To interconnect an unbalanced coaxial line with a balanced parallel two-wire line while minimizing common-mode currents and preserving impedance, 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 transformer only
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EWilkinson power divider
Answer
Correct Answer: Balun (balanced-to-unbalanced transformer)
Explanation
Introduction / Context:Connecting coaxial (unbalanced) feeds to balanced lines or antennas requires mode conversion to prevent radiation and pattern distortion.
Given Data / Assumptions:
- One port: coaxial line (unbalanced).
- Other port: parallel two-wire line (balanced).
- Goal: proper impedance transfer and suppression of common-mode currents.
Concept / Approach:
A balun converts between balanced and unbalanced modes and can also perform impedance transformation (e.g., 1:1, 4:1). It enforces equal and opposite currents on the balanced conductors, thereby mitigating feedline radiation.
Step-by-Step Solution:
1) Identify mode mismatch (unbalanced vs balanced).2) Select a device that performs mode and impedance conversion: the balun.3) Implement an appropriate balun topology (transformer, transmission-line, or ferrite choke).Verification / Alternative check:
Common TV twin-lead adapters and many antenna feeds use baluns to achieve clean patterns and low VSWR.
Why Other Options Are Wrong:
- Slotted line: Measurement tool.
- Directional coupler: Samples power; not a mode converter.
- Quarter-wave transformer only: Matches impedances but does not enforce balance.
- Wilkinson divider: Power division with isolation resistors; not a balun.
Common Pitfalls:
Assuming impedance matching alone is sufficient; ignoring mode balance leads to unwanted radiation.
Final Answer:
Balun (balanced-to-unbalanced transformer)