**7. Electromagnetic energy harvesting concept**

As presented in **Figure 3**, the energy harvesting unit consists of an antenna, rectifying diode and circuit, and a rechargeable battery. The energy harvesting unit and the radiating element provide a self-powered device. The rectifier diode converts electromagnetic energy, AC energy, to direct current (DC energy). Two types of diode rectifiers are usually used a half-wave rectifier or a full-wave rectifier [44–47]. A half-wave rectifier is presented in **Figure 18**. A half-wave rectifier converts only the half cycle of the positive voltage. It allows harvesting only one-half of the electromagnetic wave. The efficiency of the half-wave rectifier is around 41%. A full-wave bridge rectifier is shown in **Figure 19**. The bridge full-wave diode rectifier circuit converts electromagnetic energy to DC energy. The bridge rectifier consists of four diodes D1 through D4, as shown in **Figure 19**. The rectifier output DC voltage, *VODC* ¼ 2*Vm=π*. By connecting a capacitor in shunt to the resistor, as shown in **Figure 19**, the rectifier output voltage may be improved. The full-wave rectifier

**Figure 18.** *Diode voltage rectifier with a capacitor, half wave.*

#### **Figure 19.**

*Diode bridge voltage rectifier with a capacitor, full wave.*

#### **Figure 20.**

*Wearable RF system with energy harvesting unit for IoT, 5G, and healthcare systems.*

efficiency is around 81%. Electromagnetic power amount in public centers, stadiums, hospitals, and malls may range from 1 μW/cm<sup>2</sup> to 5 mW/cm<sup>2</sup> . The harvesting system efficiency increases as function of the electromagnetic power collected by the harvesting system, as listed in **Table 8**. The amplifier amplifies the input power collected by the energy harvesting system and improves the efficiency of the harvesting system. Results listed in **Table 8** are also presented by companies that manufacture commercial RF energy harvesting systems, see [2–7, 44–47]. If the RF radiating sources are close to the harvesting system, the RF power collected by the harvester will be higher.

Energy harvesting units provide green renewable energy and may eliminate the usage of power cords and the need to replace batteries frequently. Wearable medical devices with energy harvesting modules for wireless communication, IoT, and medical applications are presented in **Figure 20**. As presented in **Figure 20**, the medical device and harvesting module with a compact battery charger are located on the patient's shirt.

In 2022 almost everyone uses cellular phones, communication networks, tablets, and other RF communication systems. There is a huge increase in the amount of RF energy in the air. The expected amount of radio waves in the air in 2022 was around 60 Exa-bytes, EB, per month. The expected amount of RF energy in the air in 2025 is expected to be around 170 Exa-bytes per month. In RF energy harvesting modules, the radio frequency waves propagating in air can be received by the antennas and converted to DC power that is employed to charge batteries, sensors, and other wearable devices.
