Introduction to RFID
In general terms, RFID (Radio Frequency Identification) is a means of identifying a person or object using a radio frequency transmission. The technology can be used to identify, track, sort or detect a wide variety of objects. Communication takes place between a reader (interrogator) and a transponder (Silicon Chip connected to an antenna) often called a tag. Tags can both be active (powered by battery) or passive (powered by the reader field), and come in various forms including Smart cards, Tags, Labels, watches and even embedded in mobile phones. The communication frequencies used depends to a large extent on the application, and range from 125KHz to 2.45 GHz. Regulations are imposed by most countries (grouped into 3 Regions) to control emissions and prevent interference with other Industrial, Scientific and Medical equipment (ISM).

Most commonly used RFID frequencies for passive tags - Performance overview

	LF	HF	UHF	Microwave
Frequency Range	< 135 KHz	13.56 MHz	860 - 930 MHz [1]	2.45GHz
Standards Specifications	ISO/IEC 18000-2	ISO/IEC 18000-3 AutoID HF class 1 ISO 15693, ISO 14443 (A/B)	ISO/IEC 18000-6 AutoID class 0, class 1	ISO/IEC 18000-4
Typical Read Range	<0.5m	~ 1m	~4 -5 m[2]	~ 1m
General	Larger Antennas resulting in higher cost tags. least susceptible to performance degradations from metals and liquids	Less expensive than LF tags, Best suited for applications that do not require long range reading of high number of tags. This frequency has the widest application scope.	In volume UHF tags have the potential to be cheaper than LF or HF due to recent advances in IC design. Good for reading multiple tags at long range. More affected than LF and HF by performance degradations from metals and liquids	Similar characteristics to UHF but faster read rates. Drawback is microwaves are much more susceptible to performance degradations from metals and liquids.
Tag power source	Mainly passive using inductive coupling (near field)	Mainly passive using inductive coupling (near field)	Active and passive tags using E-Field back scatter in the far field	Active and passive tags using E-Field back scatter in the far field
Typical applications	Access Control, Animal tagging, Vehicle immobilizers	Smart cards, Access Control, Payment, ID, Item level tagging, baggage control, Biometrics, Libraries, laundries, Transport, Apparel	Supply Chain- pallet and Box tagging, Baggage Handling, electronic toll collection	Electronic toll collection, Real Time Location of goods.
Notes	Largest installed base due to mature technology. However will be overtaken by higher frequencies	Currently the most widely available high frequency world-wide due to the adoption of smart cards in transport.	Different frequencies and power allocated by different countries US 4W(EIRP) 915MHz, Europe 0.5W (ERP) 868 MHz, [2]	5.8 GHz more or less abandoned for RFID
Multiple Tag Read Rate	Slower   Faster
Ability to read near metal or wet surfaces	Better   Worse
Passive Tag Size	Larger   Smaller

[1] Japan has recently announced allocation for 950 MHz band for RFID
[2] 4 -5m is for unlicensed readers and 10m for site license in the US. In Europe with current power restrictions only around 33cm is achievable. However this is expected to improve to near 2m as power emissions increase from 0.5Watts to 2 watts.