Coupling of Very Low Frequency Through-the-earth Radio Signals to Elongated Conductors

Through-the-earth (TTE) radio signals (400-9000 Hz) have been observed to couple to elongated conductors present near the transmitter (Tx). This phenomenon, which greatly increases signal range, is poorly understood. Experimental and numerical data are combined to better understand the potential of such a communication link. A finite-difference time-domain (FDTD) code is used with a thin wire approximation to model the elongated conductors, which include railway tracks and an elevator shaft. The coupling effect, attributed to currents induced in the elongated conductor, is characterized based on measurements of the three magnetic field components in the vicinity of the conductors. The ability of FDTD to handle arbitrary material geometries makes it an effective tool not only for reconciling theoretical and experimental results, but also for predicting the viability of TTE communication links in different settings, and to optimize the placement of the Tx and receiver.