なぜアンテナ選択がモジュールの実用距離を決定するのか
Engineers evaluating a Bluetoothモジュール often focus on RF output power and receiver sensitivity — but the antenna is where most of the performance variation actually occurs. A well-matched antenna on a well-designed PCB can deliver 20–30% more effective range than the same module with a poorly implemented antenna solution. This guide covers the three primary antenna architectures used in BLE modules and provides design parameters for each.
アンテナタイプ：比較概要
| Parameter | PCB Trace (Inverted-F / Meander) | Chip Antenna (Ceramic) | External Antenna (Dipole / Whip) |
|---|---|---|---|
| Peak Gain | 1.5–3.0 dBi | 2.0–4.0 dBi | 3.0–5.0 dBi |
| Efficiency | 60–80% | 70–90% | 80–95% |
| BOM Cost | $0 (copper trace) | $0.10–0.50 | $0.30–2.00 |
| PCB Area Required | 15–25 mm × 10–15 mm | 3–7 mm × 2–4 mm | Connector + keep-out zone |
| Bandwidth (2.4 GHz) | Narrow (~50 MHz) | Moderate (100–200 MHz) | Wide (200+ MHz) |
| Tuning Required | Yes (trace length + Pi network) | Yes (Pi matching network) | Minimal (50 Ω match) |
| Human Body Effect | Significant detuning | Moderate | Low (far from body) |
PCBトレースアンテナ：Inverted-Fの設計ルール
The Inverted-F Antenna (IFA) is the most widely used PCB trace solution for BLE modules. It provides reasonable gain with zero BOM cost but demands careful PCB layout discipline.
重要な設計パラメータ
- Ground plane size: Minimum 40 mm × 20 mm below and around the antenna trace. A smaller ground plane shifts the resonant frequency upward and reduces efficiency by 10–20%.
- Trace length: For 2.4 GHz, the radiator element is approximately λ/4 = 31 mm. The exact length must be tuned based on PCB thickness (FR4 εr ≈ 4.4), trace width, and proximity to ground. Typical tuned length: 24–28 mm on 1.6 mm FR4.
- Feed point and shorting pin: The distance between feed and shorting pin controls impedance. Start with 2–3 mm spacing and adjust with a Pi matching network (series inductor + shunt capacitors).
- Keep-out zone: No ground copper, components, or traces within 3–5 mm of the antenna trace on all layers. Metal in this zone acts as a parasitic element and detunes the antenna.
- Enclosure effect: Plastic enclosures with εr > 2 shift resonance down by 50–200 MHz. Always tune the antenna with the final enclosure in place.
πネットワークチューニング手順
- Start with no matching components (open circuit at feed point).
- Measure S11 using a VNA. Target: S11 < -10 dB across 2.402–2.480 GHz.
- If resonance is low (peak below 2.4 GHz): shorten the trace or reduce PCB εr effective.
- If impedance mismatch (correct frequency but poor return loss): adjust Pi network values. Typical starting values: series L = 2.2 nH, shunt C = 1.0 pF to GND.
- Iterate in 0.1 pF / 0.1 nH steps. Document final values on the manufacturing BOM.
チップアンテナ：スペースが限られている場合
Ceramic chip antennas (e.g., Johanson 2450AT18A100, Pulse W3001) offer a compact alternative when the PCB lacks sufficient area for a trace antenna. Key considerations:
- Follow the datasheet footprint exactly: The recommended ground plane size, feed position, and clearance zone are non-negotiable. Deviations degrade efficiency.
- Ground pad connection: Chip antennas with a ground pad must connect to the ground plane with multiple vias (minimum 4, spaced evenly). Insufficient ground via count increases inductance and detunes the antenna.
- Orientation: The radiation pattern is directional relative to the chip’s long axis. Mount the chip so the primary radiation direction faces away from the largest ground plane mass.
- Cost-performance trade-off: At $0.20–0.50 per unit in volume, chip antennas cost more than PCB traces but save 15–20 mm² of board space. For devices shipping above 100K units, the PCB trace approach is almost always more economical.
外部アンテナ：最大距離のアプリケーション
External antennas (SMA/IPEX connector + whip or dipole) provide the highest gain and are essential when maximum range is required. Use cases include industrial gateways, asset trackers with no space constraints, and outdoor BLE networks.
Design rules for external antenna integration:
- Maintain a 50 Ω transmission line from the module’s RF pin to the connector. For microstrip on 1.6 mm FR4, trace width ≈ 3.0 mm.
- Place the connector at the PCB edge with maximum clearance from ground copper.
- If an IPEX/U.FL connector is used, note the insertion loss (0.3–0.5 dB per connector pair) and cable loss (0.5–1.0 dB/m at 2.4 GHz).
- For regulatory compliance (FCC/CE), the external antenna gain must be considered against the module’s conducted output power. Total EIRP must not exceed regulatory limits.
アンテナ選択の判断フレームワーク
| Use Case | Recommended Antenna | Rationale |
|---|---|---|
| Wearable / Body-worn sensor | Chip antenna | Compact, moderate body detuning tolerance |
| Smart home device (thermostat, lock) | PCB trace (IFA) | Zero cost, sufficient range for indoor |
| Industrial beacon / asset tag | PCB trace (meander) | Low cost, proven at scale |
| BLE gateway / concentrator | External dipole | Maximum receive sensitivity, omnidirectional |
| Medical patch sensor | Chip antenna (tuned for body) | Small form factor, body-tuned design available |
The antenna is not an afterthought — it is the single largest determinant of your Bluetoothモジュール‘s real-world communication performance. Invest time in proper antenna selection, layout, and VNA-verified tuning during the prototype phase, and the production device will deliver consistent, predictable range across all deployment environments.