# Low-Power Rectennas in Microwave Wireless Power Transmission
## 4 Emerging Application Scenarios  
The advancement of wireless power transmission has unlocked new possibilities for battery-free operation in various devices and systems. This WPT technology has benefited from the rapid development of rectennas, capable of harvesting ambient or directional RF energy and converting it into dc power. Depending on the source and strength of the harvested RF signals, wireless power transmission can be categorized into ambient RF energy harvesting and directive RF power transfer. Although ubiquitous and freely available, ambient RF energy is typically exhibited at low and fluctuating power levels, making it particularly suitable for devices with low-power consumption and low-duty-cycle requirements. In contrast, directive RF power transfer can provide higher and more predictable energy levels to support continuous and stable operation in more demanding applications. The practical integration of energy harvesting rectennas into real-world scenarios has been promoted by the proliferation of low-power electronics and the emergence of ubiquitous wireless communication networks such as Bluetooth Low Energy (BLE) [114; 115; 116], RFID [117; 118; 119], LoRa [120; 121; 122], and ZigBee [123; 124; 125]. These networks enable efficient data transmission with minimal energy consumption, which is essential to realizing the vision of sustainable, maintenance-free sensor networks. For example, ambient RF energy harvested from sources like LTE, GSM, or Wi-Fi signals has already been demonstrated to power microcontroller-based sensor nodes in urban environments, achieving significant efficiencies and enabling devices to operate autonomously in active, standby, and low-power modes [126; 127; 128]. In addition, integrating energy storage elements such as supercapacitors and batteries with rectenna-based power management modules smooths out intermittent energy supply, supporting long-term and reliable device operation [129; 130]. Adaptive energy-aware interfaces between power harvesting modules and sensors further optimize energy transfer based on dynamic load conditions, ensuring maximum power to the required devices even in variable environments [131; 132]. The range of applications for low-power rectennas is rapidly expanding across multiple areas. In industrial environments, wireless energy harvesting is driving the development of maintenance-free IoT sensors that enable real-time monitoring and automation in  
Figure 21: The reconfigurable transmissive metasurface and an exploded view of the unit cell [113]. smart manufacturing. Figure 22 shows IoT temperature and light sensors based on two ultrathin electrically small rectennas types. The sensor-augmented rectifier is seamlessly integrated with the antenna and printed on a single ultra-thin PCB. When the ambient temperature increases, the thermistor's resistance decreases, resulting in a rise in the output DC voltage. Once the temperature exceeds 65 \({}^{\circ}\)C, the output voltage surpasses 0.8 V, which triggers the acoustic siren. For light detection, the resistance of the photocell is nearly infinite in a dark environment. As the ambient light goes bright, its resistance decreases, causing the output voltage to rapidly rise to 5.9 V and activating the alarm. This design enables a dual-mode detection system for environmental temperature and light intensity [133]. Figure 23 illustrates a novel fully integrated planar six-sector 3D spherical coverage rectenna array designed for wireless energy harvesting applications. Each array sector contains an end-fire element and a bore-sight patch antenna element. The DC output is extracted from the center of the structure via output pins at the bottom, allowing the rectenna array to be directly mounted on IoT nodes for immediate use [134]. In healthcare, implantable medical devices powered by ambient or directive RF energy are emerging as a promising solution for continuous, uninterrupted health monitoring without frequent recharging or battery replacement constraints.