A compact wideband circularly polarized fractal slot antenna with rectangular island for X-band satellite applications

Abstract

This paper introduces a compact, circularly polarized spidron-fractal slot antenna with a rectangular island. The reflection-coefficient |S₁₁| of the proposed antenna covers the band from 7 GHz to 10 GHz. The proposed antenna is circularly polarized with an axial-ratio (AR) bandwidth that extends from 7 GHz to 10 GHz. It offers simultaneous dual circular polarization (RHCP and LHCP). The gain of the proposed antenna varies between 4 dBi and 6.9 dBi. The simulated efficiency reaches 97%. The overall dimensions of the antenna are 19.7 x 23 x 0.508 mm³ (0.55λ₀ x 0.65λ₀ x 0.014λ₀), making it suitable for CubeSats with limited surface area. The proposed antenna can be employed for both the X-band satellite downlink (from 7.25 GHz to 7.75 GHz) and uplink (from 7.9 GHz to 8.4 GHz) frequency bands. Additionally, the antenna can be utilized in small satellite Synthetic-Aperture-Radar (SAR) systems operating from 9 GHz to 10 GHz, military applications, and RFID tag tracking-equipment. A prototype of the proposed antenna has been fabricated and then measured using Vector-Network-Analyzer (VNA) and inside an Anechoic chamber. The measurement results of the proposed antenna are in excellent match with the simulated results.

Keywords:

• Slot antenna
• circular polarization
• wideband
• satellite applications

REVIEW EDITOR:

• Ai Qingsong
• Wuhan University of Technology
• Wuhan
• China

SUBJECTS:

• Electrical & Electronic Engineering
• Technology
• Engineering & Technology
• Electrical & Electronic Engineering
• Electrical Engineering Communications
• Electromagnetics & Microwaves
• Transport & Vehicle Engineering
• Aerospace & Aviation Engineering
• Satellite Technology
• Electromagnetics & Communication

1. Introduction

X-band technology is broadly used in numerous applications owing to the high rate of data transmitting, short-range features, and large bandwidths. However, designing X-band antennas to meet the specific requirements of these applications remains a challenge (Arnaud et al., 2020; El Khamlichi et al., 2021; Genovesi & Dicandia, 2022; Leszkowska et al., 2021; Varun Yadav et al., 2020). There is a growing need for compact, lightweight antennas that can be easily integrated into modern communication systems. Planar antennas, which are low-cost, low-profile, and easy to fabricate, have gained extensive utilization in advanced satellite and wireless communication devices (Adhitya et al., 2019; Imbraile, 2012). However, planar antennas have some significant drawbacks, such as small bandwidth, low power handling capacity, and low gain (Adhitya et al., 2019). Therefore, achieving wide bandwidth, acceptable gain, and small antenna size is a challenging goal. Satellites have become increasingly important in many applications, such as communication, oceanography, agriculture, astronomy, positioning, and surveillance (May, 2017; Zyl, 2013). Satellites are classified according to their size, with the smallest being femto-satellites, which have a mass smaller than 0.1 Kg (Xue et al., 2008). The design of satellite antennas differs from that of antennas used in other applications. The design constraints imposed by the satellite on size, shape, and weight are also critical factors to consider (Hwang, 1992). Modern satellite communication systems require very small, low-cost, low-profile, and high-gain antennas. Planar antennas are particularly desired for usage in satellite communication systems because of their low profile, lightweight, ease of fabrication, robustness, ability to be easily installed on various mounting surfaces and ability to be compatible with microwave-monolith-integrated-circuits (MMICs) besides optoelectronics-integrated-circuits (OEICs) technologies. Planar antennas are also very adaptable in terms of resonance frequency, polarizations, impedance and radiation patterns, making them an attractive option for many applications (Adhitya et al., 2019; Imbraile, 2012; Khac et al., 2018). Circular polarization is a highly advantageous property that finds widespread applications in various fields. One of the primary advantages of circular polarization is its ability to overcome the unwanted effects caused by multipath interference. This is due to its inherent property of rejecting signals with an opposite-handedness polarization, which makes it highly effective in mitigating the interference caused by reflected signals (Dicandia et al., 2017). In addition to its interference-rejection capabilities, circularly polarized (CP) signals are highly robust in communication links. Unlike linearly polarized radiators, CP signals are not affected by polarization misalignment between transmitting and receiving antennas. This makes them highly advantageous in situations where the relative orientation between the antennas is variable, which is the case in satellite communications (Lin & Wong, 2017).

Various antenna designs for X-band satellite applications were investigated. The first antenna, introduced in (Arnaud et al., 2020), is a telemetry antenna that is compact and has dual circular polarization. It covers a bandwidth that extends from 8.025 GHz to 8.4 GHz, with a 3-dB axial-ratio (AR) bandwidth of 370 MHz and has a maximum gain of 5 dBi. The antenna is considered relatively large, having a diameter equals to 238 mm (6.52λ₀) and a height equals to 185 mm (5.13λ₀), compared to its realized gain. The second antenna, introduced in (Leszkowska et al., 2021), is a superstrate antenna suitable for CubeSat applications. It has a bandwidth that extends from 8 GHz to 9.7 GHz, a 3-dB axial-ratio bandwidth of 580 MHz which doesn’t achieve circular polarization the whole bandwidth of antenna and a maximum gain of 14 dBi. The antenna size is 62 x 62 x 22 mm³ (1.77λ₀ x 1.77λ₀ x 0.63λ₀), which is considered a large volume especially due to the large antenna thickness. The third antenna, introduced in (Fouany et al., 2017), is a telemetry circularly polarized antenna designed for CubeSats. It has a bandwidth that extends from 8 GHz to 8.4 GHz, a 3-dB AR-bandwidth of 400 MHz, and a maximum gain of 5 dBi. The antenna size is 100 x 100 x 15 mm³ (2.74λ₀ x 2.74λ₀ x 0.41λ₀), which is considered a relatively large size compared to the achieved gain. The fourth antenna introduced in (Genovesi & Dicandia, 2022), is a metasurface (MS) based on 4 x 4 loop elements for X-band satellite communications. It has a bandwidth that extends from 7 GHz to 8.9 GHz, a 3-dB AR-bandwidth of 1.15 GHz, and a maximum gain of 8.6 dBi. The antenna has a diameter equals to 50 mm (1.3λ₀) and a height equals to 5.25 mm (0.137λ₀). The fifth antenna, introduced in (El Khamlichi et al., 2021), is a broadband substrate integrated waveguide antenna for X-band SAR applications. It has a bandwidth that extends from 9.4 GHz to 10.5 GHz, does not achieve circular polarization, and achieves a maximum gain of 9 dBi. The antenna size is 59 x 14 x 0.76 mm³ (1.96λ₀ x 0.46λ₀ x 0.025λ₀). The sixth antenna, introduced in (Anim et al., 2021), is a broadband planar antenna for X-band SAR onboard small satellites. It has a bandwidth of 9.01 GHz to 10.2 GHz, does not achieve circular polarization, and achieves a maximum gain of 8 dBi. The antenna size is 14 x 14 x 3.986 mm³ (0.45λ₀ x 0.45λ₀ x 0.126λ₀). The research gap lies in the fact that none of the antennas investigated in this literature survey have been able to achieve a sufficiently wide bandwidth and wide axial-ratio bandwidth to cater to the X-band satellite uplink, downlink, and SAR applications. Moreover, none of the antennas specifically designed for SAR applications have been successful in achieving circular polarization. Furthermore, a significant drawback of the existing designs is their large physical size, which limits their practicality, particularly in scenarios where surface area is limited such as small satellite applications. Consequently, the development of compact antenna designs capable of operating in multiple applications has become an urgent and crucial requirement.

This paper aims to focus on designing a wideband antenna that operates in the X-band. The proposed antenna design is required to cover the X-band satellite uplink from 7.25 GHz to 7.75 GHz and downlink from 7.9 GHz to 8.4 GHz for satellite communication application. The proposed antenna design is also required to cover the band extending from 9 GHz to 10 GHz for X-Band Synthetic-Aperture-Radar (SAR) application, making the antenna able to serve multiple applications. The proposed antenna is designed to achieve circular polarization all over its operating band with a wide axial-ratio (AR) bandwidth compared to the previously mentioned designs. The proposed antenna is designed to cover the band extending from 7.25 GHz to 10 GHz while achieving circular polarization in this band. It is also required to achieve gain greater than or equal 4 dBi all over the desired band, with a smaller size compared to the previously mentioned designs.

To mitigate the limitations of planar antennas, meet the specific needs of satellite communication systems, and align with the proposed requirements, it is essential for the proposed planar antenna to be compact, provide wide bandwidth, and support circular polarization. Recently, there has been growing interest in the field of fractal antenna theory. Fractal antennas are known for their compact size, lightweight construction, conformal shapes, broad bandwidths, and high gains. These antennas are based on intricate shapes that exhibit self-similarity or self-affinity in their geometric structure, such as the Spidron-fractal (Karmakar, 2020; Kwon et al., 2017; Nguyen Thi et al., 2013). The embedded islands inside slot antennas serve to enhance the antenna’s bandwidth and circular polarization by widening the axial-ratio (AR) bandwidth. Additionally, the embedded islands can enhance the antenna’s gain and directivity by providing additional resonant structures. Overall, an embedded island contribute to the improved performance and functionality of slot antennas (Abed et al., 2020; Anantrasirichai et al., 2005; Kumar et al., 2019; Trinh-Van et al., 2019).

This paper proposes a single element fractal slot antenna with a rectangular island fed by a conventional rectangular feedline from the bottom layer of the antenna. The proposed antenna provides a wide bandwidth covering the X-band uplink and downlink for satellite communication applications, along with Synthetic-Aperture-Radar (SAR) application. This research work offers several advantages due to the implementation of the spidron-fractal slot and the rectangular island. These include a wide bandwidth, the ability to achieve circular polarization with a significant axial-ratio (AR) bandwidth, and a compact antenna size. However, it is important to note a couple of limitations associated with this research work. Firstly, the complexity of the spidron-fractal structure can pose a challenge in design and manufacturing. Secondly, while the initial iterations of the spidron-fractal antenna design may yield substantial improvements in performance, such as increased bandwidth and compactness, the subsequent iterations may result in diminishing incremental enhancements that may not justify the added complexity (Anantrasirichai et al., 2005; M & Boopathi Rani, 2020). The proposed antenna provides a wide axial-ratio (AR) bandwidth covering the whole bandwidth of the antenna. The design procedure is presented in section 1. Results and analysis are introduced in section 2. The proposed antenna design and experimental results are given in section 3. Final assessment and comparison with recent X-band antenna designs are introduced in section 4. Conclusion is presented in Section 5.

2. Design procedure

The proposed antenna is composed of three layers. The top layer where a spidron-fractal slot and the rectangular island take place. The feedline exists in the bottom layer. The substrate layer exists in between as shown in Figure 1. The substrate used is Rogers-RT-Duroid-5880 with dielectric constant, ${\epsilon }_r$ of 2.2, loss tangent of 0.0009 and thickness of 0.508 mm. The overall length, L and width W of the proposed antenna are selected to be slightly greater than the height of the spidron-fractal slot as shown in Figure 1(a). The design procedure is presented in the following three steps.

Figure 1. Spidron-fractal slot antenna: (a) Front view and (b) Back view.

2.1. Step 1: spidron-fractal slot design

A spidron-fractal is composed of a number of right-angle triangles starting with a large triangle with height H_s and then reduced in size by a common factor $\mathrm{\text{tan}}(\theta )$ so that the height of the next triangle is $H_s\hspace{0.17em}\mathrm{\text{tan}}\hspace{0.17em}(\theta )$ and so on until reaching the n^th triangle whose height would be $H_s{\mathrm{\text{tan}}}^{n-1}(\theta )$ as shown in Figure 2 (Kwon et al., 2017). Each triangle is then rotated backward towards the larger triangle by a fixed angle, θ to form the shape of a spidron as shown in Figure 3. The first design step of the antenna is cutting a spidron-fractal slot of height H_s centered in the top layer of the antenna at distances S_x and S_y from the left and bottom edges of the top layer respectively as shown in Figure 1(a). The height of the spidron slot can be derived starting from the equation of the slot antenna width (Balanis, 2016; Moitra et al., 2013; Xu-Bao et al., 2012). The derivation process is outlined as follows: (1) $$H_s\mathrm{\text{tan}}\hspace{0.17em}\left(\theta \right)\cong \frac{{\lambda }_g}{2}$$(1) (2) $$H_s\cong \frac{{\lambda }_g}{2\mathrm{\text{tan}}\hspace{0.17em}\left(\theta \right)}$$(2) (3) $$H_s\cong \frac{C}{2f_r\mathrm{\text{tan}}\left(\theta \right)\sqrt{{\epsilon }_{\mathit{\text{reff}}}}}$$(3)

Figure 2. Right angle triangles forming the spidron shape.

Figure 3. Spidron shape.

2.2. Step 2: feedline configuration

The second step is the microstrip feedline configuration. The slot antenna is fed using a microstrip feedline at the bottom layer as shown in Figure 1(b). The length and width of the feedline are L_f and W_f respectively. The feedline is initially positioned in the middle of the antenna at a distance of F_L from the left side of the antenna as shown in Figure 1(b). In the next section parametric analysis is performed on the feed location, F_L to optimize its location. optimum feed location results are introduced in the next section.

2.3. Step 3: rectangular island integration

The third step is placing a rectangular island centered in the middle of the slot. The function of the embedded island is achieving a large Bandwidth along with wideband circular polarization (large 3-dB AR-bandwidth). The rectangular island is placed at a distance R_p from the left side of the spidron-fractal slot and R_g from the bottom side of the spidron-fractal slot as shown in Figure 4(a). The embedded rectangular island has a height of L_R and a width of W_R. For the rectangular island to fit within the spidron-fractal slot, it is important to choose the length L_R and width W_R to be less than $H_s\hspace{0.17em}\mathrm{\text{tan}}\hspace{0.17em}(\theta )$ by a specific percentage. The initial values of L_R and W_R are approximately equal to ${ H}_s\hspace{0.17em}\mathrm{\text{tan}}\hspace{0.17em}(\theta )/2.$ Parametric analyses are then performed on the dimensions of the rectangular island, L_R and W_R along with its horizontal position R_p. The parametric analysis is performed to achieve the widest required reflection-coefficient |S₁₁| bandwidth along with the widest required axial-ratio (AR) bandwidth. Parametric analyses are performed, and optimum results are introduced in the next section.

Figure 4. Spidron-fractal slot antenna with rectangular embedded island: (a) Front view and (b) Back view.

3. Results and analysis

The proposed design is then simulated and analyzed using CST. The dimensions of the spidron-fractal slot opening are calculated using Equation (3)(3) $$H_s\cong \frac{C}{2f_r\mathrm{\text{tan}}\left(\theta \right)\sqrt{{\epsilon }_{\mathit{\text{reff}}}}}$$(3) at angle, θ equals 35 degrees. This section consists of two parts. Part ‘a’ studies the spidron-fractal slot antenna with no island (SFSA-NI) along with deciding the optimum feed location. Part ‘b’ studies the spidron-fractal slot antenna with embedded rectangular island (SFSA-RI). In part ‘b’ parametric analyses are then performed on the embedded square island shown in Figure 4, to achieve the widest reflection-coefficient |S₁₁| bandwidth along with the widest axial-ratio (AR) bandwidth. The parametric analyses are performed in three steps. The first step is performed on the height of the embedded rectangular island, L_R. The second step is performed on the width of the embedded rectangular island, W_R. The third step is performed on the position of the embedded rectangular island, R_p.

3.1. Spidron-fractal slot antenna with no island (SFSA-NI)

First the spidron-fractal slot antenna with no island (SFSA-NI), shown in Figure 1, is simulated at different values of feed location, F_L as shown Figure 1(b) to study its effect. The value of F_L that gives the wider bandwidth and the wider 3-dB AR-bandwidth is then selected. The SFSA-NI antenna is simulated at F_L= 8.35 mm, 10.9 mm (centered) and 13.4 mm. Figure 5 shows the reflection-coefficient |S₁₁| at F_L= 8.35 mm, 10.9 mm and 13.4 mm where the bandwidth is equal to 900 MHz extending from 7.1 GHz to 8 GHz, 1 GHz extending from 7.4 GHz to 8.4 GHz and 1.4 GHz extending from 10.1 GHz to 11.5 GHz respectively. Figure 6 shows the axial-ratio (AR) at F_L= 8.35 mm, 10.9 mm and 13.4 mm where the SFSA-NI antenna achieves circular polarization only at F_L=13.4 mm with 3-dB AR-bandwidth of 600 MHz extending from 8 GHz to 8.6 GHz. The selected value of the feed location F_L is 13.4 mm where it achieves circular polarization along with the widest bandwidth. The optimized dimensions of the SFSA-NI antenna are given in Table 1. The SFSA-NI antenna is still not covering the whole required band, the 3-dB AR-bandwidth is not wide enough to achieve circular polarization for the whole required band. In addition, there is no common – 10-dB reflection-coefficient |S₁₁| and 3-dB AR-bandwidth. The – 10-dB reflection-coefficient |S₁₁| bandwidth and the 3-dB AR-bandwidth can be aligned and increased using the embedded rectangular island. In the upcoming subsection, an embedded rectangular island will be placed within the spidron-fractal slot to control the reflection-coefficient |S₁₁| and increase the bandwidth by creating additional resonant modes. Also, to control and improve circular polarization by increasing the axial-ratio (AR) bandwidth. This can be performed by adjusting the dimensions and location of the rectangular embedded island.

Figure 5. Simulated reflection-coefficient |S₁₁| of the SFSA-NI antenna at different values of F_L.

Figure 6. Simulated axial-ratio (AR)| of the SFSA-NI antenna at different values of F_L.

Table 1. optimized dimensions of SFSA-NI antenna.

Parameter	Hs	Sx	Sy	LF	WF	FL	L	W
Value (mm)	17	2.6	2.17	10.1	1.45	13.33	19.7	23

3.2. Spidron-fractal slot antenna with rectangular island (SFSA-RI)

The embedded rectangular island with width, W_R and length L_R is then placed centered in the middle of the spidron-fractal slot at a distance R_p from the left edge of the spidron-fractal slot as shown in Figure 4(a). The optimized dimensions of the SFSA-RI antenna are given in Table 2. Three steps of parametric analysis are performed to study the effect of L_R, W_R, and R_P on the reflection-coefficient |S₁₁| bandwidth and the axial-ratio (AR) bandwidth. The length, L_R and the width, W_R should be chosen to be smaller than $H_s\hspace{0.17em}\mathrm{\text{tan}}\hspace{0.17em}(\theta )$ by a percentage that ensures the rectangular island fits within the spidron slot. To begin the parametric analysis, the starting values of L_R and W_R are initially selected to be approximately equal to ${ H}_s\hspace{0.17em}\mathrm{\text{tan}}\hspace{0.17em}(\theta )/2.$

Table 2. Optimized dimensions of SFSA-RI antenna.

Parameter	Hs	Sx	Sy	LF	WF	FL	LR	WR	RP	Rg	L	W
Value (mm)	17	2.6	2.17	10.1	1.45	13.33	6.25	7.87	2.34	0.16	19.7	23

3.3. Step 1: studying the effect of length L_R

In this step, a parametric study is performed on L_R while all the other parameters are kept constant. The SFSA-RI antenna shown in Figure 4 is simulated at different values of L_R. Any variation in the length of the embedded rectangular island, L_R will change the gap length, R_g as shown in Figure 4. The reflection-coefficient |S₁₁| and the axial-ratio (AR) curves for different values of L_R are shown in Figure 7 and Figure 8 respectively. It is shown in Figure 7 that the reflection-coefficient |S₁₁| bandwidth at L_R = 3.25 mm, 4.25 mm, 5.25 mm, and 6.25 is 1.3 GHz (9.5 GHz - 10.8 GHz), 1.52 GHz (9.33 GHz - 10.85 GHz), 3.4 GHz (7.2 GHz - 10.6 GHz) and 2.7 GHz (6.7 GHz - 9.4 GHz) respectively. Figure 8 shows that the 3-dB AR-bandwidth at L_R = 3.25 mm, 4.25 mm, 5.25 mm, and 6.25 is 600 MHz (7.6 GHz – 8.2 GHz), 800 MHz (7.4 GHz - 8.2 GHz), 1 GHz (7.3 GHz - 8.3 GHz) and 2.3 GHz (7.52 GHz- 9.82 GHz) respectively. L_R = 6.25 mm achieves the largest – 10-dB reflection-coefficient |S₁₁| bandwidth which is common with the largest 3-dB AR-bandwidth, reaching 1.88 GHz. Therefore, the selected length of the rectangular island, L_R, is 6.25 mm. In the upcoming step parametric study is performed on W_R for further improvement in the reflection-coefficient |S₁₁| bandwidth and the 3-dB AR-bandwidth.

Figure 7. Simulated reflection-coefficient |S₁₁| of the SFSA-RI antenna at different values of L_R.

Figure 8. Simulated axial-ratio (AR) of the SFSA-RI antenna at different values of L_R.

3.4. Step 2: studying the effect of length W_R

After updating the value of L_R, a parametric study is performed on W_R while all the other parameters are kept constant. The SFSA-RI antenna shown in Figure 4 is simulated at different values of W_R. The reflection-coefficient |S₁₁| and the axial-ratio (AR) curves for different values of L_R are shown in Figure 9 and Figure 10 respectively. It is shown in Figure 9 that the reflection-coefficient |S₁₁| bandwidth at W_R = 6.87 mm, 7.87 mm, 8.87 mm, and 9.87 is 2.5 GHz (6.8 GHz-9.3 GHz), 3.1 GHz (6.6 GHz-9.7 GHz), 3.2 GHz (6.7 GHz-9.9 GHz) and 3.7 GHz (6.2 GHz-9.9 GHz) respectively. Figure 10 shows that the 3-dB AR-bandwidth at W_R = 6.87 mm, 7.87 mm, 8.87 mm, and 9.87 is 2.2 GHz (7.6 GHz-9.8 GHz), 2.8 GHz (7 GHz-9.8 GHz), 1 GHz (6.8 GHz-7.8 GHz) and 1 GHz (6 GHz-7 GHz) respectively. L_R = 7.87 mm achieves the largest – 10-dB reflection-coefficient |S₁₁| bandwidth that is common with the largest 3-dB (AR) bandwidth, reaching 2.7 GHz with an improvement of 43.6% compared to step 1. Therefore, the selected width of the rectangular island, W_R, is 6.25 mm. In the upcoming step parametric study is performed on R_p for further improvement in the reflection-coefficient |S₁₁| bandwidth and the 3-dB AR-bandwidth.

Figure 9. Simulated reflection-coefficient |S₁₁| of the SFSA-RI antenna at different values of W_R.

Figure 10. Simulated axial-ratio (AR) of the SFSA-RI antenna at different values of W_R.

3.5. Step 3: studying the effect of length R_p

After attuning the length, L_R, and the width, W_R of the rectangular island to achieve the largest common – 10-dB reflection-coefficient |S₁₁| and 3-dB AR-bandwidth, a parametric study is performed on the rectangular island position R_p, while all the other parameters are kept constant. The SFSA-RI antenna shown in Figure 4 is simulated at different values of R_p. The reflection-coefficient |S₁₁| and the axial-ratio (AR) curves for different values of R_p are shown in Figure 11 and Figure 12 respectively. It is shown in Figure 11 that the reflection-coefficient |S₁₁| bandwidth at R_p = 2.94 mm, 3.94 mm, and 4.94 mm is 3.2 GHz (6.5 GHz-9.7 GHz), 2.8 GHz (6.7 GHz-9.5 GHz) and 3.2 GHz (6.9 GHz-10.1 GHz) respectively. Figure 12 shows that the 3-dB AR-bandwidth at R_p = 2.94 mm, 3.94 mm, and 4.94 mm is 2.2 GHz (7.6 GHz-9.8 GHz), 2.8 GHz (7 GHz-9.8 GHz) and 3.7 GHz (7 GHz-10.7 GHz) respectively. R_p = 4.94 mm achieves the largest – 10-dB reflection-coefficient |S₁₁| bandwidth that is common with the largest 3-dB AR-bandwidth, reaching 3 GHz with an improvement of 11.11% compared to step 2. Therefore, the selected position of the rectangular island, R_p is 4.94 mm. The optimized dimensions of the SFSA-NI antenna are given in Table 2.

Figure 11. Simulated reflection-coefficient |S₁₁| of the SFSA-RI antenna at different values of R_P.

Figure 12. Simulated axial-ratio (AR) of the SFSA-RI antenna at different values of R_P.

4. Proposed design and experimental results

This section provides a detailed description of the final design of the proposed spidron-fractal slot antenna with an embedded rectangular island (SFSA-RI). The proposed antenna was fabricated using RT/Duroid 5880 substrate with dielectric constant, ε_r = 2.2, low loss tangent tan(δ) = 0.0009, and height h = 0.508 mm as shown in Figure 13. The antenna’s simulated radiation patterns and antenna parameters are presented, as well as the measurements taken from a fabricated prototype of the antenna. The final dimensions of the antenna are given in Table 2. The reflection-coefficient |S₁₁| of the antenna was measured using ROHDE & SCHARZ 20 Vector Network Analyzer (VNA) with measurement range up to 20 GHz. The radiation pattern and other antenna parameters were then measured in SATEMO Anechoic chamber as shown in Figure 14. Simulated and measured radiation patterns and antenna parameters of the SFSA-RI antenna were analyzed and compared. The simulated surface currents at 7.5 GHz, 8.15 GHz and 9 GHz are shown in Figure 15, Figure 16 and Figure 17 respectively. The surface current distribution of an antenna is an important parameter that determines its radiation characteristics. Figure 15(a–d), Figure 16(a–d) and Figure 17(a–d), are provided to describe the surface current distribution at phase angles of 0, 90, 180, and 270 degrees, respectively. The maximum surface currents at 7.5 GHz, 8.15 GHz and 9 GHz are 114 A/m, 103 A/m, and 90 A/m respectively. At 7.5 GHz, 8.15 GHz, and 9 GHz, it is observed that the surface current distributions at 0 and 90 degrees are equal in magnitude and opposite in phase to that at 180 and 270 degrees. The proposed antenna can excite LHCP and RHCP waves. The proposed antenna is a bidirectional simultaneous dual circularly polarized antenna, as it is able to generate a left-hand circular polarization (LHCP) in the positive z direction, whereas a right-hand circular polarization (RHCP) is generated in the negative z direction at the same time (Huang et al., 2019; Mohammadi et al., 2013). The simulated and measured LHCP and RHCP radiation patterns at 7.5 GHz, 8.15 GHz and 9 GHz are shown in Figure 18, Figure 19 and Figure 20 respectively. The simulated and measured radiation patterns and antenna parameters of the SFSA-RI antenna are analyzed and compared. The radiation patterns E-plane (phi = 0) and H-plane (phi = 90) at frequencies of 7.5 GHz, 8.15 GHz and 9 GHz for the simulated and fabricated antennas are shown in Figure 21(a–f), respectively. The simulated and measured reflection-coefficients |S₁₁| are shown in Figure 22 with -10 dB reflection-coefficient |S₁₁| bandwidths of 3.2 GHz extending 6.9 GHz to 10.1 GHz and 4.2 GHz extending from 6.8 GHz to 11 GHz, respectively. The simulated and measured axial-ratios (AR) are shown in Figure 23 indicating that the antenna is circularly polarized in the required band. The simulated and measured AR 3-dB bandwidths are 3.7 GHz extending 7 GHz to 10.7 GHz and 3 GHz extending from 7 GHz to 10 GHz, respectively. The simulated and measured gains are shown in Figure 24 with maximum gains of 5.5 dBi and 6.8 dBi, respectively. The simulated and measured total efficiencies are shown in Figure 25 with maximum efficiencies of 97.8% and 98%, respectively. This indicates that the proposed antenna design is well optimized and capable of achieving excellent performance in terms of efficient use of the available power. The results demonstrate that the measured parameters and radiation patterns of the fabricated antenna match the simulation results, achieving the desired design requirements.

Figure 13. Fabricated SFSA-RI antenna: (a) Front view, (b) Back view.

Figure 14. Fabricated SFSA-RI antenna measurement in the anechoic chamber.

Figure 15. Simulated surface current distribution at 7.5 GHz of the SFSA-RI antenna with embedded island at phase: (a) 0°, (b) 90°, (c) 180°, (d) 270°.

Figure 16. Simulated surface current distribution at 8.15 GHz of the SFSA-RI antenna with embedded island at phase: (a) 0°, (b) 90°, (c) 180°, (d) 270°.

Figure 17. Simulated surface current distribution at 9 GHz of the SFSA-RI antenna with embedded island at phase: (a) 0°, (b) 90°, (c) 180°, (d) 270°.

Figure 18. Simulated and measured LHCP and RHCP, radiation patterns at 7.5 GHz: (a) E-plane (phi = 0°), (b) H-plane (phi = 90°).

Figure 19. Simulated and measured LHCP and RHCP, radiation patterns at 8.15 GHz: (a) E-plane (phi = 0°), (b) H-plane (phi = 90°).

Figure 20. Simulated and measured LHCP and RHCP, radiation patterns at 9 GHz: (a) E-plane (phi = 0°), (b) H-plane (phi = 90°).

Figure 21. Measured vs simulated radiation pattern polar plot (theta vs dBi): (a) E-plane (phi = 0) at 7.5 GHz, (b) H-plane (phi = 90) at 7.5 GHz, (c) E-plane (phi = 0) at 8.15 GHz, (d) H-plane (phi = 90) at 8.15 GHz, (e) E-plane (phi = 0) at 9 GHz, (f) H-plane (phi = 90) at 9 GHz.

Figure 22. Measured vs simulated reflection-coefficient |S₁₁| of the SFSA-RI antenna.

Figure 23. Measured vs simulated axial-ratio (AR) of the SFSA-RI antenna.

Figure 24. Measured vs simulated gain of the SFSA-RI antenna.

Figure 25. Measured vs simulated antenna efficiency of the SFSA-RI antenna.

5. Final assessment and comparison with recent X-band antenna designs

This section presents a comprehensive comparison of the proposed, spidron-fractal slot antenna with rectangular island (SFSA-RI), with several recent X-band antenna designs as given in Table 3. The proposed design shows remarkable performance in terms of bandwidth, small size, and circular polarization in the required band. The table provides a detailed comparison of different antenna designs in terms of bandwidth, AR 3-dB bandwidth, maximum gain, and antenna size. It is evident that the proposed design outperforms other designs in terms of bandwidth, with a bandwidth of 4.2 GHz extending from 6.8 GHz to 11 GHz, which is exceptionally wide compared to the other designs. Moreover, the proposed design achieves a wideband circular polarization with a 3-dB AR of 3 GHz extending from 7 GHz to 10 GHz. The proposed design also demonstrates a small size with dimensions of 19.7 mm x 23 mm x 0.508 mm (0.55λ₀ x 0.65λ₀ x 0.014λ₀), which is significantly smaller than most of the other designs while still achieving a maximum gain of 6.8 dBi. The proposed antenna can be used for multiple applications unlike other designs. The antenna design being proposed is suitable for both the downlink (7.25 GHz - 7.75 GHz) and uplink (7.9 GHz - 8.4 GHz) frequency bands in the X-band satellite communication. Furthermore, it can also be used in small satellite Synthetic-Aperture-Radar (SAR) systems that operate within the frequency range of 9 GHz to 10 GHz. The results of our study demonstrate that our proposed X-band antenna design offers superior performance compared to existing designs and has the potential to advance the field of X-band applications significantly.

Table 3. Comparison with most recent designs of X-band antennas.

References	Bandwidth (Hz)	Axial-ratio 3-dB Bandwidth (Hz)	Dual Circular Polarization	Maximum Gain (dBi)	Antenna Size (mm^3)
(Arnaud et al., 2020)	8.025-8.4 GHz (370 MHz)	8.025-8.4 GHz (370 MHz)	Yes	5 dBi	Diameter: 238 mm (6.52λ0) Height: 185 mm (5.13λ0)
(Leszkowska et al., 2021)	8-9.7 GHz (1.7 GHz)	8.3-8.88 GHz (580 MHz)	No	14 dBi	(Length × Width × Height): 62 mm × 62 mm × 22 mm (1.77λ0 × 1.77λ0 × 0.63λ0)
(Fouany et al., 2017)	8-8.4 GHZ (400 MHz)	8-8.4 GHZ (400 MHz)	No	5 dBi	(Length × Width × Height): 100 mm × 100 mm × 15 mm (2.74λ0 × 2.74λ0 × 0.41λ0)
(Genovesi & Dicandia, 2022)	6.8-8.9 GHz (2.1 GHz)	7-8.15 GHz (1.15 GHz)	No	8.6 dBi	Diameter: 50 mm (1.3λ0) Height: 5.25 mm (0.137λ0)
(El Khamlichi et al., 2021)	9.4-10.5 GHz (1.1 GHz)	Not Circularly Polarized	No	9 dBi	(Length × Width × Height): 59 mm × 14 mm × 0.76 mm (1.96λ0 × 0.46λ0 × 0.025λ0)
(Anim et al., 2021)	9.01-10.2 GHz (1.19 GHz)	Not Circularly Polarized	No	8 dBi	(Length × Width × Height): 14 mm × 14 mm × 3.986 mm (0.45λ0 × 0.45λ0 × 0.126λ0)
This work	6.8-11 GHz (4.2 GHz)	7 GHz-10 GHz (3 GHz)	Yes	6.8 dBi	(Length × Width × Height): 19.7 mm × 23 mm × 0.508 mm (0.55λ0 × 0.65λ0 × 0.014λ0)

6. Conclusion

This paper presents the design, simulation, fabrication, and testing of a spidron-fractal slot antenna with embedded rectangular island (SFSA-RI). The paper introduces and demonstrates a method for enhancing the bandwidth, and axial-ratio (AR) of a fractal slot antenna by using an embedded rectangular island. The designed antenna achieved a wide bandwidth of 4.2 GHz, covering the X-band range from 6.8 GHz to 11 GHz. It also exhibited circular polarization with simultaneous dual circular polarization (RHCP and LHCP). The AR 3-dB bandwidth was measured to be 3 GHz, ranging from 7 GHz to 10 GHz. The antenna design is particularly suitable for both the X-band satellite downlink (from 7.25 GHz to 7.75 GHz) and uplink (from 7.9 GHz to 8.4 GHz) frequency bands as well as Synthetic-Aperture-Radar (SAR) on small satellites in the band from 9 GHz to 10 GHz. The proposed antenna realized a maximum gain of 6.8 dBi, with overall size of 19.7 x 23 x 0.508 mm³. The compactness and efficiency of the design are noteworthy, with a total antenna efficiency of 98%. These results show that the proposed antenna design is well-optimized and capable of delivering outstanding performance. The reliability of the design approach was verified through measurements on a fabricated prototype, further validating its effectiveness.

Data availability statement

The data that support the findings of this study are available from the corresponding author, Mostafa Mahmoud Rabie, upon reasonable request.

Disclosure statement

No potential conflict of interest was reported by the author(s).