Two of the difficulties limiting widespread acceptance of RFID are the tagging of objects with complex electromagnetic properties and tagging of objects used in extreme environments.

Standard RFID tags work very well when applied to cardboard boxes in climate-controlled warehouses but in the real world, tags are needed that work directly on metal or in metallic environments, or that work in extreme environments exposed to chemicals, invasive force or high temperature.

RFID tagging with the capability to be read on or in close proximity to metal has proven to be one of the most difficult problems in the RFID world. Simple dipoles, which represent a vast majority of the most common RFID antennas due to their ease of manufacture, perform very poorly near metal surfaces.

One solution is simply lifting the tag off the metal surface to a distance sufficient to mitigate the metals influence. This can be done inexpensively using foam, but is characterised by tag thickness of 12 mm or more, even to achieve moderate read range. Foam spacer tags are easily damaged upon contact, so for protection they are often encapsulated in plastic.

This adds significantly to the cost that defeats the purpose of this low-cost approach. Another solution is to design an antenna with a metallic ground plane as an integral part. Cellphones share a common issue with metal-mount RFID, namely needing an antenna to operate on a ground plane in a complex environment.

The most common type of antenna used in these configurations is called a Planar Inverted F Antenna (PIFA), in which the radiating element is a plane rather than a stick antenna. The appeal of PIFAs is that the impedance is easily tuned by placement of the feed.

Passive RFID tags have no battery – operation depends entirely on the energy transmitted by the interrogator. Since the power density from the reader drops off quickly with distance, the primary limitation on the read range of a tag is the efficiency with which it can transfer the power from the reader to the IC. This is a function of the impedance of the chip (which is fixed), and the impedance of the tag antenna (which will vary on the environment). The impedance of a dipole antenna will vary depending on its substrate displaying the greatest variation when in proximity to metal.

Some RFID applications require tags that withstand high temperatures. Standard RFID tags are made inexpensively and will typically not withstand high temperatures. Most RFID ICs can withstand higher temperatures, but they may not function properly when returned to room temperature.

There are two modes of operation for currently available high temperature tags. One involves embedding the tag in a ceramic material with very low thermal conductivity, which will protect the tag for as long as it takes the thermal energy to diffuse through the ceramic. An alternative, longer-term method is to combine a rugged high-temperature chip with hightemperature adhesive, bonded to a high-temperature antenna substrate.

Many high-temperature applications also fall under the category of metal-mount. High-temperature plating or painting processes of metallic containers are common. Ceramic encapsulated, metal-mount tags are sufficient where high temperatures are only present for a short period. The chip and the chip adhesive must also be capable of resisting the high temperatures. Most off-the-shelf read-on-metal tags are not suitable for high-temperature applications.