## 8 Existing Applications of RF-EH  
In this section, various rectennas were compared based on their size, operating frequency, substrate, and diode technology. Previously, a CP graphene field effect transistor (GFET)-based rectenna coupled with a monopole antenna was created to detect high-frequency RF signals [173]. A miniaturized, printed rectenna was also designed to harvest energy from ambient RF power around 2.45 GHz [174]. Meanwhile, a wearable rectenna array was introduced in which each rectenna uses a stack topology to minimize the element dimensions in the array. Cordura textile material was used to fabricate the design, which was durable, light, tough, and comfortable, making it suitable for wearable antenna design [175]. Furthermore, a transparent ultrawideband (UWB) antenna was proposed with a unique wideband rectifying circuit. The device aimed to harvest and convert EM energy from most telecommunication bands into electrical energy [176]. Additionally, the design analysis was carried out in [177] on a wideband rectenna. The study revealed that the combined antenna and rectifier supplied energy to low-powered devices, non-battery-powered sensors, and various IoT devices. Recently, a monopole wideband antenna array fed by a microstrip-to-slot-line transition feeding network was demonstrated. The feeding network was in reference to the slot-line stubs and radial microstrip to apply the microstrip-to-slot-line broadband transitions [178]. A unified, cloud-based IoT system was introduced for smart home applications [179]. A new rectifying-antenna design was introduced with an expanded power input range operating at 2.4 GHz for ambient (WEH) systems and wireless power transmission (WPT), as depicted in Figure 7[180]. A highly efficient rectifier with an input power range for EH and a wide bandwidth was also suggested [181]. In terms of compact size, low cost, and a higher voltage output provided by the realized rectifier, which was optimized for low-input power conditions, a rectenna to harvest RF energy at the ISM band of 2.4 GHz was also designed [182]. A varying topology for the rectifier, in addition to the microstrip patch antennas, was employed to terminate the power splitter at the input and a voltage adder at the output to reduce the ohmic loss and leakage. In an RF-EH device, a short traveling wave series-fed patch (SFP) was used to improve the angle range. In contrast, a design based on the periodic circuit model, which included series-coupled resonators to aid the short series-fed patch design, was introduced [183]. Besides, [PERSON] et al. [184] designed a novel dual-band Composite Right/Left-Handed (CRLH)-based rectenna to address the poor efficiency and the poor gain bandwidth in the dual-band omnidirectional antenna, as shown in Figure 8.  
Figure 7: Photographs of the fabricated Rectenna [183]. [PERSON] et al. [185] eliminated the use of an MN between the rectifier and the antenna by proposing a new approach to design the rectenna system. The ground plane supports the novel rectenna design, which consists of eight octagonal discs with a broad impedance bandwidth sandwiched between two varying layers on both sides of a single substrate. A Schottky diode was also placed on each layer next to the feeding location without an MN, resulting in a two-rectenna array. Furthermore, in ref. [186], the Huygens dipole rectenna has exceptional physical and radiation properties. Incorporated on a single Rogers 5880 substrate was a high-efficiency rectifier circuit and an ultrathin and electrically small Huygens dipole antenna (HDA). [PERSON] et al. [187] demonstrated a textile-based broadband rectenna for a fifth generation (5G) RF-EH and Wireless Power Transfer, which uses a novel wearable antenna design and a highly sensitive broadband rectifier. An RF-EH and storage module for wearable applications can harvest 8.4 mJ of energy in less than four minutes from 915 MHz industrial, scientific medical sources [188].