## 1 Introduction
Moreover, two single-band filters with common input and output coupled-feed lines can be combined to create dual-band filters [25; 26], although these filters can be challenging to debug and take up a lot of circuit space. Figure 1: Placement of bandpass filter in communication systems [6]. Some pioneered works have also been done in the design of compact and sharp triple passband filters. For example, the authors in [27; 28] used two coupled resonators and two stub-loaded resonators with the dual-mode operation to design tri-band BPFs. They achieved good selectivity by generating multiple TZs. However, the authors fail to explain the reasons behind the TZs. Using two asymmetrical step impedance resonators, a dual-band filter was created [29]. To get a triple band response, a \(\lambda/2\) UIR was placed below the transmission line to generate five T.Z.s at various locations. Various configuration of multimode resonators was used in [30; 31] to design a triband or quadband filter employing SIRs and stub-loaded resonator structures. There are two topologies for designing tri-band filters based on MMRs. One approach is to cascade multiple MMRs [32], while the other uses a single controllable MMR [33]. Several MMRs are used in the cascading tri-section SIRs technique, where each MMR's geometric arrangement can modify the frequency band. There are only two transmission zeros (TZs) close to the first passband, so this strategy can have poor selectivity in the second and third passbands [34]. A defective ground structure (DGS) can be utilized to address this issue, but this increases the difficulty of manufacturing since the circuit needs to be etched on the grounded metal surface [35; 36]. On the other hand, the single controllable MMR approach involves using a single MMR to achieve three passbands. To minimize the overall size of the circuit without compromising selectivity performance, a tri-band filter based on SLR was proposed with eight resonance modes. However, it can be difficult to individually control each frequency band in a symmetrical SLR [37; 38]. In short, cascading, and single MMR approaches have advantages and limitations in designing tri-band filters. The choice of approach depends on the specific design requirements, such as selectivity, size, and ease of manufacturing. According to [39], the filter's passbands are centred at 2.9, 5.6, and 11 GHz, employing a multi-mode surface spoof plasmon. Its insertion loss in the passband is larger than 3 dB, and its circuit area is 144 \(\times\) 40 mm. Another filter that operated at 2.4, 3.5 and 5.2 GHz was described in [40] using the idea of SIR and uniform resonators. This filter features adjustable multi-band/wideband properties and a low insertion loss of less than 1.7 dB. For operation at 1.93, 2.6, and 3.9 GHz, another triband filter also utilizes uniform resonators. The circuit measures 0.54 \(\lambda_{\text{eff}}\)\(\times\) 0.77 \(\lambda_{\text{eff}}\)[41]. Overall, while there has been considerable research on microwave bandpass filters using various techniques for obtaining size reduction, there seems to be a lot of room for improvement, particularly in sharp selectivity and performance enhancement, that presents opportunities for further investigation. In this article, a high selectivity triple passband filter is designed using a quarter wavelength SIR and embedded L-shaped structure coupled to the low impedance section of the SIR. The filter operates at resonance frequencies of 1.57 GHz, 3.57 GHz, and 4.23 GHz with a low insertion loss of 0.7 dB for the first passband, 1.2 dB for the second passband, and 1.08 dB for the third passband with a return loss greater than 25 dB for all the frequency bands. There are eight transmission zeros at different locations generated below and above the passbands resulting in a very good sharp filter.