Electronics Guide

Microwave Propagation

Microwaves occupy the electromagnetic spectrum between roughly 300 MHz and 300 GHz, corresponding to wavelengths from one meter down to one millimeter. For power transmission, frequencies in the 2.45 GHz and 5.8 GHz ISM bands are commonly used due to regulatory availability, atmospheric transparency, and technology maturity. At these frequencies, electromagnetic waves propagate through the atmosphere with relatively low attenuation under clear conditions.

Beam divergence determines how concentrated the power remains as it travels. Diffraction limits minimum beam spread to wavelength divided by aperture diameter, so larger transmitting antennas produce tighter beams. At 2.45 GHz (12 cm wavelength), a 10-meter antenna produces a beam that spreads to about 1.4 degrees. Over kilometers, even tight beams expand significantly, requiring large receiving antennas to capture transmitted power efficiently.

Atmospheric Effects

Microwave propagation through the atmosphere experiences absorption, scattering, and refraction. At the frequencies commonly used for power transmission, atmospheric absorption is minimal in clear weather. Water vapor causes some absorption that increases with frequency, while oxygen absorption creates a peak around 60 GHz. Rain significantly attenuates microwave beams, with attenuation increasing at higher frequencies and heavier rainfall rates.

Atmospheric turbulence causes beam wander and scintillation (intensity fluctuations), degrading link performance. These effects increase with path length and frequency. System design must include margins for atmospheric variations and potentially aperture averaging or diversity techniques to maintain reliable power delivery. Historical weather data informs availability calculations for specific deployment locations.

Power Beaming Efficiency

End-to-end efficiency from DC input at the transmitter to DC output at the receiver multiplies the efficiencies of all stages: DC-to-RF conversion, antenna radiation, atmospheric transmission, reception, and RF-to-DC rectification. Each stage must be optimized to achieve practical overall efficiency. Modern systems can achieve end-to-end efficiencies of 40-60% over moderate distances under favorable conditions.

The fundamental limit on power beaming is the product of transmitting and receiving aperture areas divided by wavelength squared and range squared. This relationship shows that larger antennas, higher frequencies, and shorter distances all improve efficiency. Practical considerations including antenna cost, atmospheric effects, and regulatory constraints determine optimal frequency and aperture choices for specific applications.

Microwave Sources

High-power microwave generation for power transmission uses either vacuum tube or solid-state sources. Magnetrons, the technology behind microwave ovens, provide high power at low cost but with limited frequency stability and efficiency. Klystrons offer better efficiency and frequency control for high-power applications. Traveling wave tubes provide broadband capability useful for certain system architectures.

Solid-state sources using GaN (gallium nitride) transistors increasingly compete with vacuum tubes, offering advantages in reliability, graceful degradation, and electronic beam steering capability. While individual solid-state devices produce less power than vacuum tubes, arrays of many devices can be combined to achieve required total power. The distributed nature of solid-state arrays enables sophisticated beam forming and steering through electronic phase control.

Antenna Design

Transmitting antennas must efficiently radiate microwave power into a well-defined beam. Parabolic dish reflectors fed by horn antennas provide high gain and tight beams, well-suited to single high-power sources. Phased arrays of many elements enable electronic beam steering without mechanical motion, important for tracking moving receivers or maintaining beam position despite platform motion.

Antenna size determines beam width for given frequency, with larger antennas producing narrower beams. A 10-meter dish at 5.8 GHz produces a beam about 0.4 degrees wide. Even tighter beams require larger antennas or higher frequencies. Sidelobes, the unintended radiation outside the main beam, must be suppressed to minimize wasted power and potential interference. Careful feed design and aperture tapering reduce sidelobe levels.

Beam Forming and Steering

Phased array antennas steer beams electronically by adjusting the phase of signals feeding each element. Constructive interference in the desired direction and destructive interference elsewhere create the beam pattern. Phase shifters at each element enable rapid beam repositioning without mechanical motion. Digital beam forming processes signals in software for maximum flexibility in beam shape and direction.

Retrodirective arrays automatically point beams toward a source of pilot signals, simplifying tracking of moving receivers. The receiver transmits a low-power pilot signal, which the transmitter array receives and phase-conjugates to create a return beam precisely aimed at the pilot source. This approach reduces pointing complexity and enables beam tracking without explicit position information.

Power Control

Transmitted power must be controlled to match receiver needs and maintain safety compliance. Power reduction when receivers are absent or misaligned prevents energy waste and hazards. Closed-loop control using feedback from receivers adjusts power for optimal efficiency and safety. Multiple power levels accommodate different receiver distances and power requirements.

Rapid power reduction (beam shutdown) capability is essential for safety when hazards are detected. Fail-safe designs default to no power output when safety systems are uncertain. Redundant shutdown mechanisms ensure reliable response to safety threats. System monitors detect anomalies and initiate protective response within milliseconds.

Rectenna Fundamentals

A rectenna (rectifying antenna) combines an antenna to capture microwave energy with a rectifier circuit to convert it to DC. Unlike receive antennas for communications that must preserve signal information, rectennas need only extract energy efficiently. This simplification enables straightforward designs achieving high efficiency over narrow bandwidth. Typical rectennas achieve RF-to-DC conversion efficiencies of 70-90% at optimal power levels.

Individual rectenna elements are small, typically half a wavelength in dimension. Arrays of many elements covering large areas capture power from expanded beams at distance. Array configurations range from simple planar arrangements to more complex three-dimensional structures. DC outputs from array elements combine to deliver total power to the load.

Antenna Elements

Dipole antennas are the simplest rectenna elements, consisting of a half-wavelength conductor with the rectifier at the center feedpoint. Printed dipoles on circuit boards enable low-cost mass production. Circular polarization capability, achieved through crossed dipoles or patch antennas, captures energy regardless of beam polarization and reduces losses from polarization mismatch.

Patch antennas offer advantages in planar arrays, with feed networks and rectifiers integrated on the same substrate. Compact elements enable dense arrays with small inter-element spacing. Slot antennas and other configurations provide alternatives for specific applications. Element design optimization considers impedance matching to the rectifier, pattern characteristics, and manufacturing considerations.

Rectifier Circuits

Schottky diodes dominate microwave rectification due to their low forward voltage and fast switching. Silicon Schottky diodes work well at lower microwave frequencies, while gallium arsenide devices extend operation to higher frequencies. Diode selection balances forward voltage, breakdown voltage, junction capacitance, and cost for the specific application.

Single diode rectifiers provide simple half-wave rectification but waste half the RF cycle. Voltage doubler configurations using two diodes increase DC output voltage and improve efficiency. Full bridge rectifiers further improve efficiency at the cost of additional components. Harmonic rejection filters prevent re-radiation of harmonic energy that would otherwise reduce efficiency and cause interference.

DC Combining

Large rectenna arrays must combine DC outputs from many elements to deliver total power. Series connection adds element voltages while parallel connection adds currents. Hybrid series-parallel arrangements optimize for desired output voltage and current while accommodating element variations. Mismatches between elements due to manufacturing variations or beam non-uniformity require careful array design.

Blocking diodes prevent reverse current flow through underperforming elements. Power combining networks may include local voltage regulation to handle variations across the array. Modular subarray construction simplifies manufacturing and enables field repair by replacing failed sections rather than entire arrays.

Exposure Limits

Microwave exposure limits protect people from heating effects in body tissues. International guidelines from ICNIRP specify frequency-dependent power density limits for general public exposure of 1 mW/cm2 at 2.45 GHz and 10 mW/cm2 at higher frequencies. Occupational limits are typically 5 times higher. Power transmission systems must ensure exposures remain below limits in all accessible areas.

Power densities in microwave power beams typically range from milliwatts to watts per square centimeter at the receiver, far exceeding public exposure limits. Exclusion zones, shielding, and access controls prevent human presence in high-intensity regions. Beam power reduction or shutdown when intrusion is detected provides active protection. System design must ensure safety under both normal operation and foreseeable fault conditions.

Exclusion Zones

Safety exclusion zones around beam paths prevent human exposure to hazardous power levels. Zone dimensions depend on beam power, frequency, and antenna characteristics. Ground-based exclusion zones can be fenced and monitored, while airborne beams require coordination with aviation authorities and potentially active detection of aircraft intrusion.

Beam divergence means exclusion zones expand with distance from the transmitter. For kilometer-scale power beaming, exclusion zones may cover substantial areas. Underground or elevated beam paths can reduce surface-level exclusion requirements. System geometry optimization minimizes exclusion zone impacts while maintaining power transfer capability.

Detection and Response

Safety systems must detect hazards and respond appropriately to protect people and prevent accidents. Sensors detect intrusion into exclusion zones, triggering beam power reduction or shutdown. Multiple detection technologies including radar, infrared, video, and laser scanners provide redundancy and coverage. Sensor placement ensures detection before intruders reach hazardous exposure levels.

Response time from detection to beam shutdown determines minimum safe distance from detection point to hazard zone. Faster response enables smaller exclusion zones. Safety system reliability must exceed that of the power system itself to ensure protection under all conditions. Regular testing verifies continued proper operation.

Environmental Considerations

Environmental impacts beyond human safety include effects on wildlife, electronic systems, and aircraft. Birds and other animals passing through beams may experience heating, though typical power densities are below levels causing immediate harm. Long-term effects on habitats and ecosystems near beam paths require study for permanent installations.

Electronic interference from high-power microwave beams can disrupt communications, radar, and sensitive equipment. Operation in ISM bands provides some protection through frequency coordination, but high power levels still require careful siting and shielding. Coordination with aviation authorities addresses potential impacts on aircraft systems and air traffic control radar.

Space Solar Power

Space solar power (SSP) satellites would collect solar energy in orbit and beam it to Earth as microwaves. Orbiting above weather and atmosphere, solar collectors operate 24/7 without clouds or night. Power densities of 1.4 kW/m2 in space exceed typical terrestrial solar by 5-10 times. Huge transmitting antennas would beam power to Earth-based rectennas converting it to grid electricity.

Technical challenges include launch costs for massive orbital infrastructure, kilometer-scale antenna construction in space, and efficient long-distance power beaming. Economic viability requires dramatic reduction in launch costs, potentially through reusable rockets or space-based manufacturing. Environmental concerns center on beam safety and spectrum allocation. Despite challenges, growing energy demand and climate change drive continued research.

Unmanned Aircraft

Microwave power beaming can keep unmanned aerial vehicles (UAVs) aloft indefinitely, enabling persistent surveillance, communications relay, and atmospheric research. Ground-based transmitters track aircraft and maintain power beams as they fly. Aircraft equipped with rectennas convert received power to electricity driving motors and systems. Demonstrations have achieved extended flight times, with practical deployments for specific missions beginning.

Power requirements for aircraft depend on size, altitude, and mission. Small drones might operate on hundreds of watts, while larger aircraft need kilowatts. Flight altitude affects beam losses and exclusion zone geometry. Multiple ground stations can provide coverage over extended areas, with aircraft transitioning between beams. Integration with battery storage enables operation during beam handoffs and emergency landing capability.

Remote Power

Sensors, communications equipment, and other systems in remote locations can receive power via microwave beams, eliminating battery replacement or fuel resupply logistics. Mountain-top repeaters, ocean buoys, and polar monitoring stations represent potential applications. Power levels from watts to kilowatts serve different equipment needs.

Point-to-point links between fixed locations simplify system design compared to mobile applications. Terrain features and atmospheric conditions at specific sites determine link feasibility. Reliability requirements for critical infrastructure demand backup power and redundant links. Economics compare favorably to diesel generators or helicopter battery replacement for sufficiently remote sites.

Industrial Applications

Industrial settings can use microwave power transmission to deliver energy across gaps where electrical connections are impractical. Rotating equipment, mobile robots, and sealed environments represent potential applications. Power levels from watts for sensors to kilowatts for mobile equipment span industrial needs.

Contained indoor environments simplify safety management compared to outdoor deployments. Metal shielding can contain beams within designated areas. Integration with existing industrial control systems enables coordinated operation. Cost justification requires comparison with alternative approaches including inductive power, battery operation, and trailing cables.

Link Budget Analysis

Link budget calculations determine received power for given system parameters. Starting from transmitter power, add transmit antenna gain and subtract free-space path loss, atmospheric attenuation, pointing losses, and receiver antenna gain to find received power density. Rectenna area and efficiency determine DC output power. Margins account for weather, pointing errors, and equipment degradation.

Free-space path loss increases with frequency squared and distance squared. At 2.45 GHz and 1 km range, free-space loss is about 100 dB. Antenna gains of 30-40 dBi are practical with meter-scale apertures. Atmospheric loss of a few dB under clear conditions increases substantially in rain. Complete link budgets guide system sizing and predict performance under various conditions.

Frequency Selection

Operating frequency selection balances multiple factors. The 2.45 GHz ISM band offers technology maturity, low atmospheric loss, and reasonable antenna sizes. The 5.8 GHz ISM band enables smaller antennas for given beam width but with higher atmospheric sensitivity. Higher frequencies in the tens of GHz allow very tight beams but suffer significant rain attenuation.

Regulatory availability constrains frequency choices. ISM bands allow unlicensed operation under power limits, while other frequencies require specific authorization. International coordination is necessary for cross-border or satellite systems. Spectrum allocation processes for new wireless power applications continue as the technology matures.

Reliability and Availability

System availability depends on equipment reliability and atmospheric conditions. Hardware reliability improves through redundancy, robust design, and preventive maintenance. Atmospheric availability varies by location and can be predicted from historical weather data. Combined system availability must meet application requirements, whether 99% for experimental systems or 99.99% for critical infrastructure.

Backup power provisions maintain receiver operation during beam outages. Energy storage buffers short interruptions while backup generators or grid connection handle extended outages. Graceful degradation operates at reduced power when partial system failures occur. Monitoring and diagnostics enable rapid response to problems and continuous performance optimization.

Pioneering Work

Microwave power transmission concepts emerged in the 1960s with William C. Brown's work at Raytheon. In 1964, Brown demonstrated a helicopter kept aloft by microwave power. The 1975 demonstration at the Jet Propulsion Laboratory transmitted 30 kW over 1.5 km with 82% beam-to-DC efficiency, proving large-scale microwave power beaming feasible. These experiments established fundamental techniques still used today.

Space Solar Power Studies

NASA and Department of Energy studies in the 1970s examined space solar power satellites beaming energy to Earth. Detailed reference designs specified multi-kilometer solar collectors and transmitting antennas in geostationary orbit. Economic analyses showed unfavorable comparison to terrestrial power at then-current launch costs. Renewed interest emerged in the 1990s and continues with modern launch cost reductions.

Modern Developments

Solid-state technology advances enable new system architectures with electronically steered beams and distributed power generation. Demonstrations have achieved power transmission to unmanned aircraft, rovers, and fixed receivers. International projects in Japan, United States, China, and Europe advance the technology toward practical applications. Commercial companies pursue near-term applications while space solar power remains a long-term goal.