The Effect of Parasitic Patches Addition on Bandwidth Enhancement and Mutual Coupling in 2x2 Sub-arrays

This paper presents a 2x2 sub-array design with elements having parasitic patches (PPs) for massive MIMO applications. We show that with greater number PPs, bandwidth can be increased considerably with respect to previous designs. The increase in element dimension and, consequentially, element spacings reduces the mutual coupling in the sub-array. The frequency chosen for the design is 3.5 GHz, one of the 5G frequency allocations. For each sub-array element, multiple PPs are placed on the top layer, aimed to enhance the bandwidth. A coaxial feeding probe is employed. Two types of single element antenna with 5 and 10 PPs exhibit a bandwidth of more than 600 MHz and 700 MHz or a fractional bandwidth of about 17% and 20%, respectively. Simulation results and measurements of the 2x2 sub-arrays with 5 PPs and 10 PPs elements show that the antennas meet the desired performance because the return loss is less than -10 dB in a bandwidth of 567 MHz and 730 MHz, respectively. Mutual coupling effects can be suppressed down to less than -20 dB in the frequency range of 3.202 GHz -3.934 GHz.

While the techniques in [14]- [17] do not provide wide bandwidth, the parasitic patch technique is potential to achieve wideband characteristic. In this paper, we focus on the parasitic patch technique.
Reference [18] presents an antenna design consisting of layers of a substrate with 4 parasitic patches (PPs) placed on the top layer of the antenna, while the array application uses a combination of two decoupling techniques by arranging a 2x2 antenna. The combination of the two techniques used is decoupling walls and neutralized networks. The metal wall not only makes the above two parasitic patches shortcircuited easily, but it also acts as a decoupling wall to reduce the adverse mutual coupling between the antenna elements. Furthermore, simple decoupling of short-circuit stepped impedance structures (SSISs) as a neutralizing network is added to reduce mutual coupling even further. The antenna is made up of two layers of substrate being 1.5 and 2 mm thick, respectively. All antenna structures are in a rectangular cavity formed by metal via as sidewalls. Here, two substrate layers are used to easily embed SSIS decoupling in array applications. This causes the antenna to be a cost fabrication and less compact. The design achieves a frequency range of 3.35 to 3.95 GHz or fractional bandwidth is 16%, a gain of 13.6 dB and mutual coupling below dB.
In [19], the authors designed a sub-array of two rectangular antenna elements at a frequency of 2.8 GHz.
Simulation results show that in the E field for all element spacing, the mutual coupling is less than dB. However, the mutual coupling in the H plane for all element spacing is more than dB. The bandwidth is MHz with varying spacing between elements, which are still below 100 MHz and much below the required bandwidth for 5G applications. Subsequently, the authors of [20] report on the design of a two-element sub-array using 5 PPs placed on the top layer of the antenna and one substrate layer. The design is simulated and arranged in the H-plane for 0.75 spacing resulting in mutual coupling is dB and the E-plane is dB. On the other hand, mutual coupling in the Eplane for 0.5λ spacing does not meet the maximum mutual coupling criterion. The fractional bandwidth is 17%, and the resulting bandwidth is MHz for all variations of element spacing suitable for 5G applications.
This paper presents a simple yet effective strategy in dealing with the requirements of both the wide bandwidth and low mutual coupling. We present a 2x2 sub-array design with a parasitic patch technique for use in M-MIMO 5G applications. The frequency chosen by the author is 3.5 GHz, which is one of the sub-6 GHz carriers used for 5G applications. We show that the use of more PPs can lead to a sub-array with greater bandwidth. We extend the technique of adding parasitic patches on the same layer or on the top layer to produce multiple resonant frequencies that results in a wider bandwidth [3], [21], [22] by applying ten parasitic patches. In particular, we show that by applying the 10 PPs per element, the 2x2 sub-array attains a bandwidth of 730 MHz, 163 MHz wider than that of the two element sub-array with five parasitic patch elements reported before [20], or a fractional bandwidth enhancement of 29%. With the 10 PPs, the element dimension becomes larger, increasing the length by 3%, the width by 41% and the overall size of the dimension area by 45% compared to [19]. This enforces the use of larger element spacing, in this case 0.75 , thereby reducing the mutual coupling down to below dB. However, this yields an insignificant grating lobe effect since the main beam is directed to the front. As a result, the proposed sub-array antenna is suitable for use in M-MIMO 5G applications. Our contribution here is in the use of a much larger number of parasitic patches for each element, whereas in the literature only a maximum of 4 or 5 PPs per element has been tried. The slight increase in the element dimension is paid off by the increase in bandwidth from around 600 MHz to over 700 MHz.
The 2x2 sub-array antenna design for the M-MIMO antenna system is optimized for minimum mutual coupling. The design method is first carried out by the use of a rectangular patch antenna (RMPA) design of a single element without parasitic patches. Second, the single element with 5 PPs is realized [20]. Third, another single element with the addition of 10 PPs is designed and realized in this paper. Finally, 2x2 sub-arrays of elements 5 PPs and 10 PPs with spacing of 0.75 are realized and analyzed for bandwidth and mutual coupling.
This paper is structured as follows. Section II describes the antenna design, Section III reports the evaluation of the mutual coupling effect of the 2x2 sub-array antennas, and Section IV gives the conclusions.

II. ANTENNA DESIGN
The microstrip antenna design starts with identifying the appropriate specifications based on the needs of 5G applications. 5G bandwidth requirements for 3.5 GHz are 500 MHz, according to ETSI [23]. It is rectangular in shape with PPs to increase the bandwidth. The designs include elements of 5 PPs type, i.e., a rectangular patch with five parasitic patches, and those of 10 PPs type, which involve ten parasitic patches. The structure consists of a patch, a dielectric substrate and a ground layer. The media used herein are of FR-4 (epoxy) type with a dielectric constant ) of 4.3 and a substrate thickness (h) of 1.6 mm. The main patch is fed with a coaxial probe. Expected specification parameters are a bandwidth of more than 500 MHz, a return loss (RL) of less than dB, a mutual coupling of less than dB, and a frequency of 3.5 GHz. The rectangular antenna design process involves mathematical calculations for the width and length of the patch, substrate, and ground and determining the coordinates of the coaxial probe [24], [25]. The dimensional design of the parasitic patch also uses the same mathematical equations as the rectangular patch. The parasitic patch technique is applied by adding a parasitic patch in the E-plane and H-plane [3]. The dimensions of a single element without and with PPs are displayed using CST software. Figs. 1, 2 and 3 show the resulting dimensions.

A. SINGLE ELEMENT ANTENNA
Herein we use a single rectangular element (SE) to refer to one that does not include any parasitic patch. The dimensions of the SE can be shown Fig. 1 the design of which follows the method in [24].

B. 5 PPS-TYPE ELEMENT
The dimensions of the single element with 5 PPs can be seen in Fig. 2 and which has been described in [20]. The element size of 5 PPs is 66.71 mm x 48.58 mm (0.78 x 0.36 ).

C. 10 PPS-TYPE ELEMENT
The dimensions of a single element with 10 PPs can be seen in Fig. 3. The element size of 10 PPs is 68.71 mm x 68.48 mm (0.80 x 0.79 ). The element with 10 PPs is developed from the element with 5 PPs by adding five parasitic patches. In the new element, several changes are made to the parasitic patches, especially in the dimensions Wp and Lp, Wp1 and Lp1, Wp2 and Lp2, Wp3 and Lp3, as well as the addition of parasitic patches with dimensions Wp4 and Lp4, Wp5 and Lp5, Wp6 and Lp6. The length and width of the parasitic patch as well as the placement of the parasitic patch horizontally (right and left) and vertically (up and down) determine the new resonant frequency, which increases the bandwidth [3]. The positions of parasitic patches with dimensions Wp4 and Lp4, Wp5 and Lp5, Wp6 and Lp6 are as shown in Figure 3. The parasitic patches obtain electric current due to induction from those with dimensions Wp and Lp. At first the patch of dimensions Wp and Lp are supplied with electric current, which in turn causes induction on the side radiating the parasitic patch. Due to the proximity of the parasitic patches to each other, the parasitic patch on the non-radiating side generates an electric current caused by the induction of the parasitic patch on the radiating side.

C. 2X2 SUB-ARRAY
This paper reports on two sub-array antenna designs each consisting of 5 PPs and 10 PPs elements, respectively. The 2x2 sub-arrays are simulated to evaluate the effect of mutual coupling with element spacings, defined herein as spacings between adjacent points of element feeding, of 0.75 or 64.28 mm. This article has been accepted for publication in IEEE Access. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/ACCESS.2022.3185999 The dimension of the 2x2 sub-array of 5 PPs type elements is 130.99 mm x 112.86 mm (1.53 x 1.32 ), while the one for the sub-array of 10 PPs elements is 132.99 mm x 132.77 mm (1.55 x 1.54 ). The design of the 2x2 sub-arrays can be seen in Fig. 4.

A. SURFACE CURRENT PERFORMANCE
Figs. 5(a) and (b) demonstrate the distribution of surface currents over the 2x2 sub-arrays with 5 PPs and 10 PPs elements. A significant spacing of 0.75 between adjacent antenna elements reduces the surface current crossing over the elements the 2x2 subarray. It should be noted that for this application the grating lobe is not a problem because the main lobe points forward so that the grating lobe points to the back and is of small value because the spacing between elements is 0.75 (less than ).
Meanwhile, the presence of parasitic patches in 5 PPs and 10 PPs elements suppresses even further the surface currents across the 2x2 subarray.  Fig. 6 shows the modeling results as well as measurements of RL, measured in S11, and bandwidth of single elements of 5 PPs and 10 PPs. The simulation results of a single element without PPsshow that the bandwidth is 139 MHz, the fractional bandwidth is 3.97% and RL at 3.5 GHz is dB, whereas for the single element 5 PPs the bandwidth is found to be 618 MHz, the fractional bandwidth 17.04% and RL dB at 3.5 GHz. For the singleelement antenna 10 PPS the bandwidth is 732 MHz or 20.5%, while RL at 3.5 GHz is dB.  Comparison of simulation and measurement results of SE, 5 PPs and 10 PPs at 3.5 GHz with respect to the performance parameters of a single element design are shown in Table 1. Following upon these results, simulations and measurements of the 2x2 sub-arrays with 5 PPs and 10 PPs elements are made to evaluate the effect of mutual coupling, because these two element types fulfill the desired 5G requirements in terms of wide bandwidth.

B. SCATTERING PARAMETER PERFORMANCE
The results for 2x2 sub-array of 10 PPs elements are shown in Fig. 7 and recapitulated in Table 2. They show that the sub-array meets the desired RL performance, which is less than dB. There is a shift in the working frequency range of the measured sub-array with respect to the simulated design, but the range still includes the desired 3.5 GHz band required for 5G applications.

FIGURE 7. The comparison of simulation and measurements of S-parameters 2x2 subarray of 10 PPs elements
This article has been accepted for publication in IEEE Access. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/ACCESS.2022 Simulation and measurement results of mutual coupling effects in the 2x2 sub-array of 10 PPs elements are in accordance with the desired mutual coupling performance, which is less than dB. The mutual coupling at 3.5 GHz, include S21, S23, S24, S31, S41, and S43 are shown in Table  2. Hence, mutual coupling for all 2x2 sub-array designs observed from simulations and measurements at 3.5 GHz already meets the desired performance parameters.  The results of measurements of RL for all elements, i.e., S11, S22, S33, and S44 at 3.5 GHz, show that the sub-array achieves the desired RL performance of less than dB, which is given in Table 2. The results for mutual coupling meet the desired performance, which is less than dB. The measured mutual coupling at 3.5 GHz, namely S21, S23, S24, S31, S41, and S43 are also shown in Table 2.
The mutual coupling effect depends also on the relative positions of the elements in the antenna array. Suppose the antenna array elements are spaced too close together, then significant mutual coupling can cause a decrease in efficiency and change the radiation pattern. On the other hand, if the elements are too far apart, the substrate dimensions increase and the efficiency decreases. The effective or minimum spacing between elements that are safe from the effect of mutual coupling is 0.5 [24]. In this subarray design, the element spacing is 0.75 and, therefore, the mutual coupling is suppressed.
The return losses from measurement presented as S11, S22, S33 and S44 for the 2x2 sub-array of 5 PPs elements are consistently smaller than the corresponding losses for the 2x2 sub-array of 10 PPS elements, with the average difference being 3.27 dB. This result suggests that sub-arrays constructed from elements with fewer parasitic patches have a smaller return loss. Also from the measurement, the mutual coupling indicated by S23, S31, S24 and S41 shows a lower value with an average difference of dB for sub-arrays of 5 PPs relative to those of 10 PPs. Meanwhile, the S21 and S43 measured for sub-arrays of 10 PPS elements are lower than the corresponding values for those of 5 PPs, with an average difference of dB. However, in general it can be concluded that the 2x2 sub-arrays of 10 PPs elements, each having ten parasitic patches, demonstrate a good isolation between elements, leading to low mutual coupling. These results validate [18], [20] that the implementation of additional parasitic patches can maintain low mutual coupling. In addition, Table 2 also shows that the sub-arrays with elements having 10PPs consistently show greater bandwidth than those with elements of 5 PPs. Based on the return loss for each element in Table 2, it can be safely concluded that the return loss for the overall subarray is less than -10 dB.  Fig. 9 compares the results of simulations and measurements of radiation patterns and gains of single antennas of types (a) SE (b) 5 PPs-type and (c) 10 PPs-type. The gain shown is the maximum gain. The radiation patterns displayed are H-plane (horizontal plane/azimuth) and Eplane (vertical plane/elevation). Single element gains from simulation and measurement can be seen in Table 3. The fabricated single elements of all types are each found to have gain above 2 dB.  Table 4, in which the measured gain for both types of elements in their respective sub-arrays shows a good value. The radiation pattern is represented herein by that of element 1 only, while elements 2, 3 and 4 have similar patterns. The gain shown is the maximum gain.  The proposed antenna design is compared with those from other existing literature in terms of size, operating frequency, and bandwidth, as shown in Table 5. The comparison shows that the proposed antenna, a single element with 10 PPs, while being larger in size than the others, is capable to create a broader bandwidth than the others. In the last row of Table  5 we show the size of the proposed 2x2 sub-array of elements with 10 PPs each, which is hence smaller and has a wider bandwidth increase than [18].

IV. CONCLUSION
The single element with 10 PPs shows a broader bandwidth of 722 MHz, which is equivalent to a fractional bandwidth of around 20%, an increase of 172 MHz from that of the five-parasitic patch element and a larger increase of 607 MHz from that of single element without parasitic patches. It can be concluded that the addition of parasitic patches can enhance the bandwidth. It turns out that the bandwidth improvement is also observed for the 2x2 subarrays employing elements with ten PPs relative to the subarrays of elements with five parasitic patches, i.e. increasing the bandwidth by as much as 163 MHz or 29%.
The results of simulation and measurement for the 2x2 sub-arrays of elements with 5 and 10 PPs at 3.5 GHz show that both sub-arrays meet the desired performance because the return loss is less than dB for both. Comparison of simulation results and measurement of mutual coupling of the two 2x2 sub-arrays are also in agreement with the desired performance, i.e., less than dB. Therefore, generally the increase in the number of parasitic patches added to each element yields the same order of mutual coupling in the subarray, while successfully improving the bandwidth. In overall, the antenna meets the desired specifications and is applicable as a 5G communication antenna.