Rotating permanent magnets represent a promising type of mechanical antenna suitable for environments where conventional RF wireless communication is not feasible. Their key advantage lies in enabling the miniaturization of low-frequency antennas, which is crucial for two-way through-the-ground communication. The conductive earth inevitably affects the primary electromagnetic field generated by the rotating permanent magnet due to electromagnetic induction. However, the specific mechanisms and quantitative characteristics of the earth’s conductivity on the radiation pattern of a rotating permanent magnet remain unclear. This paper investigates the influence of the earth and its conductivity on the primary field excited by a rotating permanent magnet using the synthetic field method. When the distance, earth conductivity or the frequency increases, the main direction of radiation shifts from the x-axis to the y-axis, and the asymmetry relative to the x-axis first increases and then decreases. The ratio of the magnetic field in the y-axis to the magnetic field in the air first increases rapidly at first and then slowly, while the magnetic field in the x-axis first increases and then decreases. The findings provide theoretical insights that are important for the practical application of mechanical antennas in through-the-earth communication systems.