What is a likely cause of irregular fading of signals propagated by the ionosphere?
The correct answer is D: Random combining of signals arriving via different paths. Irregular fading of signals propagated by the ionosphere is caused by random combining of signals that arrive via different ionospheric paths. These multiple paths have different lengths and propagation times, and they randomly combine at the receiving antenna.
When signals travel through the ionosphere, they can take multiple paths - some signals bounce once, others bounce multiple times, and paths can vary in length. These signals arrive at slightly different times and with different phases. As ionospheric conditions change, these paths vary, causing the signals to randomly add together (constructive interference, stronger signal) or cancel out (destructive interference, weaker signal). This creates the irregular, constantly changing fading pattern typical of HF ionospheric propagation.
Exam Tip
Ionospheric fading = Random multipath. Remember: Irregular fading on ionospherically propagated signals is caused by random combining of signals arriving via different ionospheric paths.
Memory Aid
"**I**rregular **F**ading = **I**onospheric **F**luctuating **P**aths (think 'IF = IFP' = Ionospheric Fluctuating Paths)"
Real-World Application
You're listening to a station on 20 meters that's 2000 miles away. The signal strength constantly varies - sometimes strong, sometimes weak, sometimes disappearing entirely for a moment. This irregular fading is caused by your signal taking multiple paths through the ionosphere (some signals bounce once, others bounce twice, paths vary in length). These paths randomly combine at your antenna, sometimes reinforcing and sometimes canceling, creating the constantly changing signal strength.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Faraday rotation affects polarization, not fading patterns. It doesn't cause the irregular fading described.
Option B: Incorrect. Thunderstorm interference would be more consistent and localized, not the irregular fading pattern typical of ionospheric propagation.
Option C: Incorrect. Intermodulation distortion is a transmitter/receiver issue, not a propagation phenomenon causing irregular fading.
题目解析
The correct answer is D: Random combining of signals arriving via different paths. Irregular fading of signals propagated by the ionosphere is caused by random combining of signals that arrive via different ionospheric paths. These multiple paths have different lengths and propagation times, and they randomly combine at the receiving antenna. When signals travel through the ionosphere, they can take multiple paths - some signals bounce once, others bounce multiple times, and paths can vary in length. These signals arrive at slightly different times and with different phases. As ionospheric conditions change, these paths vary, causing the signals to randomly add together (constructive interference, stronger signal) or cancel out (destructive interference, weaker signal). This creates the irregular, constantly changing fading pattern typical of HF ionospheric propagation.
考试技巧
Ionospheric fading = Random multipath. Remember: Irregular fading on ionospherically propagated signals is caused by random combining of signals arriving via different ionospheric paths.
记忆口诀
**I**rregular **F**ading = **I**onospheric **F**luctuating **P**aths (think 'IF = IFP' = Ionospheric Fluctuating Paths)
实际应用示例
You're listening to a station on 20 meters that's 2000 miles away. The signal strength constantly varies - sometimes strong, sometimes weak, sometimes disappearing entirely for a moment. This irregular fading is caused by your signal taking multiple paths through the ionosphere (some signals bounce once, others bounce twice, paths vary in length). These paths randomly combine at your antenna, sometimes reinforcing and sometimes canceling, creating the constantly changing signal strength.
错误选项分析
Option A: Incorrect. Faraday rotation affects polarization, not fading patterns. It doesn't cause the irregular fading described. Option B: Incorrect. Thunderstorm interference would be more consistent and localized, not the irregular fading pattern typical of ionospheric propagation. Option C: Incorrect. Intermodulation distortion is a transmitter/receiver issue, not a propagation phenomenon causing irregular fading.
知识点
Ionospheric propagation, Multipath, Signal fading, Random interference
Verified Content
Question from official FCC Technician Class question pool. Explanation reviewed by licensed amateur radio operators.