Passive RFID is the power of an RFID tag derived from the electromagnetic field or field of the reader. RFID tags have no battery and are usually low cost, rugged and can be used “permanently”. The tag stores the energy from the electromagnetic/induction field of the Reader and passes the information back to the Reader by modulating the radiant energy of the reader itself.  

Three frequency groups are typically used: LF (low frequency), HF (high frequency) and UHF (ultra high frequency) (LF = 125 / 134KHz, HF = 13.56MHz, UHF 850-950MHz). LF and HF systems typically use magnetic energy or inductive energy, so the operating range is low, typically less than 20 cm. UHF systems use radiated electromagnetic (RF) energy and reflective modulation similar to “radar.” The UHF RFID system has a maximum range of up to 10 meters.  

LF, HF and UHF passive tags are low cost, simple devices. Short range LF and HF readers (range 10 cm) are also available at low cost. Longer distance UHF readers (up to 10 meters) are more complex systems and are relatively expensive.  

Active RFID is where RFID tags have their own power source (usually a small battery). The tag is actually a transceiver that responds to the receive command from the Reader and “actively” returns the data. Active tags can use any ISM or licensed band, the most common being 433MHz, 850-950MHz and 2.4GHz. The range depends on the tag and reader transmitter power and receiver sensitivity. Active tags typically “sleep” for long periods of time to extend battery life. The range can be from 10 meters to several kilometers.  

Active tags can be relatively large and have a limited lifetime (batteries need to be replaced) and are much more expensive than passive tags. Active tag readers use a similar level of technology as tags and can be relatively inexpensive. Some tags can use “passive” circuits to “wake up” and then “actively” transmit data, a technology that provides longer battery life.