# Design of a Compact Multiband Monopole Antenna with MIMO Mutual Coupling Reduction
## 2 Design of Antenna Structure
### MIMO Antenna Mutual Coupling Reduction
Most of the coupling energy likely follows the second path, as Figure 8b demonstrates that the coupling energy is absorbed or re-radiated into the air by the upper meander slit on the ground. From the observed conductive current on the upper meander slit, which has maximum current in the middle and zero current at both open ends, it is evident that this condition matches the previously calculated half-wavelength slit. Furthermore, in Figure 8c,d, the conditions are similar to the previous observations, with only the excitation frequency changing to 3.47 GHz and the decoupled conductive current observed on the lower meander slit. The relationship between isolation and slit gap is shown in Figure 9. The gap of the two slits is the same size. When the gap changes from 0.1 mm to 0.4 mm (with 0 mm representing no slits), the isolation remains largely unchanged, with suppression around \(-18\) dB at 3.47 GHz. However, there is a significant change at 2.46 GHz. As the gap increases, the resonant frequency shifts from lower to higher values. Therefore, the authors chose a gap of 0.2 mm for the manufacturing parameter because it provides the correct resonant frequency at 2.46 GHz and achieves deeper suppression at \(-29.4\) dB. Figure 10 shows the radiation patterns of the MIMO antenna at 2.46 GHz. There are no slits on the common ground. When antenna 1 is excited and antenna 2 is terminated, as shown in Figure 10a, and when both antennas are excited, as shown in Figure 10b, different radiation patterns are exhibited, with gains of 2.06 dBi and 1.42 dBi, respectively. Due to the absence of slits, there is no way to suppress mutual coupling, so antenna 1 and antenna 2 interfere with each other, causing a decrease in gain. From an antenna array perspective, theoretically doubling the elements should increase the gain by 3 dB. Therefore, there is no constructive interference, only destructive interference. However, Figure 10c,d present a different situation with slits on the common ground. When antenna 1 is excited and antenna 2 is terminated, the gain is 2.51 dBi, hig  
Figure 8: The current distributions of the MIMO antenna with antenna 1 excited and antenna 2 terminated. At 2.46 GHz without slits (**a**) and with slits (**b**). At 3.47 GHz without slits (**c**) and with slits (**d**). in Figure 10a. This leads the authors to believe that the conductive current on the upper meander slit is not absorbed by the ground but, instead, radiates into the air. Furthermore, observing Figure 10d, the gain is 5.01 dBi, not 5.51 dBi (which would be the expected sum of 2.51 dBi + 3 dBi). When both antenna 1 and antenna 2 are excited in phase, their conductive currents on the upper meander slit are opposite and thus cancel each other out, as shown in Figure 12a. This results in no radiation from the slit, and the gain observed is due only to the constructive interference between antenna 1 and antenna 2, creating a more directional radiation pattern. Figure 11 shows the radiation patterns of the MIMO antenna at 3.47 GHz. These situations are similar to those observed at 2.46 GHz. However, without slits on the common ground, the increased distance between the radiators at 3.47 GHz results in less interference,  
Figure 10: The radiation pattern of the MIMO antenna at 2.46 GHz: (**a**) without slits and with antenna 1 excited and antenna 2 terminated, or (**b**) with both antennas excited; (**c**) with slits and with antenna 1 excited and antenna 2 terminated, or (**d**) with both antennas excited. Figure 9: The relationship between isolation and the slit gap. leading to no decrease in gain but rather a slight increase from 3.17 dBi to 3.36 dBi, as shown in Figure 11a,b. Figure 11b also exhibits a directional radiation pattern due to the increased distance between the two radiators, which results from the shorter wavelength. Furthermore, with slits on the common ground, a comparison of Figure 11c,d shows pattern gains of 1.47 dBi and 4.32 dBi, respectively. The increase in gain is 2.85 dB, which is quite close to the theoretical value.