A low-profile zero-phase-shift-line (ZPSL) loop antenna is proposed to achieve a directional magnetic near-field distribution for applications in ultra-high frequency (UHF) near-field radio-frequency identification (RFID). The proposed antenna consists of three coaxially-stacked components, including a driven ZPSL loop, a parasitic ZPSL loop, and a metallic plate reflector. Conventional directional antennas that involve metallic plate reflectors require an optimal separation of one-quarter of a wavelength at the operating frequency between the antenna and the plate. Decreasing the separation would result in poor impedance matching as well as a reduction in gain for far-field antennas. To realize a low-profile directional far-field antenna, a novel approach was recently proposed by adding a parasitic strip between a dipole antenna and a metallic plate reflector, such that the phase of the reflected wave can be properly controlled. In this manner, the separation between the antenna and the reflector is reduced to 0.05 of the operating wavelength, while keeping the reflected and the directly radiated fields in phase in the desired direction.
In this paper, such an approach is extended to the design of near-field antenna. Because of the geometry of the bi-directional ZSPL loop, the parasitic element is chosen as an aperiodic ZPSL loop that has the same geometry as the driven element to ensure a strong coupling between the two loops. In the design of near-field antennas, the current distributions of the antennas are the decisive factors affecting the near-field distributions. Therefore, the proposed antenna is designed based on the current distributions instead of the phase information as in the case of far-field antennas. The proposed antenna is fabricated and measured, and it proves to be an effective reader antenna for UHF near-field RFID applications.