Small Split-Ring Resonators as Efficient Antennas for Remote LoRa IOT Systems—A Path to Reduce Physical Interference

• Published in: Sensors (Basel, Switzerland) Vol. 21; no. 23; p. 7779
• Main Authors: Rohan, Cameron, Audet, Jacques, Keating, Adrian
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 23.11.2021; MDPI
• Subjects: Antenna design; Antennas; Circuit boards; Circuit design; Evaluation; Far fields; Input impedance; Internet of Things; LoRa; metamaterials; Monopoles; PCB antenna; Permeability; planar antenna; Printed circuits; Radiation; radio; Radio frequency identification; Resonant frequencies; Resonators; Sensors; Signal strength; split-ring resonator; Uncertainty analysis; Wide area networks
• ISSN: 1424-8220; 1424-8220
• DOI: 10.3390/s21237779

While wireless IOT modules can be made extremely compact, antennas typically protrude from the module, providing the potential to catch near moving/rotating equipment or transfer loads to the PCB through end forces, which can lead to failures. This work explores the use of split-ring resonator (SRR) designs to achieve a planar antenna with a maximum dimension less than a monopole working at the same frequency. The very narrow bandwidth of the SRR required detailed physical models to create printed circuit board (PCB)-based antenna designs that could be used at LoRa frequencies of 433 MHz and 915 MHz. Uncertainty analysis allowed for the impact of geometrical and physical tolerances on the resonant frequency to be evaluated. Nearfield and farfield measurements were performed allowing for the resonant frequency, directionality, and range of the antenna to be evaluated. An unbalanced SMA port was added to the SRR design to allow for the use of a network vector analyser to determine the input impedance of various designs. The optimum design achieved an input resistance of 44 Ω at a resonant frequency of 919 MHz, close to the target values (50 Ω at 915 MHz). Field measurements of the received signal strength from a planar antenna design indicated a gain of 5 dB over a conventional quarter-wave monopole antenna, in a footprint that was 40% smaller than the monopole.