# Simulation and Design of Three 5G Antennas
## 1 Introduction  
Microstrip antennas offer advantages, such as their compactness, their lightness of weight, their cost effectiveness, and their flexibility in shape [1], that make them suitable for various applications in crucial fields. However, the ever-increasing demands of wireless communication systems have rendered the conventional designs with single antenna units obsolete, as they are no longer suitable for the complex and diverse communication systems of today. To meet these evolving requirements, the approach of combining multiple antenna units in various configurations to form antenna arrays has become essential, particularly in the context of 5G microstrip array antennas, which holds profound significance. In the realm of mobile terminals, the development of wireless communication technologies has exacerbated the issue of spectrum scarcity [2]. The shortage of available spectrum resources poses limitations on the growth of wireless communication systems [3]. Consequently, the emergence of multiple-input multiple-output (MIMO) technology was timely [4]. Although the transmitter's power remains constant, MIMO technology significantly increases the channel capacity when compared with conventional single-antenna systems. To meet the demands of future 5G cellular communication systems, there is a necessity to continually boost the channel capacity of smartphones by integrating six or eight antenna units. Thus, the design of MIMO array antennas holds substantial practical value. Different feeding methods for antennas have varying impacts on microstrip antenna performance. The comparison in [5] was made to investigate the effects of embedded feedline feeding, coaxial feedline feeding, coupling feedline feeding, and probe coupling feedline feeding on antenna performance, and confirmed that embedded feedline and coupling feedline feeding yield superior results [5]. In another study, ref. [6] employed a rectangular microstrip antenna with coaxial feeding and rendered it more compact by modifying the antenna patch design. However, the size of the single antenna unit still reached 14 mm \(\times\) 14 mm \(\times\) 1.6 mm, failing to achieve genuine miniaturization. In contrast, ref. [7] utilized circular patches with embedded microstrip lines for feeding, aiming to achieve a wider impedance bandwidth. The simulated results show an impedance bandwidth of only 7.1% (5.73-6.14 GHz). Furthermore, the antenna's thickness reached 120 mm due to the use of a tapered dielectric rod, posing significant challenges during fabrication and constraints in practical use. When operating array antennas, antennas placed in proximity typically experience mutual coupling, which can significantly impact antenna performance. To address this issue, researchers have proposed various structural solutions. In [8], a series-fed millimeter-wave microstrip line array antenna was designed. When 16 antenna elements were evenly serially distributed, the antenna's gain increased to 15.7 dB. However, this led to higher sidelobe levels, thus degrading antenna performance. In another work, ref. [9] introduced a feed network composed of multiple branched microstrip lines, achieving decoupling with an 18 dB improvement in isolation. Designing MIMO array antennas suitable for the 5G frequency bands presents new challenges, primarily related to antenna miniaturization and the mitigation of the influence of metal environments on antenna performance. To address these challenges, several methods have been developed by researchers. Ref. [10] proposed an eight-element metal frame mobile MIMO antenna array operating in LTE band 42 (3.4-3.6 GHz). Within the working frequency range, the array achieved an isolation of over 12 dB, with a measured overall efficiency of 45-58%. In another study, ref. [11] investigated the use of slot antennas in metal frame mobile terminals, confirming the continued excellent performance of slot antennas in such environments. Moreover, ref.