Planar Pyramid Shaped UHF RFID Tag Antenna with Polarisation Diversity for IoT Applications Using Characteristics Mode Analysis

This paper presents a novel dual polarised tag antenna design for ultra-high frequency radio frequency identification (UHF RFID) featuring polarisation or frequency band diversity using characteristic mode analysis. The proposed tag design consists of two meandered line bowtie or dipole-like structure (resembles with planar pyramid), shorting stubs, and a ground plane. The initial design comprises of an upper patch wrapped around FR 4 substrate and shorted from all side to ground plane using shorting stubs. After running the characteristic mode analysis of the initial design, the diagonal slots are created to transform the resonant behavior of corresponding characteristic modes towards lower frequency band. However, the characteristics modes of diagonal slots-based structure still shows inductive behavior in US RFID band (902 - 928 MHz). Therefore, the parallel slots having capacitive performance are etched to realise the resonance behaviour of modes in the required RFID band. The positions and dimensions of parallel slots are optimised further to achieve a better conjugate impedance match with RFID chip. Moreover, the frequency band of one or both meandered line dipole-like structures can be tuned to European UHF RFID band by varying the length of the shorting walls. The RFID chip is directly placed at the current minima to run diversity modes. The overall dimensions of the proposed tag antenna are 50\times 50\times 2 mm³ hence making this dual polarised tag antenna low-cost and easy to fabricate due to the absence of vias, shorting pins, and any matching circuit. Additionally, the proposed tag design offers 3-dB bandwidth ranging from 900 MHz to 940 MHz (40 MHz) together with orientation insensitive 3D read range pattern. The proposed tag design achieved a read range of 5.5 m and 8.5 m on low permittivity dielectric material and 200 \times 200 mm² metal plate, respectively.