# Quad Element MIMO Antenna for C, X, Ku, and Ka-Band Applications
## 2 Antenna Geometry  
The proposed antenna element has a dual-side three-stage stair and a straight stick on the top of the stair. The partial ground plane has a small rectangle microstrip line just beneath the feed line. The introduction of a DGS on the ground plane serves to provide a wideband response. The DGS on the ground plane plays a crucial role in improving the electromagnetic interaction between the grounding surface and the top patch, contributing to the wideband characteristics of the antenna [22]. This DGS firstly functions as an extender of the antenna's current path, thereby mitigating the resonant frequency of the antenna at its fundamental mode. Secondly, it acts as a suppressor of surface currents, thereby ameliorating isolation within the frequency bands spanning OFB1, OFB2, and OFB3. The proposed DGS approach maintains a diminished influence on the antenna's intrinsic impedance characteristics, primarily since the supplementary current pathways are realized through the judicious etching of coupling slots on the ground plane. In this design, the straight stick is a 50-ohm microstrip feed line. The design is fabricated on an FR4 substrate having a thickness of 1.6 mm with a dielectric constant of 4.4, a loss tangent of 0.02, and a copper layer thickness of 35 um. The size of the single-element antenna is 14.7 mm \(\times\) 12 mm \(\times\) 1.6 mm. In this design, the following design variables are considered, which are \"a\", \"b\", \"c\" and \"d\". For optimized performance of the antenna, the design variables a, b, c, and d are considered as 1.5 mm, 2.1 mm, 3 mm, and 2.3 mm, respectively. The top and bottom views of the proposed antenna element are shown in Figures 1a and 1b, respectively. The simulated reflection coefficient is depicted in Figure 1c, where the antenna structure with a partial ground plane provides an excellent result in comparison to a structure with a full ground plane. Furthermore, the conceptual equivalent circuit model for the proposed antenna was developed (Figure 2a). The initial approximate values for the lumped components were defined from the CST simulated antenna input impedance (Zin), and the equivalent lumped circuit parameters were then optimized using Keysight advanced design system (ADS) simulations [23]. In this article, eight RLC tank circuits are connected in series due to having eight adjacent resonant frequencies overlapping with each other (Figure 1c). Note that the addition of the series capacitor Cn and inductor Ln to the RLC circuits is necessary to correctly produce the imaginary part of the antenna input impedance [24; 25]. Figure 1: \(Cont.\)The real (resistive part) and imaginary (reactance part) parts of the input impedance of the proposed MIMO antenna system are depicted in Figure 2. Figure 2b represents the real part of input impedance, obtained from CST and ADS simulation, whereas Figure 2c represents the imaginary part. Here, the proposed antenna shows good matching with the coaxial cable within all operating frequency bands. The optimized values of lumped components are shown in Table 1.  
The developed MIMO system should be applicable on n77/n78/n79 5G NR (3.3-5 GHz); WLAN (4.9-5.725 GHz); Wi-Fi (5.15-5.85 GHz); LTE5537.5 (5.15-5.925 GHz); WiMAX(5.25-5.85 GHz); WLAN (5.725-5.875 GHz); long-distance radio telecommunication (4-8 GHz; C-band); satellite, radar, space communications and terrestrial broadband (8-12 GHz; X-band); and various satellite communications (27-40 GHz; Ka-band). As mm-wave has a high carrier frequency, the signal baud rate could carry more data, resulting in a high data rate (bits/s/Hz) and a reduction in the buffering time in delivering high-quality videos [26]. The mm-wave has a limited range, this helps in cellular communication to expand the network coverage with the concept of frequency re-use through cell splitting and sectoring.