Drop SWR — Keep Return Loss and Insertion Loss

SWR was born in the vacuum-tube era as a simple, scalar indicator for a single frequency and a purely real feedline impedance. In today’s broadband, multi-stage, cascaded RF systems, it’s a poor descriptor of what actually happens. The quantities that matter—how much power comes back and how much gets through—are Return Loss (RL) and Insertion Loss (IL).

The Real Math Behind Reflections

Everything starts from the reflection coefficient |Γ|. It directly expresses how much power bounces back:

• |Γ| = 10^−RL/20
• SWR = (1 + |Γ|) / (1 − |Γ|)
• Reflected power = |Γ|²
• Mismatch loss = −10 log₁₀(1 − |Γ|²)

These are general and frequency-independent; they apply equally to cables, filters, amplifiers, and antennas.

Common Myths (and What to Say Instead)

• “We need SWR ≤ 1.5.”
  Say instead: “We need RL ≥ 14 dB from f₁ to f₂,” and specify ripple and IL. It’s clearer and maps to delivered power.
• “Cable length fixed my SWR, so the system is good.”
  You’ve added transformation and loss; the antenna didn’t change. Measure RL at the antenna plane or de-embed properly.
• “SWR tells me if my PA is safe.”
  PA stress depends on the magnitude and phase of the load seen at the PA port across power and frequency. Use load-pull contours—or at least S₁₁ phase and power limits—not SWR.
• “Balanced lines and mixed-mode? I’ll quote SWR.”
  In mixed-mode systems, SWR isn’t even defined. Use S_dd11 and S_cc11 for differential and common-mode return loss.

Minimal Math That Actually Helps

• Delivered power into a mismatched load:
  P_load = P_src × 10^{−IL_line/10} × (1 − |Γ_L|²)
• Worst-case passband ripple due to two separated reflections:
  Δ_p-p ≈ 20 log₁₀ [(1 + |Γ₁||Γ₂|)/(1 − |Γ₁||Γ₂|)]
• Quick mental math:
  20 dB RL ≈ 1% reflected; 26 dB RL ≈ 0.25%; 30 dB RL ≈ 0.1%.

How to Spec and Measure — Actionable Checklist

1. Specify RL and IL vs. frequency.
   Example: “2.2–2.6 GHz: RL ≥ 20 dB, IL ≤ 1.0 dB, ripple ≤ 0.2 dB p-p, group delay variation ≤ 0.5 ns.”
2. State the reference plane and Z₀.
   Always note where the measurement is de-embedded—e.g., “at antenna feed, 50 Ω.”
3. Use full S-parameters (magnitude + phase).
   Cascade networks, predict ripple, and include mismatch loss in link budgets.
4. Avoid SWR meters as diagnostics.
   If you must use one in the field, convert the reading to RL and interpret it in power terms—never chase cable-length “fixes.”
5. Report uncertainty and dynamic range.
   Modern VNAs reach 40–50 dB RL dynamic range easily; below that, you’re measuring noise, not reflection.

Bottom Line

SWR was a useful shortcut when systems were narrow-band, lossless, and purely resistive. Modern RF and high-speed systems are not. If you care about how much reflects back or gets through, specify Return Loss and Insertion Loss—with clear reference planes and frequency ranges. Your specifications will be unambiguous, your simulations meaningful, and your conversations between technicians, engineers, and managers closer to the physics.

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