Polarization Re-Configurable Antenna with Increase Gain for Small Satellites

Future small satellites require the development of reconfigurable antennas. Designing such antennas, especially single port patch antennas with circular polarization is a challenging task. Therefore, we propose both right-hand/left-hand circularly polarized (RHCP/LHCP) antenna which can reconfigure. The proposed antenna follows patch topology with E-shape that is single-layer and single-feed with two RF switches. The switches can alter the polarization in real-time. We also show various properties of the proposed antenna, such as radiation pattern, impedance matching, axial ratio, and bandwidth through simulations and measurements. The proposed model shows excellent performance and agrees well with the measurements. The performance of the antenna shows an effective bandwidth of 2.45 GHz–2.82 GHz with a maximum gain of 9.88 dB at 2.55 GHz. The symmetry of the antenna radiation is preserved by switching between the LHCP and RHCP polarization modes.


Introduction
With the recent advancements in satellite communication, there is an emerging trend towards small satellites and spacecraft for different missions inside and beyond the Earth's lower orbit. This has led to the development of small satellites, including micro, nano, and even pico class of satellites [1]. The small satellites are of tinier sizes and have less weight, and therefore, they are becoming the most appealing area of research in the aerospace industry. Small satellites usually weigh less than 500 Kg and are categorized accordingly as mini, micro, nano (cube satellite), pico, and femto-satellites [2]. These small satellites enable the growth of new space technologies by allowing companies, educational organizations, scientists, and engineers to have access to space at lower cost [3]. Recently, many companies and organizations have designed, manufactured, and launched the most advanced satellites and spacecraft, including SpaceX, Nano Racks, Skybox, and Pumpkin.
Small satellites have become the center of attention to a lot of researchers as they are capable of accomplishing scientific experimentations for scholastic and organizational purposes with their tiny structures [1,4]. There are around 1030 different assignments that have been propelled for the past almost 15 years. Efficient antenna designing is one of the major research areas for small satellites. These small satellites require light-weight small size antennas [5][6][7][8][9]. Small satellites operate at various frequencies depending on the applications. However, most of the missions are focused on using the 2.4 GHz free band. Hence, designing a reconfigurable antenna for the small satellites operating at this band is an interesting and challenging area of research.
In modern wireless communication systems, antennas with circular polarization (CP) plays an important role and offer several advantages over the conventional linearly polarized antennas [10][11][12][13][14][15]. It has many positive aspects such as an adjustable orientation angle between the transmitter (Tx) and the receiver (Rx), improved mobility, reduced multipath reflections, and robust to weather conditions. The circularly polarized waves are the combination of two linearly polarized modes of equal magnitude and orthogonal to each other [16]. Sometimes a dual-polarized antenna can also be designed with the generation of four modes in a rectangular patch with the same amplitude and 90 0 phase shift. Linear polarized antennas not only require the same polarizations for Tx and Rx antennas but also depends upon their precise alignment [17]. On the other hand, CP polarized antennas do not require such restrictions and can be obtained using various methods, including quasi-square/round/oval, orthogonal doubly-fed array, surface grooving, and truncated patch, respectively. The CP antennas can be widely used in various portable devices, such as GPS receivers, mobile phones, and RFID trackers.
CP with the capabilities of reconfigurability can certainly maximizes the polarization efficiency for both line-of-sight (LOS) and non-line-of-sight (NLOS) cases. Switching between the left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP) is of great importance in reconfigurable systems which is achieved with the help of switching devices [18]. However in patch antennas, the designing issue becomes more troublesome due to the narrow band nature of CP patch antennas. Moreover, another major problem of the designing is complicated accessibility to the switching devices to bias the network. Nevertheless, these issues can be addressed with the appropriate selection of antenna design where the E-shape design can provide many interesting features.
In the past, various CP patch antennas of a single-layer have been designed. However, they experience low axialratio (AR) with low bandwidths where the maximum value was near to 1% [19]. Nevertheless, some advanced broadband antenna designs including the L-probe and U-slotted have better bandwidths, i.e., nearly equal to 4% [20]. Nonetheless, these designs are complex to implement. To minimize the complexity and to improve the AR bandwidth , we propose a CP reconfigurable antenna through switching where the switching is achieved by using PIN diodes only. Major contributions of the paper are summarized as follows: • Polarization properties such as RHCP and LHCP are evaluated for an E-shaped antenna. • A circularly polarized reconfigurable antenna with improved gain is proposed for small satellites.

Related Work
Downlink communications in small satellites require high gain antennas with compact size, circular polarization (CP), robustness, and cost-efficiency [21]. In general, small satellite antennas' gain is not high enough to provide a good communication system. Such antennas require a gain of approximately 12 dBi or lower, usually depend upon the specific application requirement [21]. Among the various types of antennas, Patch antennas are considered a good match for a system having such requirements. Recent advancements in wireless communication systems enabled the increasing demand for reconfigurable antennas with multi-mode specifications [22]. Antenna's aptitude for reconfigurability can effectively adjust the radiation characteristics in terms of the radiation pattern and resonant frequency [23]. Usually, an antenna with frequency reconfigurability is considered one of the most practical options for operating at a desirable frequency. Such antennas eliminate the use of many antennas operating at different frequencies for signal transmission [24]. Additionally, antennas with multi-frequency operation can improve the overall system performance, utilizes less space, and are cost-effective. For instance, reconfigurable pattern antennas are a practical choice for applications such as tracking and surveillance [25,26].
Manipulation of patterns variability is suitable to avoid noise, lessening electronic jamming, improving energy efficiency, and enhancing the system's security. On the other hand, Polarization reconfigurable antennas can alter the polarization between linear polarization, elliptical polarization, left-hand circular polarization (LHCP), and righthand circular polarization (RHCP), respectively [27]. These attractive features enhance the importance of reconfigurable antennas for small satellite applications [28]. In literature, researchers have focused on the electrical reconfiguration of antennas with active elements, including positive -intrinsic -negative (PIN) diodes. For instance, in [28], the authors discussed different reconfigurable antennas such as polarization reconfigurable antennas and radiation pattern reconfigurable antenna. Also, the authors discussed the combination of frequency and radiation reconfigurable antenna. In [29], the authors further investigate electronic switching techniques of reconfigurable antennas and obtained some promising results. These implementations can be effectively applied to various applications such as small satellites, multiple-input multiple-output (MIMO), and cognitive radio communications.

Concept of E-Shaped Design
Antennas with E-shaped patch is one of the most important design for reconfigurable antennas which was first introduced in [30]. These broadband CP antennas have low fabrication complexity with quite a simple geometry. Hence, we consider this CP design and add both RHCP/LHCP polarization to it as illustrated in Fig. 1. Note that this method generates circular polarization with the help of RF switches.
It can be seen from Fig. 1 that the E-shaped patch antenna consists of a ground, substrate, patch, and coaxial cable. The lowest layer of antenna is ground which is followed an air space of 7mm after a layer of substrate is added. The upper most layer is the patch whereas the coaxial cable is fed in the middle. Figure 2 shows both LHCP and RHCP modes of the proposed antenna. When the upper diode is ON and the lower is OFF, LHCP is achieved while for RHCP the lower diode is kept ON and upper is turned OFF.

Polarization Reconfigurable Antenna Design and Dimensions
The proposed design comprises of RF switches, E-shaped patch which is single layered and single feed, ground, and substrate. RF switches are connected at relevant positions in the patch. Only two switches are used for changing the mode of polarization. Ground and patch is made up of pure copper. The antenna can be easily integrated with the simple direct current biasing network. The basic geometry of the proposed reconfigurable antenna is shown in Fig. 3. The dielectric substrate of RT/duroid 5880 is used to print the antenna where the thickness of the substrate is 0.787 mm. Height of the substrate is kept to 7 mm and connected to a single feed coaxial cable. Moreover, to achieve large bandwidth, the antenna is placed fairly above the ground plane with some air gap [31]. For switching purpose, we have used two RF PIN diodes connected to the patch antenna. We use the same procedure in [19] to calculate the optimal position of switches and antenna dimensions [19]. A narrow slit is integrated onto the substrate surface to remove the direct current deficit for the terminals of the diodes'. Three capacitors are embedded in the narrow slit to block DC and to keep RF stability. The relative height of patch, substrate, and ground are given in Fig. 3. Also the proposed antenna is fed through a 50 ohm coaxial cable whose dimensions are given in Fig. 3.
The proposed antenna have four possible polarization states. When all the switches are OFF, the polarization is linear. When both switches are ON still the polarization is circular [32]. When the switch 1 is ON and switch 2 is OFF the polarization is LHCP and if switch 2 is ON and switch 1 is OFF, the polarization is RHCP. All possible configurations can be seen in Fig. 4.

Simulation Results
First, we show the LHCP and RHCP in terms of surface charge density as shown in Fig. 5a and b, respectively. It is clear from Fig. 5a and b that diode 1 is conducting and diode 2 is non-conducting, resulting in LHCP where the vectors of electric field rotate in the clockwise (CW) direction [33]. Another major performance metric for antennas is the axial ratio (AR) which is the ratio between the minor and major axis of an electric field vector. The values of AR less than 3 means a CP antenna which is reflected in Fig. 6 for frequency range of 2.3 GHz to 2.9 GHz with minimum value at 2.5 GHz. Furthermore, we also calculate the gain of the proposed in antenna in Fig. 7 which is 9.88 dB. Besides that, we also calculate the s-parameters of the proposed antenna in Fig. 8. L1, L2 and L3 shows the s-parameters when the distance between the substrate and ground is 5 mm, 10 mm, and 7 mm, respectively. The best result is at 7 mm because at this value the bandwidth of 350 MHz is achieved with the gain of 9.88 dB. Figure 7 shows the antenna's gain when operating in state 3. i.e., when switch one or upper diode is in conducting state and switch two or lower diode is in non conducting state. We can say that when the antenna is left hand circularly polarized (LHCP).  Figure 8 shows bandwidth of antenna when operating with different gap-difference between the substrate and the ground. This method is also known as Electromagnet Bandgap (EBG). Initially, there was a 10 mm gap between substrate and ground; then we reduce this gap to 9 mm, then 8 mm, and so on and observe the results. Specifically, when the gap is 7 mm, we observe that bandwidth is improved from 200 MHz to 350 MHz, that can be seen from blue line in Fig. 8.
In the previous works such as [30], the maximum bandwidth of 200 MHz was achieved as shown in Fig. 9. However, our proposed design shows a considerable improvement in the bandwidth i.e., 350 MHz, which can be analyzed in Fig. 10.

Conclusions
In this paper, we proposed a reconfigurable antenna with circular polarization for small satellites. Circular polarization is accomplished by using the well-known single-feed patch antenna with E-shape. We have evaluated the performance of proposed antenna by using RF where these switches can effectively perform the circular polarization with reconfigurability. For the different modes of polarization two switches are used alternatively. A simple direct current biasing network is combined with the antenna where complex matching networks are not required, and at each mode of polarization, the antenna features preserved. The antenna has appreciable gain of 9.88 dB according to both simulations and measurements at the operating frequency of 2.55 GHz. The voltage standing waves ratio at the operating frequency of 2.55 GHz is 1.3, the axial ratio is 2.45, and the bandwidth of the E-shaped antenna is 350 MHz.
In future, the size of the antenna can be further reduced to help in high permittivity dielectric substrate, linear polarization along with LHCP and RHCP. Also, the antenna can be made a dual frequency to further enhance its effectiveness.