Broadband microstrip patch antenna at 28 GHz for 5G wireless applications

Received Jul 26, 2020 Revised Oct 7, 2020 Accepted Dec 5, 2020 In this paper, a 28 GHz broadband microstrip patch antenna (MSPA) for 5G wireless applications is presented. The Rogers RT/Duroid5880 substrate material, with a dielectric constant of 2.2, the thickness of 0.3451 mm, and loss tangent of 0.0009, is used for the studied antenna to operate at 28 GHz center frequency. The proposed design of antenna is simulated by using CST studio suite. The simulation results highlight that the studied antenna has a return loss of -54.49 dB, a bandwidth of 1.062 GHz, a gain of 7.554 dBi. Besides, radiation efficiency and the sidelobe level of the proposed MSPA are 98% and -18.4 dB, respectively. As compared to previous MSPA designs reported in the recent scientific literature, the proposed rectangular MSPA has achieved significantly improved performance in terms of the bandwidth, beam-gain, return loss, sidelobe level, and radiation efficiency. Hence, it is a potential contender antenna type for emerging 5G wireless communication applications.


THE PROPOSED DESIGN METHOD
The performance of the MSPA is mainly limited by the chosen shape of physical geometry, physical dimensions of the structures, and the material properties from which they are made. In this paper, the rectangular patch shape is chosen because it is easy to design and evaluate. Besides, it has a wider impedance bandwidth due to its broader shape as compared to other types of antenna. Moreover, Rogers RT/Duroid5880 has been selected as a dielectric substrate material having a thickness of 0.3451 mm, dielectric constant (εr) of 2.2, and tangent loss of 0.0009. Radiating copper metal thickness of 0.035 mm to function at 28 GHz center frequency has been used. Therefore, after the preliminary design parameters are selected, the remaining parameters of the proposed physical structure of the antenna, which are shown in Figure 1, are in the subsequent step calculated using general governing equations given in [24][25][26][27].
The performance of the rectangular MSPA is highly dependent on the selected dimensions of the physical structure. Hence, in this particular design, a metal patch of 3.5644 mm of length and 4.2352 mm of width is connected to a 50 Ω microstrip feed line with an inset on the top of the substrate. The overall physical dimension of the proposed rectangular MSPA is 3.5644x4.2352x0.3451 mm 3 . In this work, the effects of the antenna dimensions on its performance are analyzed using the repetitive simulation. The CST antenna simulator has been used to enhance the performance of the studied antenna in terms of beam gain, directivity, bandwidth, and radiation efficiency by tuning its parameters. The values are altered manually, and the effects are observed from the simulation results. While tuning the antenna dimension parameters, its impact on all the performance metrics is considered. The final values of the antenna parameters, which are optimal for all the performance metrics of the given design, are chosen. The initially calculated and optimized physical dimensions of the proposed antenna are tabulated in Table 1.

SIMULATION RESULTS AND DISCUSSION
The performance of the MSPA is evaluated using different metrics. Among these, the return loss, bandwidth, VSWR, beam gain, and radiation efficiency are often utilized. The return loss (S11) versus frequency plot of the proposed MSPA is given in Figure 2. From the plot, we observe that the antenna return loss is about -54.49 dB at the resonant frequency. The bandwidth of the rectangular MSPA is determined between the regions where a return loss is less than -10 dB, i.e., between 27.426 GHz and 28.488 GHz. Therefore, the -10dB return loss of the proposed rectangular MSPA bandwidth is 1.062 GHz. The return loss at the center frequency is found to be -54.49 dB. The input power delivered from the source cannot be radiated without loss due to incorrect compensations. Some of this power is reflected at the antenna, and it is returned to the transmitter, which is quantified by VSWR. The VSWR is described as a function of reflection coefficient, which expresses the power reflected from the antenna. The smaller the VSWR, the better the MSPA is matched to the feeder line and power distributed to the patch. Hence, for an ideal transmission line, the magnitude of VSWR is one, and for the practical scenarios, a magnitude of less than two is satisfactory as long as return loss is less than -10 dB [21]. Figure 3 shows that at 28 GHz, the VSWR of the studied antenna is 1.011 and very close to the ideal value. Within the -10 dB bandwidth, i.e., between 27.426 GHz and 28.488 GHz, the VSWR value of the proposed MSPA is less than two, which is within acceptable range. Another parameter that characterizes radiation properties of an antenna structure and distinguishes one antenna from the other is the radiation pattern. It is the far-field plot of an antenna described in terms of spatial coordinates. It is specified using azimuth and elevation angles. Particularly, the plot shows the amount of radiated power from an antenna per unit solid angle. The radiation pattern plot can be visualized as a 3D graph or a 2D polar or Cartesian slice of the 3D graph. It is an important parameter as it shows the antenna's directivity and gain at various points in space [21]. Figure 4 indicates the 3D radiation graph of a proposed rectangular MSPA. From the figure, the beam-gain and radiation efficiency values of the proposed MSPA structure are found to be 7.554 dBi and 98 %, respectively. Also, the antenna's sidelobe level is -18.4 dB, as depicted in Figure 5.  Table 2. Obviously, as the electromagnetic wave travels to different portions of the antenna, they encounter different impedance at each interface. However, in any case, at whatever point there is imperfect impedance matching at any of the interfaces, it causes a portion of the electromagnetic waves to return back to the source. Therefore, at the feeding network, perfect impedance matching is necessary to transfer considerable amount of power from the port to the feeder networks, which is connected to it. The proposed rectangular MSPA has been excited using the microstrip inset feed line.
At the interface of the feed-point and the patch edge, the impedance mismatch is significantly minimized by tuning the dimension of the inset-feed, patch width, and width of the microstrip transmission line. As a result, a huge amount of input power is transmitted to the feeding networks with very low return input power as a return loss. Therefore, from Table 2, it is clearly apparent that the studied antenna shows lower return loss as compared to the designs reported in [10,[12][13][14][15][16][17][18][19][20]22, 23] and it gives lower VSWR than the structures introduced in [10,13,19,20,22,23].
The dimension of the patch width determines the range of antenna bandwidth and radiation efficiency of the antenna. Then again, the chosen ground plane dimensions and substrate thickness plays a significant role in determining the performance characteristics of the antenna. Accordingly, in this study, both the ground plane dimension and the substrate thickness of the proposed rectangular MSPA are carefully designed and optimized. Extensive and meticulous tuning of the antenna parameters is indispensable for its performance improvement. Consequently, the surface wave, spurious feed radiation, and the reflection of the input power are considerably reduced. Besides, the input impedance mismatch of the MSPA is significantly reduced by using different impedance matching techniques and optimizing the patch width dimension. As a result, the proposed antenna has achieved better performance in terms of radiation efficiency, beam-gain, and sidelobe level. From Table 2, we note that the antenna radiation efficiency is higher than the designs reported in [10-13, 19, 22], the bandwidth is wider as compared to the antennas cited in [19,20,22]. In terms of beam gain, the rectangular MSPA in this paper outperforms the designs reported in [14,20,22]. Generally, the proposed single element rectangular MSPA gives an exceedingly competitive performance as compared to similar structure antennas reported in the scientific literature.

CONCLUSION
In this paper, a 28 GHz broadband rectangular MSPA has been proposed for 5G wireless applications. The conducted simulations highlight that the proposed rectangular MSPA resonates at 27.992 GHz having a return loss of -54.49 dB. The achieved maximum beam-gain is 7.554 dBi, bandwidth is 1.062 GHz, VSWR is 1.011, the radiation efficiency is 98%, and the sidelobe level is -18.4 dB. As compared to other similar works, the proposed rectangular MSPA has achieved significantly higher performance in terms of bandwidth, beam-gain, return loss, and radiation efficiency. This improved performance has been achieved because of the reduced impedance mismatch between the microstrip feeder line interface and the patch edge using the inset-feed and quarter-wavelength impedance matching techniques. In addition to these impedance matching techniques, the antenna parameters (physical dimensions) are optimized by considering the performance trade-off between the parameters. The proposed rectangular MSPA has a compacted size; therefore, it is suitable for mobile devices with space constraints, and also, it can be considered a potential candidate to be used in an array of 5G communication systems.