Which of the following is a reason to use shielded wire?
The correct answer is C: To prevent coupling of unwanted signals to or from the wire. Shielded wire (coaxial cable or shielded cable) is used to prevent coupling of unwanted signals - both keeping external interference from affecting the signal inside the wire, and preventing the signal from radiating and interfering with other equipment. The shield (braided or foil conductor surrounding the inner conductor) acts as a barrier, blocking electromagnetic fields. For amateur radio operators, shielded wire is essential for RF connections, microphone cables, and sensitive signal paths to prevent RF interference, reduce noise pickup, and comply with regulations by minimizing unwanted radiation. Understanding when to use shielded wire is crucial for proper station setup and interference reduction.
Exam Tip
Shielded wire prevents unwanted signal coupling. Think 'S'hield = 'S'tops 'S'ignals from coupling. It blocks interference both ways - in and out. The shield is for isolation, not resistance or current capacity.
Memory Aid
"Shielded wire = prevents coupling. Think 'S'hield = 'S'tops 'S'ignals from coupling. Blocks interference in both directions - keeps noise out and signals in."
Real-World Application
When running a microphone cable from your transceiver to your operating position, you use shielded wire. The shield prevents RF from your transmitter from coupling into the microphone circuit (which would cause feedback), and also prevents external interference from affecting your audio. Without the shield, you'd get RF feedback and noise problems. The shield acts as a barrier around your signal.
Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Shielded wire doesn't decrease DC resistance - the resistance depends on the conductor material and size, not the shield. The shield adds no DC resistance benefit.
Option B: Incorrect. The shield doesn't increase current-carrying capability - that depends on the inner conductor size. The shield is for signal isolation, not current capacity.
Option D: Incorrect. Shielded wire prevents coupling, it doesn't couple signals. The shield blocks unwanted coupling, which is the opposite of coupling signals.
题目解析
The correct answer is C: To prevent coupling of unwanted signals to or from the wire. Shielded wire (coaxial cable or shielded cable) is used to prevent coupling of unwanted signals - both keeping external interference from affecting the signal inside the wire, and preventing the signal from radiating and interfering with other equipment. The shield (braided or foil conductor surrounding the inner conductor) acts as a barrier, blocking electromagnetic fields. For amateur radio operators, shielded wire is essential for RF connections, microphone cables, and sensitive signal paths to prevent RF interference, reduce noise pickup, and comply with regulations by minimizing unwanted radiation. Understanding when to use shielded wire is crucial for proper station setup and interference reduction.
考试技巧
Shielded wire prevents unwanted signal coupling. Think 'S'hield = 'S'tops 'S'ignals from coupling. It blocks interference both ways - in and out. The shield is for isolation, not resistance or current capacity.
记忆口诀
Shielded wire = prevents coupling. Think 'S'hield = 'S'tops 'S'ignals from coupling. Blocks interference in both directions - keeps noise out and signals in.
实际应用示例
When running a microphone cable from your transceiver to your operating position, you use shielded wire. The shield prevents RF from your transmitter from coupling into the microphone circuit (which would cause feedback), and also prevents external interference from affecting your audio. Without the shield, you'd get RF feedback and noise problems. The shield acts as a barrier around your signal.
错误选项分析
Option A: Incorrect. Shielded wire doesn't decrease DC resistance - the resistance depends on the conductor material and size, not the shield. The shield adds no DC resistance benefit. Option B: Incorrect. The shield doesn't increase current-carrying capability - that depends on the inner conductor size. The shield is for signal isolation, not current capacity. Option D: Incorrect. Shielded wire prevents coupling, it doesn't couple signals. The shield blocks unwanted coupling, which is the opposite of coupling signals.
知识点
Shielded wire, RF interference, Signal isolation, Coaxial cable
Verified Content
Question from official FCC Technician Class question pool. Explanation reviewed by licensed amateur radio operators.