## 3 Antenna Performance Enhancement Techniques
### Effect of Slots in Radiator with Different Geometrical Shapes  
In [31], a patch antenna in a rectangular shape with a triangular slot loaded on the surface of the radiator on the right side from top to bottom is presented for wide-band applications. The combined effect of the triangular slot and partial ground plane excellently increases the impedance bandwidth more than 90% efficiency and 12 dBi gain is achieved. The impact on bandwidth is shown in Figure 11.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **Performance** & **Advantages** & **Disadvantages** \\ \hline \multirow{6}{*}{[45,46]} & \multirow{6}{*}{Substrate Choice} & Substrate with low permittivity characteristics have a significant impact on the performance of an antenna and provide enhanced gain, wide bandwidth and high efficiency. Having high permittivity characteristics improves the value of return loss. & Substrate with low permittivity is costly and not easily available. \\  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \hline \multirow{6}{*}{[47, 48, 49]} & \multirow{6}{*}{Multielement} & Excellently improves the impedance matching and directly enhances both the gain and efficiency. Mutual coupling reduction techniques also reduce the size of antenna. & Mutual reduction has an impact on antenna designing and increases the complexity. \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \hline \multirow{6}{*}{[50,51]} & \multirow{6}{*}{Multielement} & It significantly improves the return loss, bandwidth and radiation efficiency, besides these properties, it also effectively reduces the side- and back-lobe levels. & For such techniques, the feeding network is a difficult task to design and makes complexity to some extent. \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Performance Enhancement Techniques: Advantages and Disadvantages.  
Figure 11: Proposed patch antenna with triangular slot loaded. (**a**) Top view. (**b**) Enhancement in bandwidth due to slot in blue [31].  
An ultra-wideband antenna with a truncated \"U\"-shaped slot in the patch is presented in [32]. A compact size 2.9-23.5 GHz and a LTY-5 Taconic \(\varepsilon\tau=2.2\) substrate is used; the lower corner of the patch is modified with a semicircle. For the wide-band characteristics' realization, a slot in U shape is etched at the center of the radiator to ensure achievement of the notch band. A Structure in the semi ring shape in [26] has excellent impact on bandwidth enhancement.  
In [24], for enhancement of impedance bandwidth, leaky wave half width microstrip patch antenna is presented. The enhancement of bandwidth is realized by etching four circular slots on the surface of the radiating patch. The main beam with a wide scanning range of +12 degrees and +70 degrees is achieved where range frequency sweep is 4.28 GHz to 7.13 GHz with peak gain of 10.31 dBi at 5 GHz. A high radiation efficiency with main-beam continuous scanning capabilities in forward direction only by changing the operating frequency is achieved. Due to the circular slots in the radiation element, the level of cross-polarization decreases.  
In [57], a multimode characteristic E-shaped patch antenna for 5G mm wave application is presented. A slot is introduced in the proposed design and a 45.4% wide impedance with low loss polarization and stable radiation pattern is achieved.