Leaf-shaped solar cell antenna for Energy Harvesting and RF Transmission in ku-band

A R T I C L E I N F O A B S T R A C T Article history: Received: 31 August, 2017 Accepted: 13 November, 2017 Online: 10 December, 2017 In this paper, a hybrid system solar cell antenna was studied. Our contribution lies in the combination of both optical and radio-frequency signals in a solar cell antenna. Hence, we propose a new method to merge antenna and solar cell. The leafshaped patch solar cell antenna was dedicated at a time to the energy harvesting and the RF transmission. A performance analysis of the solar cell antenna was conducted using Advanced Design System (ADS) software. Simulation results showed a resonance at a frequency of 15.77 GHz with an effective return loss of -27.62dB and a gain of 5.77dBi.

In this paper, we describe the solar cell antenna in the second section; we detail the proposed approach as well as the effects of optical transparency on the antenna parameters. In the third section we explain the proposed solar cell antenna structure by giving a model of the mesh patch with optimized geometry. The simulations results of the solar cell antenna are presented. A solar cell antenna array has also been studied in this section by giving their parameters. The solar cell antenna equivalent circuit as well as the RF / DC decoupling circuit have been studied in order to eliminate the influence of the RF signal on the DC signal and to show that this hybrid system will be both for the recovery of a solar cell, energy and RF transmission. Finally, we conclude the paper.

Approach
Several attempts to integrate solar cells with patch antennas have been studied, but they did not pay attention to the antenna patch when it is placed above a solar cell. To remedy the drawbacks, especially of limited surface area, encountered during the integration of patch antenna with a photovoltaic cell, we ASTESJ ISSN: 2415-6698 propose here new types of antennas based on optically transparent photovoltaic cells with mesh patches.
Optically transparent mesh patch antennas are antennas that have a certain level of optical transparency. These transparent antennas, for example, are potentially suitable for integration with solar cells for small satellites. Traditional patch antennas used on small satellites compete with solar cells for the surface. However, a mesh patch antenna can be placed directly on the solar cells and solve the surface limitation problem. For such integration, a high optical transparency of the patch antenna is required from the solar cells point of view, since solar cells require sufficient sunlight to generate adequate electrical power. On the other hand, the antenna must have at least acceptable electrical properties at the same time so that it can radiate correctly and efficiently [7][8] and [9][10][11].
The hybrid system studied in this paper was devoted at a time to the energy harvesting and the RF transmission for satellite communication. This structure was made of a photovoltaic cell in which the front grid was designed to have a miniature antenna suited to the band transmission and to minimize the power loss of the cell devoted to energy conversion. The received electrical energy was used to operate the complete system.

Optical transparency effects on the antenna parameters
The openings in the conductor make it possible to pass the light while the mesh can still be conceived as an efficient radiator. Mesh patch antennas, compared to their solid counterparts, have a higher surface resistance due to much lower conductor coverage. Consequently, it is normal, when the optical transparency increases, to have a low performance of the antenna.
The antenna optical transparency is given most often by the ratio of the mesh patch surface to the patch total area (eq.1). 1 .100% It has been demonstrated that, when the line width is fixed, we notice that during the patch transparency increase the frequency of resonance decreases. Since t the patch physical length is the main determinant of the resonance frequency as noted by Clasen [7], the increase in transparency introduces the possibility of minimizing the antenna design. Using the mesh patch increases the fringing fields; therefore, increase the antenna electrical length [12]. Also from Figure 1 one is able to see that increasing the transparency decreases the bandwidth. Bandwidth is defined by the following equation: With fh the highest frequency at which the voltage standing wave ratio (VSWR) is below 2, fl the lowest frequency at which the VSWR is below 2, and fr is the frequency of the minimum VSWR.
Also as seen in Figures.2, 3 and 4, the increase in transparency decreases the antenna efficiency, gain and directivity.

Effect of the line width
The study of optically transparent antennas is based essentially on the optimization of the mesh patch dimensions. In this paper, antennas with an optical transparency of 70%, 80% and 90% are generated. Each transparency depended of the line width and the number of mesh lines. Simulation results present the efficiency of the mesh antenna as a function of the line width for each transparency is plotted in figure 5. We interpret that for a given transparency, efficiency of a mesh antenna can be improved by refining the line width. From fabrication standpoint of view, the line width realization of 0.1 mm or more is simple. The efficiency and transparency are adequate for this hybrid system of solar cell antenna for satellite and terrestrial communications. From the simulation results (see Figure 5), we conclude that with a line width of 0.1 mm, it is possible to achieve more than 60% efficiency with an antenna of 90% transparency, 63% efficiency with an antenna of 80% transparency and 67% efficiency with an antenna of 70% transparency. Efficiency is inversely proportional with optical transparency, this is true because traditional patch antennas are more efficient but when they are integrated with solar cells they screen these and can reduce the collected electrical power. Our method allows us to propose a hybrid solar cell antenna system for energy harvesting and RF transmission for satellite and terrestrial communications.

Structure of solar cell antenna
We propose a solar cell antenna with mesh patch. A mathematical model that we have already studied to minimize the power losses of the solar cell antenna and improve the conversion efficiency as a solar cell [13][14]. Optimization of the maximum electrical power collected as a function of finger width was determined. The optimal width Wf has been used for the mesh patch design or front face collection grid.
Improving the performance of a solar cell depends not only on the materials and structure but also on the design of the metal grid front face.
With this mesh patch structure two types of waves can exist. The optical waves that will be absorbed by the silicon (semiconductor) and the RF waves that will be collected by the mesh patch metal whose width of fingers Wf is optimized. The structure designed was a solar cell antenna printed on a multilayered substrate Figure 6. The silicon on an insulating layer SiO2 confers to the components that are realized, a higher operating frequency, an ability to operate at low voltage and low power consumption and an insensitivity to the effects of ionizing radiation. When using a thin-film substrate we observe that the simulations are complex and time consuming. The meshed patch of solar cell antenna proposed is given in Figure 7. The dimensions of the proposed patch of solar cell antenna are given in the following table (Table  1).  .
where Wf is the mesh line width, Lf length of each mesh line, Wb is the busbars line width, Lb the busbars length, W is the InsetFeed width, L is the InsetFeed length, Wpatch is the patch width and Lpatch is the patch length (see Figure7).

Antenna Parameters
As an antenna, the electromagnetic performances of the solar cell antenna were simulated by using the ADS software. The simulated S11 parameter was presented in Figure 8. An antenna should be perfect radiator, rather than perfect absorber. The amount of radiated power returned back through the port can be calculated for finding return loss at that resonating frequency. For the resonant frequencies the return loss should be less than -10dB i.e. S11<-10 dB. Simulation results show that the designed antenna can be used as a frequency antenna with an effective return loss of -27.62dB at 15.77GHz. To the normal scale $S11=10-2.762, ie only 0.17% of the emitted power will be reflected as well as the impedance Za=49.82 Ω which confirms the adaptation of the antenna. The adaptation of our antenna is substantiated by the return loss low value S11. Similarly, its operation can be assessed by the VSWR curve which is another performance measurement parameter at the input. It indicates the signal reflection due to a mismatch between the impedance of the antenna and that of the source. The reflected signal leads to the appearance of the standing wave voltage in the feed line which, in turn, destroys the reliability of the antenna. The ideal value of VSWR is 1 which means that 100% of the delivered power is accepted (no reflection). But, practically speaking, it is tolerable up to 2 [15]. The VSWR obtained for this antenna equal to 1.0036. Figure 8: Reflection coefficient S11 The radiation of our antenna is gathered in the main lobes which have a wide beam width at -3 dB and whose amplitude is much higher than that of the side lobes. This reflects a very directive radiation by offering a total directivity equal to 7.26 dB at 15.77 GHz. The radiation pattern of this antenna at a frequency of 15.77GHz is shown in Figure 9. The polarization of the radiated field was linear. When designing an antenna, the gain must be taken into consideration as it is an important metric. The good values of S11 and VSWR are not enough to confirm a good radiation. Antenna gain describes how much power is transmitted in the direction of peak radiation to that of an isotropic source. The gain of this antenna was 5.77dBi.

Array of antennas
In order to increase the gain, we proposed an array of antennas shown in Figure 10. Simulation results of S11 parameter and radiation pattern are given, respectively, in Figures 11 and 12. This array of antennas was excited by a microstrip line of La=16.82 mm in length and Wa=0.228 mm in width and of characteristic impedance equal to 50 Ω. S11 parameter shown in Figure 11 of a dual resonant frequency at 15.72GHz and 15.77 GHz with an effective return loss respectively of -20.18 dB and -22.81dB. Figure 11: Reflection parameter S11 of array antennas The radiation pattern of this array of antennas is shown in figure 12. It is gathered in the main four lobes. The gain at the resonance frequency of 15.77GHz is 9.76 dB and the directivity is 11.24 dB. The designed array antennas are showing excellent gain and directivity at the resonating frequencies with high radiation efficiency.

Equivalent circuit of solar cell antenna
Solar cell equivalent circuit is specially simulated in Advanced Design System for measuring power (P), current (I) and voltage (V). Microstrip antenna equivalent circuit is separately simulated and the same output is measured. Finally both microstrip and solar cell circuits are combined and output is measured. While comparing both outputs, combination of microstrip and solar cell equivalent circuit produces high voltage (V), current (I) and power (P). The simulation of SOLAN in ADS provides the following values of current (I) =872.8mA Voltage (V) =8.728mV and Power (P) =7.6177984mW were measured as shown in Figure 13.

Decoupling RF/DC in solar cell antenna proposed
Since the solar cell has a DC circuit, the direct current path must be decoupled from the RF signal path in such a way that the DC load has no influence on the RF properties of the antenna. The decoupling can be realized by means of concentrated reactive elements and distributed elements, respectively. In this case, we propose an inductive and capacitive RF/DC decoupling circuit shown in Figure 14. The decoupling circuit was simulated (see Figure 15) and values are compared to the S11 parameter shown in Figure 8. The simulation of the reflection parameter S11 of decoupling circuit provides the same result of resonant frequency of the proposed solar cell antenna in Layout with a slight increase of return loss.
After RF/DC decoupling, this solar cell antenna can meet the purpose for which it is being studied energy harvesting for the relevant telecommunication system power supply and RF transmission. Based on the simulations results obtained, the solar cell antenna proposed here can be used for ku band satellite communication. Figure 16 explains well the operation of this solar cell antenna.

Conclusion
In this paper, we presented a solar cell antenna with leafshaped patch. This optically transparent antenna is particularly well adapted for energy harvesting and for the RF transmission in Ku band with a large gain. Good performances are obtained for this solar cell antenna, as a PV cell with its collected electrical power and as an antenna with its gain 5.77dBi and 9.76 dBi of array of antennas, directivity and efficiency. We conclude that the hybridized method where the patch element of a patch antenna has been replaced by a solar cell is more efficient than a simple integration where the patch antenna was placed directly on the solar cell is what which limited the available area of the cell and consequently a loss of electrical energy collected.