In the last fifty years, the universe has suffered rapid changes in climate. It is obvious in 2020 to most of the world population that green energy is crucial to save the universe from climate changes. The rapid spread of diseases and macro-parasites, and the extinction of species are the direct result of climate changes. In the last twenty years, the idea of using free space energy in the forms of light, heat, vibration, electromagnetic waves, muscle motion and other types of energy, has become useful and challenging. A number of methods to produce electricity from these different types of energy sources have been investigated and developed    . Energy harvesting systems provide green energy and may eliminate the need to replace batteries every day and the usage of power cords. Batteries and cables waste pollute the environment. To use as much free space energy as possible it is important to harvest the electromagnetic power from wideband range of wireless communication systems. In these cases, we should use ultra-wideband antennas. Moreover, a programmable array with two to four antennas can harvest energy from 100 MHz to 18 GHz. The energy harvesting antenna must satisfy the requirements related to the system application. Due to considerably low-power electromagnetic energy densities in free space, highly efficient antennas are significant. The antennas should operate at a specific frequency range and polarization. The antenna radiation pattern should have a wide beam width or omnidirectional radiation pattern. Several printed antennas were employed for harvesting energy applications    . Patch and slot antennas are widely used in communication and medical system  - . Wideband printed compact slot and notch antennas are good choices to function in wideband harvesting energy systems. Slot and notch antennas are compact, low cost, flexible, and efficient. Moreover, a compact low-cost energy harvesting network and matching network may be achieved by integrating the system RF components with the antennas on the same substrate. Printed compact antennas are widely presented in the literature in the last twenty years as referred in  - . Human body effect on the electrical performance of wearable medical system and antennas at microwave frequencies should be considered. Electrical properties of human tissues have been presented in several papers such as  . Several wearable antennas were presented in books and papers in the last years as referred in  - . Printed notch and slot antennas for harvesting energy applications are rarely presented in the literature. New ultra-wideband wearable antennas for 5G, IoT and medical RF systems with energy harvesting units are presented in this paper.
2. Energy Harvesting Systems for 5G, IoT, Medical and Computer Industry
In the last decade, there is a significant increase in the amount of electromagnetic energy in the air. Almost every person has a cellular phone tablet and other communication devices. In electromagnetic energy harvesting systems, the electromagnetic waves propagating in free space may be received by harvesting antennas and converted to electric energy that is used to charge batteries and for other devices. The expected amount of radio wave in the air in 2019 was 33 Exa-bytes, EB, per month. However, the expected amount of radio wave in the air in 2025 is expected to be 164 Exa-bytes per month, see Table 1. Table 1 presents the expected amount of radio wave in the air for 2G, 3G, 4G, and 5G networks. 5G is forecast to account for 45% of global mobile data traffic by 2025. Computations per KWh from 1985 to 2018 are listed in Table 2. Energy sources used in harvesting systems are listed in Table 3. Communication services such as television, GSM, wireless local area networks, WLAN, and Wi-Fi covers the frequency range from 0.2 GHz to 5.4 GHz. Wireless communication systems operate in the frequencies from 700 MHz to 270 0MHz. Medical systems operate in the frequencies from 200 MHz to 1200 MHz. WLAN systems operate in the frequencies from 5400 MHz to 5900 MHz.
RF energy is inversely proportional to distance and therefore drops as the distance from a source is increased. Harvested power from RF energy sources is lower than 0.1 mW/cm2. Electromagnetic energy harvesting system is shown in Figure 1. A programmable array with 3 to 4 antennas can harvest RF energy from 0.15 GHz up to 18 GHz. The received RF energy may be combined after transformed to DC power.
Table 1. Expected amount of radio wave in free space from 2015 up to 2025.
Table 2. Computations per KWh from 1985 to 2018.
Table 3. Energy sources used in harvesting systems.
Figure 1. Dual mode programmable array energy harvesting system.
The RF energy harvesting system consists of antennas, matching and feed networks, rectifying circuit, and a rechargeable battery. The harvesting energy system operates as a dual mode RF harvesting system. The harvesting unit can be part of a medical, IoT, computer, and smartphone. The LNA DC bias voltages are supplied by the receiving system The Low Noise Amplifier is part of the. The energy coupled to the transmitting built in test, −20 dB, may be harvested and used to charge a battery. We can calculate the energy harvesting link budget by using Equation (1) if the antennas are matched and there are no losses in the medium. However, if the antennas are not matched and the RF energy propagates in a lossy media, we can calculate the energy harvesting link budget by using Equation (2). Wireless smartphone using standard 802.11 can transmit up to 1 Watt. Free Space Loss (Lp) represents propagation loss in free space. Losses due to attenuation in atmosphere, , should be accounted for in the transmission equation.
The attenuation constant is . Where, . The received power may be given as: . Losses due to polarization mismatch, , should also be accounted. Losses associates with receiving antenna, Lra, and with the receiver, , should be accounted in computation of transmission budget. Losses associates with the transmitting antenna as written as, . Where: is the reflection coeficient of the receiving antenna.
is the reflection coeficient of the transmitting antenna. .
Pt = Transmitting antenna power. Lt = Loss between power source and antenna.
EIRP = Effective isotropic radiated power. Where, EIRP = PtGt. Where,
G, Gain in dB; and L, Loss in dB are written as: , .
The received power Pr in dBm may be calculated by using Equation (5). The received power Pr is referred to as the “Carrier Power”. Personal Computer Memory Cards using standard 802.11 can transmit up to 100 mW.
Figure 2 presents a wideband receiving system, 500 MHz - 18 GHz, with energy harvesting units.
Figure 3 presents a wideband transmitting system, 500 MHz to 18 GHz, with energy harvesting units.
Figure 2. Wideband harvesting receiving programmable array system.
Figure 3. Optimized harvesting transmitting programmable array system.
Three wideband antennas are employed in the receiving and transmitting systems presented in Figure 2 and Figure 3. The first antenna covers frequencies from 150 MHz to 0.5 GHz. The second antenna covers frequencies from 500 MHz to 6 GHz. The third antenna covers frequencies from 6 GHz to 18 GHz. The receiving and transmitting systems can harvest energy from 150 MHz to 18 GHz. Almost in every transmitting or receiving channel part of the energy, −10 dB to −20 dB, is coupled to a built in test port. This RF energy can be transformed to DC energy and can be used to charge electrical devices. In the receiving channel the received RF energy from 150 MHz to 6 GHz is transferred via a wideband combiner or SPDT to a second SPDT that is connected to the 6 GHz to 18 GHz antenna. The output of the second SPDT is connected to a third wideband SPDT via a wideband LNA. The LNA is part of the receiving channel. The output of the third SPDT transfer the receiving RF energy to the receiving channel or to the harvesting system. A wideband amplifier transfers the received RF signal to the receiver. The received RF energy is coupled, −10 dB to −20 dB, is coupled to a Built in Test port. This RF energy is transformed into DC energy and may be used to charge electrical devices. A wideband switching matrix connects three power amplifiers to three transmitting antennas. The first antenna covers the frequency range from 150 MHz to 0.5 GHz. The second antenna covers frequencies from 500 MHz to 6 GHz. The third antenna covers frequencies from 6 GHz to 18 GHz. A DC unit supplies the bias voltages to the amplifiers and controls the switching matrix state. The harvesting system may consist one to three antennas according to the system requirements. The antennas may cover the frequency range from 150 MHz to 6.4 GHz only.
3. Wideband 150 MHz to 0.4 GHz, Energy Harvesting Antenna
RF energy in the frequency range from 150 MHz to 500 MHz may be harvested by a wideband compact metamaterial antenna as presented in Figure 4. The antenna is a dual polarized printed dipole and slot antennas with Split Ring Resonators and metallic strips. The microstrip loaded dipole antenna with SRR provides horizontal polarization. The slot antenna provides vertical polarization. The dipole feed network is printed on the first layer. The radiating dipole with SRR is printed on the second layer. The thickness of each layer is 0.8 mm. The length of the antenna with SRR is 19.8 cm. The SRR ring width is 1.4 mm the spacing between the rings is 1.4 mm. The antennas have been analyzed by using electromagnetic software. The matching stubs and metallic strips locations and dimensions have been optimized to get the best VSWR results. The S11 parameter of the metamaterial antenna with metallic strips is presented in Figure 5. The antenna bandwidth is around 75% for VSWR better than 2.3:1.
Figure 4. Wideband antenna with SRR and Metallic strips.
Figure 5. S11 for antenna with SRR and metallic strips.
Figure 6. Radiation pattern for antenna with SRR and metallic strips.
Figure 7. 3D Radiation pattern for antenna with SRR.
4. Wideband 400 MHz to 6.4 GHz, Energy Harvesting Slot Antenna
RF energy in the frequency range from 400 MHz to 6.4 GHz may be harvested by a wideband compact T shape slot antenna as presented in Figure 8. The slot antenna is printed on a dielectric substrate with dielectric constant of 2.2 and 1.2 mm thick. The dimensions of the slot antenna shown in Figure 8 are 116 × 70 × 1.2 mm. The antenna electrical parameters were calculated and optimized by using ADS software . The S11 parameter of the wideband T shape slot antenna is presented in Figure 9. The radiation pattern of the wideband T shape slot antenna at 1.5 GHz is shown in Figure 10. The antenna gain is 3 dB.
5. New Compact Ultra-Wideband Harvesting Notch Antenna 5.8 GHz to 18 GHz
A wideband notch antenna with fractal structure has been designed. The antenna is printed on a dielectric substrate with dielectric constant of 2.2 and 1.2 mm thick. The notch antenna is shown in Figure 11. The notch antenna dimensions are 11 × 7.7 mm. The antenna center frequency is 12 GHz. The antenna bandwidth is around 100% for S11 lower than −5 dB, as presented in Figure 12. The notch antenna VSWR is better than 3:1 for more than 90% of the frequency range from 5.8 GHz to 18 GHz. The antenna beam width is around 84˚. The antenna gain is around 3.5 dBi. Figure 13 presents the radiation pattern of the wideband notch antenna with fractal structure at 8GHz. The antenna matching network was optimized to get better S11 results at 16 GHz to 18 GHz. The length and width of the stubs were tuned to get better S11 results at 16 GHz to 18 GHz.
6. Wideband Energy Harvesting Systems and Applications
As presented in Figure 1 the programmable array energy harvesting system consists of wideband programmable antenna network, a rectifying circuit, and a rechargeable battery. A rectifier is a circuit that converts electromagnetic energy, alternating current AC, to direct current (DC). Half wave rectifier or full wave rectifier are used to convert electromagnetic energy to DC electrical energy. A Half wave rectifier with a capacitor is presented in Figure 14. A half-wave rectifier conducts only during the positive voltage half cycle. It allows only one half of the electromagnetic waveform to pass through the load. The rectifier output DC voltage, VODC, is given in Equation (5). The rectifier output voltage may be improved by connecting a capacitor in shunt to the resistor. The time constant should be lower than T.
Figure 8. A ultra-wideband T shape printed harvesting slot antenna.
Figure 9. S11 of a wideband printed T Shape energy harvesting slot antenna.
Figure 10. Radiation pattern of a wideband wearable printed T shape slot antenna at 1.5 GHz.
Figure 11. A wideband 5.8 GHz to 18 GHz notch antenna with fractal structure.
Figure 12. A wideband 5.8 GHz to 18 GHz notch antenna with fractal structure, S11 results.
Figure 13. Radiation pattern of the wideband notch antenna with fractal structure at 8 GHz.
Figure 14. Half wave voltage rectifier with a capacitor.
where, . The half wave rectifier efficiency is 40.6%. Only 40.6% of the input AC power is converted into DC power. Where rf is the diode resistance is negligible as compared to R.
A Half wave bridge rectifier is shown in Figure 15. The bridge full wave rectifier circuit is used to convert AC energy to DC energy and for DC power suppliers. The bridge rectifier consists of four diodes D1 through D4, as shown in Figure 15. During the positive input half cycle, terminal A will be positive and terminal B will be negative. Diodes D1 and D2 will become forward biased and D3 and D4 will be reversed biased. The rectifier output DC voltage, , The rectifier output voltage may be improved by connecting a capacitor in shunt to the resistor as shown in Figure 15.
The half wave rectifier efficiency is 81.2% as presented in Equation (7). This means that only 81.2% of the input AC power is converted into DC power. The harvested voltage from the three antennas is VTotal as written in Equation (8). The total DC energy is PTotal as written in Equation (9). The actual harvested energy is P as written in Equation (10).
A Schottky diode may be used in the rectifier circuit. Schottky diodes are semiconductor diodes which have a low forward voltage drop and a very fast switching action. There is a small voltage drop across the diode terminals when current flows through the diode. The voltage drop of a Schottky diode is usually between 0.15 and 0.4 volts. This lower voltage drop provides better system efficiency and higher switching speed. A normal diode has a voltage drop between 0.6 to 1.7 volts. RF energy harvesting systems can be used to charge wearable devices. Figure 16 presents a wearable harvesting system with a wearable battery charger attached to a patient shirt. A wideband energy Harvesting panel with three antennas is presented in Figure 17.
The panel dimensions are 20 × 12 × 0.02 cm. This panel can harvest energy in frequencies from 150 MHz to 18 GHz as part of communication, medical, and IoT systems.
Figure 15. Full wave bridge voltage rectifier with a capacitor.
Figure 16. Medical wearable harvesting system.
Figure 17. Wideband RF energy harvesting panel.
Figure 18 presents a photo of a wideband metamaterial antenna with metallic strips, 150 MHz - 0.5 GHz. Figure 18(a) presents the antenna feed network and the metallic strips. Figure 18(b) presents the dual polarized metamaterial antenna with four SRRs. Figure 19 presents a photo of a wideband T shape slot antenna, 420 MHz - 6.4 GHz. These antennas can be used to harvest RF energy to charge wearable devices and sensors. The UHF dual polarized antenna can be attached to the patient stomach or back. The wideband T shape slot can be attached to the patient stomach or back. The wideband compact notch antenna can be also attached to the patient stomach or back. These antennas provide a wideband wearable communication system with a wideband RF harvesting system. Wearable Body Area Network with energy harvesting system for medical applications is presented in Figure 20.
A comparison of computed and measured results of compact wearable antennas for medical, 5G and IoT systems is listed in Table 4. Printed dipoles with and without SRR were presented in  .
Table 4. Comparison of electrical characteristics of energy harvesting antennas  .
Figure 18. Wideband Energy harvesting metamaterial UHF antenna. (a) The antenna feed network and metallic strips. (b) Dual polarized UHF energy harvesting metamaterial antenna with four SRRs.
Figure 19. A photo of wideband energy harvesting slot antenna, 400 MHz - 6 GHz.
Figure 20. Wearable Body Area Network with energy harvesting system for medical applications.
This paper presents a new Ultra-Wideband energy harvesting system and antennas in frequencies ranging from 0.15 GHz to 18 GHz. Three wideband antennas cover the frequency range from 0.15 GHz to 18 GHz. A wideband metamaterial antenna with metallic strips covers the frequency range from 0.15 GHz to 0.42 GHz. A wideband slot antenna covers the frequency range from 0.4 GHz to 6.4 GHz. A wideband fractal notch antenna covers the frequency range from 6 GHz to 18 GHz. The electromagnetic energy is converted to DC energy that may be employed to charge batteries, wearable medical devices, IoT, laptop batteries and commercial Body Area Networks, BANs. The harvesting energy system operates as a dual mode RF harvesting system. The harvesting unit can be part of a medical, IoT, computer, and smartphone. The notch and slot antennas were analyzed by using 3D full-wave software. Harvested power from RF transmitting links is usually lower than 0.1 µW/cm2. All antennas presented in this paper can operate also as active antennas. Active antennas may improve the energy harvesting system efficiency. The wideband RF energy harvesting system consists of wideband antenna, DC and control unit, a rectifying circuit, and a rechargeable battery. The harvesting energy system operates as a Dual Mode Energy harvesting system. The active devices DC bias voltages are supplied by the communication system. The wideband programmable energy harvesting panel can harvest energy in frequencies from 150 MHz to 18 GHz as part of communication, medical, and IoT systems. The antennas presented in this paper provide a wideband wearable communication system with a wideband RF harvesting system. There is a good agreement between computed and measured results.
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