjoys the benefit of being a near-field communication that uses a transceiver that essentially powers the RFID tag due to its close proximity.  

For far-field applications, we need to harvest energy from broadcast transmissions. Broadcast transmissions are nearly everywhere, with service from televisions, cell signals, and radio. Capturing energy from radio frequencies versus other forms is particularly difficult, as RF signals have the smallest energy density of all harvesting techniques. The capturing of RF signals is based on having the appropriate antenna to capture a frequency band. Typical frequency bands used are in the 531 to 1611 kHz range (all in the AM radio range).