## 1 Introduction  
The antenna is an important component of the telecommunications system because it is the final part of the process of sending and receiving data. Presently, there is a continuous increase in data transmission hence research for improving antenna performance continues to rise. Many antenna studies have been carried out for microwave imaging [1, 2, 3] military or electronic warfare [4, 5], Wifi [6, 7], 5G telecommunications [8, 9, 10, 11], vehicle communication [12, 13, 14], Maritim, airborne [15, 16], and RADAR applications [17, 18, 19]. The studies conducted on RADAR applications including GPR applications usually work in the low frequency between 500 MHz to 3 GHz [20, 21]. Antenna research that discusses its usage in an extensive frequency has been conducted [22; 23; 24], however, research on planar antennas that perform at low frequencies below 1 GHz is still quite restricted. An antenna that has higher bandwidth would contribute immensely to the high imaging resolution of the radar system. UWB imaging system is designed to detect and see the object of interest in a structure that can be classified as through-the-wall imaging biomedical imaging and ground-penetrating radar. The higher the bandwidth the better the image resolution. The lower the frequency the larger the wavelength, so a radar with a very large wavelength will not be able to locate anything very precisely. The lower frequency the poorer the resolution. On the other hand, the lower frequency has its advantage, particularly when using the radar for GPR or through-the-wall imaging applications as this would ease the high material losses that will be encountered when using a higher frequency range and also guarantee deeper penetration of the electromagnetic wave. Furthermore, radar and telecommunications applications require antennas that work with wide bandwidth and high gain. There have also been studies on how to boost its bandwidth and gain by changing its feeding, ground plane, and radiator, including the inclusion of corrugated slots. Vivaldi is one of the planar antennas that can work in a wide bandwidth. This antenna produces high gain coverage by providing corrugated [25], metamaterial [26; 27], lens [28; 29], fractal structures [30], and others. In addition to modifying the antenna elements, the increasing gain can be done by arranging the antenna in the form of an array and MIMO [31] as discussed in [32; 33; 34; 35; 36]. MIMO (Multiple Input Multiple Output) uses multiple antennas in the transmitter and receiver to improve its gain and data output. The MIMO antenna can be applied for radio astronomy applications, GPR, through-wall imaging applications [37], microwave tomography [38], and 5G applications [39]. The antenna arrangement in the form of MIMO on wideband antennas must pay attention to the mutual coupling between neighboring components in the low-end frequency because it affects the scattering characteristics. Research on the method of reducing mutual antenna coupling, especially for Vivaldi, has been discussed by [40] using the corrugated slot technique, in [41] by multiple notch structure in the ground plane, and [42] by a triangular director. Mutual coupling is a significant problem in the E-plane because surface current flows in the neighboring element by the continuous patch, causing coupling between elements, but in the H-plane, the coupling is caused by an electric field flowing through the air. Mutual coupling antenna with very wide bandwidth will be risky at low-end frequencies while for high frequencies there is usually no problem. This is at low frequencies, the antenna has a longer wavelength. Therefore, to reduce mutual coupling between elements, the antenna must be more than half the wavelength long. If the antenna is arranged in MIMO for wideband antennas, there will be a trade-off between the mutual coupling performance and the radiation pattern performance at low and high frequencies.