What is the main effect that makes long-distance radio communication possible?

Long-distance radio communication primarily relies on the reflection of radio waves from the ionosphere. The ionosphere is a layer of the Earth's atmosphere that contains a high concentration of ions and free electrons, which can reflect certain radio frequencies back to Earth. This characteristic allows radio waves to travel much farther than they would if they were only sent directly through the atmosphere.

When specific frequencies of radio waves are transmitted, they can bounce off the ionosphere and return to Earth at a distance much greater than the line-of-sight range. This phenomenon extends the reach of communication across vast distances, making it possible to connect with receivers that are beyond the normal range dictated by the curvature of the Earth.

Using high-powered transmitters can enhance signal strength but does not guarantee the ability to cover long distances without relying on reflection. Similarly, direct line-of-sight transmission is limited to shorter distances and can be obstructed by geographical features. Multiple frequency signaling can improve transmission quality and reduce interference but does not facilitate long-distance communication in the same way the ionosphere does.

Therefore, the reflection of radio waves from the ionosphere is the crucial factor enabling effective long-distance radio communication.