Quad-Port Circularly Polarized MIMO Antenna with Wide Axial Ratio

This article studies a quad-port multi-input-multi-output (MIMO) circularly polarized antenna with good isolation properties. Using characteristic mode analysis (CMA), the first six distinct modes of the asymmetric square slot with an inverted L-strip are analyzed. In this study, modal parameter extraction is carried out for circular polarization (CP) radiation. A simple annular ring microstrip feed is excited to obtain broadband CP based on CMA. The single-unit feeding structure is replicated orthogonally four times to achieve a CP MIMO antenna. This antenna provides port isolation of more than 21 dB without the use of an additional decoupling element. The quad-port CP-MIMO antenna is simulated with a total dimension of 50 × 50 mm2. The antenna attains impedance matching (S11 < −10 dB) from 5.37 GHz to beyond 11 GHz with an axial ratio bandwidth (ARBW) of 4.65 GHz (5.61 GHz to 10.26 GHz). The peak realized gain of the MIMO antenna is measured at 5.69 dBi at 8.4 GHz. Additionally, the diversity performance parameters of the MIMO structure are computed. The advantages of the proposed structure have been evaluated by comparing it to previously reported MIMO structures. A prototype of the MIMO structure measurements was found to match the simulation results.


Introduction
The rapid advancement of wireless communications services is attributed to the increasing demand for higher transmission rates and significant transmission capacity. However, it has become apparent that the limited frequency spectrum is the primary barrier to the advancement of wireless communication. To make out of the confined frequency spectrum resource overhead, the multiple-input multiple-output (MIMO) technology has been developed and intensively investigated [1,2]. The MIMO antenna has the potential to significantly increase channel capacity and thus improve spectral efficiency and reliability [3]. In the design of a MIMO antenna, the four most important parameters such as space, operating bandwidth (BW), polarization, and mutual coupling are considered.
When designing the MIMO antenna, the antenna elements should fit within the limited space available. This is a very difficult task for a practical communication system. Because of the limited space, the antenna elements are tightly packed, so that the elements are strongly coupled together. In addition, there is a potential for interference between two antenna elements, especially around feeding ports, due to their location on the same ground plane [4]. To address this issue, many studies have concentrated on decoupling methods [5][6][7][8][9][10][11][12][13][14][15][16][17][18]. The insertion of different slits into the ground plane is the easiest way to suppress the coupling [5][6][7]. Alternatively, parasitic elements [8,9] can be placed between or in proximity to MIMO elements. Here, feeding techniques or the use of multiple-layer methods increase the complexity. In addition, the use of a defective ground (DG) [10] UWB slot antenna was proposed. However, the structure comprises only two element MIMO antennas. In addition, metamaterials [11][12][13], electromagnetic bandgap (EBG) [14][15][16], and Unlike the above-mentioned MIMO structures, this work addresses a simple quadport CP-MIMO antenna with CMA. To take the advantage of CMA, the antenna features can be extracted in an appropriate way. In addition, it highlights the lack of physical understanding in the development of the basic antenna design before any feed is applied for excitation [37]. The CMA gives details about antenna resonances that can be predicted by extracting modal parameters. These parameters can be considered to be modal significance (MS), and characteristic angle (CA). Based on MS and CA, a simple coplanar waveguide slot antenna design is proposed for examining CP radiation. More precisely, the MS and CA help to generate CP radiation [38,39]. These parameters make it possible to optimize the IPBW and ARBW. Further, based on the single-slot antenna, the design is extended to a four-port MIMO configuration without impacting the parameters of an individual element.
The four-port CP-MIMO provides IPBW (S 11 < −10 dB) of >65% (5.37 GHz to beyond 11 GHz) and ARBW (<3 dB) of 58.6% (5.61 GHz to 10.26 GHz). The measured isolation between the ports is observed to be greater than 15 dB. The envelope correlation coefficient (ECC) and diversity gain (DG) are observed below 0.003 and 10 dB, respectively. The total active reflection coefficient (TARC) (0.0001 dB) and mean effective gain (MEG) (−3 dB) within the band are extracted. The antenna supports C-band uplink and X-band(military) both uplink and downlink applications(military). In addition, the diversity parameters also show little dependence on the excitation phase in the design. The computer simulation tool is used to generate all simulation results throughout the overall design process. The organization of the paper is divided into the following sections: Section 2 presents the design and evaluation of a four-port MIMO antenna. This section covers the evaluation of CP radiation using CMA followed by a four-port antenna design procedure. Section 3 presents the results and discussion associated with various simulated and measured parameters, followed by a comparison of the related literature. Section 4 deals with the conclusion of the proposed design.

Evolution of CP-Radiation Using CMA
This section describes the evolution of the modal behavior for employing CP behavior without feeding into the design, as shown in Figure 1. To achieve the designated structure shown in Figure 1, various other sequential steps have been analyzed. Those are not represented here for the sake of brevity. The CMA is analyzed for the first six modes as shown in Figure 2. From the MS parameter, significant orthogonal modes such as modes 2, 4, 5, and 6 are observed at 5.32 GHz, 6.80 GHz, 9.61 GHz, and 10.40 GHz, respectively. In addition, the MS indicates that modes 1 and 3 are no longer significant within the specified band. Broader frequency bandwidth is achieved through significant modes. Furthermore, the intersection point of the significant modes offers to analyze the CP behavior of the antenna. From Figure 2, it is observed at 6.19 GHz, 8.4 GHz, and 9.9 GHz between mode 2 and mode 4, mode 4 and mode 5, and mode 5 and mode 6, respectively. The MS value at the above three frequencies is reported as 0.81, 0.98, and 0.76. Moreover, the CA observed simultaneously at these frequencies are 71.7 • , 73.3 • , and 77.7 • , respectively. From all the intersecting points except mode 5 and mode 6, the CA stabilizes to more than 70 • , which contributes to CP radiation by providing a proper feeding in the design with an additional phase shift.

Evolution of CP-Radiation Using CMA
This section describes the evolution of the modal behavior for employing CP behavior without feeding into the design, as shown in Figure 1. To achieve the designated structure shown in Figure 1, various other sequential steps have been analyzed. Those are not represented here for the sake of brevity. The CMA is analyzed for the first six modes as shown in Figure 2. From the MS parameter, significant orthogonal modes such as modes 2, 4, 5, and 6 are observed at 5.32 GHz, 6.80 GHz, 9.61 GHz, and 10.40 GHz, respectively. In addition, the MS indicates that modes 1 and 3 are no longer significant within the specified band. Broader frequency bandwidth is achieved through significant modes. Furthermore, the intersection point of the significant modes offers to analyze the CP behavior of the antenna. From Figure 2, it is observed at 6.19 GHz, 8.4 GHz, and 9.9 GHz between mode 2 and mode 4, mode 4 and mode 5, and mode 5 and mode 6, respectively. The MS value at the above three frequencies is reported as 0.81, 0.98, and 0.76. Moreover, the CA observed simultaneously at these frequencies are 71.7°, 73.3°, and 77.7°, respectively. From all the intersecting points except mode 5 and mode 6, the CA stabilizes to more than 70°, which contributes to CP radiation by providing a proper feeding in the design with an additional phase shift. Table 2 summarizes the modal parameters.

Evolution of CP-Radiation Using CMA
This section describes the evolution of the modal behavior for employing CP behavior without feeding into the design, as shown in Figure 1. To achieve the designated structure shown in Figure 1, various other sequential steps have been analyzed. Those are not represented here for the sake of brevity. The CMA is analyzed for the first six modes as shown in Figure 2. From the MS parameter, significant orthogonal modes such as modes 2, 4, 5, and 6 are observed at 5.32 GHz, 6.80 GHz, 9.61 GHz, and 10.40 GHz, respectively. In addition, the MS indicates that modes 1 and 3 are no longer significant within the specified band. Broader frequency bandwidth is achieved through significant modes. Furthermore, the intersection point of the significant modes offers to analyze the CP behavior of the antenna. From Figure 2, it is observed at 6.19 GHz, 8.4 GHz, and 9.9 GHz between mode 2 and mode 4, mode 4 and mode 5, and mode 5 and mode 6, respectively. The MS value at the above three frequencies is reported as 0.81, 0.98, and 0.76. Moreover, the CA observed simultaneously at these frequencies are 71.7°, 73.3°, and 77.7°, respectively. From all the intersecting points except mode 5 and mode 6, the CA stabilizes to more than 70°, which contributes to CP radiation by providing a proper feeding in the design with an additional phase shift. Table 2 summarizes the modal parameters.    Based on the above analysis, it is observed that all three orthogonal modes are appropriate for CP by considering the additional phase shift from the feed network. To achieve the additional phase shift, the parametric variation of the feed length is carried out by excitation from the bottom center of the slot. By varying the feed length, the AR is observed around 6 GHz, with narrow ARBW. In the next step to improve the ARBW, an annular ring is attached to the feeder structure. Because of this annular ring, the extra phase shift is intended for other higher-order modes that generate the wide ARBW. To understand the CP behavior, the surface current distribution analysis is analyzed at some arbitrary frequency (7.5 GHz) within the band. The surface current analysis with different phase instances is shown in Figure 3. It can be shown that surface current has similar magnitudes, but quadrature is in phase, indicating that CP attributes are predicted. As shown, the top left of the asymmetric patch rotates counterclockwise (CCW) with a phase difference of 90 • at all phases. In addition, there is a 90 • -phase differential in all phase variations from 0 • to 270 • at these positions. Moreover, it is also observed that the current is distributed clockwise (CW) at the point of intersection of the upper edge between the annular ring and the feed line. With CCW rotation on the asymmetric slot patch and CW rotation on the feedline, it is concluded the antenna presents left-hand CP. Further, to investigate the MIMO antenna, the single CP antenna is comprised of four-identical antennas located orthogonally to each other. The analysis and design process is presented in the following sections.  Based on the above analysis, it is observed that all three orthogonal modes are appropriate for CP by considering the additional phase shift from the feed network. To achieve the additional phase shift, the parametric variation of the feed length is carried out by excitation from the bottom center of the slot. By varying the feed length, the AR is observed around 6 GHz, with narrow ARBW. In the next step to improve the ARBW, an annular ring is attached to the feeder structure. Because of this annular ring, the extra phase shift is intended for other higher-order modes that generate the wide ARBW. To understand the CP behavior, the surface current distribution analysis is analyzed at some arbitrary frequency (7.5 GHz) within the band. The surface current analysis with different phase instances is shown in Figure 3. It can be shown that surface current has similar magnitudes, but quadrature is in phase, indicating that CP attributes are predicted. As shown, the top left of the asymmetric patch rotates counterclockwise (CCW) with a phase difference of 90° at all phases. In addition, there is a 90°-phase differential in all phase variations from 0° to 270° at these positions. Moreover, it is also observed that the current is distributed clockwise (CW) at the point of intersection of the upper edge between the annular ring and the feed line. With CCW rotation on the asymmetric slot patch and CW rotation on the feedline, it is concluded the antenna presents left-hand CP. Further, to investigate the MIMO antenna, the single CP antenna is comprised of four-identical antennas located orthogonally to each other. The analysis and design process is presented in the following sections.   Figure 4 illustrates the structure of an annular ring microstrip feed quad-port CP MIMO antenna. It occupies the dimensions of 50 × 50 mm 2 . It is designed on a single-layer FR-4 substrate with a 0.4 mm thickness. Of the four ports, a single-element design consists of an asymmetrical square slot with an inverted L-strip. The annular ring is attached to the feed line to achieve CP radiation. The designed single port overall dimensions are depicted in Table 3.  The design process of the quad-port CP-MIMO antenna from a single port is described in three different stages, as shown in Figure 5. In the first stage, a single port CP antenna is designed with 25 × 25 mm 2 , as illustrated in Figure 5a. In the second stage, the single port antenna is mirrored with an 180° phase shift, as shown in Figure 5b. It occupies a size of 50 × 25 mm 2 . At the final stage, it is reproduced on 50 × 50 mm 2 dimensions with  Figure 4 illustrates the structure of an annular ring microstrip feed quad-port CP MIMO antenna. It occupies the dimensions of 50 × 50 mm 2 . It is designed on a single-layer FR-4 substrate with a 0.4 mm thickness. Of the four ports, a single-element design consists of an asymmetrical square slot with an inverted L-strip. The annular ring is attached to the feed line to achieve CP radiation. The designed single port overall dimensions are depicted in Table 3.  Figure 4 illustrates the structure of an annular ring microstrip feed quad-port CP MIMO antenna. It occupies the dimensions of 50 × 50 mm 2 . It is designed on a single-layer FR-4 substrate with a 0.4 mm thickness. Of the four ports, a single-element design consists of an asymmetrical square slot with an inverted L-strip. The annular ring is attached to the feed line to achieve CP radiation. The designed single port overall dimensions are depicted in Table 3.  The design process of the quad-port CP-MIMO antenna from a single port is described in three different stages, as shown in Figure 5. In the first stage, a single port CP antenna is designed with 25 × 25 mm 2 , as illustrated in Figure 5a. In the second stage, the single port antenna is mirrored with an 180° phase shift, as shown in Figure 5b. It occupies a size of 50 × 25 mm 2 . At the final stage, it is reproduced on 50 × 50 mm 2 dimensions with  The design process of the quad-port CP-MIMO antenna from a single port is described in three different stages, as shown in Figure 5. In the first stage, a single port CP antenna is designed with 25 × 25 mm 2 , as illustrated in Figure 5a. In the second stage, the single port antenna is mirrored with an 180 • phase shift, as shown in Figure 5b. It occupies a size of 50 × 25 mm 2 . At the final stage, it is reproduced on 50 × 50 mm 2 dimensions with a quadrature phase, as depicted in Figure 5c. These three antennas from Figure 5a-c are denoted as Ant @ 1, to Ant @ 3, respectively. Their simulated S 11 and AR are shown in Figure 6. The Ant @ 1 offers an IPBW < −10 dB from 5.41 GHz to 10.54 GHz, and ARBW from 5.92 GHz to 9.50 GHz. While the IPBW and ARBW are observed for Ant @2 from 5.56 GHz to beyond 11 GHz and 5.69 GHz to 10.05 GHz, respectively. The final stage antenna named Ant @3 provides IPBW beyond 11 GHz from 5.59 GHz and ARBW from 5.54 GHz to 10.31 GHz. Additionally, a slight deviation of the ARBW from the broadside direction at higher frequencies was observed. The design provides isolation of more than 18 dB without employing special isolation improvement techniques. Additionally, it is noticed that the realized peak gains (simulated) of single-port, dual-port, and quad-port antennas are 3.66 dBi, 4.55 dBi, and 5.48 dBi, respectively. The comparative analysis of the foregoing discussion is summarized in Table 4. a quadrature phase, as depicted in Figure 5c. These three antennas from Figure 5a-c are denoted as Ant @ 1, to Ant @ 3, respectively. Their simulated S11 and AR are shown in Figure 6. The Ant @ 1 offers an IPBW < −10 dB from 5.41 GHz to 10.54 GHz, and ARBW from 5.92 GHz to 9.50 GHz. While the IPBW and ARBW are observed for Ant @2 from 5.56 GHz to beyond 11 GHz and 5.69 GHz to 10.05 GHz, respectively. The final stage antenna named Ant @3 provides IPBW beyond 11 GHz from 5.59 GHz and ARBW from 5.54 GHz to 10.31 GHz. Additionally, a slight deviation of the ARBW from the broadside direction at higher frequencies was observed. The design provides isolation of more than 18 dB without employing special isolation improvement techniques. Additionally, it is noticed that the realized peak gains (simulated) of single-port, dual-port, and quad-port antennas are 3.66 dBi, 4.55 dBi, and 5.48 dBi, respectively. The comparative analysis of the foregoing discussion is summarized in Table 4. a quadrature phase, as depicted in Figure 5c. These three antennas from Figure 5a-c are denoted as Ant @ 1, to Ant @ 3, respectively. Their simulated S11 and AR are shown in Figure 6. The Ant @ 1 offers an IPBW < −10 dB from 5.41 GHz to 10.54 GHz, and ARBW from 5.92 GHz to 9.50 GHz. While the IPBW and ARBW are observed for Ant @2 from 5.56 GHz to beyond 11 GHz and 5.69 GHz to 10.05 GHz, respectively. The final stage antenna named Ant @3 provides IPBW beyond 11 GHz from 5.59 GHz and ARBW from 5.54 GHz to 10.31 GHz. Additionally, a slight deviation of the ARBW from the broadside direction at higher frequencies was observed. The design provides isolation of more than 18 dB without employing special isolation improvement techniques. Additionally, it is noticed that the realized peak gains (simulated) of single-port, dual-port, and quad-port antennas are 3.66 dBi, 4.55 dBi, and 5.48 dBi, respectively. The comparative analysis of the foregoing discussion is summarized in Table 4.   Figure 7a shows the fabricated dual-and four-port CP-MIMO antenna. The simulated S-parameters of dual and quad port results are validated with measured results using a vector network analyzer. Figure 7b,c shows the S parameter comparison of dual-and quadport MIMO antenna, respectively. The measured reflection parameters of dual-port antenna are seen from 5.40 GHz to over 11 GHz. This will result in a deviation of 0.16 GHz in the lower spectrum bands. The transmission parameter (S 12 ) isolates more than 21 dB and is observed in simulation at more than 18 dB at 7.1 GHz. Similarly, the quad-port antenna measured |S 11 | results are seen beyond 11 GHz from 5.37 GHz. This result in a deviation of 0.22 GHz to a lower frequency. In addition, it can be seen that the isolation difference between simulation to measurement is greater than 3 dB. In the simulation, it is noted as more than 18 dB and when measured as 21 dB. This will indicate the antenna provides better performance in the MIMO system. Furthermore, this antenna covers military X-band uplinks and downlinks as well as C-band downlinks applications.   Figure 7a shows the fabricated dual-and four-port CP-MIMO antenna. The simulated S-parameters of dual and quad port results are validated with measured results using a vector network analyzer. Figure 7b,c shows the S parameter comparison of dualand quad-port MIMO antenna, respectively. The measured reflection parameters of dualport antenna are seen from 5.40 GHz to over 11 GHz. This will result in a deviation of 0.16 GHz in the lower spectrum bands. The transmission parameter (S12) isolates more than 21 dB and is observed in simulation at more than 18 dB at 7.1 GHz. Similarly, the quad-port antenna measured |S11| results are seen beyond 11 GHz from 5.37 GHz. This result in a deviation of 0.22 GHz to a lower frequency. In addition, it can be seen that the isolation difference between simulation to measurement is greater than 3 dB. In the simulation, it is noted as more than 18 dB and when measured as 21 dB. This will indicate the antenna provides better performance in the MIMO system. Furthermore, this antenna covers military X-band uplinks and downlinks as well as C-band downlinks applications.     Figure 8b, the difference in gain achieved with the measurement values is observed at 0.21 dBi and 0.17 dBi for quad and dual ports. The peak measured gain is observed as 5.69 dBi and 4.72 dBi, respectively. The foregoing discussion is summarized in Tables 5 and 6.  The direction of CP in the far field is measured at 6.0 GHz, 7.5 GHz, and 10.0 GHz for the quad port antenna in the left-hand CP (LHCP) and right-hand CP (RHCP). Figure  9 illustrates the comparison of simulation and measurement patterns in the x-z (0°) and y-  The direction of CP in the far field is measured at 6.0 GHz, 7.5 GHz, and 10.0 GHz for the quad port antenna in the left-hand CP (LHCP) and right-hand CP (RHCP). Figure 9 illustrates the comparison of simulation and measurement patterns in the x-z (0 • ) and y-z (=90 • ) planes. Deviations in results are due to fabrication and measurement errors. According to the radiation patterns, the magnitude of both planes is identical to their respective frequencies. From the main lobe direction, the magnitude of the simulated LHCP in the x-z plane is observed to be 2.92 dBi, 3.12 dBi, and 4.28 dBi at 6.0 GHz, 7.5 GHz, and 10.0 GHz, respectively, whereas the magnitude of LHCP in the y-z plane is 1.15 dBi, 3.76 dBi, and 3.16 dBi in the direction of the main lobe, respectively. Additionally, the magnitude difference between LHCP and RHCP in the x-z plane between 0° and 180° is 0.03 dBi, 0.06 dBi, and 0.1 dBi, respectively, for 6.0 GHz, 7.5 GHz, and 10.0 GHz. The magnitude difference in the y-z plane at the above frequencies is 0.01 dBi, 0.07 dBi, and 0.11 dBi, respectively. In all frequencies, the RHCP component is somewhat larger than the LHCP component, and the magnitude difference is within acceptable limits. A pattern at lower and medium frequencies is almost in boresight, whereas a pattern at higher frequencies is slightly off in the broadside direction.

Result and Discussion
Further, this article includes other surpassing parameters for the quad-port antenna in order to demonstrate its effectiveness regardless of the excitation phase. An essential parameter in this regard is the ECC. Essentially, it is a measurement of the correlation between the different antenna elements in a MIMO structure. An ECC is calculated from Additionally, the magnitude difference between LHCP and RHCP in the x-z plane between 0 • and 180 • is 0.03 dBi, 0.06 dBi, and 0.1 dBi, respectively, for 6.0 GHz, 7.5 GHz, and 10.0 GHz. The magnitude difference in the y-z plane at the above frequencies is 0.01 dBi, 0.07 dBi, and 0.11 dBi, respectively. In all frequencies, the RHCP component is somewhat larger than the LHCP component, and the magnitude difference is within acceptable limits. A pattern at lower and medium frequencies is almost in boresight, whereas a pattern at higher frequencies is slightly off in the broadside direction.
Further, this article includes other surpassing parameters for the quad-port antenna in order to demonstrate its effectiveness regardless of the excitation phase. An essential parameter in this regard is the ECC. Essentially, it is a measurement of the correlation between the different antenna elements in a MIMO structure. An ECC is calculated from S-parameters or a far-field Equations (1) and (2). In the case of any two-element MIMO antenna, it is defined as follows [40,41]: where → R i (θ, φ) refers to the radiation pattern when the ith port of the mn MIMO antenna system is energized. Additionally, the • symbol denotes the Hermitian product.
As shown in Figure 10, S-parameters are used to compute ECC in simulated environments. Ideally, the value should be less than 0.5. For all port combinations in the designated range (ECC 12 , ECC 13 , ECC 14 , ECC 23 , ECC 24 , and ECC 34 ), the proposed four-port CP-MIMO antenna is less than 0.003. It is essential to maintain this low-value requirement to prevent the degrading of the signal-to-noise ratio under diverse circumstances. There is also another parameter involved in this condition called DG. This is the parameter used to detect multiple path signals that are transmitted by the same transmitter. Equation (3) is used to calculate it [42]. S-parameters or a far-field Equations (1) and (2). In the case of any two-element MIMO antenna, it is defined as follows [40,41]: where ( , ) R i  refers to the radiation pattern when the ith port of the mn MIMO antenna system is energized. Additionally, the  symbol denotes the Hermitian product.
As shown in Figure 10, S-parameters are used to compute ECC in simulated environments. Ideally, the value should be less than 0.5. For all port combinations in the designated range (ECC12, ECC13, ECC14, ECC23, ECC24, and ECC34), the proposed four-port CP-MIMO antenna is less than 0.003. It is essential to maintain this low-value requirement to prevent the degrading of the signal-to-noise ratio under diverse circumstances. There is also another parameter involved in this condition called DG. This is the parameter used to detect multiple path signals that are transmitted by the same transmitter. Equation (3) is used to calculate it [42]. For the proposed CP-MIMO antenna, DG is less than 10 dB for all cases, as shown in Figure 11. It demonstrates that the antenna performs adequately in terms of diversity. The above-mentioned parameters of the MIMO antenna are not taken into account with the impact of the input signal phase. From this, another key parameter called TARC should be taken into consideration. It measures the reflecting coefficient of the signal phases. TARC is calculated for a multiport antenna system using the following formula (4) [43]. For the proposed CP-MIMO antenna, DG is less than 10 dB for all cases, as shown in Figure 11. It demonstrates that the antenna performs adequately in terms of diversity. The above-mentioned parameters of the MIMO antenna are not taken into account with the impact of the input signal phase. From this, another key parameter called TARC should be taken into consideration. It measures the reflecting coefficient of the signal phases. TARC is calculated for a multiport antenna system using the following Equation (4) [43].
where b i and a i are the reflected and incident signals, respectively. These are related to the S parameters matrix as b = Sa where Figure 11. Simulated diversity gain of quad-port antenna.
Considering a two-port MIMO antenna, TARC is evaluated as (7).
The TARC values for the four-port antenna are shown in Figure 12   Another important MIMO diversity parameter is called MEG, which quantifies the mutual interaction between antenna elements and the statistical properties of the propagation environment. The MEG is helpful in understanding the power imbalance from various parameters such as gain and total efficiency. MEG is the ratio of the MIMO antenna receiving power to the total power incident. It is calculated using Equation (8) [44]. Considering a two-port MIMO antenna, TARC is evaluated as (7).
The TARC values for the four-port antenna are shown in Figure 12  Considering a two-port MIMO antenna, TARC is evaluated as (7).
The TARC values for the four-port antenna are shown in Figure 12   Another important MIMO diversity parameter is called MEG, which quantifies the mutual interaction between antenna elements and the statistical properties of the propagation environment. The MEG is helpful in understanding the power imbalance from various parameters such as gain and total efficiency. MEG is the ratio of the MIMO antenna receiving power to the total power incident. It is calculated using Equation (8)   Another important MIMO diversity parameter is called MEG, which quantifies the mutual interaction between antenna elements and the statistical properties of the propagation environment. The MEG is helpful in understanding the power imbalance from various parameters such as gain and total efficiency. MEG is the ratio of the MIMO antenna receiving power to the total power incident. It is calculated using Equation (8) [44].
where k = number of antennas, I = antenna under construction. Therefore, The MEG difference of any two antenna elements should not be more than −3 dB. Figure 13 shows the MEG ij of the four-port antenna. This demonstrates the diversity performance of the proposed antenna. Finally, to show the advantage of the proposed design, Table 7 presents the comparison of the four-port CP-MIMO antenna with the MIMO antenna designs in the literature. The comparison includes size, number of ports, polarization type, IPBW, ARBW, and peak gain. In comparison with the literature except [34], the proposed antenna provides wide ARBW. Compared to the proposed structure, this structure is relatively large.
The MEG difference of any two antenna elements should not be more than −3 dB. Figure 13 shows the MEGij of the four-port antenna. This demonstrates the diversity performance of the proposed antenna. Finally, to show the advantage of the proposed design, Table 7 presents the comparison of the four-port CP-MIMO antenna with the MIMO antenna designs in the literature. The comparison includes size, number of ports, polarization type, IPBW, ARBW, and peak gain. In comparison with the literature except [34], the proposed antenna provides wide ARBW. Compared to the proposed structure, this structure is relatively large.

Conclusions
A simple quad-port CP-MIMO antenna is fabricated, characterized, and compared to its simulation results. A single antenna element is arranged orthogonally across 360 • to form four identical antenna elements for quad port MIMO. The proposed CP-MIMO antenna design provides an IPBW > 64.9% and ARBW of 58.6%. The antenna covers both the C-band uplink and X-band uplink and downlink applications. The CP-MIMO antenna maintains isolation greater than 21 dB in the operating band. The final fabricated antenna achieves a gain of 5.69 dBi. It is obvious that the antenna presented is well represented in terms of diversity (ECC < 0.001, DG 10 dB, TARC 0.0001 dB, MEG −3 dB), insulation, impedance correspondence, and AR. From a future perspective, the CMA approach to antenna design becomes attractive if CP antennas can satisfy the requirements of long-distance communication as done by high gain linearly polarised antennas. This approach is also useful in demonstrating concurrent MIMO operations and boosting channel capacity and throughput. Further research is being conducted on eight port antennas to improve other important parameters, such as gain, efficiency, compactness, and dual polarization for 5G and mm-wave applications.