Question T3C11
From subelement T3 - T3C
Why is the radio horizon for VHF and UHF signals more distant than the visual horizon?
Why is this correct?
Radio waves travel farther than line-of-sight because the atmosphere refracts (bends) them slightly downward as they travel. This atmospheric refraction allows VHF and UHF signals to follow Earth's curvature a bit longer before being blocked, extending the radio horizon beyond the visual horizon. Options A and D are factually incorrect - radio waves travel at light speed and aren't significantly affected by dust particles. Option B is true but irrelevant to horizon distance.
Memory tip
Look for 'atmospheric effects' in propagation questions - the atmosphere almost always influences radio wave behavior through refraction, reflection, or absorption. When comparing radio versus visual horizons, remember that radio waves can bend around obstacles that completely block light waves.
Learn more
This atmospheric refraction is why VHF/UHF repeaters on mountaintops can serve much larger coverage areas than expected from pure line-of-sight calculations. Radio frequency engineers use a '4/3 Earth radius' rule when calculating radio horizons, accounting for standard atmospheric refraction. This same principle explains why you might receive weak VHF signals from stations slightly beyond the theoretical radio horizon during stable weather conditions.
Think about it
Why do you think atmospheric refraction affects radio waves differently than it affects visible light when both are forms of electromagnetic radiation?