# A Review on SIW and Its Applications to Microwave Components
## 9 SIW Filter  
The modern-day EM spectrum is becoming overcrowded and heavily inhabited with numerous wireless signals and parasitic interferers in connection with transmission and sensing services. Progressively more advanced RF, microwave, and millimetre-wave filters are essential to facilitate the passing and/or stopping of particular signals [1]. A bandpass filter selects frequency signals in a particular channel while discarding all other frequency signals external to the channel. The main function of a bandpass filter in the transmitter is to reduce the bandwidth of the output signal to the channel allocated for the transmission. Due to this, the transmitter is blocked from intruding on other stations. In the receiver, a channel filter allows signals within a specified range of frequencies to be received and interpreted, while preventing signals at unwanted frequencies from going through. SIW filters have recently being receiving particular attention and numerous design techniques and topologies have been reported in the literature. All reported SIW filter design methods involve some sort of compromise in choosing which crucial design specifications to prioritise, including size, selectivity, power handling capability, quality factor, cost, sensitivity to environmental effects, and other performance metrics (e.g., in-band and out-band performance metrics). It is practically challenging, if not impractical, to concurrently achieve all these conflicting design requirements. Attaining higher band selectivity, for example, normally involves the use of additional resonators which implies greater insertion loss along the communication channel [67]. A collection of SIW filter design techniques were recently suggested as modifications of the conventional substrate integration waveguide filter shown in Figure 7 and reported in [67]. These variations including the dual-mode SIW filters [68; 69], the wideband SIW filters [68], the multi-band SIW filters [70; 71], and the reconfigurable SIW filters [72]. Dual-mode SIW filters are primarily developed to decrease the filter volume by at least 50% [68] when compared to the conventional SIW filter [67]. This means that a reduced filter transmission loss is guaranteed due to the halved number of resonating SIW cavities. Dual-mode SIW filters could further achieve compactness when implemented on half-mode SIW cavities as reported in [49] and shown in Figure 8a. A dual-mode SIW filter with flexible responses has also been suggested in [69]. The proposed technique achieved many  
Figure 7: Simulation and measurement responses of a conventional three-pole SIW filter including the image of the fabricated filter device. adjustable transmission zeros. It also maintains the integrity and shielding of the SIW without increasing the design complexity [69]. Wideband substrate-integrated waveguide filters have been proposed and achieved with the transverse electromagnetic (TEM) mode transmission line because of their dispersion-free, broadband, and single-mode performance. A substrate-integrated waveguide allows only the propagation of TE modes which make the design of wideband SIW filters simpler and more achievable without the design consideration of parasitic TM modes. To increase  
Figure 8: Simulation and measurement responses and the image of the fabricated SIW filters: (**a**) four-pole half-mode SIW bandpass filter. Reprinted/adapted with permission from Ref. [49]. 2021, [PERSON] et al. ; (**b**) third-order triple-band bandpass filter [72]. the coupling for wideband applications, a zigzag filter topology with 28% bandwidth for European ultrawideband (UWB) applications was reported in [70]. Additional controllable cross-coupling networks are realised by both physical and nonphysical approaches to realise sharper results and more flexible alteration of the transmission zeros. A quick and correct full-wave EM analysis technique based on the boundary integral-resonant mode expansion technique was proposed and established to design the filter [68].