Antenna S11 and Bandwidth
The most fundamental antenna measurement is S11 (or Γ), which describes how well the antenna is matched to the feed line. The impedance bandwidth is defined as the frequency range over which S11 remains below a specified threshold, most commonly −10 dB (VSWR < 1.92).
Standard Bandwidth Definitions
| Criterion | S11 Threshold | VSWR | Return Loss | Reflected Power |
|---|---|---|---|---|
| Strict (cellular) | −15 dB | 1.43 | 15 dB | 3.2% |
| Standard (most) | −10 dB | 1.92 | 10 dB | 10% |
| Relaxed | −6 dB | 3.0 | 6 dB | 25% |
Antenna Types and Typical Bandwidths
| Antenna Type | Typical −10 dB BW | Resonance Mechanism |
|---|---|---|
| Half-wave dipole | 8–12% | λ/2 resonance |
| Patch (single layer) | 2–5% | λ/2 cavity mode |
| PIFA (phone) | 5–10% | Folded λ/4 |
| Log-periodic | 10:1 ratio | Frequency-independent |
| Spiral | >10:1 ratio | Self-complementary |
| UWB (FCC) | 3.1–10.6 GHz | Multiple modes |
Reading Antenna S11 in RF View
- Measure the antenna with a VNA, save as .s1p file
- Open the .s1p file in RF View
- The S11 magnitude plot shows the resonance dip(s)
- Use the marker to find the resonance frequency (deepest dip)
- Use two markers to find the −10 dB bandwidth: frequencies where S11 crosses −10 dB
- View the Smith chart to see the impedance trajectory through the band
Smith Chart Interpretation for Antennas
On the Smith chart, a well-matched antenna resonance shows:
- A loop or arc that passes through (or near) the chart center at resonance
- Capacitive locus below resonance (loop below center line)
- Inductive locus above resonance (loop above center line)
- The tighter the loop, the narrower the bandwidth
Multi-Band Antenna Analysis
Modern devices require multi-band operation. A dual-band antenna shows two separate resonance dips in the S11 plot. RF View's marker system lets you characterize each resonance independently: note the center frequency, minimum S11 value, and −10 dB bandwidth of each dip. The batch SNP feature lets you compare multiple prototype iterations simultaneously.
Detuning Effects
Hand effects, ground plane variations, and enclosure proximity all shift antenna resonance. S11 measurements in different hand grip positions, saved as separate .s1p files, can be batch-loaded in RF View to visualize the worst-case detuning range.