What is the radio horizon?

The radio horizon refers to the distance at which radio signals, due to the curvature of the Earth, can travel between two points before being blocked. This concept is crucial for ham radio operators because it influences how far and effectively they can communicate. When the Earth's curvature obstructs the line-of-sight between an antenna and the receiver, it limits the distance that signals can propagate effectively.

This phenomenon is particularly important for VHF and UHF signals, which are typically limited to line-of-sight communication. Operators must account for the radio horizon when planning communications, such as when setting up repeaters or coordinating in emergencies, as understanding this limitation helps in optimizing the positioning of antennas for maximum coverage.

The other answer choices do not accurately capture the concept of the radio horizon. Understanding the physical limitations posed by the Earth's curvature is essential for effective radio communications, which is why the correct choice is the distance at which signals are effectively blocked by the Earth's curvature.