# Magnetic Field Enhancement of an Electromechanical–Magnetic Antenna for ELF Cross-Medium Communication via a Parallel Configuration

## TL;DR

A compact antenna system for ELF communication was developed, enabling efficient cross-medium transmission with improved performance over traditional methods.

## Contribution

A scalable parallel-EMA system was introduced, enhancing magnetic radiation for compact ELF communication.

## Key findings

- A single EMA has a volume of 3.3 cm³ and a length of 12 cm, much smaller than conventional ELF antennas.
- The parallel-EMA system achieved 114 pT magnetic flux density at 20 m in seawater, doubling the performance of a single EMA.
- Optimal separation distance between EMAs was determined to maximize cooperative radiation efficiency.

## Abstract

Extremely low-frequency (ELF, 3–30 Hz) signals are effective for cross-medium transmission, yet conventional implementations are hindered by their large size and low efficiency. To address these limitations, a compact electromechanical–magnetic antenna (EMA) was developed and experimentally validated for ELF magnetic communication. The basic unit of the antenna, a single-EMA, consists of a stacked magnetostrictive composite beam, piezoelectric ceramic plates, and tip-mounted permanent magnets. The total envelope volume of a single EMA is only 3.3 cm3 with a maximum length of 12 cm, representing a substantial reduction compared with conventional ELF antennas. Building on this compact architecture, two EMAs were operated in parallel to form a parallel-EMA system, which significantly enhanced magnetic radiation through constructive magnetic coupling. Moreover, the optimal separation distance between the two EMAs was identified, ensuring efficient cooperative radiation. When driven at 50.2 mW, the parallel-EMA configuration generated a magnetic flux density of 114 pT at a transmission distance of 20 m in seawater. This performance demonstrates nearly a twofold improvement over a single-EMA unit, validating the scalability of parallel operation for stronger magnetic radiation. The compact form factor of the single EMA combined with the enhanced radiation performance of the parallel-EMA system enables portable ELF magnetic communication across diverse cross-medium scenarios, including air-to-sea and underground-to-air links.