A 2.45 GHz FSS Loaded Rectifying Antenna

In this paper the development of a rectenna system has been presented for 2.45 GHz wireless power transfer application. The receiving element of the rectenna (or the antenna) has been designed to possess spurious free response at least up to 10 GHz to improve the RF-DC conversion eciency. It was found that the gain of the antenna is not sucient for rectenna application. Therefore, to improve the gain of the antenna, it has been loaded with an angle and polarization insensitive FSS. The FSS loaded antenna achieved 7.7 dB gain, 85% radiation eciency, and single operating band at 2.45 GHz; which is suitable for developing a rectenna for wireless power transfer. To convert the received RF energy into DC voltage a 2.45 GHz matched rectier circuit has been designed. L-type matching network has been used to match the complex rectier impedance with the 50 Ω antenna impedance. 1.52 V output voltage was obtained for 7 dBm input power and 3 k Ω load. Achieved maximum eciency is 66.13% for 1.1 mW received power. It has been shown that the FSS loading of the antenna has the capacity of drastically improve the eciency of the rectenna system.


Introduction
Rectifying antenna, or rectenna, generally consists of antenna, lter, matching circuit, recti er, DC pass lter, and load [1]. The recti er can produce unintended frequencies, which may couple to the antenna and cause interference with nearby RF devices, distortion of received signal, and reduction of e ciency.
Therefore, a lter is used [2]. But it increases size and insertion loss. Therefore, spurious free antennas are preferred. High gain is also required for receiving low power.
Suppression of spurious bands and simultaneously increasing antenna gain is an existing challenge.
Rectenna design was rst proposed by Brown [15]. He designed a rectenna at 2.45 GHz. He also developed a 35 GHz rectenna with 33% conversion e ciency (CE) [16]. Hong [17] reported a rectifying antenna using a nite ground CPW circuit which had 68.5% CE for 270 Ω load. Riviere Fig. 1(a)). 3 kΩ load is considered. Complex input impedance of the recti er is plotted in Fig. 1 Therefore, η max remains almost constant with load variation. E ciency and output voltage (V DC ) of the matched recti er have been plotted with frequency in Fig. 3(b) for R L = 3 kΩ and P in = 3 dBm. It reveals maximum E ciency of 73.03% at 2.45 GHz. Corresponding V DC is 1.7 V.
The recti er has been fabricated on a Rogers Duroid 5880 substrate of height 1.575 mm, tan(δ) = 0.0009, ε r = 2.2, and copper thickness 0.035 mm, as depicted in Fig. 4(a). Figure 4(b) depicts details of fabricated recti er circuit. The return loss measurement has carried out using a calibrated Keysight N5221A PNA. The measured frequency responses are shown in Fig. 5(a). The recti er was measured to nd the variation of V DC and η with input power P in . The 2.45 GHz frequency was generated using an Agilent E8257D signal generator and the load was set to 2.7 KΩ (nearest commercially available resistor of 3 kΩ). V DC was measured using a volt meter. E ciency was calculated for various P in levels using Eq. (1) and η max was found to be 53.03% at 3 dBm input power. Corresponding V DC is 1.69 V. Measured V DC and η plots are shown in Fig. 5(b). The difference between the simulated and measured e ciency is due to variations in diode parameters, parasitic effects of solder joints, and connecting wires that were not considered during simulation.

Rectenna Design Using Circular Shaped Antenna
In order to develop the rectenna the spurious free 2.45 GHz antenna, reported in [20], is considered. The schematic diagram of the antenna is shown in Fig. 6. The rectangular ring slot on ground is used to suppress the spurious bands. The antenna is printed on a FR4 substrate of thickness 1.6 mm, area 30 35 mm 2 , ε r = 4.4, and tan(δ) = 0.02. The rectenna circuit is shown in Fig. 7. The rectenna has been measured to nd output voltage (V o ) and e ciency variations with input/received powers in an anechoic chamber. Agilent E8257D signal generator was used as the 2.45 GHz signal source. The signal was ampli ed by a mini circuit ZX60-14012L RF ampli er (gain 11.9 dB) and fed to a horn antenna (gain 8 dBi at 2.45 GHz) using a cable (cable loss 3 dBm). The circular shaped antenna was used as the receiving element, which was kept at a far eld distance (60 cm). Gain of the receiving antenna is 2.3 dBi at 2.45 GHz. Output voltages of the rectenna were measured by a volt meter across a 2.7 kΩ load for different transmitter powers (P in ) and measured data are plotted in Fig. 8(a). It reveals that V o increases with P in and the maximum output voltage of the recti er is 190 mV for 10 dBm transmitter power. E ciency of the rectenna have been plotted with incident/received power (P r ) in Fig. 8

Rectenna Design Using Fss Loaded Antenna
In this section, an attempt has been made to increase rectenna e ciency by loading it with a frequency selective surface (FSS). The concept is to design an FSS that is highly re ective at 2.45 GHz and place it at an optimized distance below the ground so that the backscattered eld can be re ected by it and add with the broadside eld at the same phase to increase the broadside gain. The increased gain will increase received RF voltage at the base of the antenna and hence e ciency of rectenna system.

A. Design of Frequency Selective Surface
The unit cell of the proposed FSS is shown in Fig. 9(a). Size of the unit cell is 12.9×12.9 mm 2 (0.105λ 0 × 0.105λ 0 , where λ 0 is free space wavelength at 2.45 GHz). Optimized dimensions of the unit cell are, d 1 = 10.2 mm, d 2 = 5.6 mm, and G = 0.65 mm. The FSS unit cell was simulated using CST Microwave studio (Version 14) with periodic boundary conditions ( Fig. 9(b)) and incident EM wave from z-direction. For unit cell simulation the required boundaries are PEC on yz-planes and PMC on xz-plane. Wave ports are used on the plane of FSS (xy-plane). The nal FSS structure is a 7×7 element periodic array of the unit cell and has dimensions 90×90 mm 2 . 1.6 mm thick FR4 substrate with copper thickness 0.035 mm and ε r = 4.4 is used to design the FSS. The FSS is four-fold symmetric and exhibits similar stop band response for TE and TM polarizations, as shown in Fig. 9(c). Transmission coe cient of the FSS has been simulated for TE and TM polarizations and are plotted in Fig. 10(a) and Fig. 10(b), respectively. They show angular insensitive performance of the FSS for both the polarizations. It is observed that both resonances are insensitive up to 70 0 incidence angle with a maximum frequency shift of 7.9 % only. × Equivalent circuit of the FSS is shown in Fig. 11(a). Values of the lumped elements can be found using [27] equations (2) and (3) and simulated |S 11 | response of the unit cell in Fig. 11(b). In the equivalent circuit, inductances represent metallic strips whereas the capacitances represent gap of the structure.
Transmission line with characteristic impedance Z 0 represents the free space.
where ω 0 is angular resonance frequency, B 3dB is the 3 dB bandwidth, S 11 (jω 0 ) is the value of S 11 at ω 0 , and η 0 is the free space impedance. The values of the lumped elements can be found as L 1 = 0.47651 nH, C 1 = 8.8559 pF, L 2 = 0.11989 nH, and C 2 = 2.2134 pF. The simulated |S 11 | responses of the unit cell and of its equivalent circuit are also plotted in Fig. 11(b) for comparison purpose.

B. FSS Loaded Circular Shaped Antenna
The FSS is placed below the antenna using four Bakelite rods, as shown in Fig. 12(a). Distance between the FSS and the antenna was determined using parametric analysis, shown in Fig. 12(b). It reveals that as H decreases from 35 mm to 20 mm, the antenna gain response improves till H = 25 mm and after that it deteriorates. Therefore, optimized distance 'H' is considered as 25 mm (or 0.204λ 0 ). Radiation e ciencies of the antenna, with and without FSS loading, are plotted in Fig. 12(c) and is found to be 80.5% for unloaded antenna and 85% for loaded antenna. Comparison of the |S 11 | responses of the antenna with and without the FSS layer is shown in Fig. 13(a). It reveals that the loading of the FSS layer has minor effect on the |S 11 | response. The 10 dB RL bandwidth of the antenna covers 2.34-2.66 GHz. Sparameter and far eld measurements of the antenna are done using a calibrated Keysight N5221A VNA. Comparison of the simulated and measured |S 11 | responses is provided in Fig. 13(a). They are in close agreement. The slight discrepancies at the higher frequencies are due to the parasitic effects of soldier joint of connector with antenna. Figure 13(a) reveals a 10 dB RL bandwidth of 2.20-2.51 GHz (around 6.58% on each side of the center frequency). Comparison of the gains of the antennas (with and without FSS layer) are provided in Fig. 13(b). It shows that within the 10 dB RL bandwidth, the gain of the FSS loaded antenna remains almost constant around 6 dBi with a maximum of 7.7 dBi at 2.45 GHz. It also reveals that the antenna gain is almost 3.34 times higher than that of antenna without FSS layer.
The normalized radiation patterns of the antenna, with and without FSS loading, on both the orthogonal planes at 2.45 GHz are provided in Fig. 14. Suppression of the back lobe is observed, as expected.
Simulated half-power beamwidths at xz and yz planes are found to be 84 0 and 68 0 , respectively. It also reveals that the cross-pol is more than 35 dB down than the co-pol at the broadside direction on both the orthogonal planes.

C. Development of the rectenna circuit
The fabricated rectenna is shown in Fig. 15. E ciency of the fabricated rectenna can be calculated using the relation 4 where R L is the load, P r is the power received by the receiving antenna, and V dc is the output DC voltage across load. The received power P r can be calculated using the relation 5 where P t is the power at the input of the transmitting antenna, G t is the transmitting antenna gain, G r is the receiving antenna gain, λ is the free-space wavelength at 2.45 GHz, and R is the distance between the transmitting and receiving antenna.
The rectenna have been measured in an anechoic chamber to nd the output voltage and e ciency variations with the transmitter (p in )/received (P r ) powers. Output voltages of the rectenna was measured by a volt meter across a load resistance 2.7 kΩ for different transmitter powers and measured data are plotted in Fig. 16(a). It reveals that output voltage of the rectenna increases with transmitter power till 7 dBm (output power of the signal generator) and after that it saturates. It further reveals that maximum output voltage of the rectenna is 1.52 V. The e ciencies of the rectenna have been calculated using Eqs. (4) and (5) and are plotted with received powers in Fig. 16(b). It reveals that the maximum e ciency of the rectenna is 66.13% for 1.1 mW received power (incident power at the antenna). This corresponds to a signi cant improvement in e ciency of the rectenna presented in section III, which has maximum e ciency of only 1.24 %.
Characteristics of the proposed rectifying antenna is compared with few other rectennas, available in literature, in Table I. It reveals that the proposed rectifying antenna has a high conversion e ciency at low input power than others.

Conclusion
This paper presents the development of a 2.45 GHz rectenna for WPT application. Initially a matched recti er was designed at 2.45 GHz using a L-section matching network. The output voltage of the recti er at load was found to be 1.69 V. The maximum e ciency of the recti er was measured to be 53.03% for an input power 3 dBm. Next, the recti er was connected with a circular shaped antenna of gain 2.3 dBi to form the rectenna. The rectenna showed only 1.24% maximum e ciency. Since this e ciency is not acceptable for WPT application, next, an attempt was made to increase the rectenna e ciency by increasing the gain of the antenna. To accomplish this a 2.45 GHz re ector type FSS structure was developed and placed below the ground of the antenna at an optimized distance. The FSS re ected the backscattered wave which was then added with the radiated eld in broadside direction in phase to result in gain enhancement. The FSS back antenna has 7.7 dBi gain and 85% radiation e ciency. The antenna was connected with the recti er to form a new rectenna. The developed rectenna provided an output voltage of 1.52 V across the load resistor at 7 dBm input power (output power of the signal generator). The maximum achieved conversion e ciency was found to be 66.13%, which is much higher than the 1.24% maximum e ciency achieved earlier.