Microstrip-Based Segmented Coupling Reader Antenna For Near-Field UHF RFID Applications

The radio-frequency identification (RFID) technology is of growing interest to commerce, industry, and academia, especially due to the recent declines in cost and increase in reading distance because of improved design and associated signal processing. Conventional radiatively coupled ultra-high frequency radio-frequency identification (UHF RFID) tags have certain limitations:they are physically large and do not operate well in or close to aqueous fluids.Recently, the use of tags configured to use inductive instead of, or as a supplement to, radiative coupling has been advanced as an alternative application model for UHF RFID. Inductive coupling systems are preferred in most applications since most reactive energy is stored in magnetic field and only affected by objects with high magnetic permeability.The mutual inductance between two loop antennas is strongly influenced by the size of antenna. A small loop produces a strong magnetic field along its axis, but the field falls rapidly. The extent of field of a large loop is large, but as the antenna diameter approaches the series resonant size (approximatelyλ/π) the current distribution shifts to produce a change in sign of the current flow. In most segmented loop antenna designs, lumped matching network is designed to connect the balanced port and SMA connector. However, the loss caused by low frequency lumped elements will reduce the read distance and shift the frequency, and the high price of radio frequency lumped elements increase the cost a lot.In order to address the problems cited above, a microstrip-based segmented coupling(MSC) near-field UHF RFID reader antenna is proposed and designed for item level identification. Because of the miacroarip-based structure,it provides 13.5cm reading distance and no reading nulls between 4cm and 13.5 cm. Based on microstrip line structure, the antenna can be matched with tuning stubs conveniently. The measured bandwidth of this antenna is around 55MHz (895～950MHz) under the condition of Voltage Standing Wave Ratio (VSWR) less than 2.0, which can cover the FCC standard (902MHz-928MHz) completely. And the fabricated antenna matches well with the simulated one.