A Low-Profile Top-Loaded Monopole Antenna for On-Metal RFID Tag Design

For the first time, a novel low-profile top-loaded monopole antenna is proposed for on-metal omnidirectional tag design. Here, a square patch is loaded on top of a short monopole antenna to improve its input impedance and radiation efficiency. Subsequently, the tag antenna is shunt-fed (using both the direct-feeding and proximity-coupled-feeding methods) to further enhance its input impedance and omnidirectionality. As a result, the antenna impedance, radiation resistance, and radiation efficiency have all been enhanced significantly, as compared with a typical short monopole antenna. A C-shaped feedline, which is to shunt-feed the tag antenna, is also introducing additional inductance to the antenna impedance for attaining good impedance matching level. Despite having a profile of not more than one hundredth of the operating wavelength, the proposed tag antenna is still able to exhibit a strong and stable omnidirectional radiation pattern when attached on a metal. The proposed tag antenna, which has a size of
$0.1069 \lambda \times 0.1069 \lambda \times 0.0098~\lambda $
, can provide a constant read range of ~9.2 m in all azimuthal directions, with a minimal fluctuation of 0.5 m at 0.910 GHz. The tag’s resonance remains stable and independent of the size and shape of the backing conductive platform.