How to Measure Return Loss (S11) with a VNA

Equipment Required

• Vector Network Analyzer (VNA): Keysight, Rohde & Schwarz, Copper Mountain, or low-cost NanoVNA
• Calibration standards: SOLT (Short, Open, Load, Thru) matched to your connector type (SMA, 2.4mm, etc.)
• RF cables: phase-stable, matched to frequency range
• DUT (Device Under Test) with appropriate connectors

Step 1: VNA Setup

  Frequency range: cover full operating band ±20% margin
    Example for 2.4 GHz WiFi: 2.0 to 3.0 GHz (or 1 to 6 GHz for broader context)

  Number of points: 401–1601 (balance detail vs sweep speed)
  IF bandwidth: 1–10 kHz (narrower = lower noise, slower sweep)
  Power: −10 to 0 dBm (low enough not to compress DUT, high enough for good SNR)
  Reference impedance: 50 Ω (standard)

Step 2: SOLT Calibration

Calibration removes VNA port errors (directivity, source match, reflection tracking). Perform at the cable ends that will connect to the DUT:

1. Open: Connect open standard to port 1 cable end. Measure. RF View exports this as S11 ≈ 0 dB at 0°.
2. Short: Connect short standard. S11 ≈ 0 dB at 180° (±3°).
3. Load (50 Ω): Connect precision 50 Ω termination. S11 < −40 dB.
4. VNA computes 3-term (or 12-term for 2-port) error correction coefficients.

Calibration quality check: after cal, reconnect Load → S11 should read < −35 dB across full band. If not, check cable, connector cleanliness, or replace calibration standards.

Step 3: Connect DUT and Measure

  1. Connect DUT port (ANT, IN, or port 1) to VNA port 1 calibrated cable
  2. Terminate unused DUT ports in 50 Ω loads (critical for S11 accuracy)
  3. Trigger sweep → VNA measures S11 across frequency range
  4. Observe S11 trace in dB, VSWR, or Smith chart view

Step 4: Extract Key Parameters

  Return loss at design freq: S11(f₀) in dB
    → Set marker at f₀, read value: e.g., S11 = −18.5 dB

  VSWR: VSWR = (1 + |Γ|)/(1 − |Γ|), |Γ| = 10^(S11_dB/20)
    → S11 = −18.5 dB → |Γ| = 0.119 → VSWR = 1.27:1

  Frequency range where S11 < −10 dB (VSWR < 2:1):
    → Use BW marker or manually find crossing points

  Impedance at specific frequency:
    Z = 50 · (1+Γ)/(1−Γ)  (read Γ = |Γ|∠θ from marker)

Step 5: Save and Export

  Save as Touchstone: File → Save → .s1p (1-port) or .s2p (2-port)
  Ensure options line: # GHz S DB R 50  (or MA or RI)

  Load into RF View to view on phone for:
  - Smith chart analysis
  - Impedance matching design
  - Comparison with other measurements

Common Measurement Errors

Related Topics

← Back to Tutorial  ·  RF View Home