Design solutions that pursue the sound quality and experience you need
Bluetooth headsets use three antenna types. The PCB inverted-F antenna (IFA) is etched directly on the top copper layer, adding zero BOM cost and suiting rapid iteration. Ceramic chip antennas at approximately 2×1.2 mm fit TWS in-ear models with 0–1 dBi gain. Metal spring antennas leverage the headband frame as a radiator, achieving above 2 dBi gain. Liwei Electronics uses a PCB IFA covering 2.4–2.48 GHz.
The antenna requires 50 Ω impedance matching. On FR-4 substrate, a 1.6 mm board yields a 50 Ω microstrip width of roughly 3 mm; 0.8 mm yields about 1.5 mm. Simulate the S11 curve in 3D electromagnetic software first, then verify return loss with a VNA, targeting S11 below −10 dB. Liwei Electronics converges trace parameters through this simulation-plus-measurement loop.
The IFA radiator arm requires a keep-out zone prohibiting copper pour and signal traces, extending 3–5 mm beyond the antenna outline. Amplifier traces crossing this zone absorb RF energy. Liwei places the antenna at the PCB short edge, physically isolating it from the audio amplifier and mic-bias circuitry. Battery-to-antenna distance must exceed 2 mm.
RF and audio signals coexist on the PCB. Amplifier PWM harmonics (1–2 MHz) can fall near 2.4 GHz; mic-bias ripple couples through the ground plane to the antenna feed line. Mitigation: dedicated amplifier power routing, decoupling capacitors near IC pins, ferrite bead isolation on mic bias, and feed lines routed away from amplifier outputs. The bottom-layer ground plane should remain uninterrupted.
Q: What if the TWS earbud has no room for a full antenna keep-out zone?
A: Choose a ceramic chip antenna mounted on a flexible FPC bent along the inner shell wall. The keep-out zone can shrink to 2–3 mm, but you must verify S11 compliance with a VNA and compensate by adjusting trace length if needed.
Q: How long should the PCB antenna trace be?
A: The quarter wavelength at 2.4 GHz is approximately 31 mm, but the actual length depends on substrate dielectric constant, ground plane size, and nearby metal. Start with a simulation, then fine-tune with a VNA; meander trace total length typically falls between 28 and 34 mm.
Q: How do you ensure Bluetooth signal in a metal-shell headset?
A: A metal shell blocks RF. The antenna radiator must extend beyond the shell edge, or a non-metal window (plastic insert molding) must be opened so the antenna radiates into unshielded space.
Liwei Electronics has specialized in audio headset solutions for 12 years, delivering complete turnkey PCBA solutions. Our Bluetooth dual-mode designs employ VNA-verified antenna layouts. Contact us for a customized solution.
Keywords: Bluetooth antenna, PCB layout, impedance matching, TWS RF
