What is the only method of communicating over long distances from one point of the earth to another without a relay, satellite, or connecting cable?

The correct answer is HF radio because it utilizes the ionosphere to reflect radio waves over long distances. HF, or High Frequency, band typically ranges from 3 to 30 MHz and is particularly effective for long-range communication as waves can be bounced off the ionosphere back to the ground, allowing signals to travel beyond the horizon without the need for a satellite, relay, or physical connecting cable. This characteristic makes HF radio ideal for situations where direct line-of-sight communication is not possible.

In contrast, VHF radio operates at lower frequencies (30 to 300 MHz) and is usually limited to line-of-sight communication. Satellite communication relies on satellites orbiting the Earth to relay signals between two points, making it an indirect method. Cellular communication depends on a network of ground-based cell towers, which also does not qualify as direct communication between two remote points. Thus, HF radio is unique in its ability to achieve long-distance communication through the reflective properties of the ionosphere.