Chapter 2
Wireless power transfer (WPT) has to solve four problems at once: efficiency falls as distance grows, alignment is never perfect, high-power fields must stay safe around people and metal, and equipment from different vendors must still work together. WIA-COMM-009 treats them as one engineering envelope that runs from millimetres to kilometres and from milliwatts to megawatts. The sections below follow the standard's own formulas, tables and terminology.
The standard defines WPT as electromagnetic field coupling between a transmitter and a receiver. The coupling mechanism changes with distance, measured against the wavelength λ and, for radiating systems, the aperture diameter D. WIA-COMM-009 separates three regimes:
| Regime | Distance | Dominant mechanism | Efficiency scaling |
|---|---|---|---|
| Near-field | d < λ/2π | Reactive coupling | ∝ (1 + d²/r²)⁻³ |
| Fresnel | λ/2π < d < 2D²/λ | Transition | ∝ 1/d |
| Far-field | d > 2D²/λ | Radiation | ∝ 1/d² |
Here r is the coil or plate radius. The cubic fall-off in the near field is why a charging pad that works at contact distance becomes useless a few coil radii away. The far-field regime degrades more gently, but a beam spreads and the receiver captures only a small fraction of it.
Two calculations show what the boundaries mean in practice. At 2.45 GHz the wavelength is about 12 cm, so λ/2π is only about 2 cm. Any microwave link longer than that is outside the reactive near field. For a 1 m aperture, 2D²/λ is about 16 m, so a 100 m beam is solidly far-field. At the 87 kHz used in the SDK's Qi example, λ is roughly 3.4 km and λ/2π is roughly 550 m. Inductive charging is therefore not limited by leaving the near-field region. It is limited by how quickly the coupling coefficient k collapses as the coils separate.
The standard's summary table puts the resulting engineering choices side by side:
| Technology | Range | Efficiency | Power | Frequency |
|---|---|---|---|---|
| Inductive (Qi) | 0–5 cm | 70–90% | 5–15 W | 80–300 kHz |
| Resonant | 10 cm–2 m | 40–80% | 1–11 kW | 6.78 MHz |
| Microwave | 1–1000 m | 10–50% | 1 W–100 kW | 2.45–24 GHz |
| Laser | 100 m–1000 km | 10–40% | 1 W–1 MW | IR/Optical |
One discrepancy deserves attention. This README-level table gives the Qi range as 0–5 cm, while the Qi profile in the specification (section 3.3.1) gives 0–5 mm. Implementers should read the profile-level figure as the tighter, device-specific statement and the table as a coarse technology comparison.
For near-field magnetic coupling, the specification starts from mutual inductance. A transmitter current I₁ produces flux Φ₂₁ = M × I₁ at the receiver and an induced voltage V₂ = −M × dI₁/dt. For two coaxial circular coils the specification gives:
M = μ₀ × π × r₁² × r₂² × N₁ × N₂ / (2 × (r₁² + d²)^(3/2))
k = M / √(L₁ × L₂)The coupling coefficient k runs from 0 to 1. The specification gives three rules of thumb: k = 0.6–0.9 with excellent alignment and d < r, 0.3–0.6 with moderate alignment and d ≈ r, and below 0.3 with poor alignment and d > r. Alignment is therefore a first-order efficiency variable, not a cosmetic one.
The second lever is the quality factor, Q = ωL/R = 2πfL/R, the ratio of reactive to resistive power in a resonant circuit. For loosely coupled coils the efficiency is:
η = k² × Q₁ × Q₂ / (1 + k² × Q₁ × Q₂)This expression shows that weak coupling can be compensated by high Q. The product k²Q₁Q₂ is the figure of merit. With k = 0.02 and Q₁ = Q₂ = 50 the product is 1 and the ideal link efficiency is 50%. With k = 0.1 and the same Q values the product is 25 and efficiency rises to about 96%. When the product is much larger than 1, η approaches the theoretical limit of 1. This is why resonant systems can work at 10 cm to 2 m with a modest k. It is also why anything that lowers Q, such as a metal object absorbing field energy, hurts a resonant link so badly.
The specification's coil-design guidance targets Q directly:
Far-field systems use a different model. The Friis equation, P_rx / P_tx = G_tx × G_rx × (λ / 4πd)², governs the link. Path loss is L_path [dB] = 20 × log₁₀(4πd/λ). Between isotropic antennas at 2.45 GHz and 100 m, this gives about 80 dB. Real systems recover most of that loss with aperture gain, which is why the standard lists beam steering among its efficiency-optimisation techniques.
The wireless link is also only one stage of the chain. Every WPT system follows DC → AC (inverter) → wireless link → AC → DC (rectifier). The specification gives 92–98% for the inverter, 30–95% for the link depending on distance, and 85–95% for the rectifier. The overall figure is the product of all stages. Even with an 80% link, the end-to-end result is only about 63% to 75%, so headline link efficiency always overstates what the user sees at the wall socket.
The standard gives no market-size figures, but its structure shows how the field divides into segments. Consumer electronics occupy the low-power, near-contact corner of the technology table: Qi and AirFuel for phones, wearables, laptops and tablets. Electric vehicles use the kilowatt-class inductive corner, governed by SAE J2954. Industrial WPT covers robotics, AGVs and cranes, where the load moves or the environment is hard on connectors. At the far end, microwave, laser and space solar power address remote or orbital energy. The segments differ so much in power, range and risk that no single product spans them, which is why the standard keeps them in one framework with shared terminology.
Each application domain maps to a profile with its own frequency, power and tolerance envelope. The specification gives these three standards-body profiles in section 3.3:
| Profile | Frequency | Power | Range | Link / communication |
|---|---|---|---|---|
| Qi (WPC) | 80–300 kHz baseline, 2 MHz extended | 5–15 W baseline, up to 100 W extended | 0–5 mm | ASK modulation (2 kbps) |
| AirFuel Resonant | 6.78 MHz (ISM band) | 5–50 W | 0–50 mm | Bluetooth Low Energy |
| SAE J2954 (EV) | 85 kHz | WPT1 3.7 kW, WPT2 7.7 kW, WPT3 11 kW, WPT4 22 kW | Air gap 100–250 mm | Efficiency > 85% grid to vehicle |
The EV profile shows how alignment becomes a specification item. SAE J2954 tolerates ±100 mm misalignment in X and ±75 mm in Y. These tolerances are wide because a driver cannot park to the millimetre. The coils, compensation and control must therefore keep acceptable efficiency across the whole range of k values that offset produces. The standard also covers dynamic roadway charging, where the vehicle moves over the transmitter. Its CLI includes an ev-dynamic simulation that takes speed, power and efficiency as inputs.
Capacitive power transfer (CPT) fills niches where inductive coupling struggles. It couples through an electric field across a dielectric, with C = ε₀ × εᵣ × A / d and power P = (2πfC) × V². The specification's comparison:
| Feature | Inductive | Capacitive |
|---|---|---|
| Tolerance to metal | Poor | Good |
| Tolerance to dielectric | Good | Poor |
| EMI | Higher | Lower |
| Coil/plate cost | Higher | Lower |
| Typical efficiency | 70–90% | 60–80% |
Underwater charging, with waterproof dielectric isolation, is the application the specification names first for CPT.
At the long-range end, microwave transfer uses a rectenna (rectifying antenna) to convert RF to DC. The SDK example uses the 2.45 GHz ISM band. Laser beaming covers 100 m to 1000 km, and the CLI example uses a 1064 nm wavelength. Space solar power combines orbital generation with wireless transmission. In all three, the Friis relationship and the pointing accuracy of the beam determine how much of the transmitted power reaches the receiver.
A wireless charger deliberately puts a strong field into the space around a person. The specification therefore treats safety as a core requirement alongside efficiency, and it groups the requirements into four areas:
| Concern | Requirement or mechanism in the standard |
|---|---|
| Tissue heating | SAR (Specific Absorption Rate) below 2 W/kg (FCC/ICNIRP) |
| Interference | EMI/EMC compliance: FCC Part 15, EN 55011 |
| Metal in the field | Foreign Object Detection (FOD): Q-factor monitoring, thermal sensing |
| Biological exposure | IEEE C95.1 exposure limits |
FOD follows from the physics in section 2.2. A coin or foil between the coils absorbs energy and lowers the effective Q of the resonant circuit, so monitoring Q reveals the object. The same object can heat up under the field, which is why the standard pairs Q-factor monitoring with thermal sensing. The terminology also defines LOP (Living Object Protection), which extends the idea from inanimate metal to people and animals entering the field.
SAR matters most in the radiating regimes. The SDK's safety analysis command takes power, distance and frequency as inputs, which are the same variables that drive field strength at a given point:
wia-comm-009 safety --power 1000 --distance 2 --frequency 2.45e9Conformity to the standard is also layered. Near-field consumer devices lean on WPC and AirFuel profiles, vehicles on SAE J2954, and all systems on emissions rules such as FCC Part 15 and EN 55011. Anything that radiates into occupied space must additionally satisfy the exposure limits above.
The TypeScript SDK (@wia/comm-009) exposes one class per technology: InductiveWPT, ResonantWPT, MicrowaveWPT, LaserPowerBeaming and EVDynamicCharging. The design carries the challenges above into the interface. An inductive charger takes its physical parameters and the FOD switch at construction:
const qiCharger = new InductiveWPT({
standard: 'Qi',
frequency: 87000, // Hz
maxPower: 15, // Watts
coilDiameter: 0.05, // meters
foreignObjectDetection: true
});
const session = await qiCharger.startTransfer({
deviceId: 'phone-12345',
requestedPower: 10, // W
alignment: { x: 0, y: 0, z: 0.005 } // meters
});
console.log(`Transfer efficiency: ${session.efficiency}%`);
console.log(`Power delivered: ${session.powerDelivered} W`);The alignment offsets in metres correspond to the misalignment tolerances discussed in section 2.3. The efficiency and powerDelivered values in the returned session let an application see how far a real transfer falls short of the ideal. A far-field transmission has the same shape, with beam steering enabled at construction and a target location and required power supplied at transmit time:
const microwaveWPT = new MicrowaveWPT({
frequency: 2.45e9, // Hz (2.45 GHz ISM band)
transmitPower: 1000, // Watts
antennaGain: 20, // dBi
beamSteering: true
});
const beam = await microwaveWPT.transmit({
targetLocation: { latitude: 37.7749, longitude: -122.4194 },
distance: 500, // meters
powerRequired: 100 // W
});Engineers can run the same models offline from the CLI before building hardware. The efficiency command takes technology, distance and coupling:
wia-comm-009 efficiency --technology resonant --distance 0.5 --coupling 0.6Practical guidance follows from the earlier sections. Size coils for the k range that real misalignment produces, not for the ideal case. Budget the whole inverter-link-rectifier chain. Enable FOD in any system where metal may enter the gap. Run the safety calculation for any radiating system before it is deployed.
The standard's feature list points to where the hardest unsolved problems lie. Dynamic roadway charging must hold efficiency while k changes continuously as the vehicle moves. Microwave and laser beaming depend on accurate beam steering over long, changing paths. Space solar power joins orbital generation to wireless delivery over the longest distances in the framework. In every case the two constraints of this chapter still apply. Efficiency is bounded by geometry and Q, and field exposure is bounded by SAR and the IEEE C95.1 limits. Progress means widening the usable distance without crossing either limit.
This chapter framed WPT as a trade among distance, efficiency, alignment tolerance and safety. Near-field inductive and capacitive links are governed by the coupling coefficient k and the quality factor Q. Their efficiency follows k²Q₁Q₂ / (1 + k²Q₁Q₂), and the inverter and rectifier then reduce the end-to-end figure further. Far-field microwave and laser links follow the Friis equation and path loss. The standard's profiles (Qi, AirFuel Resonant, SAE J2954) fix frequency, power, range and tolerance for each market. Safety requirements (SAR, FOD, LOP, EMI/EMC, IEEE C95.1) bound what any of them may do around people. The SDK and CLI turn these quantities into parameters such as alignment, power, frequency and distance.
The next chapter moves from the problem statement to the Phase 1 data format. It covers how a WPT system, its resonators, microwave and laser links, safety data and space-solar installations are described as structured envelopes, so that the quantities introduced here can be exchanged unambiguously between devices and tools.