## 1 Introduction
At low frequencies, The Mutual coupling performance of the antenna is good if it has the mutual scattering parameter \(S_{21}<-20\) dB or the isolation of more than 20 dB. When the distance between elements is too large relative to the wavelength at high frequencies, a free lobe will emerge, thereby deteriorating the performance of the radiation pattern. Many different types of antennas may be used in radar and communications applications, including patch antennas [3], monopole antennas [6; 8], and 3D antennas [5]. However, some of those antennas have an omnidirectional radiation pattern, or if they do have a directional radiation pattern, the directivity is minimal. Vivaldi antenna has advantages such as a planar antenna, wide bandwidth, and directional radiation pattern. There are several types of Vivaldi antennas including Coplanar Vivaldi Antenna [43; 44], Antipodal Vivaldi Antenna (AVA) [45], and Balanced Antipodal Vivaldi Antenna (BAVA) [46]. The AVA is frequently mentioned in contrast to the CVA, even though the CVA also offers benefits in its gain performance. Additionally, the Vivaldi antenna's elements are explained in greater detail than those of the MIMO antenna. As a consequence, a discussion about the performance of the CVA antenna in MIMO is also suggested. According to the preceding explanation, many radar applications require antennas that can operate at low frequencies, and the size of antennas that operate at low frequencies is often large. Meanwhile, the production of huge antennas and MIMO setups is prohibitively expensive. Aside from that, other researchers have done studies on Vivaldi antennas with several models, but very few have compared multiple models with the same substrate size. And antenna designers are occasionally perplexed about which antenna model to utilize to optimize certain performances. As a result, a discussion about the performance of the Vivaldi antenna is required, including return loss, mutual coupling, beam squint, and a beamwidth of the antenna of the same size, so that the antenna performance results can be used as a recommendation for which antenna design accommodates the required performance. As a result, our contribution is to create 15 different slot structures to the Palm Tree CVA to investigate the effect of slot shape on mutual coupling reduction as well as the effect of different slot structure shapes on the performance of the radiation pattern using the same substrate width. We only discuss mutual coupling in the E-field in this discussion because mutual coupling in the E-field is a major problem, particularly at low frequencies. After all, surface currents can flow directly to adjacent elements, whereas mutual coupling in the H field can occur due to electric field induction through the air. We discovered the position and shape of the truncated slot that provides the best performance in terms of return loss performance, the wavy slot shape that provides the best performance in terms of mutual coupling at low frequencies, and the size of the elliptical structure that can provide 0\({}^{\circ}\) beam squint performance at all entire frequencies (0.5-4.5 GHz) while also providing the best beamwidth. Furthermore, we developed a metamaterial in the form of a split ring resonator that can boost gain by 4.451 dBi when compared to the RPT-CVA without giving a slot structure, and we evaluate it with measurement data, apply it to radar, and compare it to relevant research. In this study, we compared the performance of the return loss, mutual coupling, side lobe level, beam squint, and a beamwidth of 15 types of 1 \(\times\) 2 MIMO Palm Tree Coplanar Vivaldi Antenna using the same feeding shape and the substrate width in the E-plane.