Which are the two principal ways in which electromagnetic (radio) energy travels from a transmitting antenna to a receiving antenna?

The two principal ways electromagnetic (radio) energy travels from a transmitting antenna to a receiving antenna are indeed through ground waves and sky waves. Ground waves propagate along the surface of the Earth and are used in lower frequency transmissions, allowing signals to travel over obstacles and follow the curvature of the Earth. This mode can deliver signals over long distances, particularly at lower frequencies.

On the other hand, sky waves are transmitted at higher frequencies and utilize the ionosphere to refract radio waves back to Earth. This allows for long-distance communication, especially in cases where direct line-of-sight transmission is not possible, such as over the horizon.

Understanding these two propagation methods is essential for effective communication system design and operation, as they influence factors like range, coverage, and frequency selection for radio transmissions. The other choices do not accurately describe the primary modes of electromagnetic wave propagation, focusing instead on different concepts or categories of wave behavior.