Which propagation method reflects off the atmosphere?

• A. Direct wave
• B. Sky wave
• C. Space wave
• D. Ground wave

Answer: (B)

This question tests how radio waves can travel by interacting with the atmosphere. A sky wave is a radio signal sent toward the ionosphere that is refracted and reflected back to Earth, allowing communications beyond the line of sight. The ionosphere is a layer of ionized particles high in the atmosphere that can act like a mirror for certain frequencies, bending the signal back down to earth. That reflection is what enables long-range transmission without satellites or repeaters.

In contrast, a direct wave travels straight from transmitter to receiver and is limited by the horizon, while a space wave essentially follows the same line-of-sight path through the air into space. A ground wave travels along the Earth's surface, following its curvature due to diffraction and ground conductivity, and doesn’t reach far beyond the horizon in most conditions.