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?

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:

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)