Why is the radio horizon for VHF and UHF signals more distant than the visual horizon?
The correct answer is C: The atmosphere refracts radio waves slightly. The radio horizon for VHF and UHF signals is more distant than the visual horizon because the atmosphere refracts (bends) radio waves slightly. This bending extends the effective range of radio signals beyond what you can see.
Radio waves are refracted by the atmosphere due to changes in air density and moisture content with altitude. This refraction bends the radio waves slightly downward, following Earth's curvature more than light does. This extends the radio horizon beyond the visual horizon - you might be able to communicate with stations you can't see. The effect is more pronounced at lower frequencies and under certain atmospheric conditions. This is why your radio can 'see' farther than your eyes can.
Exam Tip
Radio horizon > Visual = Atmospheric refraction. Remember: The radio horizon is more distant than the visual horizon because the atmosphere refracts (bends) radio waves slightly, extending their effective range.
Memory Aid
"**R**adio **H**orizon = **R**efracted **H**orizon (think 'RH = RH' = Refracted Horizon)"
Real-World Application
You're on a hill and can see about 10 miles to the horizon. However, you can communicate with a station 15 miles away on 2 meters, even though you can't see them. This is because atmospheric refraction is bending your radio signals slightly, following Earth's curvature and extending your radio horizon beyond your visual horizon. The radio waves are being refracted by the atmosphere, allowing communication beyond line-of-sight.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Radio signals don't move faster than the speed of light. All electromagnetic waves travel at the speed of light.
Option B: Incorrect. While radio waves aren't blocked by dust, this isn't why the radio horizon is more distant. It's due to atmospheric refraction.
Option D: Incorrect. Radio waves aren't blocked by dust particles in a way that extends the horizon. Dust would actually reduce range, not extend it.
题目解析
The correct answer is C: The atmosphere refracts radio waves slightly. The radio horizon for VHF and UHF signals is more distant than the visual horizon because the atmosphere refracts (bends) radio waves slightly. This bending extends the effective range of radio signals beyond what you can see. Radio waves are refracted by the atmosphere due to changes in air density and moisture content with altitude. This refraction bends the radio waves slightly downward, following Earth's curvature more than light does. This extends the radio horizon beyond the visual horizon - you might be able to communicate with stations you can't see. The effect is more pronounced at lower frequencies and under certain atmospheric conditions. This is why your radio can 'see' farther than your eyes can.
考试技巧
Radio horizon > Visual = Atmospheric refraction. Remember: The radio horizon is more distant than the visual horizon because the atmosphere refracts (bends) radio waves slightly, extending their effective range.
记忆口诀
**R**adio **H**orizon = **R**efracted **H**orizon (think 'RH = RH' = Refracted Horizon)
实际应用示例
You're on a hill and can see about 10 miles to the horizon. However, you can communicate with a station 15 miles away on 2 meters, even though you can't see them. This is because atmospheric refraction is bending your radio signals slightly, following Earth's curvature and extending your radio horizon beyond your visual horizon. The radio waves are being refracted by the atmosphere, allowing communication beyond line-of-sight.
错误选项分析
Option A: Incorrect. Radio signals don't move faster than the speed of light. All electromagnetic waves travel at the speed of light. Option B: Incorrect. While radio waves aren't blocked by dust, this isn't why the radio horizon is more distant. It's due to atmospheric refraction. Option D: Incorrect. Radio waves aren't blocked by dust particles in a way that extends the horizon. Dust would actually reduce range, not extend it.
知识点
Radio horizon, Atmospheric refraction, VHF, UHF
Verified Content
Question from official FCC Technician Class question pool. Explanation reviewed by licensed amateur radio operators.