Wideband Passive and Active Wearable Energy Harvesting Systems for Medical and IOT Applications

Demand for green energy is in continuous growth in the last years. Compact efficient antennas are crucial for energy harvesting portable systems. Small antennas have low efficiency. The efficiency of communication and energy harvesting systems may be improved by using efficient passive and active antennas. The system dynamic range may be improved by connecting amplifiers to the small antenna feed line. Novel passive and active portable harvesting systems are presented in this chapter. Printed patch, notch and Slot antennas are compact and have low volume. The active antennas may be employed in energy harvesting wearable systems. The antennas and the harvesting system components may be assembled on the same printed board. The printed notch and slot antennas bandwidth are from 40 to 100% for VSWR better than 3:1. The slot antenna gain is around 3 dBi with efficiency higher than 85%. The antennas ’ electrical parameters were computed in free space and near the human body. The active notch antenna gain is around 23 (cid:1) 3dB for frequencies ranging from 200 to 900 MHz. The active notch antenna gain is 13 (cid:1) 3 dB for frequencies ranging from 1 to 3 GHz. The active notch and slot antenna noise figure is 0.5 (cid:1) 0.3dB for frequencies ranging from 200 MHz to 3 GHz.


Introduction
In the last decade, the idea of employing free space energy in the forms of heat, light, vibration, electromagnetic waves, muscle motion, and other types of energy has become useful and attractive. A number of methods to produce electricity from these different types of energy sources have been developed [1][2][3]. Energy harvesting systems may eliminate the need to replace batteries everyday and the usage of power cords. In order to use as much free space energy as possible, it is important to collect the electromagnetic power from several wireless communication systems. In these cases, we should use wideband or multiband antennas. The energy harvesting antenna must satisfy several specific requirements related to the system application. Due to considerably low-power densities, highly efficient radiators are crucial. 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 [4][5][6]. Patch and slot antennas are widely used in communication and medical system [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. Wideband compact slot and notch antennas are good choice to function in wearable harvesting energy systems. Slot and notch antennas are compact and flexible and have low production cost. Moreover, a compact low-cost feed network may be achieved by integrating the amplifiers with the antennas on the same substrate. Printed wearable antennas are widely presented in the literature in the last decade as referred in [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. The human body effect on the electrical performance of wearable antennas at microwave frequencies is not always presented in the literature. Electrical properties of human tissues have been investigated in several papers such as [27,28]. Several wearable antennas have been presented in papers in the last decade as referred in [22,[28][29][30][31][32][33][34][35][36]. Wearable printed notch and slot antennas for harvesting energy applications are rarely presented in the literature. A new class of wideband passive and active wearable antennas for harvesting energy applications is presented in this chapter. The system efficiency and dynamic range may be improved by connecting amplifiers to the antenna feed line. The active antenna gain is around 23 dB, and the active antenna noise figure is 0.3 dB for frequencies from 200 to 600 MHz.

Energy harvesting systems
In RF energy harvesting systems, electromagnetic waves propagating in free space are captured, stored, and used to charge batteries and for other applications. There is a significant increase in the amount of electromagnetic energy in the air. The expected amount of radio wave in the air in 2013 was 1.5 exabytes per month. However, the expected amount of radio wave in the air in 2017 was 11 exabytes per month (see Table 1). Today we can do more computations per kWh as listed in Table 2. Energy sources used in harvesting systems are listed in Table 3. Wireless communication systems operate in the frequencies from 700 to 2700 MHz. Medical systems operate in the frequencies from 200 to 1200 MHz. WLAN systems operate in the frequencies from 5400 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 around 0.1 μW/cm 2 . Harvested power from RF energy sources in malls and stadiums may increase to around 1 mW/cm 2 . RF energy harvesting concept is shown in Figure 1.
The RF energy harvesting system consists of an antenna, a rectifying circuit, and a rechargeable battery. The harvesting energy system operates as a dual mode energy harvesting system. The low-noise amplifier is part of the receiving system. The LNA DC bias voltages are supplied by the receiving system. We can calculate the energy harvesting link budget by using Eqs.  Table 1. Amount of radio wave in free space.
Free-space loss (Lp) represents propagation loss in free space. Losses due to attenuation in atmosphere, La, should also be accounted for in the transmission equation, where, L p ¼ 4πR λ À Á 2 . The received power may be given as P r ¼ P t G t G r L p . Losses due to polarization mismatch, L pol , should also be accounted. Losses associate with receiving antenna, L ra , and with the receiver, L r , cannot be neglected in  Table 3. Energy sources used in harvesting systems. computation of transmission budget. Losses associate with the transmitting antenna as written as L ta .
where P t = transmitting antenna power; L t = loss between the power source and antenna; EIRP = effective isotropic radiated power.
The received power P r in dBm is given in Eq. (4). The received power P r is commonly referred to as "Carrier Power." Wireless smart phone using standard 802.11 can transmit up to 1 W. PCMCIA cards using standard 802.11 can transmit around 10 mW up to 100 mW.

Wideband notch antenna, 2-7.8 GHz, for energy harvesting applications
A compact notch antenna was printed on a dielectric substrate with dielectric constant of 2.2. The antenna dimensions are 116.4 Â 71.4 Â 1.2 mm, as presented in Figure 2. The antenna bandwidth for VSWR better than 2.5:1 is around 90-100% (see Figure 3). The notch antenna has VSWR better than 3:1 at frequencies from 2.1 to 7.8 GHz. The antenna beam width is around 80°at 5 GHz. The antenna gain is around 2.5 dBi at 3 GHz. The notch antenna radiation pattern at 3.5 GHz is presented in Figure 4. The electromagnetic energy is converted to DC energy by a rectifying circuit connected to the antenna input. A rechargeable battery is connected to the output of the rectifying circuit.

New wideband active 0.5-3 GHz energy harvesting notch antenna
Harvested power from RF transmitting links is usually lower than 0.1 μW/cm 2 . Active antennas may improve the energy.  Improve harvesting system efficiency. A wideband active notch antenna with fractal structure was printed on a 1.2 mm thick with dielectric constant of 2.2. The compact active notch antenna is shown in Figure 5. The notch antenna dimensions are 74.5 Â 57.1 mm. The antenna center frequency is 1.75 GHz. The active antenna bandwidth is around 150-200% for VSWR better than 3:1. The notch antenna gain, S21 parameter, is presented in Figure 6. The active antenna gain is 23 AE 3 dB for frequencies from 200 to 900 MHz.
The active notch antenna VSWR is better than 3:1 for frequencies from 0.5 to 3 GHz. The antenna beam width is around 84°at 1 GHz. A compact E-PHEMT LNA, low-noise amplifier, is connected to the notch antenna via an input matching network. An output matching network connects the amplifier port to the rectifying circuit. A printed compact DC voltage bias network supplies the bias voltages to the harvesting system. The amplifier specification is listed in Table 4. The amplifier   Table 7.
The active antenna gain is 12 AE 3 dB for frequencies from 1 to 3 GHz. The active notch antenna noise figure is 0.5 AE 0.3 dB for frequencies ranging from 300 to 3.0 GHz as presented in Figure 7. The active notch antenna output VSWR is better than 3:1 for frequencies from 0.5 to 3 GHz. All antennas presented in this paper can operate as passive and active antennas.

New fractal active 0.4-3 GHz energy harvesting antenna
A compact notch antenna with fractal structure is shown in Figure 8. The antenna is printed on a 5580 Duroid substrate, 1.2 mm thick, with dielectric constant of 2.2. The notch antenna dimensions are 52.2 Â 36.8 Â 1.2 mm. The antenna center frequency is 1.7 GHz. The antenna bandwidth is around 100% for VSWR better than 3:1. The active notch antenna VSWR is better than 3:1 for frequencies from 0.4 to 3 GHz. The antenna beam width is around 82°at 1 GHz. An LNA is connected to the antenna feed line. The antenna is connected to the LNA via an input matching network. An output matching network connects the amplifier output port to a rectifying circuit. A compact DC network supplies the required voltages to the active antenna. The amplifier specification, S parameters, and the F-GHz S12 S12°S22 S22°0    amplifier noise parameters are listed in Tables 4-7. The active antenna gain is 21 AE 3 dB for frequencies from 400 MHz to 1.3 GHz, as presented in Figure 9. The active antenna gain is 12 AE 3 dB for frequencies from 1.3 to 3 GHz. The active notch antenna noise figure is 0.5 AE 0.3 dB for frequencies from 300 MHz to 3.0 GHz, as presented in Figure 10. The notch antenna output VSWR is better than 3:1 for frequencies from 0.5 to 3 GHz.

New wideband active 0.8-5.4 GHz energy harvesting slot antenna
A wideband T shape wearable slot antenna for energy harvesting applications is shown in Figure 11. The antenna electrical parameters were computed by using momentum software [38]. The volume of the T-shape slot antenna is 7 Â 7 Â 0.12 cm. The slot antenna center frequency is around 3 GHz. The computed S11 parameters  are presented in Figure 12. The antenna bandwidth is around 100% for VSWR better than 3:1. The antenna beamwidth is around 138°at 1 GHz as shown in Figure 13. The antenna gain is around 2.5 dBi. The antenna was designed also as an active antenna as shown in Figure 14. The slot antenna is connected to the LNA via an input matching network. The output matching network connects the amplifier output port to a rectifying circuit. For example, a MMIC LNA with 16 dB gain and 1 dB noise figure has DC power consumption of less than 18 mW in the frequency range from 70 MHz to 1 GHz. The system DC bias network supply the required voltages to the energy harvesting system. The amplifier specification is listed in Table 5. The amplifier complex S parameters are listed in Tables 5 and 6. The active slot antenna gain, S21 parameter, is presented in Figure 15. The active antenna gain is 23 AE 3 dB for frequencies from 200 to 900 MHz. The active antenna gain is 13 AE 3 dB for frequencies from 1 to 3 GHz. The active slot antenna noise  figure is 0.5 AE 0.3 dB for frequencies from 200 MHz to 3.0 GHz. The computed S11 parameters of the T-shape slot on human body are presented in Figure 16. The dielectric constant of human body tissue was taken as 45. The antenna was attached to a shirt with dielectric constant of 2.21 mm thick.

Energy harvesting systems for medical and IOT applications
The notch and slot antennas' electrical performance near the human body was investigated by using the model shown in Figure 17. Properties of human body tissues are listed in Table 8 [27,28]. These properties were employed in the antenna design. Up to four energy harvesting antennas may be assembled in a belt and attached to the patient body as presented in Figure 18. The bias voltage to the active elements is supplied by a compact recorder battery. The DC cables from each harvesting antenna are connected to a rechargeable battery. The electromagnetic energy is converted to DC energy that may be employed to charge medical or commercial body area networks (BANs).

Energy harvesting system
As presented in Figure 1, the energy harvesting system consists of an antenna, 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 may be used to convert electromagnetic AC energy to DC electrical energy. A half-wave rectifier is presented in Figure 19. A half-wave rectifier conducts only during the positive half cycle. It allows only one half of an AC waveform to pass through the load. The rectifier output DC voltage, V ODC , is  Table 8 Electrical properties of human body tissues. given in Eq. 5. The rectifier output voltage may be improved by connecting a capacitor in shunt to the resistor as presented in Figure 20.
The improved half wave rectifier is shown in Figure 20.
The time constant τ should be lower than T, where, τ ¼ RC ≪ T. The half-wave rectifier efficiency is 40.6% as given in Eq. 7. Only 40.6% of the input AC power is converted into DC power. Where rf the diode resistance is negligible as compared to R. Figure 19.
The bridge full-wave rectifier circuit is used usually for DC power supplies. It consists of four diodes, D1-D4, as shown in in Figure 21, connected to form a bridge. 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, V ODC ¼ 2V m =π, may be improved by connecting a capacitor in the shunt to the resistor. The improved halfwave rectifier is presented in Figure 22.
The half-wave rectifier efficiency is 81.2% as presented in Eq. 8. This means that only 81.2% of the input AC power is converted into DC power.
A capacitor is used in the improved rectifier to get flat output voltage variation as function of time. The capacitor may be a voltage-controlled varactor diode. Varactors are voltage variable capacitors designed to provide electronic tuning of electrical devices. The output voltage ripple (see Eq. 6) of the improved rectifier may be tuned as function of the frequency of the received signal or of the load resistance R.
A Schottky diode may be used in the rectifier circuit. Schottky diodes are semiconductor diodes which has 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 V. This lower voltage drop provides better system efficiency and higher switching speed. A normal diode has a voltage drop between 0.6 and 1.7 V. Electrical characteristics of Schottky diode and standard PN diodes are listed in Table 9. Typical I-V curves of commercial Schottky diodes are shown in Figure 23.

Conclusion
This paper presents new ultra-wideband wearable passive and active energy harvesting systems and antennas in frequencies ranging from 0.4 to 8 GHz. The antennas are inserted in a belt and attached to the body. The antennas are compact and can be attached to the body. The antennas allow the patients' easy movement (running, jumping, and working).
The electromagnetic energy is converted to DC energy that may be employed to charge batteries, wearable medical devices, and commercial body area networks. The passive and active 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/ cm 2 . Active antennas may improve the energy harvesting system efficiency. All antennas presented in this paper can operate also as passive antennas. The active notch and slot antenna bandwidth are from 50 to 100% with VSWR better than 3:1. The slot antenna gain is around 3 dBi with efficiency higher than 90%. The antenna electrical parameters were computed near the human body. The active slot antenna gain is 24 AE 2.5 dB for frequencies ranging from 200 to 900 MHz. The active slot antenna gain is 13 AE 3dB for frequencies from 1 to 3.3 GHz. The active wearable antennas may be used in energy harvesting systems for wireless communication and medical applications. The RF energy harvesting system consists of an antenna, a  rectifying circuit, and a rechargeable battery. The harvesting energy system operates as a dual mode energy harvesting system. The low-noise amplifier is part of the receiving system. The LNA DC bias voltages are supplied by the receiving system.