The Smallest RFID Antenna

by Richard Gaughan

Radio Frequency Identification is a shorthand way of referring to the technology that allows objects to be tagged with unique identifying information, then have that information read and delivered into the computerized digital world. Although there is a push for making RFID components smaller and cheaper, that has to be counterbalanced by the other technological requirements.

Elements of an RFID System

Readers and tags each have electronics and an antenna.

There are two basic active components in an RFID system: the reader and the tag. The reader is a permanent or semi-permanent device that puts out a radio-frequency signal and reads the signal returned from a tag. The tag is a small, often disposable element that detects a radio frequency signal and then sends a return signal to the reader. Both the reader and the tag are composed of two different parts: an electronic circuit and a radio-frequency antenna. Readers can be in a toll booth, at a library checkout desk, or at the grocery store; tags can be in a little box in your car, glued to the inside cover of a library book, or built into a box of cereal.

Electronics and Antennas

The electronic circuit in the reader is responsible for creating an electrical signal to drive the antenna, then detecting the return radio signal and pulling out the digital data carried by the signal. The tag electronics contain identifying information about the object they're attached to. When the tag electronic circuit is triggered, it formats a signal containing the identifying information and sends it to the antenna. The antennas on the reader and the tag are there to make certain the two devices can "hear" each other. Although all RFID antennas detect and send radio-frequency signals, there is a broad range of frequencies that fall in that range. The reader and the tag must have antennas that work in the same relatively narrow frequency range.

Antenna Design

The most efficient antenna is as long as the wavelength of the radiation it's designed to receive. Radio-frequency waves are several meters long, however, so practical RFID antennas can't be designed to be that long. Instead, RFID antennas are loop antennas, conceptually like a tiny coil of wire that winds around in a circle many times. The size of the antenna, the material it's made of, and the number of loops all influence the frequency range it can detect. So if you have an antenna that works, you can't just decide to use the same design and make it smaller – it won't work in the same frequency range and your tags and readers won't be able to talk with each other.

Making Reader Antennas Small

RFID reader antennas are designed to carry more electrical current than their counterpart tags, and they're also designed to be more sensitive than the tag antennas. Those two requirements limit the possible size reduction for reader antennas. A typical "small" reader antenna is about 45 mm square, and about 10 mm in thickness. About the smallest you'll find is 18 mm square and 4 mm thick, although you pay a performance penalty to get that small.

Small Tag Antennas

Printed RFID antennas are very thin, but usually several square centimeters in area.

There are a few different technical approaches to reducing the size of tag antennas. Murata Manufacturing Company has a 3.2 mm square, 0.7 mm thick ceramic module tag. Texas Instruments markets a 2 mm diameter, 12 mm long tag antenna. Printable tags are paper-thin, but usually have a larger area, tens of mm square. Each of those different form factors carries different performance characteristics related to their frequency and antenna gain, as well as other important parameters.

About the Author

First published in 1998, Richard Gaughan has contributed to publications such as "Photonics Spectra," "The Scientist" and other magazines. He is the author of "Accidental Genius: The World's Greatest By-Chance Discoveries." Gaughan holds a Bachelor of Science in physics from the University of Chicago.

Photo Credits

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