# Low-Power Rectennas in Microwave Wireless Power Transmission
## 3 Low-Power-Density Rectenna
### Advanced Functional Antenna Structures  
Conformal antennas are designed to conform to or follow a specific shape, typically mounted on cylinders, cones, hyperboloids, or other complex geometries. By closely adhering to the surface of the supporting platform, conformal antennas offer excellent flexibility and portability and have been widely adopted in both civilian and military applications [101; 102]. Figure 19 illustrates a conformal meandered dipole rectenna array designed for Internet of Things (IoT) water meter applications. The antenna elements are wrapped around four faces of the water meter enclosure to enable omnidirectional radio frequency energy harvesting in the environment. When the ambient RF power density reaches 500 \(\upmu\)W/cm\({}^{2}\), the antenna array can deliver a direct current (DC) output power of 7.4 mW [103]. An ultra-wideband implantable conformal antenna for wireless capsule endoscopy is proposed in [104]. Utilizing a bent-line technique, a compact planar antenna is conformed to a cylindrical capsule with a radius of 3 mm, achieving an operating frequency range of 284 MHz to 825 MHz with a maximum gain of \(-31.5\) dBi. To further improve the adaptability of conformal antennas for wireless power transfer in complex electromagnetic environments, a design methodology assisted by deep reinforcement learning (DRL) is introduced in [105]. The overall system consists of a decision module (DRL), a control module (microcontroller unit, MCU), and a radiation module (conformal patch array). By taking the current phase states and gain of the antenna as inputs, the system employs a deep Q-network (DQN) algorithm to determine the phase adjustment actions. The experimental results demonstrate that the measured performance of the array, after DQN training, closely matches the simulated results for various beam directions, greatly simplifying the design, tuning, and deployment of conformal antennas. Figure 19: Conformal rectenna array on water meter case [103]. Figure 18: Photograph of tracking phased array [100]. Metamaterials are a novel class of materials engineered with artificially designed and periodically arranged structures, featuring electromagnetic properties beyond those of conventional natural materials. These unique characteristics enable more precise manipulation of light and electromagnetic waves, thereby attracting significant attention from researchers across various fields. By introducing resistive loads into the gaps of metamaterial unit cells, their impedance can be matched to that of free-space wave impedance. This allows metamaterials to function as alternatives to conventional antenna structures for the purpose of wireless energy harvesting (WEH) [106]. Based on such resonator units, various WEH metasurfaces have been proposed, which can achieve high-efficiency RF energy collection under arbitrary polarization angles and across a wide range of incident angles [107; 108; 109]. Inspired by rectennas, increasing research efforts have focused on the design of rectifying metasurfaces. Figure 20 presents a two-sided rectifying metasurface for low-power energy harvesting. The front side consists of metasurface units designed based on the electric-inductive-capacitive (ELC) structure, and the backside integrates a voltage-doubling rectifying circuit, separated by a ground plane and interconnected via through-holes. The designed rectifying metasurface achieves an RF-dc conversion efficiency of 76.8% at an input power of 0.4 dBm [110]. To enhance the adaptability of metasurfaces for diverse application scenarios, [PERSON] et al. proposed the concept of digital metasurfaces, in which two materials with different dielectric constants are encoded as \"0\" and \"1.\" These coding bits are arranged in mixed patterns to form arrays, enabling flexible modulation characteristics [111]. In the same year, [PERSON] et al.