# Energy Harvesting Methods for Transmission Lines: A Comprehensive Review
## 4 State-of-the-Art Energy Harvesting Methods for Transmission Lines
### Radio-Frequency (RF) Energy Harvesting  
RF enables energy harvesting from RF sources such as base transceiver stations (BTS), TV towers, Wi-Fi, radio broadcast [93], or from dedicated RF transmitters [36] through the use of receiving antennas. These signals are then converted to DC power to supply sensor nodes [14]. RF power transmitters are broadly classified as dedicated units [94] and ambient RF power sources [95]. Ambient RF energy is found in a wide range of frequency bands, mainly in the MHz and GHz ranges [96]. Due to the widespread installation of communication systems, RF energy harvesting has become a widely analyzed technology, although it is still a challenge for low-power regions [7]. RF energy harvesting can provide a regular energy source in the presence of strong RF signals due to dedicated RF power transmitters. Unlike wind and solar energy, local weather conditions do not affect RF sources [7], RF EH presents notable features such as predictability, reliability, or the possibility of simultaneous power supply to different WSNs [96]. Additionally, RF signals can be found almost everywhere, 24 h a day, offering low-cost implementation, compactness, and a wireless nature [97; 98]. The major drawback is related to the amount of power that can be harvested [99] due to the decay of the radiated power with the inverse square relationship with the distance to the radiation source [100]. The power density near powerful transmitters is relatively low, in the order of tens to hundreds nW/cm\({}^{2}\)[101; 95] to some few uW/cm\({}^{2}\)[102; 103; 104], so power-dedicated transmitters are often required to supply WSNs using inductive, capacitive, or radiative tag readers [11]. In addition, the input signal may not be permanently available due to random service usage or the duty cycle (mobile phones or Wi-Fi routers) or the size of the antenna required (AM radio waves) [105; 106]. Therefore, automatic frequency tuning could be useful to maximize the output power [104]. As shown in Figure 7, a conventional RF EH circuit includes an antenna to convert the incoming RF signal into a voltage difference or vice versa, a rectifier to convert the  
Figure 6: Electric-field energy harvesting concept in power conductors, adapted from [92]. (**a**) Schematics of the energy harvester. (**b**) Single-layer structure. (**c**) Two-layer structure. high-frequency signal to DC, a voltage multiplier that produces a higher DC output voltage, and an energy storage device, such as a battery or a supercapacitor [96]. In [108], a BLE device was combined with a wireless RF energy harvester equipped with a 50 mF capacitor, charged by a GSM mobile, while in [109], a WSN prototype using an RF EH system from Wi-Fi transmissions was proposed. In [110], an ultra-high-frequency (UHF) RF energy harvesting solution was proposed to supply the electronics to monitor power lines. A planar monopole antenna was used for better resonator frequency performance. In [102], a multi-hop displacement of an array of magnetic antennas was proposed to harvest ambient energy for power cord monitoring. A far-field RF EH approach was also proposed for the same purpose. Batteryless operation of RF-powered WSNs can be achieved by integrating the sensor into a radio-frequency identification (RFID) tag using an external RFID transmitter receiver [111]. Batteryless RF sensors can receive a steady energy flow from a nearby RFID reader during the entire measurement cycle, sufficient to supply the sensing and communication tasks [36]. Wake-up radio (WuR) is an energy-saving technique being applied in WSNs based on saving power by transiting into a low-power sleeping mode and turning off their main radios [112]. Devices implementing WuR usually maintain a low-power Wake-up Receiver waiting for any incoming Wake-up Signal (WuS). They can also sometimes include a wake-up transmitter capable of sending WuSs. The WuS wakes up the receiver device, which connects its main radio, so it is ready to communicate with other nearby devices [113].