Which of the following is a disadvantage of FM compared with single sideband?
The correct answer is B: Only one signal can be received at a time. A disadvantage of FM compared with single sideband is that only one signal can be received at a time due to FM's capture effect. When two FM signals are on the same frequency, the stronger signal completely captures the receiver, and the weaker signal is not heard at all. With SSB, you can often hear both signals (though with interference). For amateur radio operators, this FM characteristic means you can't copy a weak signal if a stronger one is present, which can be a problem in crowded conditions or when trying to work weak DX.
Exam Tip
FM disadvantage = only one signal at a time (capture effect). Think 'F'M = 'F'ully 'M'onopolizes receiver. Strong signal captures receiver completely, weaker signal not heard. SSB allows hearing both signals (with interference). FM has better voice quality and easier tuning than SSB.
Memory Aid
"FM disadvantage = only one signal at a time. Think 'F'M = 'F'ully 'M'onopolizes receiver. Capture effect means stronger signal completely suppresses weaker one. SSB allows hearing both signals."
Real-World Application
On a busy FM repeater, if two stations transmit simultaneously, you only hear the stronger one - the weaker signal is completely suppressed. On SSB, you might hear both signals (though garbled). This FM capture effect means you can't work a weak DX station if a stronger local station is also transmitting on the frequency.
Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. FM actually has better voice quality than SSB due to its wider bandwidth and noise immunity. Voice quality is an FM advantage, not a disadvantage.
Option C: Incorrect. FM signals are actually easier to tune than SSB - FM is more forgiving of slight frequency errors. SSB requires more precise tuning.
Option D: Incorrect. Since A and C are wrong, 'all of the above' cannot be correct. Only the capture effect (one signal at a time) is a true FM disadvantage.
题目解析
The correct answer is B: Only one signal can be received at a time. A disadvantage of FM compared with single sideband is that only one signal can be received at a time due to FM's capture effect. When two FM signals are on the same frequency, the stronger signal completely captures the receiver, and the weaker signal is not heard at all. With SSB, you can often hear both signals (though with interference). For amateur radio operators, this FM characteristic means you can't copy a weak signal if a stronger one is present, which can be a problem in crowded conditions or when trying to work weak DX.
考试技巧
FM disadvantage = only one signal at a time (capture effect). Think 'F'M = 'F'ully 'M'onopolizes receiver. Strong signal captures receiver completely, weaker signal not heard. SSB allows hearing both signals (with interference). FM has better voice quality and easier tuning than SSB.
记忆口诀
FM disadvantage = only one signal at a time. Think 'F'M = 'F'ully 'M'onopolizes receiver. Capture effect means stronger signal completely suppresses weaker one. SSB allows hearing both signals.
实际应用示例
On a busy FM repeater, if two stations transmit simultaneously, you only hear the stronger one - the weaker signal is completely suppressed. On SSB, you might hear both signals (though garbled). This FM capture effect means you can't work a weak DX station if a stronger local station is also transmitting on the frequency.
错误选项分析
Option A: Incorrect. FM actually has better voice quality than SSB due to its wider bandwidth and noise immunity. Voice quality is an FM advantage, not a disadvantage. Option C: Incorrect. FM signals are actually easier to tune than SSB - FM is more forgiving of slight frequency errors. SSB requires more precise tuning. Option D: Incorrect. Since A and C are wrong, 'all of the above' cannot be correct. Only the capture effect (one signal at a time) is a true FM disadvantage.
知识点
FM capture effect, FM vs SSB, Signal reception, Modulation comparison
Verified Content
Question from official FCC Technician Class question pool. Explanation reviewed by licensed amateur radio operators.