Fujitsu develops slim RFID tag that can maintain signal strength when attached to metal
December 17, 2014
A compact and slim RFID tag that can be affixed to ID cards, wearable devices, metal parts and other objects that have limitations with regards to signal reception has been developed by Japan-based Fujitsu Laboratories.
It is difficult to make RFID tags compact and slim enough to be affixed to metals or ID cards worn by people, as these hinder signal reception. For that reason, to ensure a communications range of two meters, for example, radio wavelength constraints meant that the tag needed to be at least 75mm long or about 5mm thick.
Fujitsu has now developed a technology that uses a looped structure of thin plastic to emit radio waves, resulting in what it claims is the world's most compact and slim RFID tag, measuring 30mm long and 0.5mm thick, which can be attached to a wide range of objects.
This technology enables RFID tags to be used in various applications, such as managing machine parts and in ID cards that people wear as a means of access control.
The range of communications is dependent on the radio frequency. Frequencies in the UHF band, near that used by televisions and mobile phones, with relatively long communications ranges of a few meters, are widely used in RFID systems.But there is a need for RFID tags sufficiently slim and compact to be attached to any kind of object, at nearly any point on the object.
Radio waves have difficulty travelling through metal and the human body, however, when RFID tags are directly attached to a metal object or an ID card carried on a person's body. Accordingly, up until now, to create space between the RFID tag and the object to which it is attached, spacers have been inserted to reduce the impact of the metal or human body, enabling communications ranges of a few meters.
For example, when using a spacer that is 1mm thick, for communications to travel a distance of two meters or more, the RFID tag itself needs to be at least 75mm long (one quarter the radio wavelength of approximately 300mm).
Conversely, to enable communications to travel the same distance, it is possible for the length of the RFID tag to be reduced to 33mm (one eighth the radio wavelength), but then the spacer needs to be at least 5mm thick. So there has been a trade-off between RFID tag length and spacer thickness that is dictated by the radio wavelength, making it difficult to achieve tags that are both compact and slim.
Fujitsu has developed a technology that emits radio waves using a looped structure in which the RFID tag is wound around rubber or plastic so that its ends overlap, overcoming the limitations imposed by wavelengths.
When tags of this type are affixed to metal, a large current (loop current) will flow in a way that follows the shape of the loop, and a portion of that current will leak through to the metal surface to which the tag is attached. The original radio waves generated by the loop current combine with the new radio waves being generated by the current leaking through to the adjacent metal, resulting in a combined signal that is emitted on the surface of the metal. Furthermore, in accordance with the desired target size of the tag, by adjusting the length and thickness of the overlap at the ends of the RFID tag, the degree of leaked current (the ratio of the two radio waves that are combined) can be optimised to increase the communications distance. In other words, the metal to which the tag is attached and the RFID tag act as a coordinated source of current, functioning as an antenna emitting their combined radio waves, and thereby enabling communications to distances of several meters.
On the other hand, when the RFID tag is attached to a plastic ID card or cardboard instead of metal, there are only the radio waves generated by the loop current. This enables communications equivalent to metal as the radio signal efficiently propagates in a loop shape outward from the RFID tag, due to the absence of a metal object that would hinder the radio signal.
In addition, as the human body has a significant water component and is structured to carry easily an electrical charge, it can be treated the same as metal. An ID card with an embedded tag will act the same as when attached to metal, so the negative effects of being carried on the body are reduced.