Reception of high frequency (HF) radio waves beyond 4000 km is generally made possible by:

High Frequency (HF) signals travelling beyond 4000 kilometers rely on the ionosphere to refract the signal back to Earth. This type of propagation is known as an ionospheric wave (often used interchangeably with sky wave). Through single or multiple hops between the Earth and the ionosphere, global communication becomes possible. Other propagation modes cannot achieve these distances at HF. Space waves travel in a straight line and are limited by the visual horizon, while ground waves are absorbed by the Earth's surface and typically die out after a few hundred kilometers.