Which propagation method travels in a straight line?

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

Answer: (C)

Line-of-sight propagation means the wave travels directly through the air along a straight path from transmitter to receiver. This straight-line path is the hallmark of the direct or space-wave mode, which is dominant when the two ends are within each other’s radio horizon and there are no reflections from the ground or ionosphere. Ground waves hug the Earth’s surface and follow its curvature, not a straight line. Sky waves are bent by and reflect off the ionosphere, returning to Earth after an angled path, also not a direct line. Space wave is a broad term for waves propagating through space, typically in line-of-sight, but the specific straight-line path between transmitter and receiver is most precisely described by the direct wave.