A novel dumbbell-shaped magnetoelectric antenna with reduced operating frequency via first-order longitudinal resonance

This study proposes a three-dimensional (3D) dumbbell-shaped magnetoelectric antenna that utilizes an acoustic resonance mechanism to circumvent fundamental dimensional constraints inherent in conventional electromagnetic radiators and to address the critical demand for operating frequency reduction in compact very low frequency (VLF) antennas for lossy-media communications. The proposed architecture features a custom constricted central cross section geometry that is engineered to downshift the operational frequency band through the manipulation of topological mass and equivalent stiffness distribution. Tapering the central segment intrinsically reduces the equivalent structural stiffness, breaking the low-frequency limitations of standard cylindrical geometries while simultaneously enhancing stress-mediated electromechanical coupling. A composite bar vibration theory-based Mason equivalent network model clarifies this structural reconfiguration and confirms highly efficient energy conversion and radiation performance. Prototype experimental evaluations demonstrate a reduction in the fundamental longitudinal resonance frequency to 13.8 kHz through the dumbbell-shaped geometry. This reduction constitutes an effective frequency downshift compared to standard cylindrical benchmarks without expanding the overall physical volume. The geometric design simultaneously induces a stress enhancement effect that stabilizes electromechanical responses and increases the radiation resistance required for long-distance VLF transmission. This designed geometric structure presents a feasible strategy for addressing space-constrained antenna limitations and serves as a potential design reference for integrating high-performance VLF wireless systems into mobile and underwater platforms.