IoT Sub-GHz RF Frequencies
| Frequency | Standard | Application |
|---|---|---|
| 433 MHz | ISM (Europe) | Remote controls, keyfobs, basic LoRa |
| 868 MHz | SRD (Europe) | LoRaWAN EU868, SigFox, Z-Wave EU |
| 915 MHz | ISM (Americas) | LoRaWAN US915, Zigbee, Z-Wave US |
| 2.4 GHz | ISM (global) | Zigbee 2.4G, BLE, Thread, Matter |
Typical IoT Frontend: CC1101 + Chip Antenna at 868 MHz
TX chain: CC1101 PA output (Z_out ~10Ω) → LC match → antenna
RX chain: antenna → LC match → CC1101 LNA input
CC1101 output impedance at 868 MHz (from datasheet .s2p):
Z_out = 12 + j8 Ω (from Smith chart S22 view)
L-network design to transform 12+j8 → 50 Ω:
Q = √(50/12 − 1) = 1.93
Cancel +j8: series C = 1/(2π·868MHz·8) = 22.9 pF → pick 22 pF C0G
Transform 12Ω: shunt C = 1/(2π·868MHz·1.93×12) = 7.94 pF → pick 8.2 pF
Series L = 1.93×12/(2π·868MHz) = 4.24 nH → pick 4.3 nH
Chip Antenna VSWR Optimization for 868 MHz PCB
- Load chip antenna .s1p file (from manufacturer, e.g., Taoglas FXP73)
- Note S11 at 868 MHz: typically −4 to −8 dB (needs matching)
- Smith chart: antenna is inductive at 868 MHz on 70×70 mm PCB
- RF View Auto Match → synthesizes 1-element (series C or shunt L) match
- Verify matched S11 < −10 dB across 863–870 MHz (SRD EU868 band)
RF View IoT Design: Download chip antenna .s1p from manufacturer's website, load into RF View, and design the optimal matching network for your PCB size and target frequency band. Free on Android.