The effect of changing the formation of multiple input multiple output antennas on the gain

In this paper, different 2×1 and 2×2 multiple input multiple output (MIMO) antennas were investigated with changing substrate shapes and changing the placing of the patches on the substrate, all the investigated antennas based on FR-4 substrate are characterized by 𝑝𝑒𝑟𝑚𝑖𝑡𝑡𝑖𝑣𝑖𝑡𝑦 = 4.4 , and loss 𝑡𝑎𝑛𝑔𝑒𝑛𝑡 = 0.02 , with a partial ground. The original antenna covered 3.4 to 13.5 GHz. The best simulation results of the proposed 2×1 MIMO antenna received for 2×1 inverted with high ultra-wideband (UWB) with bandwidth up to 40 GHz, the received maximum gain was up to 6.51 dB, with an average gain of more than the original single antenna at about +1.27 dB. The best of eight 2×2 MIMO antennas configurations that give good results were shown. The best-received gain compared with a single antenna gain were at 4.2 GHz about +2.73, +1.17, and +0.92 dB for plus-shaped, loop, and chair-shaped respectively. A comparison between the proposed MIMO antennas and other reported works were done. The proposed MIMO antennas give a good maximum gain and are suitable for different narrow bands within the UWB such as wireless local area network (WLAN), worldwide interoperability for microwave access (WiMAX), aeronautical radio navigation (ARN), International Telecommunication Union 8-GHz (ITU-8), and X-Band applications with the ability to give high gain without the need to increase the radiated power of the transmitter antenna.


INTRODUCTION
Nowadays, ultra-wideband (UWB) micro strip antenna design plays an important role in modern microwave antenna design; researchers give it more attention after UWB (3.1 to 10.6) GHz was licensed by the Federal Communication Commission (FCC) [1].The importance of using micro strip antenna is due to its several advantages compared with other microwave antennas such as low profile, light weight, low cost, capability of many frequency operations, and ease of integration with microwave integrated circuit.Many simulation tools were introduced to microstrip antenna design such as: CST, HFSS, FEKO, IE3D, and others that gave the researchers the possibility to work in this direction and obtain results close to the reality of the antenna to be manufactured and adjust and improve it before manufacturing, to reach the required antenna with accuracy and effectiveness.Some different parameters were investigated to determine the operated, mismatched, and rejected bands such as those which were investigated Soliman et al. [2].Moreover, to study the effect of inserting different slots within the patch or ground or both of them, Dwairi et al. [3] proposed ISSN: 2088-8708  The effect of changing the formation of multiple input multiple output antennas on … (Majed Omar Dwairi) 533 [24] used metamaterial photonic bandgap techniques to reduce mutual coupling which is simple and effective.Alibakhshikenari et al. and Wang et al. [25], [26] an effective technique is used for suppressing the mutual coupling based on a metamaterial (MTM) electromagnetic bandgap.Alsaif [27] a UWB microstrip antenna has been proposed for portable wireless devices based on a simplified composite right/left-hand transmission line, this antenna was implemented using F and T-shaped slots in the patch radiator, the operating bandwidth operates from 0.6 to 9.2 GHz giving relative bandwidth of about 173.6% and gains about 4 dBi.Proposed and investigated important antennas works based on metamaterials, for UWB portable microwave devices and wireless communications [28]- [30].
In this paper, an UWB antenna based on [8], was optimized to operate at UWB, then the authors inserted some slots within the patch and the feed line to reject WLAN and X-bands.The proposed antenna in [8] is a good candidate for our investigation to be modified to MIMO scenarios with 2×1 and 2×2, with different modifications to compare these MIMO antennas and discover which gives the best gain.The remainder of this work has been organized as follows: section 2 single antenna design, next section 3 simulation results of 2×1 and 2×2 MIMO antennas; section 4 discussion of the received results.Finally, conclusions and future work were discussed in section 5.

ANTENNA DESIGN
The investigated UWB micro strip patch antenna based on [8], is presented in Figures 1(a) to (c), the substrate is built on FR4 with relative permittivity equal to 4.4 and loss tangent equal to 0.02.The antenna dimensions are listed in Table 1.In order to improve the patch antenna bandwidth and its matching inserted an arc cut of radius R mm at the corners of the rectangular patch antenna and the cutting polygonal slot in the ground, which in turn neutralized the capacitive and inductive reactance receiving a pure resistive input impedance [8].The best slot cut circles at the rectangular angles are R1=R2=R3=R4=2 mm.The polygonal dimensions are listed in Table 1.This produced antenna will be modified to different 2×1 and 2×2 MIMO antennas and all the comparisons of the investigated MIMO antennas will be referred to this original antenna in point of view return loss and the released gain.

SIMULATION RESULTS
The simulation of the investigated single antenna and the all-proposed MIMOs antennas has been carried out using CST-EM simulator 2018, where all the investigated antennas are based on the reference antenna introduced in [8].The investigation started with the simulation of the original antenna followed by the three proposed 2×1 MIMO antennas designed and the results.Finally, an investigation of the eight proposed 2×2 MIMO antennas designed and results.

Single antenna results
Equations should be placed at the center of the line and provided consecutively with equation numbers.The investigated single antenna based on [8], shown in Figure 1.The simulation results of the return loss (S11), voltage standing wave ratio (VSWR), and the released gain are shown in Figures 2 to 4. The operating bandwidth for all proposed antennas has been from 2 to 15 GHz.

2×1 MIMO antenna results
Three different 2×1 antenna configuration was investigated based on changing the dimensions of the substrate and the location of the patch antenna on the substrate.The investigated are shown in Figures 5(a) to 5(c), where the first 2×1 antenna was introduced as inverted MIMO, the second 2×1 was introduced as mirrored MIMO, and finally, the third 2×1 antenna was introduced as nearby MIMO.For all the investigated 2×1 MIMO antennas we found the return loss and the gain; which will be later compared in the discussion section.

2×1 Inverted MIMO antenna
The proposed 2×1 inverted MIMO antenna is the modified single antenna substrate dimension (2Ws+2mm×Ls×hs) mm 3 , as shown in Figure 5(a).The two antennas are the same dimensions inverted 180 degrees with 2 mm inserted in the substrate width to separate the ground of the two antennas.The simulation results of the return loss S11, S22, S12, and S21 (where S12, and S21 are the coupling return loss), and the released gain are shown in Figures 6 and 7 respectively.In Figure 7, it is seen that S11 and S22 are identical and S12 and S21 are identical as well.From S-parameters, it is notable that the bandwidth is received up to 40 GHz without any mismatched or rejected band; this is the only case that received these results.From Figure 7, the overall released gain of the bandwidth is good.

2×1 mirrored MIMO antenna
The 2×1 mirrored MIMO antenna is shown in Figure 5(b), the modification is done by doubling the substrate length, with the same width.The second patch antenna is the same dimensions but mirrors the first.The simulation results of the return loss S11, S22, S12, and S21, and the released gain are shown in Figures 8  and 9, respectively.

2×1 nearby MIMO antenna
The 2×1 nearby MIMO antenna is shown in Figure 5(c), the modification is done by doubling the substrate width and adding 2mm to separate the ground of antennas, with the same antenna length.The second patch antenna is the same dimensions nearby the first.The simulation results of the return loss S11, S22, S12, and S21, and the released gain are shown in Figures 10 and 11 respectively.

2×2 MIMO antennas results
Eight 2×2 antenna configuration designs were a comparison will investigated with different antenna location on the substrate as shown in Figures 12(a

2×2 loop MIMO antenna results
The 2×2 loop MIMO antenna configured as shown in Figures 12(a) front view and 12(b) back view.Where the dimension of the substrate is increased to (Ws+Ls) squared.The next antenna is placed 90 degrees left to the previous one and shifted up to a substrate length, as shown in Figure 13(a).Figures 16 to 18 show return loss, return loss of coupling antenna, and released gain respectively.Figure 18 showed both gains for single antenna and 2×2 loop MIMO, to demonstrate the difference in one figure.

2 ×2 Mirrored MIMO antenna results
The 2×2 Mirrored MIMO antenna configured as shown in Figures 12(c) front view and 12(d) back view.Where the dimension of the substrate is changed to double length (2Ls) and the width is changed to the double of the substrate width with adding 2 mm, in order separate the ground of the patches from each other (2 Ws+2 mm).The configuration of this antenna is done by inserting two patches nearby with 2 mm separation and mirroring the other two antennas.Figures 19, it is evident that all the return losses are identical.Figure 21 showed both gains for single antenna and 2×2 loop MIMO, to demonstrate the difference in one figure.

2×2 T-Mirrored MIMO antenna results
The 2×2 T-Mirrored MIMO antenna is configured as shown in Figures 13(a) front view and 13(b) back view.The shape of this MIMO antenna is T-shaped, where the dimension of the substrate is changed to double length (2Ls) and the width is changed to the triple of the substrate width with adding 4 mm (3 Ws+4 mm).The upper antennas are in the same direction but are mirroring the base antenna.Figures 22 to 24 show return loss, return loss of coupling antenna and released gain respectively.From Figure 22 it is evident that S22 and S33 are identical, whereas S11 and S44 are different.Figure 23 showed both gains for a single antenna and 2×2 T-shaped MIMO.

2×2 T-Mirrored center inverted MIMO antenna results
The proposed simulation 2×2 T-Mirrored center inverted MIMO antenna is configured as shown in Figures 13(c) front view and 13(d) back view.The shape and dimensions of this MIMO antenna is the same as T-mirrored Figures 13(a) and 13(b), with one difference where outer upper antennas are in the same direction as the lower base antenna.Figures 25 to 27 show return loss, return loss of coupling antenna and released gain respectively.From Figure 25 it is evident that S33 and S44 are identical, whereas S11 and S22 are different.Figure 27 shows both gains for a single antenna and 2×2 T-shaped MIMO.In Figure 28 a comparison of released gain was made between T-shaped and this antenna; as evident, this T-mirrored center inverted MIMO antenna configuration gives better gain over the operating UWB.

2×2 nearby MIMO antenna results
The 2×2 nearby MIMO antenna is configured as shown in Figures 14(a) front view and 14(b) back view.The shape and dimensions of this MIMO antenna is done by increasing the width of the substrate four  29 it is evident that S11 and S22 are identical, and that S33 and S44 are identical as well.Figure 30 shows both gains for a single antenna and 2×2 nearby MIMO.

2×2 nearby-inverted MIMO antenna results
The proposed 2×2 nearby inverted MIMO antenna is configured as shown in Figures 14(c) front view and 14(d) back view.The shape and dimensions of this MIMO antenna are the same as 2×2 nearby MIMO antenna except that every antenna is an inverted version of the nearby one.Figures 32 to 34 show return loss, return loss of coupling antenna and released gain respectively.From Figure 32 it is evident that S11 and S22 are identical, and that S33 and S44 are identical as well.Figure 35 shows a comparison between the released gain of 2×2 nearby and 2×2 nearby-inverted MIMO antenna.

2×2 plus-shaped MIMO antenna results
The proposed 2×2 plus-shaped MIMO antenna is configured as shown in Figures 15(a) front view and 15(b) back view.The shape of this MIMO antenna is done like a plus-shape where the dimensions are formed in a way that the width is twice the original substrate length plus the substrate width (2Ls+Ws), and the length is the original substrate antenna width plus 40 mm (Ws+40 mm).Figures 36 to 38 shown return loss, return loss of coupling antenna, and released gain respectively.From Figure 36 it is evident that S11 and S22 are identical and that S33 and S44 are identical as well.Figure 38 shows both gains for a single antenna and 2×2 nearby MIMO.

2×2 chair-shaped MIMO antenna results
The 2×2 chair-shaped MIMO antenna is configured as shown in Figures 15(c) front view and 15(d) back view.The shape of this MIMO antenna is done like a chair-shape where the dimensions are formed that the width is twice the original substrate length plus 2 mm (2 Ws+2 mm), and the length is twice of the original substrate (2Ls).Figures 39 to 41 show return loss, return loss of coupling antenna and released gain respectively.From Figure 39 it is evident that S11 and S22 are identical and that S33 and S44 are identical as well.Figure 41 shows both gains for a single antenna and 2×2 chair-shaped MIMO. Figure 42 shows a Comparison between the released gain of 2×2 nearby and 2×2 nearby-inverted MIMO antenna.

DISCUSSIONS OF THE RECEIVED RESULTS
The main purpose of this paper is to use the MIMO patch antenna to increase the released gain without the need to increase the power radiated from the antenna.The investigation was done on three 2×1 MIMO antennas, and eight 2×2 MIMO antennas.The discussion will be as follows: first, comparison of the original single antenna with all the MIMO antennas from the point of view of the operating UWB bandwidth; then, comparison of the single antenna released gain with 2×1 MIMO antennas; comparison of original antenna with 2×2 MIMO antennas; compare our results with other reported works, and finally, evaluation of the best-released gain

UWB operating bandwidth
A composition between the original antenna and all other investigated MIMO antennas from the point of view of the operating UWB bandwidth, resonant frequencies, and relative bandwidth, is listed in.The operating bandwidth of the original single antenna ranges between 3.4-13.5GHz; all the investigated MIMO antennas are laying around this bandwidth.Most of the investigated MIMOs start before and end after this bandwidth, and the relative bandwidth of all investigated antennas is around the relative bandwidth of the original single antenna which is 119.5.Some of their values exceed the previously mentioned value, reaching the maximum for the 2×1 inverted MIMO at about 169.6, whereas for the 2×2 nearby inverted MIMOs it reaches 141.2.As seen form Table 2, 2×1 inverted MIMO gives the best UWB bandwidth with the best relative gain, which gives a perspective to use this type of antenna for higher operating frequencies.In this work, we are not investigating enhancement of the bandwidth, but this antenna could be studied separately for this type of research.

Relative gain
Figure 43 shows the comparison between the original single antenna and 2×1 MIMO antennas, where the worst case was received for mirrored MIMO.While the inverted MIMO gives the best results, portions of bandwidth 8.82 to 10.1 GHz and 13.45 to 15 GHz received gain is less than the single antenna.In Table 3, gain comparison for selected frequencies is listed.
From Table 3, it is evident that the maximum gain improvement is about 1.37 dB.Next, there is a comparison between the single antenna and all 2×2 MIMO antennas as shown in Figure 44, to evaluate the worst and the best-released gain received.A T-shaped MIMO antenna received the worst one as its gain was equal or less than the single antenna except for portions of bandwidth 7.24 to 8.65 GHz, and 13.9 to 15 GHz, where the maximum improvement in these portions was about 0.75 dB.The gain received for MIMO antennas was better than a single antenna until 9.1, 9, 8.7, 9.67, 9.1, 9.22, and 8.9 GHz for plus-shaped, chair-shaped, mirrored, nearby, nearby inverted, loop, and T-shaped inverted respectively.For the best visualization of the received results of the improved gain comparison between all MIMO antennas and the single antenna, the received gain for a selected frequency is listed in Table 4.
From Table 4, it is evident that the worst released gain improvement was received for T-shaped and T-shaped inverted MIMO antennas followed by nearby inverted, mirrored, and nearby respectively.From Table 4, it is notable that the best-released gain received was for plus-shaped, loop, and chair-shaped respectively.The maximum gain improvement received was 3.07 dB at resonant frequency 8.2 GHz for chair-shaped, whereas at this frequency 1.65 dB was received for both plus-shaped and loop antennas.The best gain for plus-shaped received was 2.73 dB at 4.2 GHz, whereas 1.17 and 0.92 dB were received for loop and chair-shaped respectively.Finally, the best-released gain for loop received 2.4 dB at 3 and 3.85 GHz resonant frequencies at these frequencies plus-shaped received 1.24, 2.4 dB respectively; however, chair-shaped received 0.38 and 0.25 dB respectively at these frequencies.The average improvement received was 1.81, 1.45, and 1.27 dB for plus-shaped, loop, and chair-shaped respectively.
From the received simulation we conclude that the plus-shaped, loop and the chair-shaped MIMO antennas give the best results of released gain.Depending on the purpose of using the antenna in terms of the required narrowband frequencies within the UWB range and the purpose of this use, any of the antennas can be used to increase the gain for narrow frequencies without the need to increase the radiated power.In Table 5, comparisons are made with other works to compare the received results.From Table 5, it is evident that the proposed antennas are perspective, essentially 2×1 inverted MIMO as its bandwidth is the largest without any mismatches in the whole interval; moreover, it could have more operating bandwidth than shown (not simulated more than 40 GHz).The other three best 2×2 MIMO antennas give good peak gain concerning the reported works, which seems to be good to use for UWB high gain without the need to increase the radiated power.

CONCLUSION
A different configuration of UWB 2×1 and 2×2 MIMIO micro strip patch antennas was proposed and compared with the single UWB patch antenna to increase the released gain without the need to increase the input power radiation, using different substrate dimensions and patch antenna placing.The best simulation results of the three proposed 2×1 MIMO antenna received was for the 2×1 inverted with high UWB and a bandwidth up to 40 GHz without any mismatches, in addition to a maximum gain up to 6.51 dB, with an average gain of more than a single antenna of about 1.27 dB.For 2×2 MIMO antennas, the best received gain compared with a single antenna gain were at 4.2 GHz about +2.73, +1.17, and +0.92 dB for plus-shaped, loop, and chair-shaped respectively.The proposed MIMO antennas are suitable for narrowband within the UWB such as WLAN, WiMAX, ARN, ITU-8, and X-Band without the need to increase the radiated power of the transmitter antenna.

Figure 1 .
Figure 1.The investigated antenna: (a) side view, (b) front view, and (c) back view

Figure 2 .Figure 3 .Figure 4 .
Figure 2. The simulated return loss for the single antenna

Figure 43 .
Figure 43.Comparison of the released gain between the single antenna and all 2×1 MIMO investigated antennas

Table 2 .
Comparison between a single antenna and all investigated antennas

Table 3 .
Gain comparison between single and 2×1 inverted MIMO antennas

Table 4 .
The improved gain comparison between all MIMO antennas and the single antenna

Table 5 .
Comparison between the proposed MIMO antennas and recent works