# Multiport Single Element Mimo Antenna Systems: A Review
## 4 Multiport Single Element System
### Single Element Two-Ports MIMO Antenna System
This size reduction was achieved by two antenna elements sharing the single radiator and the ground size is reduced using an ACS-fed structure as shown in Figure 8a. The operating bandwidth and the isolation between the two ports are enhanced by attaching a rectangular patch on the bottom side and etching a I-shaped slot in the radiator. A modified common rectangular radiator connected with two antenna elements is fed by two microstrips placed perpendicular to each other and the ground plane connected by a short strip as shown in Figure 8b [66]. The current flow between the two antenna elements is reduced and the isolation is improved through the insertion of slots in the radiator. This configuration disturbs the coupling by focusing the current flow on the edges of the slots. Good isolation of less than -15 dB is achieved with a reflection coefficient of better than 10 dB from 3 GHz to 11 GHz. The MIMO antenna design shown in Figure 9b as an ultra-compact size of 22 \(\times\) 24.3 mm\({}^{2}\), which is smaller as compared to the designs presented in [65; 66; 67]. Figure The design discussed in [68], the single shared radiating element is fed with two perpendicular meandered microstrip lines. Good isolation is obtained for the entire bandwidth through the placement of an open shunt stub on the radiator and a T-shaped slot in the middle of the radiator. The use of meandered slots reduces the total length while maintaining the electrical length of the microstrip lines which helps to achieve compact size of the antenna [69] but increases the energy leakage. Hence, a partial ground plane of small strips is placed on the bottom of the substrate and the stub connected to the ground provides the current a path to the stub providing good isolation between the two ports. The antenna presented in [70] consists of a common radiating element with leaf-shaped structure, shared evenly as shown in Figure 10, using two microstrip lines and a common ground plane with curved shaped slot etched on the bottom side of the substrate. Isolation less than -15 dB is obtained for the  
Figure 8: (**a**) Top layer of ACS-fed structure with common radiator (**b**) Bottom layer [66]. Figure 7: Shared radiator with circular cut at ground [63]. operating frequency band (2.4-12.75 GHz) with the help of end-loaded meandered line connected to the ground plane. The two-port antenna presented in [71] comprises a shared radiator with two shorting strips connected to the ground plane via holes at their end. High isolation is achieved for the impedance bandwidth from 3.3 GHz to 3.7 GHz through appropriate adjustment of the two feeding ports' distance and by proper placement of shorting strips on the radiator. Compared to [71], the dual-feed MIMO antenna designed with one shared radiator and three shorting pins achieves a good isolation of less than \(-25\) dB for the impedance bandwidth from 3.4 to 3.6 GHz [72]. Due to the insertion of shorting pins between two feeding ports and on the other sides as shown in Figure 11, the current distribution takes place everywhere on the antenna, which leads to good isolation. In [73], a two-port PIFA with a common radiator placed at a height of 5 mm from the substrate having two feed plates placed perpendicular to each other as shown in Figure 12. A slot is etched on the ground plane which reduces the current flow between the two ports which results in improved isolation. The impedance bandwidth achieved is around 800 MHz from around 2.1 to 2.9 GHz. The ECC is below 0.005 and the MEG of the two ports is  
Figure 11: Fabricated two port shared radiator with shorting pins [72]. Figure 9: (a) Slots etched in the common radiator antenna design [67], (b) Open shunt stub placement and etched T-shaped slots in the common radiator antenna design [68]. In [74], a common single circular patch radiator designed for UWB application with dimensions of 45 \(\times\) 45 mm\({}^{2}\) was proposed.