DURING THE DESIGN of a radio-frequency-identification (RFID) reader antenna, several aspects must be prioritized: small size, simple structure, and ease of integration with a portable reader. At Korea's Hanyang University, a miniaturization method for an ultra-high-frequency (UHF) RFID reader antenna using an artificial magneto-dielectric has been proposed by Haeil Chung, Youngki Lee, and Jaehoon Choi. Such miniaturization was realized using a split-ring resonator (SRR), which stores electromagnetic energy. Its magnetic field is perpendicular to the surface of the ring, which acts as an inductor.
The gap in the SRR structure acts as a capacitor to store energy. As a result, the SRR can be modeled in the same way as an LC resonant circuit. After the resonance frequency is determined by both the capacitance and inductance of its geometry, the SRR array structure can be designed to provide high permeability at a specific frequency band.
The antenna's microstrip feed line also uses a Wilkinson power divider, square radiating patch, and ground plane. The square radiating patch measures 68 x 68 mm2 m down from its original 78 x 78 mm2. The magneto-dielectric material, which is realized using the SRR, is inserted to reduce the size of this patch. When the researchers compared a high-permittivity and magneto-dielectric substrate, both were found to generate the same resonant frequency. Using a magneto-dielectric substrate, however, antenna size can be reduced without a bandwidth reduction. From 908.5 to 914.0 MHz, the proposed antenna has 10 dB return loss. It provides a measured peak gain of 3.75 dBi. See "Miniaturization of an UHF RFID Reader Antenna Using an Artificial Magneto- Dielectric," Microwave And Optical Technology Letters, September 2010, p. 1926.
