Radio Frequency Testing

AR glasses (augmented reality eyewear) — integrating waveguide displays, Wi-Fi 6/6E, Bluetooth LE, and increasingly UWB spatial positioning — are classified as composite intentional radiators under FCC rules. The FCC ID certification for AR glasses differs from smartphone or tablet certification in two critical respects: body-worn SAR (Specific Absorption Rate) testing at near-zero separation distance (the glasses are worn on the face), and the unique challenges of embedding multiple antennas within a temple/ frame geometry that offers far less volume than a typical handset. This article examines the FCC certification framework for AR glasses and the engineering implications of wearable form-factor RF compliance.

FCC Regulatory Classification — Composite Device with Wearable-Specific Requirements

AR glasses fall under multiple FCC rule parts in a single FCC ID application:

FCC Part 15C §15.247 — Wi-Fi 2.4 GHz + Bluetooth LE　Measured per ANSI C63.10: conducted output power ≤ 1 W (30 dBm), 6 dB bandwidth ≥ 500 kHz, PSD ≤ 8 dBm/3 kHz. The challenge is the antenna placement — a temple-mounted antenna has a ground plane area limited to approximately 20 mm × 5 mm, which constrains efficiency to roughly 30-40% compared to 60-70% for a handset antenna with a full PCB ground plane. The low antenna efficiency directly reduces ERP/EIRP at the FCC compliance measurement — the conducted power must compensate, but is bounded by the 30 dBm Part 15.247 limit.

FCC Part 15E §15.407 — Wi-Fi 6E (6 GHz)　For AR glasses operating as low-power indoor (LPI) clients in the 5.925-7.125 GHz band, FCC rules limit conducted power to 24 dBm and PSD to 11 dBm/MHz. No AFC (Automated Frequency Coordination) is required for client devices. The 6 GHz antenna faces extreme miniaturization constraints — the λ/4 monopole at 6.5 GHz is only 5.8 mm, which is physically achievable in the glasses temple, but the ground plane area requirement remains the efficiency bottleneck.

FCC Part 15B — Unintentional Emissions　The AR glasses' waveguide projector (micro-LED or LCoS microdisplay), camera sensors (SLAM tracking), and SoC processor all fall under unintentional radiator requirements per Part 15B Class B. Radiated emissions measured at 3 m from 30 MHz to 40 GHz.

Head-Worn SAR Evaluation — The Regulatory Distinction

AR glasses worn on the face require SAR evaluation per FCC KDB 447498 and KDB 648474 (body-worn and extremity SAR). The key parameters:

• Separation distance: The temple-mounted antenna is at essentially 0 mm from the head tissue — the outer housing wall (~0.5-1.0 mm plastic) is the only physical separation. This is a "touch" SAR configuration, which triggers the strictest SAR limits: ≤ 1.6 W/kg averaged over 1 g of tissue (FCC) for head SAR.
• Simultaneous transmission: If Wi-Fi 2.4 GHz, Bluetooth, and UWB radios transmit simultaneously (a common AR glasses use case: video streaming over Wi-Fi + BLE for controller + UWB for spatial anchoring), SAR must be evaluated in simultaneous TX mode — the combined SAR values from all active transmitters are summed and compared to the 1.6 W/kg limit.
• SAR test reduction: If measured SAR at the 0 mm separation is ≤ 0.4 W/kg across all frequency bands and operating modes, SAR testing at larger separation distances is not required per KDB 447498 §4.3.1. This is a critical early-stage measurement — achieving ≤ 0.4 W/kg at 0 mm with a temple-embedded antenna is challenging and requires careful antenna placement and power management.

Antenna Design Constraints in AR Glasses Form Factor

The AR glasses temple houses multiple antennas in a volume roughly 60 mm × 8 mm × 5 mm — this creates three engineering constraints for FCC compliance:

• Ground plane limitation: The temple PCB ground area is approximately 3-4 cm² — far smaller than the λ/4 ground plane area (≈ 36 cm² at 2.4 GHz). The antenna design must use ground-independent structures (PIFA variants, meandered monopoles, or chip antennas) that do not rely on a resonant ground plane.
• User body loading: The antenna's impedance and radiation pattern shift significantly when the glasses transition from "on-bench" to "on-head" — head tissue (εr ≈ 40, σ ≈ 1.4 S/m at 2.4 GHz) loads the antenna and detunes it by 50-100 MHz. Antenna matching must be optimized for the on-head condition, not free-space — this is a common error in early-stage AR glasses development.
• Co-site interference: Wi-Fi 2.4 GHz and Bluetooth sharing the same ISM band require ≥ 20 dB isolation between their antenna ports to prevent receiver desensitization. In a 60 mm temple, achieving 20 dB isolation at λ/2 separation (≈ 6 cm at 2.4 GHz) without spatial separation is difficult — polarization diversity (orthogonal antenna orientations) and a shared front-end with TDD scheduling are the practical engineering approaches.

Frequently Asked Questions

This content is for informational reference only. Specific certification requirements must be evaluated by professional engineers. For inquiries, please contact us.

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