Paper #3

Impact of Radial Grounding Model Granularity on Directivity of 433 MHz Monopole Antennas with Flat and Inclined Radials for ISM IoT Applications

Jinfeng Li and Haolin Zhou

Abstract: Grounding is a critical factor in the performance of quarter-wave monopole antennas. Previous studies have explored finite, continuous grounding configurations with one- and two-dimensional variations in size for a 433 MHz vertical monopole antenna, identifying optimal geometries that maximize directivity while minimizing material costs and grounding size. However, these findings are not directly applicable to mission-critical environments (e.g., space, airborne, underwater, or ground-based applications) where continuous metallic grounding may be unavailable. This study extends the investigation to discretized grounding configurations, specifically employing radial monopoles formed by metal rods arranged in sparse or dense radial patterns. Both flat planar and inclined configurations of radial rods are analysed, with a focus on understanding the influence of design parameters, such as radial length and inclination angle, on antenna directivity and radiation patterns—key factors affecting signal reception in wireless communication systems, particularly in applications such as the internet of things (IoT). Using the Method of Moments (MoM) for simulation, the study provides practical guidelines for optimizing the design of 433 MHz monopole antennas constrained by finite and discrete grounding structures. The results indicate that an elevated radial configuration, consisting of five radials inclined at 5° from the monopole plane (equivalent to 85° from the horizontal plane) with a radial length of 2.5 meters, achieves a directivity of 9.23 dBi at 433 MHz. This represents a significant improvement over the flat planar configuration, which achieves a directivity of 6.23 dBi under the same conditions. These findings are particularly relevant for mobile communication, Internet of Things (IoT) devices, and radiofrequency (RF) systems requiring high performance from vertical monopole antennas in challenging grounding environments.

Keywords: Antenna directivity; Electromagnetics; Grounding in monopole antennas; Image theory; Internet of Things; ISM band; Radial monopole; 433 MHz.