This work proposes, builds, and manufactures a small, high-gain MIMO antenna for radar applications. Using a Parasitic elements such as director & reflector show a simple method for increasing the gain and bandwidth of a Yagi-Uda Antenna. Using a Parasitic elements such as director and reflector show a simple method for increasing the gain and bandwidth of a Yagi-Uda Antenna. Using a nearby parasitic director and reflector. Also, it shows a simple method for increasing the gain and bandwidth of a Yagi–Uda antenna. A strip with a parasitic director is placed close to the top dipole element to improve the gain as well as bandwidth attributes of the Yagi–Uda antenna. The antenna performance parameters are investigated for 1 × 2 MIMO, cross dipole MIMO antenna (1 × 4), and an array configuration (two 1 × 4). It has high gain and operates in the S-band (2 GHz–4 GHz). This antenna supports three bands and has a highest maximum directive gain of 12.5 dBi at 2.5 GHz and a bandwidth of 0.21 GHz. It is made of FR4 substrate. The suggested structure is 120 mm × 70 mm in size. Due to its high gain and less return loss, this antenna is better suited to radar applications.

1. Introduction

An antenna is a device that converts electrical power into radio waves and the other way around. An antenna is essential in wireless communications. MIMO (multiple input multiple output) is a wireless antenna system in which many antennas are used at the transmission and reception. In comparison to conventional radar, MIMO radar allows for simultaneous transmission and reception by utilizing multiple antennas. The SISO channel’s performance is insufficient, and fading will have a greater impact on the system than a MIMO antenna. The addition of some arms to the basic dipole element enhances the bandwidth while improving the gain [1]. Because of its simple structure, improved bandwidth, and stable gain, a series-fed two-dipole antenna array was used in the base station antenna elements. Series-fed dipole pairs can produce broadband and dual-band characteristics. By changing the dimension of the dipole elements, the operating frequency could be varied, and the return loss could be controlled by varying the separation between the dipole elements [2]. The use of a parasitic strip pair director to enhance the bandwidth of a series-fed dipole pair (SDP) antenna is described [3]. A wideband planar Yagi–Uda antenna suitable for MIMO system design is presented, with working frequencies in the range from 1.8 GHz to 3.5 GHz [4].

For satellite application, a variety of antennas are designed using various techniques. A compact 4 × 4 MIMO antenna for UWB applications is built on a Rogers RT5870 substrate, but it has a low gain of 3 dBi [6]. A dual-band WLAN/UWB broad slot microstrip-fed antenna with FR4 substrate with reduced mutual coupling, reduced envelope correlation, and radiation efficiency of 85–95% is developed [7, 8]. A simple, small dual-polarized UWB MIMO antenna for Wi-MAX and WLAN systems is presented. A tiny MIMO antenna for WLAN, Wi-MAX, and UWB applications is given. The ramped ground stub protrudes vertically to increase isolation [9, 10].

A MIMO band-notched antenna with a small footprint of 22 × 36 mm2 is presented. Two meander lines, a connection line, and a short parasitic line are used to improve isolation between the two input ports. In order to create the band-notched characteristics, an open stub in the radiator was used. The antenna structure was somewhat difficult and needed more fabrication accuracy [11]. The antenna operates in the ISM band and has a center frequency of 2.45 GHz. There are eight Yagi–Uda antennas arranged in a circle. Each Yagi_Uda antenna consists of a bent reflector, abent driven and a bent director. PIN diodes [12] share the bent reflector for different Yagi–Uda antennas. For indoor applications, a dense packed coplanar waveguide (CPW)-fed MIMO antenna was developed. The antenna is 31 × 43 mm2 in size. The antenna covers a frequency range of 3.9 to 8.1 GHz. The mutual coupling between the antennas was less than −15 dB for practically the whole bandwidth, peaking at −31 dB at 5.4 GHz [13]. A wideband four-port MIMO antenna [25], flexible four-port MIMO antenna [26], and four-port ground-coupled MIMO antennas [27] were designed.

According to the literature review, the isolation, gain, and return loss are all low. The array MIMO antenna has a large size. To address the aforementioned concerns, a MIMO antenna using a cross dipole array MIMO antenna is developed in this attempt. The existing antennacharacteristics including Bandwidth and directivity are less. Several initiatives have been taken to realize the structure for S-band and UWB operations, where the array elements are joined serially or parallelly. As a solution, the cross dipole has been researched as an exhibit component in 1 × 2, 1 × 4, and two 1 × 4 MIMO cluster topologies. Different cluster structures have exhibited high directivity and an upgraded signal-to-noise proportion.

This paper describes a revolutionary small size crossed dipole Yagi–Uda MIMO microstrip patch antenna for S-band radar applications. A cross dipole is utilized related to a balun to accomplish high gain and adjusted feed activity. The balun is introduced and it matches the input impedance of a radio wire to the 50 Ω feed line [5]. To increase overall gain, a crossed dipole Yagi–Uda MIMO antenna (1 × 4) and an array arrangement (two 1 × 4) were studied for 1 × 2 MIMO. It is suited for radar communication because of its high gain, compact size, and balanced operation.

The rest of the paper is organized as follows. Section 2 describes the structure and design technique for the microstrip patch antenna. Section 3 shows how to build the crossed dipole Yagi–Uda MIMO antenna in detail. Section 4 explains the patch’s array implementation, followed by Section 5 which is devoted to the study of the measurements and the simulated results. Section 6 brings the paper to a close.

2. Construction of Antenna

On a FR4 substrate, the antenna presented here is 120 mm 70 mm in dimensions. Table 1 displays the antenna parameters and data. The antenna’s structural parameter values are as follows. The patch’s length (L) is 70 mm, and its width (W) is 120 mm. W3 = 35 mm between the ground plane and the first dipole (D2), and W2 = 35 mm between the first dipole (D2) and the next dipole (D1). W4 = 20 mm ground plane height, and L2 = 20 mm for the width of the coplanar strip line (WCPL). The substrate’s height (H) is 1.6 mm, and the balun’s length (WF) is 40 mm. The director’s length and width are W0 and W1, respectively.

As illustrated in Figure 1, this antenna is made up of four dipole antennas that are crossed to produce a cross dipole Yagi–Uda MIMO antenna. The size is scaled up by 0.8 to function at a lower operating frequency. FR4 is used due to its inexpensive raw material prices, simplicity of fabrication, and ability to give great performance in a broad range of purposes [14]. Table 2 describes the design specifications.

3. Design Procedure

3.1. Design of a Single Element Antenna

A new structure has been proposed to improve the operation of the antenna in the existing system. A novel construction is implemented and adjusted to build a single element antenna in order to obtain a high gain and efficiency. A single antenna is made up of multiple parallel elements in a line with two parts: Driven and Parasitic. l. The reflector element of the antenna is slightly longer than the driven dipole element, and the directors are slightly shorter as shown in Figure 2. When compared to a conventional antenna, this type of design provides a higher antenna gain, bandwidth, and efficiency. On a FR4 substrate (dielectric constant = 4.4, tanδ = 0.02), the proposed antenna is printed. Its operating frequency is 2.45 GHz. The structure’s feed line is fixed at the bottom of the patch, and a 50 Ω feed line is to be provided. The proposed antenna was designed; simulation of the antenna was performed using CST Studio Suite (simulation tool).

3.2. Design of Balun

A balun is required for the feeding mechanism needed to restore balance to an unbalanced transmission line. The balun is constructed in such a way that its input impedance equals 50 Ω, allowing the intended antenna to be supplied through the hole. An incorporated balun is constructed between the microstrip (MS) and CPS lines to match the antenna’s input impedance with a range of 50 Ω feed lines. The feed position is used to short the end of the MS line with a shorting pin. L2 represents the breadth of the CPS line.

3.3. Design of MIMO Antenna (1 × 2)

To make (1 × 2) MIMO, the two antennas are placed in series manner as shown in Figure 3.

3.4. Cross Dipole MIMO Antenna Design (1 × 4)

A cross dipole Yagi–Uda MIMO antenna is suggested to improve the gain and bandwidth of a series-fed two-dipole antenna (STDA). Antennas 1 and 2 are designed in series, followed by antennas 3 and 4. The antenna’s two pairs are cross-coupled to each other. Figures 4(a) and 4(b) show the top cut first pair of antennas and the top cut second pair of antennas. It converges each other to create the cross dipole MIMO antenna; as delineated in Figure 1, the general aspect is brought down in this work by joining the double elements to frame a cross dipole antenna. At the center, a pair of antennas is joined. When we cross-coupledthe antennas, it occupied a little space. MIMO antenna is installed on FR4 substrate. It uses four cross-coupled antennas to operate at different resonant frequencies. The four antennas have the same dimensions. The cross dipole antenna is made up of eight dipole elements that are same but phase opposite. The balun supplies two ports to each single dipole element. When formed into a cross dipole, this type of structure takes up less space.

The design steps of 1 × 4 MIMO antenna are illustrated in Figure 5. The first step in designing a single element is to design two circular patch dipoles with different lengths and a ground plane. The second step is to design a balun for balance and an unbalanced transmission line. The circular patches are connected to the coplanar strip (CPS) line using smooth shape progress. A pair of parasitic directors is appended above the radiating element to improve the gain, return loss, and bandwidth of antenna. In step 3, a two-element MIMO antenna is formed. The two-element MIMO antenna operates at two frequencies: 1.8 GHz and 2.45 GHz. Step 4 depicts the assembly of 1 × 4 MIMO antennas with dual frequencies of 2.18 GHz and 2.52 GHz in the S-band. When the two antennas are cross shaped, the 1 × 4 MIMO antenna takes up less space. It provides high gain.

3.5. Fabrication

Figure 6 depicts a photograph of a constructed cross dipole Yagi–Uda MIMO antenna. Figure 4(b) depicts the front view of the constructed MIMO antenna with a single top cut element, whereas Figure 4(a) shows the rear view of the constructed MIMO antenna with a single top cut element. The antenna’s entire size is approximately 120 mm × 70 mm = 8400 mm2.

4. Formation of Array

The Two (1 × 4) MIMO array is depicted in Figure 7. The distance between the two elements is 36 mm, as calculated by equation W/2 [15]. It consists of eight components and eight ports, which are labelled as S11, S22, S33, …, and S88, S11 for input port voltage reflection, S22 for output port voltage reflection, S21 for forward voltage gain, and S12 for reverse voltage gain. The input-output relationships between the ports are described by the scattered parameters.

5. Results and Discussion

5.1. The Single Antenna’s Return Loss (S11), Gain, and Directivity

Figure 8 shows a graph of the single antenna’s simulated and observed return loss (S11). At 2.5 GHz, the return loss of a single antenna is −33.85 dB. Figure 9 displays the 1 × 2 MIMO design’s simulated and measured S parameter (S11) findings. As demonstrated in Figure 9, the antenna has achieved broadband impedance matching in the S-band frequency range. The greatest return loss was around −45 dB. A dual resonance in the S-band was produced and calculated, as expected.

The simulated and measured results of the S parameter (S11) for the developed 1 × 4 MIMO antenna design are shown in Figure 10. This MIMO antenna has a return loss of −30 dB at 2.1 GHz and −44 dB at 2.5 GHz. It has dual-band capability. The simulation results of the S parameter (S11) for the developed the two (1 × 4) MIMO antenna design are depicted in Figure11. This MIMO antenna has a return loss of −30 dB at 2.1 GHz and −44 dB at 2.5 GHz. It has dual-band capability.

5.2. Parametric Analysis

A parametric analysis is carried out to demonstrate the fine adjustment of the proposed antenna’s port isolation and S parameters. The simulated S parameters of port 1 for varying the length of dipole 2 (L4) are shown in Figure 12. L4 results ranging from 62 mm to 66 mm are presented. Figure 12(a) shows very minor effects on S11. The antenna’s low band shifts to lower frequencies as the length of dipole 2 increases for S12 of two adjacent ports shown in Figure 12(b). S13 in Figure 12(c) exhibits better isolation.

The simulated S parameters of port 1 for varying the length of dipole 1 (L3) are shown in Figure 13. The results for L3 ranged from 32 mm to 36 mm. Figure 13(a) shows very minor effects on S11. The antenna’s high band is shifted to higher frequencies as the length of the dipole 1 increases for S12 of two adjacent ports shown in Figure 13(b). S13 in Figure 13(c) exhibits better isolation.

5.3. Radiation Pattern

Figures 14(a)14(d) depict the three-dimensional radiation pattern of a single antenna and 1 × 2, 1 × 4, and two 1 × 4 MIMO antennas. The simulated gain values of all antennas are displayed. The proposed antenna has a gain of 6.37 dBi for a single antenna, 9.32 dBi for (1 × 2) MIMO, 9.22 dBi for (1 × 4) MIMO and 12 dBi for two (1 × 4) MIMO antennas.

5.4. Gain

The proposed antenna emanates omnidirectional radiation and works in the S-band. Thus, the radiations are communicated and recuperated without misfortune in each open region, demonstrating that the radiation design gathered from the suggested MIMO antenna is the amount of the relative multitude of examples made by each exhibit part Figure 15 shows the simulated and measured radiation pattern of the single antenna at 2.4 GHz with the gain of 6.37 dBi which is being matched to the simulated pattern. Figure 16 shows the recreated and genuine radiation example of the 1 × 2 MIMO antenna with an increase of 9.32 dBi at 2.4 GHz.

5.5. Outcomes and Examination of the MIMO in Arrays

The proposed antenna beats traditional S-band radio wires regarding gain and input reflection coefficient (S11). The proposed antenna shows an improved performance in terms of return loss, gain over the existing antennas in an S-band. The comparison of gain and return loss between the proposed single antenna and MIMO array antennas are analysed. It is observed that there is a great variation in the antenna parameters between the single antenna and the formed array antennas. The performance of the antenna is significantly improved while there is an increase in the number of array elements, even at the cost of an increasing in size and complexity. The gain increases linearly when the radiating elements in the antenna are increased. The proposed antenna provides an efficient output by radiating each element By emanating every component, the proposed antenna produces an effective result.

5.6. Radiation Patterns of the Proposed Four-Port MIMO Antenna Array

Figure 17 depicts the measured and simulated radiation pattern in the E and H planes. The 2D patterns in the H plane are similar between antennas 1-2 and 3-4, whereas the E plane patterns are 180 degrees out of phase. Similarly, the antenna’s E plane patterns are similar. The H plane patterns are 180 degrees out of phase with antennas 1-4 and 2-3. This demonstrates that the proposed antenna has achieved pattern diversity, ensuring no interference during reception and maintaining an omnidirectional radiation pattern in both the E plane and the H plane. It is also noted that the deviation between the simulated and measured results is very small, and any discrepancies may be due to fabrication inaccuracies or unavoidable conductor losses in the cable used during measurement analysis.

5.7. Gain and Efficiency of the Proposed Four-Port MIMO Antenna Array

Figure 18 depicts the proposed four-port MIMO antenna array’s gain and efficiency. Because the other three antenna elements (antenna 2 to antenna 4) have identical structures, the gain and efficiency of antenna 1 are only measured here. The gain and efficiency results of antenna 1 measured from the proposed four-port MIMO antenna array are better than those of the two-port MIMO antenna array, as shown in this figure. The measured gain and efficiency are well above 9.22 dBi and 87%, respectively.

5.8. Gain and Return Loss Examination for Various Cluster Setups

The gain of a single antenna, 1 × 2 MIMO, 1 × 4 MIMO, and two 1 × 4 MIMO array antennas for S-band applications is dissected and looked at. The proposed antenna beats existing S-band antenna in terms of return loss and gain. From Table 3, analyzes the proposed antenna to exhibit array antennas. From the Table 3, it is noticed that there is a significant variation in the antenna parameters between the single antenna and the differently formed array antennas with various MIMO configurations. The performance of the antenna always improves with an increase in the number array elements, though it leads to an increase in size and complexity. VSWR is less than 2 for all the antennas.

5.9. Isolation

It utilizes the design of the type of antenna components to ascertain how much power is connected among neighboring antenna components. The simulated and measured isolation graph of the presented four-component MIMO antenna is displayed in Figure 19. The isolation between ports 1 and is not completely resolved, nor is the isolation between ports 1 and 3, ports 1 and 4, ports 2 and 3, and ports 3 and 2.

5.10. Envelope Correlation Coefficient

It is used to assess the feasibility of the suggested MIMO antenna. A low ECC necessitates the use of a MIMO antenna. The correlation between the branch signals acquired by the various elements is computed by ECC, and a lower ECC implies higher pattern variety [24]. The ECC between i and j is calculated using the following equation, assuming that the designed antenna is a lossless antenna in an isotropic scattering environment. The measured and simulated ECC values are given in Figure 20.

Between port 1 and port 2, port 1 and port 3, port 1 and port 4, port 2 and port 3, port 2 and port 4, and port 3 and port 4, the EEC is calculated. The ECC of 1 × 4 structure is less than 0.0001.

5.11. Diversity Gain (DG) of the Proposed Four-Port MIMO Antenna Array

The calculated DG values from the S parameters and far-field patterns are shown in Figure 21. The DG values are observed to be greater than 9.22 dB. This ensures that the proposed four-port MIMO antenna array has good diversity performance.

5.12. Total Active Reflection Coefficient (TARC)

The TARC is a parameter used to validate a MIMO antenna’s performance in a single attempt. TARC is the ratio of total incident power to total radiated power of a MIMO antenna array. The measured TARC values are shown in Figure 22. The TARC values are nearly constant across the operating bands, indicating that the proposed MIMO antenna system has achieved high port isolation.

Table 4 compares the functional parameters of the developed MIMO antenna with those of the reported antenna. The table shows that the return loss (S11), gain, isolation, and ECC are enhanced than the reported ones. Table 4 shows that our proposed scheme provides a better gain improvement. Above all, the proposed MIMO antenna provides a significant improvement in performance in all aspects. According to the comparison table, the proposed antenna outperforms the reported one. As a result, it could be integrated into real-time on-demand applications.

6. Conclusion

The proposed Yagi–Uda MIMO antenna is intended to outperform traditional MIMO antennas. In Microwave 13 Studio with CST, two prototype antennas are designed and then fabricated and tested for comparison. The size of the antenna is 120 mm × 70 mm. The proposed antenna has a return loss of −30 dB at 2.1 GHz and a loss of −44 dB at 2.5 GHz. At S-band, the antenna resonates at two frequencies. Furthermore, the two 1 × 4 element MIMO array antennas have a maximum gain of 12.1 dBi. For many radar applications, the suggested antenna is a strong competitor. MIMO cluster antenna with different dipole plans can accordingly be utilized for S-band broadband radar correspondence.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.