Planar CP RFID R/W antennas based on EM Coupling

This paper presents the new approach to design planar circularly polarized (CP) antennas for RFID read/write (R/W) applications in UHF band. The objective of this work has been to design an RFID R/W antenna having simple unobstructed layout and single feeding port yet demonstrating good CP performance. Two RHCP antennas utilizing the principle of EM coupling from dipole element to the parasitic structure have been realized. The first design is the loop coupled dipole antenna (LCDA) where the pair of rectangular loops is placed in close vicinity of a dipole element. The second one is the dipole coupled dipole antenna (DCDA) which includes a pair of parasitic strips coupled to a dipole element. The operational principle of producing the CP radiation has been analyzed and illustrated in the detail. Both simulations and prototype measurements data demonstrating the required impedance characteristics and CP axial ratio have been obtained.