Radio-frequency-identification (RFID) systems comprise a read/write device, such as a tag, and a transponder. Because it is attached to the object to be identified, the tag must maintain a very low profile while satisfying demands for low cost and small size. These requirements make it critical for the tag's antenna to offer wide impedance bandwidth, omnidirectionality, and high gain. It also must have a planar and compact structure. At Taiwan's National Formosa University, Wen-Chung Liu developed a simple coplanar-waveguide (CPW)-fed patch antenna for RFID applications.
This development focuses on a planar monopole antenna. Because it is composed of a slotted patch and a CPW feed structure, the antenna only requires a single-layer substrate. The CPW-fed, slotted U-shaped monopole antenna is printed on one side of an inexpensive FR4 dielectric substrate that is 1.6 mm thick with permittivity of 4.4. The other side is without any metallization. With the insert of the symmetrical folded slot to the patch, the antenna can excite 5.8-GHz resonance mode with an impedance bandwidth of 490 MHz, average antenna gain of >4.1 dBi, and monopole-like radiation patterns. It is only 15 X 10 mm2. This design also is well suited for 5.8-GHz wireless-local-area-network (WLAN) systems.
With electromagnetic simulation software, the antenna's performance also is investigated based on method of moments. The equivalent-circuit model for determining the antenna's input impedance is established as well. See "A Coplanar Waveguide-Fed Folded-Slot Monopole Antenna for 5.8-GHz Radio Frequency Identification Application," Microwave and Optical Technology Letters, Jan. 2007, p. 71.