For a well-designed balun (balanced-to-unbalanced transformer), what should the standing wave ratio (SWR) look like at its two ports under proper matching conditions?

Introduction:A balun converts between balanced and unbalanced lines (for example, a dipole and a coax). The key performance metric is how well it passes power without reflections, which is quantified via SWR (or return loss). This question checks whether you recognize that a properly matched balun should present low SWR on both sides.

Given Data / Assumptions:

• Two-port passive device (balun).
• Source and load impedances are chosen to match the balun’s specified impedances (e.g., 50 Ω to 200 Ω).
• Standard definition: SWR ≈ (1 + |Gamma|) / (1 − |Gamma|), where Gamma is the reflection coefficient.

Concept / Approach:

Low SWR indicates minimal reflections and good impedance matching. In an ideal balun, each port is matched to its intended impedance, yielding SWR close to 1 at both ports across the design bandwidth.

Step-by-Step Solution:

Verification / Alternative check:

Measured return loss S11 and S22 on a VNA: values > 10 dB (preferably > 15–20 dB) indicate good matching, which corresponds to SWR near 1.3 or better.

Why Other Options Are Wrong:

• High SWR at either port implies mismatch and reflections.
• Mixed high/low SWR indicates only one side is matched, wasting power and distorting patterns.
• Fixed SWR = 2 is arbitrary and not a design goal.

Common Pitfalls:

Confusing impedance transformation ratio with SWR. A 1:4 balun can still have low SWR if both sides are matched to their intended impedances.

Final Answer:

low SWR at both ports