Design of a Compact Tri-band L-Shaped Antenna for Underground Communication and Sensing

This paper presents a compact tri-band L-shaped antenna designed for underground communication and sensing. The combination of an L-shaped monopole with horizontal branches and a triangular structure enables operation in three industrial, scientific, and medical (ISM) frequency bands: 430 910 MHz, and 2.4 GHz, with relative bandwidth of 52.5%, 39.6%, and 35.8%, respectively. The size of the antenna is 100 $\mathrm{mm} \times 100 \mathrm{~mm}\left(0.13 \lambda_{0} \times 0.13 \lambda_{0}\right)$, making it suitable for deployment in confined underground spaces. The antenna design approach initially entailed creating an L-shaped monopole structure to generate resonance points at 430 MHz and 2.4 GHz. Subsequently, a horizontal branch was added to the right side of the antenna to achieve a resonance point at 910 MHz. Finally, to broaden the bandwidth at the 430 MHz frequency band, a triangular structure was appended to the left side of the L-shaped antenna. Simulations of the antenna operating in soil demonstrate that its radiation can be effectively directed towards the soil, with minimal scattering in the air behind the antenna.