# A Review on SIW and Its Applications to Microwave Components
## 12 SIW Antennas  
Antennas are the key components in wireless communication systems. Their main function is to transmit and receive signals. Further to the increasing trend concerning simplicity and miniaturisation of communication systems, it is necessary to merge antennas and filters into a solitary device that simultaneously achieves both radiating and filtering functionalities, as shown in Figure 11. This integrated device is known as a filtering antenna (or filtenna). The filtenna lessens the pre-filtering condition and advances the noise performance of the system.  
Substrate-integrated waveguide filtering antennas are currently receiving a wide range of interest and have been achieved using different design techniques. Studies [84; 85; 86] proposed single band SIW filtennas with controllable radiation nulls. The radiation nulls enhance the radiation features of the filtering antennas. Electric and magnetic combined coupling structures and basic modes were used to generate two radiation nulls as explained in [84]. The two radiation nulls may be independently operated to attain elevated selectivity and flexibility in the suggested filtenna, as explained in [85]. Dual-band SIW filtering antennas have also been reported [87; 88; 89]. These type of filtennas have the added advantage of improved out-of-band suppression, with a multifunctional single slot. The etched multi-functional slot helps to improve the out-of-band suppression and the radiation gain. SIW filtennas have also been proposed and designed for millimetre-wave applications [90; 91; 92; 93]. A performance-improved filtenna was exploited in [90] to act as the source of a 60 GHz  
Figure 10: Simulation and measurement responses and images of the fabricated SIW filtering power divider: (**a**) with equal power division; (**b**) with unequal power division. Reprinted/adapted with permission from Ref. [82]. 2020, [PERSON] et al.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & \(f_{\mathbf{0}}\) & **Filtering** & **No. of** & **Insolation** & **Return** & **Dimension** \\  & **(GHz)** & **Property** & **Bands** & **(dB)** & **Loss (dB)** & **(\(\boldsymbol{\lambda_{o}\times\lambda_{o}}\))** \\ \hline
[79] & 3.5/4.7/6.3 & No & 3 & \(>\)14 & \(>\)20 & \(0.03\times 1.0\) \\ \hline
[79] & 3.3/4.8/6.2/7.7 & No & 4 & \(>\)12 & \(>\)19 & \(0.02\times 1.0\) \\ \hline
[80] & 5.9/6.4 & Yes & 2 & \(>\)21 & \(>\)20 & \(0.77\times 1.0\) \\ \hline
[81] & 1.7/2.85 & Yes & 2 & \(>\)13 & \(>\)15 & \(0.28\times 0.28\) \\ \hline
[82] & 11.8 & Yes & 3 & \(>\)16.8 & \(>\)18 & \(1.47\times 0.87\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Performance comparison of published works on SIW power dividers/combiners.  
Fabry-Perot Cavity antenna. This greatly improved the filtering function of the proposed filtenna. Compact size and low insertion loss was achieved in [91] by using eight-mode SIW cavities fully shielded by metallised vias. Multilayer SIW structures have also been employed in the design of antennas as reported in [92]. The structure consists of an SIW feed with a coupling slot, differential-fed L-shaped probes, and radiating patches. The authors of [93] proposed an antipodal linearly tapered slot filtenna with diverse split-ring resonators. The proposed design achieved a lower band suppression between 23.5 and 27.5 GHz. An SIW antenna based on negative order resonance is reported in [94]. The major difference between the antenna reported in [94] and those reported in [84; 85; 86; 87; 88; 89; 90; 91; 92; 93] is the absence of the integrated filtering property in [94]. This simply means that, while the antennas in [84; 85; 86; 87; 88; 89; 90; 91; 92; 93] will radiate energy as well as filter out unwanted frequencies, the antenna reported in [94] will only radiate energy, while depending on separately designed filters for frequency filtering. A performance comparison of published works on SIW antennas is given in Table 4.