SURFACE WAVES travel along the contour of the Earth by diffraction.
SPACE WAVES can travel through the air directly to the receiving antenna or can be reflected from
the surface of the Earth.
SKY WAVES, often called ionospheric waves, are radiated in an upward direction and returned to
Earth at some distant location because of refraction.
NATURAL HORIZON is the line-of-sight horizon.
RADIO HORIZON is one-third farther than the natural horizon.
The IONOSPHERE consists of several layers of ions, formed by the process called ionization.
IONIZATION is the process of knocking electrons free from their parent atom, thus upsetting
RECOMBINATION is the opposite of ionization; that is, the free ions combine with positive ions,
causing the positive ions to return to their original neutral atom state.