Tecnologia a Microonde Basata sul Trasferimento di Potenza Wireless

What is wireless power transfer? Microwave-based wireless power transfer is revolutionizing how energy reaches remote and hard-to-access devices, enabling maintenance-free operation for IoT sensors, electric vehicles, and drones. Let's explore the latest advancements in wireless energy transmission technologies and their vital role in powering the connected world of tomorrow.  

Microwave Power Transfer: Past and Present

Microwave energy transfer received considerable attention almost a decade after its invention during World War II. In 1958, researchers initiated studies on wireless energy transfer for solar-powered satellites, supported by funding from Raytheon, NASA, and the US Air Force. Since 1980, all sponsors have investigated microwave energy transfer, with NASA as the primary sponsor.

Diverse techniques for wireless energy transmission have been established, encompassing Radio Frequency (RF), electromagnetic induction, ultrasonic-guided waves, and laser beams. The most advanced method for energy transfer is wireless transmission using radio frequencies. The microwave power transmission system is a technique that conveys energy to almost inaccessible locations. Maintaining the self-sufficiency of Internet of Things (IoT) devices necessitates a harvesting circuit for power storage. 

Wireless Power Transfer

Figure 1 illustrates the considerable variability of wireless power methods, influenced by numerous characteristics such as frequency, distance between transmitter and receiver, and specific applications. Electric vehicles, drones, consumer electronics, and wearable electronics can all employ WPT for near-fi eld applications, namely MC-WPT (Magnetic Coupling Wireless Power Transfer) and EC-WPT (Electric-Field Coupling Wireless Power Transfer). Far-fi eld  applications encompass advanced technologies and military uses, such as MPT (Microwave  Power Transmission) and LPT (Laser Power Transmission).
  Figure 2 displays the block diagram of the wireless power transmission system.
  The wireless energy transmission structure consists of three parts:

• Microwave energy source,
• Traveling wave area,
• Rectenna (Rectifying Antenna) or Cyclotron wave rectifier (CWC).

The necessity for a restricted distance between coils hinders the advancement of electromagnetic induction technology. Currently, four modalities of wireless energy transmission are employed. We enumerate four categories of electromagnetic radiation in ascending order of frequency:

• Microwaves (Long-range distance),
• Lasers (long-range distance),
• Electromagnetic induction (Short-range distance),
• Ultrasonic waves (Short-range distance).

Wireless power transfer technology can be divided into electromagnetic-based WPT technology and mechanical-based WPT technology according to indirect energy, as shown in Figure 3. Electromagnetic-based WPT technology can be classified into the magnetic field principle, electric field-principle WPT, microwave-principle WPT, and laser-principle WPT. Mechanical-based WPT technology can be classified into ultrasonic-principle WPT and mechanical vibration-principle WPT. The WPT technologies in aviation include inductive wireless power transfer, magnetic coupling resonant, laser, and microwave wireless power transfer. Inductive and conventional cognitive radios are classified as near-field WPT technologies, and laser and microwave are far-field WPT technologies.
  Energy harvesting, a crucial component of wireless power transmission, is an innovative technology that delivers electricity to historically unreachable locations. This method, known as RF harvesting, is especially significant for its ability to energize electronic devices with minimal power requirements in remote and complex settings. It is also referred to as radio frequency harvesting. It can wirelessly energize electronic gadgets with minimal power usage.

Researchers have recently attracted significant interest because they can wirelessly charge sensors in complex environments. Much research has examined wireless power charging by RF energy harvesting. The fundamental principle of this technology is to capture the RF energy surrounding the antenna or receiver to energize sensors. Consequently, the IoT sensors must possess battery-free power supply methods. In the future, substantial resources will be required to sustain IoT devices due to the prevalence of numerous sensors in wireless sensor networks. Consequently, each sensor must be battery-free to ensure maintenance-free operation. Wireless power transfer, or RF energy harvesting, is a promising technique because it can supply power to IoT devices over greater distances than alternative methods, as illustrated in Figure 4.
  Numerous distinguished researchers have advanced RF energy harvesting through various approaches and strategies. Chang-Yeob Chu et al. investigated system design for electric car charging, considering a broad spectrum of coupling coefficient variations due to coil misalignment. Koichiro Ishibashi and colleagues proposed the radio frequency characteristics of rectifying devices for ambient energy harvesting.

Mohamed Zied et al. examined the influence of wireless power transmission on the future of warfare globally and its implications for ranking nations by military might. The investigation considered two factors: the impact of the microwave power sources and the effect of the distance on the attenuation. They investigated establishing energized IoT sensors through RF energy recovery. Mohamed Zied et al. concluded that all parameters efficiently supply energy to wirelessly power 5 W LED bulbs at distances exceeding five meters.

Nermeen A. Eltresy et al. studied a CPIFA antenna to harvest the RF energy at three different frequency bands: GSM 900, GSM 1800, and Wi-Fi 2400. According to their analysis, the aim of designing a matching circuit is to match the proposed antenna with a rectifier to get maximum power transfer and minimum loss. They successfully harvested an output of 624 mV at 0 dBm input power.

Diffa Pinto et al. evaluated the performance and established a relationship between process parameters to harvest energy and the number of rectifier circuits. They developed a framework for a Wi-Fi energy harvesting system based on the 7-stage Villard rectifier voltage multiplier circuit, which was analyzed and simulated using Agilent Design Systems (ADS). The output of the multiplier circuit is given to the power management unit circuit (PMU). With the increase in applications related to smart home communication, the Internet of Things (IoT), intelligent health care, and environmental monitoring, the demand for low-power electronic devices has increased significantly.

This solution ensures two essential components for IoT in smart homes: a reliable electrical supply for the wireless charging of electronic device batteries and ongoing self-energization of any IoT sensor utilizing RF energy, employing signals such as Wi-Fi, 4G, WiMax, 5G, or any ambient radio frequency. This technology can power numerous electrical devices using IoT’s integrated and stationary harvesting systems. This aspect illustrates the effectiveness of wireless power harvesting in smart cities, enhancing intelligence and safety in daily life. The rectifier circuit efficiently converts the received RF energy through Schottky diodes, a power management circuit, and a low-pass filter, all directly connected to the IoT device.

More on Wireless Power Transfer

For more information about wireless power transfer, refer to Mohamed Zied Chaari's book, Wireless Power Design (Elektor 2025), from which this article (Ch. 2) is sourced.