Chapter 6
Phase 1 of this standard defines the envelopes that describe a power link, and Phase 2 exposes them over HTTPS. Phase 3 covers what happens when energy actually crosses a gap, and when that crossing must be agreed on by parties that do not fully trust each other. The medium may be a magnetic field, an electric field, a microwave beam or a laser beam. Each medium has its own failure modes, and each needs its own handshake before power flows and its own interlock to stop it.
The Phase 3 protocol text is organised by transfer regime rather than by message type. The regimes differ in operating band, in how the receiver is found, and in what the regulator wants to see. The table condenses the operating envelopes defined in the protocol annex.
| Regime | Operating band | Control concerns | Status under this standard |
|---|---|---|---|
| IPT (inductive) | LF (kHz) for EV and industrial; HF (MHz) for spatial-freedom consumer use | Tx current envelope, Rx rectifier and DC-DC envelope, alignment tracking, multi-receiver coordination, per-cycle FOD | Operating protocol |
| CPT (capacitive) | Operator-selected | Plate-pair field coupling, safety distance from accessible plates, impedance matching | Operating protocol |
| Resonant | Typically 6.78 MHz (AirFuel-class ISM) | High-Q tank, BLE/Wi-Fi/proprietary side channel, per-receiver decoupling | Operating protocol |
| Microwave / laser beaming | GHz / optical-IR | Beam acquisition, interlock chain, regulatory authorisation | Operating protocol |
| Dynamic EV, space solar | Various | Segment handover, orbit-and-beam coordination | Research track only |
Three ideas recur across all rows:
The protocol makes sense only against the physics it controls. The two long-range regimes, microwave and laser, are where protocol and physics are most tightly coupled, because a small pointing error changes the received power.
Microwave transfer treats the link as an antenna problem. Antenna gain follows from effective aperture, and received power follows from the two gains and the free-space spreading loss:
G = (4π × A_eff) / λ²
S = (P_tx × G_tx) / (4π d²)
P_rx = S × A_eff = P_tx × G_tx × G_rx × (λ / 4πd)²Two consequences shape the protocol. First, the λ/(4πd) term means the distance enters squared, so the link budget is only workable with large apertures and tight pointing. Second, the energy at the receiver is concentrated in a narrow beam, so a misdirected beam is a safety event and not just an efficiency loss.
The receiver is a rectenna, a rectifying antenna that converts RF to DC. It has four stages:
The specification gives antenna efficiency of 80–95% and rectifier efficiency of 70–90%. Together they give 60–85% overall. State-of-the-art figures are 82% at 2.45 GHz (40 mW/cm²), 85% at 5.8 GHz (60 mW/cm²) and 70% at 24 GHz (100 mW/cm²). The protocol annex quotes the same 60–85% range as the per-rectenna design-frequency envelope.
| Band | Frequency | Wavelength | Typical use |
|---|---|---|---|
| S-band | 2.45 GHz (ISM) | 12.2 cm | Drones, IoT |
| C-band | 5.8 GHz (ISM) | 5.2 cm | Mid-range power |
| K-band | 24 GHz (ISM) | 1.25 cm | High-density links |
| W-band | 94 GHz | 3.2 mm | Emerging research |
Steering is done with a phased array, where element n receives a phase offset set by element spacing d and steering angle θ:
Φₙ = 2π × d × sin(θ) / λA retrodirective array removes the need to compute θ. It uses phase conjugation to point the beam back toward the source of a pilot signal. The pilot signal is the same beacon the protocol uses for acquisition, so steering and presence verification share one mechanism.
The beaming sequence in the annex has a fixed order:
1. Receiver transmits a transponder challenge (IFF-style) to prove presence
2. Transmitter servos beam pointing onto the receiver's beacon
3. Rectenna conversion runs inside its design-frequency envelope
4. Independent radar / LiDAR / camera scanners watch the beam volume
5. Any intrusion detected -> interlock chain gates the beam off immediately
6. Authorisation (ITU-R + national regulator + site clearance) is a precondition, not a stepA laser beam spreads far less than a microwave beam. A Gaussian beam diverges with a half-angle θ = λ / (π × w₀), where w₀ is the beam waist radius. Beyond the waist, the spot size grows with distance, and the intensity profile falls off from the axis as exp(−2r²/w²). The benefit is that the receiver aperture can be small. The cost is that the atmosphere matters: absorption, scattering, cloud blockage and turbulence-driven beam wander all appear in the specification's list of disadvantages.
The end-to-end efficiency is a product of three stages, so every stage must be good:
η_total = η_laser × η_transmission × η_PVWith wall-plug laser efficiency of 30–60%, atmospheric transmission of 60–90% and PV conversion of 25–50%, the specification puts overall efficiency at 5–25%. Receiver choice is a matching problem between laser wavelength and cell material:
| PV cell | Peak wavelength | Efficiency | Cost |
|---|---|---|---|
| Silicon | 800–900 nm | 25–30% | Low |
| GaAs | 850 nm | 35–40% | Medium |
| InGaAs | 1064–1550 nm | 30–35% | High |
| Multi-junction | 400–1800 nm | 40–50% | Very high |
Wavelength selection trades atmospheric transmission, receiver match and eye hazard against one another. A 450 nm blue diode suffers Rayleigh scattering and is an eye hazard. The 808 nm and 980 nm diodes transmit well but are also eye hazards. Nd:YAG at 1064 nm transmits excellently and is rated "eye safe-ish". A 1550 nm fibre laser transmits excellently and is eye safe, which makes it attractive wherever people may stray into the beam. The protocol side of laser beaming mirrors microwave: beacon acquisition, a photovoltaic receiver envelope in place of the rectenna, and a safety envelope of either Class 1M or strictly controlled Class 4 under IEC 60825-1.
The specification does not publish market-size forecasts, and this chapter does not invent any. It does give enough cost and economics data to show where each regime is commercially plausible today.
The gap between these rows explains the protocol's structure. Where capital cost is low and volumes are high, interoperability matters most. Where capital cost is extreme, the priority is documented safety and coordination.
Applications are grouped by how much the protocol has to coordinate. The EV case is the best defined, so it is covered first.
Stationary EV charging. The vehicle parks over a ground pad. Alignment is assisted by parking radar and a visual cue, and the vehicle and charger exchange sideband messages over LIN/CAN per SAE J2847/6. SAE J2954 defines four power classes, and charge times in the table are for a 60 kWh pack:
| Class | Power | Use case | Charge time (60 kWh) |
|---|---|---|---|
| WPT1 | 3.7 kW | Overnight | ~16 hours |
| WPT2 | 7.7 kW | Home/workplace | ~8 hours |
| WPT3 | 11 kW | Public charging | ~5.5 hours |
| WPT4 | 22 kW | Fast charging | ~2.7 hours |
The ground assembly coil is 400–600 mm in diameter with ferrite backing for flux shaping and shielding to reduce EMI. The vehicle assembly is 300–500 mm, under 10 kg, and sits at a ground clearance of 100–250 mm. Alignment tolerance is ±100 mm laterally and ±75 mm longitudinally. These tolerances are why the protocol includes alignment-assist and why power stays low or off until coupling is good.
Dynamic charging (research track). Road segments of 1–5 metres, each with its own ground pad, are activated by the vehicle-side controller as the vehicle approaches and deactivated as it passes. Billing is per pad, with inter-vendor settlement. The averaging arithmetic in the specification works out as follows:
Duty Cycle = L_segment / L_spacing
Power_avg = Power_rated × Duty_cycle × η
100 km/h, 3 m segments, 20 kW rated, 85% efficiency:
time per segment = 3 m / 27.8 m/s ≈ 0.108 s
energy per segment = 20 kW × 0.108 s ≈ 2.16 kJ
average power = 20 kW × 0.85 = 17 kW (duty cycle of 1)The 17 kW figure holds when a pad is always active under the pickup. If segments are spaced apart, the duty cycle falls below 1 and the average falls with it. Each activation lasts only about a tenth of a second at highway speed, so activation timing is a protocol problem. Dynamic charging is explicitly not certified for commercial deployment under this standard.
Drones, sensors, relief and military. Microwave beaming supports in-flight power for drones and UAVs, cable-free power for remote IoT sensors, disaster relief, and forward operating bases.
Laser-specific niches. Laser beaming suits space power relay (satellite to satellite, orbit to ground), lunar base power to polar night regions, helicopter-to-ground relief, and hazardous areas where there must be no sparks or EMI. Underwater use is another niche, since light penetrates water better than RF.
Space solar power (research track). The coordination envelope covers satellite handover within a constellation, rectenna-farm active-element selection and on-orbit phased-array calibration. Atmospheric loss is modelled with ITU-R P.676 for frequency-dependent absorption and ITU-R P.838 for rain fade. The ground segment defines a rectenna-farm exclusion zone with a documented field-strength contour.
Compliance in wireless power is mostly about people and neighbouring radios. The exposure limits, emissions limits and detection requirements below are the checks that gate a link. Interoperability standards decide whether two vendors' products can share one.
Human exposure. SAR is computed as σ|E|²/ρ in W/kg. The regulatory limits cited are 1.6 W/kg (1 g average) under the FCC and 2.0 W/kg (10 g average) under ICNIRP. Power density limits for the general public per ICNIRP are:
| Frequency | Limit |
|---|---|
| 1–10 MHz | 0.92 W/m² |
| 10–400 MHz | 2 W/m² |
| 400–2000 MHz | f/200 W/m² |
| 2–300 GHz | 10 W/m² |
Occupational limits are five times higher. For space solar power the specification targets a ground power density below 10 W/m², matching the top row of the table.
EMC. Conducted and radiated emissions fall under FCC Part 15 Class B and EN 55011 Class B (CISPR 11 Group 1). EV systems add ground-fault protection (GFCI). Resonant systems carry the regulatory references listed in 6.1, and their strict frequency stability is both a compliance requirement and a way to avoid cross-link interference at dense sites.
Foreign object detection (FOD). A metal object in the field heats up, so the system must find it quickly. The specification compares methods by principle and response time:
| Method | Principle | Detects | Response |
|---|---|---|---|
| Q-factor | Coil resistance change | Metallic | < 100 ms |
| Thermal (IR) | Temperature rise | All conductive | 1–5 s |
| Capacitive | Capacitance change | All | < 50 ms |
| Radar/Lidar | Reflection | All | < 10 ms |
| Visual | Image processing | All | 100–500 ms |
The Qi power-loss method compares transmitted and received power. If ΔP = P_tx − P_rx exceeds a threshold (for example 1 W), the transmitter reduces or stops power. The protocol runs this check per cycle for IPT.
Living object protection (LOP). Detection may use motion sensing (camera, PIR), thermal signature, biometric cues such as heartbeat and breathing seen by IR imaging, or reflection of a pilot tone from tissue. The required action is to reduce power below 1 W when a living object is detected.
Interoperability. Qi (WPC) is inductive at 80–300 kHz. AirFuel Resonant is resonant at 6.78 MHz. SAE J2954 (2020) covers EV IPT at 85 kHz, ISO 19363 (2020) covers EV wireless charging, and IEC 61980 (2015) covers EV inductive charging.
A practical build order starts with safety and works outward. First, implement the interlock path and verify it can cut energy without the control path's cooperation. Second, add presence verification (beacon, challenge or coil coupling, depending on regime). Third, add the control loops and side channel. Last, integrate billing and settlement where multiple parties share infrastructure.
POST /links.POST /links. A tampered or missing safety history therefore blocks new links until it is repaired.Two research tracks carry the longest horizon. Dynamic in-road charging is an instrumented-road concept whose economics hold only on high-traffic routes. Space solar power is a concept built on GEO orbit, where sunlight is available about 99% of the time and the hemispherical coverage suits continuous power. A 1 km² array at 30% efficiency yields about 408 MW in the specification's example, and a 2.45 GHz link with a 1 km transmit aperture and a 5 km receive rectenna is estimated at 40–60% link efficiency. Realisation depends on beam safety, international spectrum coordination and public acceptance, as well as launch cost.
Nearer term, the specification points to lunar base power (beaming across polar night regions), satellite-to-satellite relay and Mars surface power from orbital solar. W-band (94 GHz) remains in the emerging-research row of the band table.
Phase 3 treats wireless power as a family of protocols. Each regime has its own physics, handshake and safety envelope. Microwave beaming rests on the Friis relation, rectenna efficiency and phased or retrodirective steering. Laser beaming rests on beam divergence, a three-factor efficiency chain and wavelength-matched receivers. Near-field and EV systems depend on alignment, FOD and LOP. In every regime, presence verification comes first and an independent interlock sits beside the control path.
The next chapter moves from protocol exchanges to Phase 4, where these links are integrated with the wider ecosystem. There, the questions are how a single deployment reuses identity, signing and audit transport across neighbouring standards, and how operators put the safety and settlement records from this chapter to work.