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?

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:

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)