Why are simplex UHF signals rarely heard beyond their radio horizon?
The correct answer is C: UHF signals are usually not propagated by the ionosphere. Simplex UHF signals are rarely heard beyond their radio horizon because UHF frequencies are generally too high for ionospheric propagation. The ionosphere typically doesn't refract UHF signals back to Earth.
UHF signals (300-3000 MHz) are above the maximum usable frequency (MUF) for normal ionospheric propagation. While the ionosphere can sometimes propagate lower UHF frequencies under exceptional conditions, normal UHF communications are line-of-sight. UHF signals travel in straight lines and are blocked by the curvature of the Earth beyond the radio horizon. This is why UHF is excellent for local communications but doesn't provide the long-distance capabilities of HF.
Exam Tip
UHF beyond horizon = No ionospheric propagation. Remember: UHF signals rarely go beyond the radio horizon because they're usually not propagated by the ionosphere. UHF is primarily line-of-sight.
Memory Aid
"**U**HF **H**orizon = **U**nable **H**igh **F**requency (think 'UH = UHF' = Unable High Frequency, too high for ionosphere)"
Real-World Application
You're operating on 70 centimeters (UHF) and trying to contact a station 100 miles away. Even with high power, you can't make contact because the station is beyond your radio horizon, and the ionosphere doesn't refract your UHF signal. UHF signals travel in straight lines and are blocked by Earth's curvature. This is why UHF is great for local communications but doesn't provide HF-style long-distance capabilities.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. UHF signals aren't necessarily too weak - they can be quite strong. The issue is propagation, not power.
Option B: Incorrect. FCC regulations don't limit UHF range to 50 miles. The limitation is physical propagation, not regulatory.
Option D: Incorrect. The D region absorbs lower frequencies, not UHF. UHF isn't significantly absorbed by the D region.
题目解析
The correct answer is C: UHF signals are usually not propagated by the ionosphere. Simplex UHF signals are rarely heard beyond their radio horizon because UHF frequencies are generally too high for ionospheric propagation. The ionosphere typically doesn't refract UHF signals back to Earth. UHF signals (300-3000 MHz) are above the maximum usable frequency (MUF) for normal ionospheric propagation. While the ionosphere can sometimes propagate lower UHF frequencies under exceptional conditions, normal UHF communications are line-of-sight. UHF signals travel in straight lines and are blocked by the curvature of the Earth beyond the radio horizon. This is why UHF is excellent for local communications but doesn't provide the long-distance capabilities of HF.
考试技巧
UHF beyond horizon = No ionospheric propagation. Remember: UHF signals rarely go beyond the radio horizon because they're usually not propagated by the ionosphere. UHF is primarily line-of-sight.
记忆口诀
**U**HF **H**orizon = **U**nable **H**igh **F**requency (think 'UH = UHF' = Unable High Frequency, too high for ionosphere)
实际应用示例
You're operating on 70 centimeters (UHF) and trying to contact a station 100 miles away. Even with high power, you can't make contact because the station is beyond your radio horizon, and the ionosphere doesn't refract your UHF signal. UHF signals travel in straight lines and are blocked by Earth's curvature. This is why UHF is great for local communications but doesn't provide HF-style long-distance capabilities.
错误选项分析
Option A: Incorrect. UHF signals aren't necessarily too weak - they can be quite strong. The issue is propagation, not power. Option B: Incorrect. FCC regulations don't limit UHF range to 50 miles. The limitation is physical propagation, not regulatory. Option D: Incorrect. The D region absorbs lower frequencies, not UHF. UHF isn't significantly absorbed by the D region.
知识点
UHF propagation, Ionosphere, Radio horizon, Line-of-sight
Verified Content
Question from official FCC Technician Class question pool. Explanation reviewed by licensed amateur radio operators.