Patch Antenna S-Parameter Characteristics
A rectangular microstrip patch antenna resonates at a frequency determined by its length (L ≈ λg/2) and substrate properties. S11 shows a single deep notch at resonance with narrow bandwidth, typically 1–5% fractional bandwidth for a single patch on low-εr substrate.
Key Patch Antenna S11 Metrics
| Parameter | Typical Value | Note |
|---|---|---|
| Resonant frequency f_r | Deepest S11 dip | Target: within ±2 MHz of design freq |
| S11 at resonance | −15 to −30 dB | Depends on feed point matching |
| −10 dB bandwidth | 10–50 MHz (1–4%) | Low-εr substrate → wider BW |
| Impedance at resonance | Near 50 Ω on Smith chart | Feed point at 50 Ω inset or coaxial |
| In-band VSWR | <2:1 | Minimum spec for most applications |
GPS L1 Patch Antenna Specification (1575 MHz)
Load GPS patch .s1p → verify: f_r: 1575.42 ± 3 MHz (GPS L1 center) S11 at f_r: <−15 dB (good feed match) −10 dB BW: ≥10 MHz (must cover GPS C/A signal) Axial ratio (circular polarization): requires 3D pattern measurement → Cannot measure from S11 alone; need measurement from RF chamber Example measurement in RF View: BW Marker → f_r = 1574.8 MHz ✓, IL = −24.3 dB ✓, BW = 18.2 MHz ✓
Effects of Substrate on Patch S11
Higher εr substrate: - Smaller patch size (L ≈ λg/2 = λ0/(2√εr)) - Narrower bandwidth (higher Q) - More sensitive to substrate tolerance Lower εr substrate: - Larger patch - Wider bandwidth (lower Q) - Less sensitive to tolerance For GPS patch on RO4003C (εr=3.55): L ≈ c/(2f√εr) = 3e8/(2×1575MHz×√3.55) ≈ 50.5 mm Actual: shorter due to fringing field correction (~47–48 mm)
RF View Patch Antenna: Load .s1p file from VNA measurement. BW Marker finds resonant frequency and bandwidth instantly. Smith chart shows feed point impedance locus. Auto Match corrects minor detuning. Free on Android.