A Nail-Shaped Ferrite Antenna for Optimizing VLF Radiation Characteristics

As a more mature alternative to emerging mechanical antennas, rod-shaped ferrite antennas (RFAs) are widely employed to enhance magnetic flux density ( B ) in very low frequency (VLF) near-field communications. However, they induce detrimental magnetic interference to nearby electronic components in compact devices. To address this, we present a nail-shaped ferrite antenna (NFA) that integrates a shielding structure to optimize VLF radiation characteristics. The proposed shielding structure effectively guides magnetic flux, suppressing backward interference while simultaneously enhancing forward radiation. Experimental results demonstrate that the NFA reduces backward B by 66.00% at a distance of 5 cm compared to a conventional RFA, while enhancing forward B by an average of 50% within a 5 to 50 cm range. Furthermore, the NFA exhibits superior tuning capability, achieving a resonant frequency range 7.69 times that of the RFA for the same coil displacement. A compact active antenna prototype (10×10×15 cm) based on the NFA was developed, achieving a transmission distance of 120 m with an input power of 8.64 W. Building penetration experiments further verified the system’s capability to transmit signals through four reinforced concrete floors, highlighting its potential for portable applications in challenging environments such as emergency and mining communications.