To couple a coaxial (unbalanced) line to a parallel two-wire (balanced) line with minimal common-mode currents and good impedance transfer, which device should be used?
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ASlotted line section
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BBalun (balanced-to-unbalanced transformer)
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CDirectional coupler
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DQuarter-wave impedance transformer
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EWilkinson power divider
Answer
Correct Answer: Balun (balanced-to-unbalanced transformer)
Explanation
Introduction / Context:Coaxial lines are unbalanced, whereas parallel two-wire lines are balanced. Directly interconnecting them causes common-mode currents and radiation. A balun solves this by converting between balanced and unbalanced interfaces.
Given Data / Assumptions:
- One side is a coaxial cable (unbalanced).
- Other side is a two-wire line (balanced).
- Goal: Proper coupling and impedance transfer with minimal radiation.
Concept / Approach:
A balun provides both a current/voltage transformation (if designed as an impedance transformer) and the balanced-to-unbalanced conversion to suppress common-mode currents.
Step-by-Step Solution:
1) Identify line types: coax (unbalanced) and twin-lead (balanced).2) Choose a device that enforces equal and opposite currents on the balanced ports while isolating the shield from radiating.3) A balun satisfies these conditions and can also provide impedance transformation if needed.Verification / Alternative check:
Practical RF systems (e.g., TV twin-lead to coax) employ 1:1 or 4:1 baluns to ensure proper mode conversion and matching.
Why Other Options Are Wrong:
- Slotted line: Measurement tool, not a mode converter.
- Directional coupler: Samples power; not a balanced/unbalanced converter.
- Quarter-wave transformer: Matches impedances but does not perform mode conversion.
- Wilkinson divider: Power split/combining; not a balun.
Common Pitfalls:
Attempting to match impedance only without addressing mode conversion; ignoring common-mode radiation on the feedline.
Final Answer:
Balun (balanced-to-unbalanced transformer)