How does a ferrite bead or core reduce common-mode RF current on the shield of a coaxial cable?
The correct answer is A: By creating an impedance in the current's path. How a ferrite bead or core reduces common-mode RF current on the shield of a coaxial cable is by creating an impedance in the current's path. Ferrite presents high impedance to common-mode current, blocking it. For amateur radio operators, this is a common RFI fix. Understanding this helps when troubleshooting RF interference.
Exam Tip
Ferrite reduces common-mode = creates impedance in current's path. Think 'F'errite = 'F'orms 'I'mpedance. Ferrite presents high impedance to common-mode current, blocking it. Not conversion, not cancellation, not expulsion - just impedance blocking.
Memory Aid
"Ferrite reduces common-mode = creates impedance in current's path. Think 'F'errite = 'F'orms 'I'mpedance. Ferrite presents high impedance to common-mode current, blocking it. Effective RFI fix."
Real-World Application
Common-mode RF current flows on the shield of a coaxial cable, causing interference. You place a ferrite bead on the cable. The ferrite presents high impedance to the common-mode current, blocking it and preventing interference. This is how ferrites reduce common-mode RFI - by creating impedance.
Key Concepts
Why Other Options Are Wrong
Option B: Incorrect. Ferrite doesn't convert common-mode to differential mode - it blocks common-mode current, it doesn't convert it. Conversion isn't the mechanism.
Option C: Incorrect. Ferrite doesn't create out-of-phase current to cancel - it blocks current by creating impedance, not by cancellation. Cancellation isn't the mechanism.
Option D: Incorrect. Ferrites don't expel magnetic fields - they concentrate magnetic fields. Expulsion isn't the mechanism.
题目解析
The correct answer is A: By creating an impedance in the current's path. How a ferrite bead or core reduces common-mode RF current on the shield of a coaxial cable is by creating an impedance in the current's path. Ferrite presents high impedance to common-mode current, blocking it. For amateur radio operators, this is a common RFI fix. Understanding this helps when troubleshooting RF interference.
考试技巧
Ferrite reduces common-mode = creates impedance in current's path. Think 'F'errite = 'F'orms 'I'mpedance. Ferrite presents high impedance to common-mode current, blocking it. Not conversion, not cancellation, not expulsion - just impedance blocking.
记忆口诀
Ferrite reduces common-mode = creates impedance in current's path. Think 'F'errite = 'F'orms 'I'mpedance. Ferrite presents high impedance to common-mode current, blocking it. Effective RFI fix.
实际应用示例
Common-mode RF current flows on the shield of a coaxial cable, causing interference. You place a ferrite bead on the cable. The ferrite presents high impedance to the common-mode current, blocking it and preventing interference. This is how ferrites reduce common-mode RFI - by creating impedance.
错误选项分析
Option B: Incorrect. Ferrite doesn't convert common-mode to differential mode - it blocks common-mode current, it doesn't convert it. Conversion isn't the mechanism. Option C: Incorrect. Ferrite doesn't create out-of-phase current to cancel - it blocks current by creating impedance, not by cancellation. Cancellation isn't the mechanism. Option D: Incorrect. Ferrites don't expel magnetic fields - they concentrate magnetic fields. Expulsion isn't the mechanism.
知识点
Ferrite bead, Common-mode current, Impedance, RFI reduction
Verified Content
Question from official FCC General Class question pool. Explanation reviewed by licensed amateur radio operators.