3D-structure coupled monopole designs with lower frequency resonance

In this study, we investigated three-dimensional configurations involving a monopole-coupled wired ring, a hollow metallic cylinder, and wired helix spring to achieve resonance lower than quarter-wave resonance. The resonance frequency of a quarter-wave monopole, originally 2.05 GHz, was significantly lowered by 54.53% through effective coupling with a wired ring, and a quarter-wave monopole properly coupled with a hollow metallic cylinder experienced a large decrease of 64.34%. The primary incentive for connecting a wired ring to a hollow metallic cylinder is to significantly reduce resonance frequency. Efforts were also made to lower resonance frequency in a wired helix spring with a length close to that of the wired ring, resulting in a substantial 78.83% reduction in resonance frequency. The −10 dB bandwidth was 14.63% for the quarter-wave monopole, 27.21% for the monopole-coupled wired ring, 33.71% for the monopole-coupled hollow metallic cylinder, and 4.69% for the monopole-coupled helix spring. The ka values for the monopole-coupled wired ring, hollow metallic cylinder, and helix spring were 0.35, 0.27, and 0.16, respectively. Hence, these antennas can be classified as electrically small antennas. The achieved resonant frequencies have applications in various wireless communication scenarios. The experimental results from the fabricated prototype agree well with the simulation results.