Abstract
Circularly polarized (CP) transparent microstrip reflectarray antenna is integrated with solar cell for small satellite applications at 10 GHz. The reflectarray unit cell consists of a perfect electric conductor (PEC) square patch printed on an optically transparent substrate with the PEC ground plane. A comparison between using transparent conducting polymers and using the PEC in unit-cell construction has been introduced. The waveguide simulator is used to calculate the required compensation phase of each unit cell in the reflectarray. The radiation characteristics of 13 × 13 CP transparent reflectarray antenna are investigated. A circularly polarized horn antenna is used to feed the reflectarray. The solar cell is incorporated with the transparent reflectarray on the same area. The solar-cell integration with the reflectarray reduces the maximum gain by about 0.5 dB due to the increase in the magnitude of the reflection coefficient. The results are calculated using the finite integral technique (FIT).
1. Introduction
Recently, high-gain reconfigurable microwave antennas have emerged in many applications as radar and satellite communications [1]. The reflectarray has several advantages such as low profile, light weight, less cost, and the fact that it can work at millimeter-wave frequencies without suffering from insertion losses associated with the phased array. The disadvantage of the reflectarray is narrow bandwidth, which generally cannot exceed much beyond 10%, depending on its element design and its aperture size, such as the microstrip patch element [2–4]. Various techniques are employed to enhance the reflectarray bandwidth by using thick substrate for the patch, stacking multiple patches [5, 6], and aperture coupled microstrip patches [7]. A reflectarray antenna combines some of the best characteristics of the parabolic reflector antennas and phased array antennas [8]. The reflectarray consists of a planar surface of many units cells with perfect electrical ground plane (PEC) illuminated by a primary feeding source, usually horn antenna [9]. The feed signal is reflected from the reflectarray structure, transforming the spherical feed wavefront to a plane wave. Microstrip reflectarrays are very attractive aperture antennas because of their planar structure and a simple feed system [10, 11].
Recently, transparent conducting polymers have been used instead of copper in microstrip patch antenna fabrications. Transparent conducting films (TCFs) are suitable to be implemented with clear substrates for emerging applications such as security, aesthetics [12, 13], and vehicles [14] or can be integrated with solar cells to save surface area of small satellites [15]. TCFs allow the transmission of electric currents and keep back the optical transparency [16]. There are three types of TCFs used by researchers for antenna developments, namely, indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and silver coated polyester films (AgHT) [17].
By employing the integration technologies, the antennas are used for data communication and the solar cells are used for energy harvesting, saving the available surface area of emerging applications [18–24]. Integrating both the antennas and solar cells yields significant area savings and improves the economic viability of the renewable energy. In open literature, there are three types of antenna integration techniques that have been reported. The first type is to place a patch microstrip antenna under solar cells [10]. The second type is to use slot patch antennas and deposit solar cells directly on top of them [11, 25]. The third type is to integrate antennas that are transparent to light immediately on top of commercial solar cells [8].
In this paper, a design of 13 × 13 circularly polarized transparent microstrip patch reflectarray antenna is investigated. The reflectarray antenna consists of 169 unit cells; each is a square patch printed on an optically transparent dielectric substrate and a PEC ground plane. A comparison between the radiation characteristics of the reflectarray consists of PEC microstrip patches and TCFs patch has been investigated. An integration of the solar cell with the microstrip reflectarray antenna for small satellite applications has been presented. The finite integral technique (FIT) [26] is employed to investigate the radiation characteristics of the optically transparent reflectarray integrated with the solar cell.
2. Numerical Results
2.1. Unit-Cell Design
The detailed construction of the reflectarray unit cell is shown in Figure 1. The unit cell is a PEC square patch with arm length, , printed on a square optically transparent dielectric substrate with dielectric constant , and dimensions 14.6 × 14.6 × 1.95 mm3. A square PEC ground plane is printed on the back side of the dielectric substrate. The unit cell is designed to operate at 10 GHz. To calculate the required reflection coefficient compensation phase shift, the unit cell is placed in a waveguide simulator [18]. The waveguide simulator is used to simulate an infinite array approximation by applying perfect electric and magnetic boundary conditions to the sides of the waveguide (i.e., image theory). A linearly polarized plane wave is used to normally excite the unit cell. The infinite array approach has several limitations. In the first approach, all elements of the reflectarray are identical, which is not the case in the real reflectarray. Secondly, the reflectarray itself is not infinite in extent. Figure 2(a) shows the variation of the reflection coefficient magnitude and phase versus the PEC patch arm’s length, , at 10 GHz. The reflection coefficient magnitude achieves nearly 0 dB due to the high conductivity of the PEC patch and ground plane. The phase of the reflection coefficient varies from 0 to 300° for patch arm length varying from 1 mm to 7 mm. In order to improve the optical transparency of the reflectarray unit cell, an optically transparent conducting polymer TCF with σ = 5 × 105 S/m is used instead of the PEC patch and the ground plane. The variation of the reflection coefficient magnitude and phase with the patch arm length at 10 GHz is shown in Figure 2(b). The same behavior of the transparent conducting polymer and the PEC unit cell is achieved.
(a) 3D view
(b) Top view
(a) The PEC unit cell
(b) The optically transparent unit cell
2.2. Theory of Reflectarray
Consider a 2D array placed in the - plane and is illuminated by a feed horn located at (, , ) from the array aperture as shown in Figure 3(a). The required compensation phase shift for each unit cell in the reflectarray is obtained aswherewhere is the propagation constant in free space, (, ) are the coordinates of reflectarray unit cell, and (, ) is the desired direction of the main beam. Figure 3 shows the geometry of a 13 × 13 unit-cell reflectarray antenna covering an area of 18.98 × 18.98 cm2 and placed on the - plane at 10 GHz. A circularly polarized (CP) horn antenna is used to feed the reflectarray located at a distance = 18.98 cm from the array aperture. The horn has diameter = 40 mm and length = 40 mm and is fed via two orthogonal coaxial probes with 90° phase shift to produce CP field. The left- and right-hand CP radiation patterns, components of the horn antenna at 10 GHz in E-plane and H-plane, are shown in Figure 4. The horn antenna introduces a maximum gain of 12 dB with side lobe level (SLL) of −22 dBi in E- and H-planes.
(a) 3D view
(b) Top view
(a) E-plane
(b) H-plane
A 13 × 13 CP microstrip reflectarray antenna is designed using the PEC patch and compared to that designed using the TCFs. The radiation characteristics of both reflectarrays are calculated using a full-wave simulator CST-Microwave Studio based on the FIT. The solution technique takes into consideration the effect of the mutual coupling between the array elements. The radiation pattern for the PEC microstrip reflectarray and the TCFs microstrip reflectarray in different planes at 10 GHz is shown in Figure 5. The PEC patch reflectarray has first side lobe levels (SLL) of approximately-16.3 dBi in both the E-plane and H-plane below the main beam. The TFCs reflectarray has SLL of −13.5 dBi in both the H- and E-planes with nearly the same half-power beam width (HPBW) of 5 degrees.
(a) E-plane
(b) H-plane
The gain variations against the frequency for the PEC/TFCs microstrip reflectarrays are shown in Figure 6(a). The PEC microstrip reflectarrays introduce peak gain of 24.43 dB with a 1 dB gain bandwidth of 1.15 GHz (11.22%). The TCFs microstrip reflectarray introduces a peak gain of 24.44 dB with 1 dB gain bandwidth of 0.39 GHz (4%). A reduction in the TCFs reflectarray gain bandwidth is due to the reduced conductivity of the TCFs polymer compared to the PEC. The axial ratio variations versus frequency of the PEC/TFCs microstrip reflectarray is shown in Figure 6(b). Both reflectarrays produce CP radiation patterns with AR < 3 dB and cover a frequency range of 3 GHz compared to that for the horn antenna.
(a) Gain
(b) Axial ratio
2.3. Solar-Cells Integration with the Microstrip Reflectarray
Solar-cells integration with the microstrip reflectarray occupies the same area. The effect of solar cell position relative to the radiation element on the radiation characteristics of the transmit array is studied in [27]. The unit cell consisting of TCFs square patches printed on a dielectric substrate as described in Section 2.1 is integrated with a solar cell placed on the top of the TCFs patch as shown in Figure 7(a). The solar cell has a square shape with arm length, , thickness mm, and dielectric properties of and . Again the variation in the arm length of both the patch and the solar cell is used to adjust the needed reflection coefficient compensation phase of each element. The reflection coefficient magnitude and phase variation versus patch arm’s length of the unit cell is shown in Figure 7(b). The minimum value of the reflection magnitude experienced by the structure is −1.8 dB, while the reflection coefficient phase covers approximately 300 degrees. The solar cell has an effect on increasing the magnitude of the reflection coefficient while keeping the same reflection coefficient phase variation due to the losses introduced in the solar-cell layer. Figure 8 shows the configuration of 13 × 13 TCFs microstrip reflectarray integrated with the solar cell covering an area of 18.98 × 18.98 cm2. The E- and H-plane, left-hand and right-hand CP radiation patterns for the TCFs microstrip reflectarray integrated with the solar cell at 10 GHz, are shown in Figure 9. The copolar/cross-polar ratio is lower than −30 dBi with SLL of −13.5 dBi/−14.5 dBi in the E-/H-planes. The gain and axial ratio variations versus frequency is shown in Figure 10. A maximum gain of 24 dBi is achieved. The 1 dB gain bandwidth variation is 1.5 GHz with a reduction in the maximum gain by about 0.5 dB due to the increase in the magnitude of the reflection coefficient. The array introduces circular polarization with AR < 3 dB and covers 3 GHz.
(a) The unit cell with solar cell
(b) The reflection coefficient variation
(a) E-plane
(b) H-plane
(a) Gain
(b) Axial ratio
3. Conclusion
The paper introduces the radiation characteristics of a microstrip patch reflectarray antenna for small satellite applications at 10 GHz. The reflectarray antennas are used for data communication and the solar cells are used for energy harvesting, saving the available surface area of emerging applications. A unit cell consists of PEC patch printed on an optically transparent substrate with the PEC ground plane. A comparison between the PEC patch and the TCFs polymer unit cell has been introduced. The same behavior of the transparent conducting polymer and the PEC unit cell is reached. The magnitude of the reflection coefficient achieves 0 dB nearly with 300° reflection coefficient phase using patch arm’s length. A 13 × 13 CP microstrip reflectarray antenna is designed using the PEC patch and compared to that designed using the TCFs. A circularly polarized horn antenna is utilized to feed the reflectarray. The peak gain is 24.43 dB with a 1 dB gain bandwidth of 1.15 GHz (11.22%) for the PEC patch reflectarray and 24.44 dBi with a 1 dB gain bandwidth of 0.39 GHz (4%) for the TCFs reflectarray. The solar cell is integrated with the TCFs reflectarray for area reduction. The solar-cell integration with the reflectarray reduces the maximum gain by about 0.5 dB due to the increase in the magnitude of the reflection coefficient.
Competing Interests
The authors declare that there is no conflict of interests regarding the publication of this article.