What factor contributes to the ADF's ability to receive long-distance signals?

The ability of the Automatic Direction Finder (ADF) to receive long-distance signals is influenced significantly by the curvature of the Earth. As radio waves travel, they can be affected by the Earth's surface, particularly in terms of how they propagate over distances. The curvature of the Earth plays a crucial role in radio wave propagation and line of sight.

For long-distance reception, particularly at the frequencies typically used by ADF systems (which operate in the low frequency and medium frequency bands), the curvature can impact how signals are refracted and can affect the ground wave and sky wave propagation modes. Ground waves follow the contour of the Earth, while sky waves can reflect off the ionosphere and return to the Earth's surface. The curvature allows these waves to bend, potentially enabling them to travel beyond the horizon and maintain a signal, making it possible for the ADF to function effectively over long distances.

While other factors such as antenna type, altitude, and even time of day can also affect signal reception, they do not have the same direct impact on long-distance capabilities stemming specifically from the geological shape of the Earth.