What is the area called that is bounded by the outer edge of usable ground wave propagation and the point nearest the antenna at which the sky wave returns to earth?

The area described is known as the skip zone. This term refers to the region between the range of the ground wave and the point where the sky wave, after being reflected back to the earth by the ionosphere, returns to the ground. In this zone, radio signals can be weak or absent due to the nature of how ground waves and sky waves behave in radio propagation.

Ground waves typically travel along the surface of the earth and are limited in range due to obstacles and the curvature of the earth. On the other hand, sky waves can travel long distances by reflecting off the ionosphere. However, there is a distance beyond which the sky wave is no longer effective at the ground level before it has a chance to reflect back. This creates a gap in coverage known as the skip zone.

Understanding the skip zone is crucial for radio engineers and operators in planning transmission systems to ensure that coverage is optimized and to identify where signal losses may occur.