Dual-band uhf-rfid/wlan circularly polarized antenna for portable rfid readers

A compact, low-profile, two-port dual-band circularly polarized (CP) antenna for portable RFID readers is presented. The radiating element for the UHF-RFID band (902-928 MHz) has been realized through a circular array of four inverted-F meandered monopoles, where the array elements are excited with a 90$\circ phase offset (sequential rotation feeding technique) through a microstrip feeding network. In order to also provide the portable reader with a wireless web access, a miniaturized CP patch has been added for the WLAN IEEE 802.11b,g radio link, by exploiting the available space between the four array elements, without increasing the overall antenna volume. The meandered monopoles and the patch are both etched on the same layer of an FR4 substrate, so getting a significant antenna compactness $(60\ {\rm mm}\times 60\ {\rm mm}\times 7\ {\rm mm})$. The patch loading effect on the monopoles can be compensated by adjusting the meandered monopole length. Reflection coefficient, port isolation, axial ratio and radiation patterns are evaluated by numerical simulations and compared with measurements on an antenna prototype.