UHF-RFID desktop reader antennas: performance analysis in the near-field region

In this letter, a metric is proposed to qualitatively predict the achievable reading performance of a near-field (NF) UHF-RFID (865-928 MHz) desktop reader antenna, through a numerical analysis. Specifically, since the typical far-field antenna parameters (gain, radiation pattern, axial ratio) are not effective in the near-field region, the normalized power density is here proposed as a near-field antenna performance parameter. For a given NF antenna, the mean value of the normalized power density is evaluated on an area parallel to the antenna surface, at a set of different distances. Then, its decay rate is studied as a function of the distance from the antenna surface, which is useful to compare reading range performance of different antenna layouts. Additionally, the probability density function of the normalized power density at a given distance from the antenna surface is considered to investigate the field uniformity characteristics, the latter being important to guarantee tag reading independently on its location with respect to the reader antenna. The proposed analysis is applied to a set of NF UHF-RFID printed antennas, and conclusions are validated through a set of experimental tests.