# Multiport Single Element Mimo Antenna Systems: A Review
## 4 Multiport Single Element System
### Single Element Four-Ports MIMO Antenna System  
Two compact co-radiators with dual polarization, where each radiator shares two antenna elements, are discussed in [76]. A metal branch, expanded and etched in the two antenna elements at symmetric axis and which can be viewed as a reflector, decreases the current flow on the ground plane and reduces the electromagnetic coupling. The combination of a T-shaped slot on the patch with a pair of slits on the ground plane, and a stub attached to the ground plane results in good isolation. Further, the two slots placed in the ground plane stops the current in the ground from flowing to the other port. Dual polarization is also achieved by exciting the shared radiator with two perpendicular feeds. The four ports' semi-elliptical monopole antennas with inverted L-shaped ground layer on the same plane are presented. The isolation is improved through polarization diversity and by inserting narrow slits to the degenerated ground plane, improves isolation at low frequencies. By implementation of inverted 'L' shaped ground layer, the ground plane acts as a reflector and operates as a directional antenna element as shown in Figure 14.  
In [77], asymmetric coplanar strip (ACS)-fed multi-port antenna is designed with four antenna elements and two radiators in which the two antenna elements share a single radiator to provide overall dimensions of 36 \(\times\) 36 mm\({}^{2}\). Good isolation between the antenna elements is achieved and the operating bandwidth is also broadened by placing two I-shaped slots at the middle of the radiator and a rectangular patch attached on the bottom of the substrate, as shown in Figure 15. The surface current flowing between the ports is blocked by the stubs and reduces the mutual coupling. Since the ground plane presented here is asymmetric, the radiation pattern gets weaker at the higher frequency range. In [78], a microstrip patch antenna having multi-cut design with four ports sharing a common radiator and the stepped ground is considered, as shown in Figure 16a. The energy released as surface current from the ports enters in the undesired directions. This causes mutual coupling and is reduced by etching multiple saw-tooth c  
Figure 14: Compact co-radiator with dual polarization [76]. Figure 15: ACS-fed 4 port antenna [77]. shared rectangular patch. The overall size of the antenna is 75 \(\times\) 75 \(\times\) 1.6 mm\({}^{3}\) and covers the frequency range from 4.96 to 5.5 GHz with gain varying between 2.4 and 5.5 dB in the proposed band. A four-port shared radiator is further improved with better isolation and overall size of the antenna is 75 \(\times\) 75 \(\times\) 1.6 mm\({}^{3}\) is presented in [79] as shown in Figure 16b. The designed stepped ground and the multi-cuts at the radiator patch controls the surface current direction which subsequently reduces the mutual coupling. The signal to noise ratio and back lobe radiation is also reduced through stepped rectangular design geometry. Good isolation is achieved for the entire frequency range from 4.4 GHz to 6.4 GHz. Low ECC of <0.04 is obtained and the gain of the designed antenna varied between 3.0 and 6.1 dBi. In [80], four microstrip feedline monopole antenna connected to a shared radiator and ground plane is discussed. To achieve wideband, the rectangular-shaped common radiator is modified with stepped lines at the four corners as shown in the Figure 17a, to provide a better bandwidth of 1100 MHz. The four slots etched in each corner of the modified ground plane disturbs the current flow to achieve good isolation of better than 15 dB at the expected frequency range from 1.8 GHz to 2.9 GHz. The port coupling is also reduced via slot-loaded technique. Low measured ECC of < 0.1 and DG of about 10 dB are achieved. In [81], the common element MIMO antenna with a notch-loaded circular radiator is fed by four identical modified feedlines, suitable for Wi-Fi applications, is proposed as shown in Figure 17b.