Wireless Standards and Protocols IoT and LPWAN Informational

How do I design a low cost antenna for a sub-GHz ISM band IoT device?

Q: How do I simulate the antenna before prototyping?

Use a 3D EM simulator: ANSYS HFSS, Dassault CST, or Altair FEKO. Model the complete PCB (copper, substrate, ground plane, components). Include the enclosure (plastic housing, screws, battery). The simulation predicts: resonant frequency, bandwidth, gain, efficiency, and radiation pattern. For quick estimates: use chip vendor antenna design tools (Silicon Labs Antenna Design Guide, TI SmartRF Studio). Always validate simulation results with a VNA measurement on the first prototype.

How do I design a low cost antenna for a sub-GHz ISM band IoT device? Antenna design for sub-GHz IoT is challenging because the wavelength is large relative to the device size (lambda = 345 mm at 868 MHz), making it difficult to achieve an efficient antenna in a compact form factor: (1) Antenna options by cost and performance: PCB trace antenna (zero cost): meander-line, inverted-F, or monopole printed on the PCB. Size: 20-40 mm (electrically short; lambda/4 = 86 mm). Efficiency: 20-60% (depends on ground plane size). Gain: -5 to +1 dBi. Best for: cost-sensitive, medium-range applications. Design: use free reference designs from chip vendors (TI DN038, Silicon Labs AN1138, Nordic DevZone). Chip antenna ($0.20-0.80): ceramic-loaded miniature antenna (Johanson 0868AT, Abracon ACAG-868). Size: 5-15 mm. Efficiency: 30-70%. Gain: -3 to +2 dBi. Best for: space-constrained designs where PCB area is limited. Helical antenna ($0.10-0.50): wire wound in a helix, mounted on the PCB or external. Length: 30-60 mm. Efficiency: 50-80%. Gain: 0 to +3 dBi. Best for: devices where a protruding antenna is acceptable. Whip/monopole ($0.05-0.30): quarter-wave wire. Length: 82-86 mm at 868 MHz. Efficiency: > 90%. Gain: +2 to +5 dBi. Best for: maximum range (gateways, outdoor sensors). (2) Ground plane: the most critical factor for sub-GHz antenna performance. Minimum: 40 × 40 mm copper ground plane on the PCB. Smaller ground plane: resonant frequency shifts up, impedance becomes more reactive, efficiency drops by 10-30%. Antenna must be placed at the edge of the ground plane (not in the center) for best performance. Keep a clearance zone (no copper, no components) around the antenna: 5-10 mm minimum. (3) Matching network: most compact antennas are not inherently 50 ohms. A 2-3 component LC matching network tunes the antenna to 50 ohms at the operating frequency. Use a VNA to measure S11 and design the matching network. The matching must be re-tuned after final assembly (enclosure, battery, cables all affect the antenna impedance). (4) Design flow: start with a reference design from the chip vendor. Modify the PCB layout for your device form factor. Prototype and measure S11 on a VNA. Tune the matching network for best return loss (< -10 dB) at the operating frequency. Measure antenna efficiency in a test chamber or via range testing.

Category: Wireless Standards and Protocols

Updated: April 2026

Product Tie-In: IoT Modules, Filters, Antennas

Low Cost Sub-GHz IoT Antenna

The antenna is often the make-or-break factor for IoT device range but is frequently underestimated in design time and budget.

Common Design Mistakes

(1) Antenna too close to metal: battery, screws, and metal enclosure components near the antenna detune and absorb the signal. Keep metal at least 10 mm from the antenna element. Use a ferrite sheet between the antenna and metal (adds 1-3 dB but prevents severe detuning). (2) Ground plane too small: a 20 × 20 mm ground plane at 868 MHz loses 3-5 dB compared to a 40 × 40 mm ground plane. The entire range budget suffers. If the PCB is too small: use an external antenna (helical or whip) with a coaxial feed. (3) Not re-tuning after enclosure: the plastic enclosure changes the antenna impedance and adds 1-3 dB of detuning. Always perform final antenna tuning with the final enclosure in place. (4) Regulatory compliance: the antenna gain is included in the EIRP calculation. If using a high-gain antenna (+5 dBi), the conducted TX power must be reduced accordingly to stay within the regulatory limit.

Sub-GHz Antenna Options

λ at 868 MHz = 345 mm, λ/4 = 86 mm
PCB antenna: 20-40 mm, 20-60% efficiency, $0
Chip antenna: 5-15 mm, 30-70% efficiency, $0.20-0.80
Monopole: 82-86 mm, >90% efficiency, $0.05-0.30
Ground plane: minimum 40×40 mm at 868 MHz

Common Questions

Frequently Asked Questions

Which antenna type gives the best range per dollar?

The PCB trace antenna ($0) provides the best range per dollar for devices with > 40 × 40 mm PCBs. With proper design and tuning: a PCB inverted-F antenna achieves -1 to +1 dBi (only 2-3 dB less than a full-size monopole). For smaller PCBs: a chip antenna is more predictable and easier to tune (worth the $0.30-0.50 cost). For gateways and maximum range: a quarter-wave whip at $0.10 from a wire provides the best absolute performance.

Can I use the same antenna for 868 and 915 MHz?

Yes. 868-915 MHz is a 47 MHz range (5.3% fractional bandwidth). A well-designed antenna can cover both with < 1 dB efficiency variation. Tune the matching for the center (approximately 890 MHz) and verify performance at both band edges. This enables a single global hardware design with software-selectable frequency.

How do I simulate the antenna before prototyping?

Use a 3D EM simulator: ANSYS HFSS, Dassault CST, or Altair FEKO. Model the complete PCB (copper, substrate, ground plane, components). Include the enclosure (plastic housing, screws, battery). The simulation predicts: resonant frequency, bandwidth, gain, efficiency, and radiation pattern. For quick estimates: use chip vendor antenna design tools (Silicon Labs Antenna Design Guide, TI SmartRF Studio). Always validate simulation results with a VNA measurement on the first prototype.

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