A color map showing the current distribution around an asymmetrically holed and symmetrically slotted antenna for UHF applications.
© 2010 IEEE
Radiofrequency identification (RFID) is a technology that grants users the ability to track the origin, destination and characteristics of any RFID-tagged product in a warehouse simply by using an RFID reader. The ideal handheld RFID reader should be small, but the limit of miniaturization for such devices is often restricted by the size of the reader’s antenna, which must be able to receive signals in the ultrahigh-frequency (UHF) range of 840 to 960 MHz. Nasimuddin and co-workers at the A*STAR Institute for Infocomm Research have now demonstrated a new, compact antenna design with superior sensitivity for UHF signals.
The antenna consists of one to four circular holes cut out of a square patch (see image). By giving each of the holes a different radius, Nasimuddin and his co-workers made the antenna extremely sensitive to circularly polarized radiation. This allowed the antenna to operate without maintaining any particular orientation relative to a signal source, in this case the RFID tag. In addition to the variously sized circular holes, the antenna has four long, uniform and symmetrically arranged slits that can be customized to achieve the desired operating frequency range without affecting the antenna’s polarization sensitivity.
The final, constructed antenna was designed to operate at 900 MHz; it had circular holes of between 6 and 12 mm in radius, a side length of 90 mm and thickness of about 5 mm, making the antenna smaller than other antenna designs suitable for similar frequencies, including designs that rely on truncated corners. Part of the reason for the reduced size is the long, meandering path taken by the current flowing across the antenna’s surface, which brings the operating frequency closer to the UHF range without requiring a physical increase in antenna size.
The antenna was able to sustain reading ranges of up to 96 cm, depending on the dielectric constant of the substrate that was used. It was also able to detect circularly polarized radiation over a wide range of incoming angles with an angular width of 100°. In addition, by varying the slit widths, the research team was able to tune the operating frequency over a range of tens of megahertz.
The combination of these strong performance characteristics and the antenna’s tunability and small size make the asymmetrical design particularly versatile and attractive. “Our proposed antenna design may become useful for portable and handheld wireless devices, such as handheld RFID readers, medical implant device readers and small portable wireless devices,” says Nasimuddin.
The A*STAR-affiliated researchers contributing to this research are from the Institute for Infocomm Research.
