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RFID License Plates: A Successful In-Metal RFID Application

University of Bremen researchers explain how a vehicle's license plate can function as an antenna for a passive UHF tag, outperforming smart labels on windshields, as well as on-metal tags.
By Dieter Uckelmann and Dennis Brandwein
Feb 27, 2012Not long ago, the use of passive ultrahigh-frequency (UHF) RFID tags was said to be problematic in proximity to metal, since UHF radio waves bounce off metallic objects. Recent tests, however, have shown that the read ranges of UHF tags mounted to metal can match those of smart labels used within other environments. Nonetheless, even greater read ranges can be achieved if the metal structure itself is used—for example, through a slot antenna design within the metal surface—a method that could be called in-metal RFID. The BIBA Institute, at the University of Bremen, working together with J.H. Tönnjes E.A.S.T. GmbH & Co. (a developer and manufacturer of license plates) and Kathrein Sachsen GmbH (an RFID reader and antenna supplier), has developed and tested new passive RFID license plates that outperform smart labels on windshields, as well as on-metal passive tags for vehicle identification.

Dieter Uckelmann
Electronic vehicle identification (EVI) is a promising new development to enhance such applications as vehicle access control, electronic toll collection or complex traffic control. To date, active transponders operating in the UHF and microwave frequency ranges have been employed. However, passive transponders can be more compact, and are available at significantly lower costs. Moreover, they are more durable and environmentally friendly, since passive transponders do not require a battery. Then again, the determining requirement in toll collection and traffic control is having a read range of 5 meters (16.4 feet) and beyond, as well as readability at speeds of up to 250 kilometers per hour (155 miles per hour).

J.H. Tönnjes E.A.S.T. focused its strategy on the development and design of passive UHF RFID license plates. The idea was to move beyond using standard on-metal RFID tags, as these are still expensive compared with RFID smart labels. Instead, the company considered an integrated design, in which the license plate itself would function as an antenna by utilizing a slot-antenna layout.

Dennis Brandwein
First, prototypes were produced, which offered promising results comparable to the performance of on-metal RFID tags. However, J.H. Tönnjes E.A.S.T. wanted to go beyond the current state-of-the-art, so it launched a series of tests and improvements in partnership with the University of Bremen and Kathrein Sachsen.

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