Question
How does a ferrite bead or core reduce common-mode RF current on the shield of a coaxial cable?
Answer Options
- A) By creating an impedance in the current’s path
- B) It converts common-mode current to differential mode current
- C) By creating an out-of-phase current to cancel the common-mode current
- D) Ferrites expel magnetic fields
Correct Answer: A
Explanation
Common-mode RF current is unwanted current that flows in the same direction on all conductors, typically on the outside of a coaxial cable shield, causing the shield to act as an unintended antenna. A ferrite bead or core is slipped over the cable to suppress this current.
The ferrite material’s high magnetic permeability works specifically on the common-mode current (but ignores the desired differential mode current inside the coax). It effectively creates an impedance in the current’s path by adding resistance and inductive reactance. This high impedance chokes off the unwanted common-mode RF current, preventing it from radiating or entering station equipment.
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