Design of Circularly Polarized Conical RFID Reader Antenna Based on Natural Boundary Conditions

A novel design approach to circularly polarized (CP) conical RFID reader antenna is presented in this paper. First, operational principle of the proposed antenna is revealed by exploring the distribution of natural boundary conditions. It is demonstrated that due to the introduction of a shorting pin, the proposed antenna is dominated by two resonate modes, i.e., the monopole mode and loop mode, as the symmetrical property of conventional planar conical antenna is disturbed. As a result, the respective natural boundary conditions from these two modes will be sequentially excited with an interval of one-quarter of temporal period, thus the surface current becomes rotated and a CP wave is radiated. Prototypes with CP bidirectional and unidirectional radiation centered at 2.4 GHz are then fabricated and tested. It is shown that they can achieve a 3-dB axial ratio bandwidth about 5.38% and 2.84% in +y direction, respectively, which is further demonstrated that the design approach can be employed to effectively generate CP radiation without the use of orthogonal structures, modified ground plane as well as extra polarizer.