Antenna S11 Fundamentals
S11 is the input reflection coefficient of an antenna — it tells you how much of the RF power sent to the antenna is reflected back instead of being radiated. A perfect antenna has S11 = −∞ dB (nothing reflected), but real antennas always reflect some power, especially at frequencies away from resonance.
Antenna S11 vs VSWR Specification Table
| System | S11 Requirement | VSWR | % Power Radiated |
|---|---|---|---|
| Smartphone (LTE/5G) | <−10 dB across band | <1.92:1 | >90% |
| Base station antenna | <−14 dB | <1.5:1 | >96% |
| WiFi AP antenna | <−10 dB, 2400–2484 MHz | <2:1 | >90% |
| GPS patch (1575 MHz) | <−15 dB | <1.43:1 | >97% |
| IoT sensor (BLE) | <−10 dB at 2.4 GHz | <2:1 | >90% |
Bandwidth vs Resonance
An antenna meets S11 < −10 dB only within a frequency band. Outside this band, S11 rises → increasing VSWR → more reflected power. Example smartphone penta-band PIFA: Band 1 (2110–2170 MHz): S11 < −10 dB ✓ Band 3 (1805–1880 MHz): S11 < −10 dB ✓ Band 7 (2620–2690 MHz): S11 < −10 dB ✓ Band 8 (925–960 MHz): S11 < −10 dB ✓ Band 28 (758–803 MHz): S11 < −10 dB ✓ → All 5 bands simultaneously covered by single PIFA This is achieved by antenna designers through careful shape optimization, notch elements, and tunable matching networks.
Ground Plane Effect on S11
Small antennas (chip antennas, meandered monopoles) on PCBs are extremely sensitive to ground plane size. The ground plane acts as a counterpoise — part of the antenna. Reducing ground plane shifts resonance and degrades S11 bandwidth.
RF View Antenna Analysis: Load antenna .s1p file to view S11/VSWR vs frequency, identify resonance and bandwidth, and design matching network with one-tap Auto Match. Free on Android.