RFID Antennas for Difficult Environments

Abstract

The research work explored the fundamental ideas and techniques required to design an antenna placed on different common target objects for radio frequency identification. This was achieved by studying the effect of antenna conductivity in a wide range and the substrate relative permittivity from 1 to 10. The effect on wire meander antenna properties are different to that for a straight linear dipole antenna; the increase in wire radius of the meander antenna increases the resonant frequency but increases in wire radius of a straight dipole antenna decreases the resonant frequency. This study concluded that the approximate antenna bandwidth needed for common target object materials is 28.5%. A genetic algorithm GA code was written to optimize the meander antenna structure to obtain a larger bandwidth. The GA optimized each vertical element and horizontal element for radius and length. The optimized antenna has 14.5% bandwidth which covers the entire UHF RFID frequency. As conductivity is not changed further, the properties of antennas on ungrounded substrates with different permittivity and thickness were investigated. The solution for this problem involved transforming a circular cross section wire antenna structure to a planar strip antenna structure. A number of theoretical approaches were assessed to solve for the effective permittivity when the substrate material is thin. Surface impedance and slab waveguide propagation techniques were compared to a capacitive solution and an insulated wire antenna. The insulated wire method gives most accurate results (< 3.5% error) and was verified using numerical modelling and experimental work. Measurements on a planar straight dipole on FR4 (fc = 1.50GHz) compare favourably with the antenna modelled without the substrate and scaled using the insulated wire technique at (fc = 1.49GHz).