Overview: What S11 Tells You About an Antenna
An antenna's S11 reveals its resonant frequency (where it's closest to 50 Ω), operating bandwidth (range where S11 < −10 dB), and complex impedance at any frequency. All this comes from a single .s1p file measured with a VNA.
Step 1: Load .s1p and Read S11 dB Plot
RF View → S-Parameter Plot → load antenna.s1p What to look for in S11 dB plot: Resonance: deepest dip in S11 (most negative value) Operating BW: frequency range where S11 < −10 dB Example: Deepest S11 dip: −18.5 dB at 2432 MHz → resonance at 2432 MHz S11 < −10 dB from 2380–2490 MHz → BW = 110 MHz (4.5%)
Step 2: VSWR View for Specification Check
Switch to VSWR display:
At resonance: VSWR = 1.29:1 (corresponding to −18.5 dB RL)
At band edges:
2380 MHz: VSWR = 1.90:1 (just below 2:1 spec)
2490 MHz: VSWR = 1.87:1 (just below 2:1 spec)
→ Antenna meets VSWR < 2:1 across full 2.4 GHz WiFi band ✓
Step 3: Smith Chart for Impedance Analysis
Switch to Smith chart: At 2432 MHz: dot is close to chart center (good match, ~50Ω) At 2380 MHz: dot drifts to slightly inductive region (upper half) At 2490 MHz: dot drifts to slightly capacitive region (lower half) → Antenna impedance locus sweeps through 50Ω at resonance
Step 4: Design Matching Network If Needed
If S11 > −10 dB at the target frequency:
- Note antenna impedance from Smith chart marker (e.g., Z = 30 − j25 Ω)
- Open Matching tab → set target frequency
- Tap Auto Match → RF View synthesizes network
- Verify matched S11 < −10 dB across target band
- Check bandwidth: matched antenna may have narrower BW than unmatched
Step 5: Temperature and Location Variation
Load antenna .s1p measured in different environments (free air, inside enclosure, near human hand) to see how environment affects resonance. Multi-file overlay shows all variations simultaneously.
RF View Antenna Analysis: S11 dB, VSWR, Smith chart, BW Marker, Auto Match — complete antenna analysis workflow in one free Android app. Load .s1p from NanoVNA or lab VNA in seconds.