Microwave or Radio-Frequency (RF) wireless power transfer uses high-frequency electromagnetic (EM) waves in the range of 300 MHz to 300 GHz to wirelessly transfer energy between a transmitter and a receiver. It can operate both on the near- and far-field depending on the distance and frequency. RF WPT generates RF energy at a transmitter, radiating it through space and capturing it at a receiver where it is converted back into usable DC power. The transition between these regions is defined by the Rayleigh distance, previously discussed in chapter 2.1. Figure 10 shows a typical RF WPT system.

An RF generator converts DC power into a high-frequency alternating signal, in the radio frequency range. This generator is commonly based on a phase-locked loop or crystal oscillator. This ensures frequency accuracy and phase stability. The signal then goes through a power amplifier, which increases the amplitude of the RF signal to a level suitable for wireless transmission. After that, the signal goes through an impedance matching network. This network is composed of reactive components such as inductors and capacitors arranged in specific ways to minimize power reflection and ensure maximum energy transfer from the amplifier to the antenna. The transmitter antenna converts the electrical power into propagating electromagnetic waves. Depending on the type of system, the antenna may be a dipole, microstrip patch, horn, or even an array to support directional beamforming, which will be discussed later in this chapter. Some transmitters can also adjust frequency, beam direction, or output power. This is useful in dynamic systems.

Choosing the right frequency is critical. It affects system performance, antenna size, transmission efficiency, and regulations compliance. Lower frequencies (hundreds of MHz) offer better penetration and lower free-space path loss but require larger antennas and may limit rectifier efficiency due to lower voltage levels. Higher frequencies (2.45 GHz or 5.8 GHz) allow for more compact antennas and better spatial resolution but suffer from increased path loss and may be more sensitive to obstacle obstruction or environmental absorption. Frequencies in the ISM bands are commonly used since they are license-free in many regions and widely supported by commercial RF components.

The receiver side is designed to efficiently capture the transmitted RF energy and convert it back into usable electrical energy. The first part is the receiving antenna, which is modified to match the operating frequency and may be a simple dipole, a microstrip structure or an integrated array. A matching circuit is used to optimize the power delivery to the rectifier. If the system demands energy availability at any time, a storage element can be added such as supercapacitors or rechargeable batteries to buffer energy and provide power during interruptions.

Because RF power radiates in all directions when no focusing is used, multiple receivers can simultaneously receive energy from the same transmission, as long as they are within range. A few things should be considered: the total available transmitted power must be divided among the receivers, and spatial separation can lead to non-uniform power distribution due to differences in antenna gain, distance, and propagation path losses. To maintain optimal performance, the system can implement power allocation strategies, frequency-division schemes, and adaptive impedance matching techniques that respond to varying load and positioning conditions.

RF WPT systems also commonly apply Simultaneous Wireless Information and Power Transfer (SWIPT). It is a technique where both energy and data are transmitted over the same radio-frequency signal. This allows a receiver not only to collect energy from the incoming RF wave but also information, such as commands or configuration data. SWIPT is especially useful in low-power applications like IoT networks, where devices require both wireless energy and communication.