This paper presents a quad-port triplexer structure designed for next-generation wireless portable devices. The proposed triplexer consists of three bandpass filters isolated from one another. Filters 1 and 2 are designed using identical coupled line structures with varying physical properties, while filter 3 is developed with a novel coupled line structure. The modified coupled line enables a compact triplexer design, enhances isolation and bandwidth, and reduces the insertion loss. A comprehensive analysis of the proposed triplexer is conducted, and the S-parameter response demonstrates that the triplexer operates at 2.91, 4.43, and 1.20 GHz, with fractional bandwidths of 30.2$\%$, 22$\%$, and 33.3$\%$, respectively. The in-band return loss is better than 12.3 dB across the entire passband. Additionally, the triplexer features three transmission poles for the first two passbands and two for the third. The in-band insertion loss is less than 0.63 dB for all three passbands. The measured performance of the fabricated triplexer closely corresponds to the simulated results, validating its effectiveness in achieving the desired specifications. This makes the triplexer highly suitable for 5G networks and IoT-based wireless communication devices that demand multiband, high-performance filtering.

With the rapid development of communication technology, the researches of multi-band filtering circuits have become more and more important. Multimode resonator (MMR) is one of the vital methods to provide multi-resonant modes within a single design. In this paper, a dual-band ultra-wideband bandpass filter (UWB-BPF) using stepped impedance stub-loaded resonators (SI-SLR) is presented. The main advantage of using SI-SLR is to have better performance with multimode behavior and more parameters to control resonant modes. SI-SLR combines the advantages of SIR and SLR structures, which gives a compact, high-performance multiband filter. The proposed filter design has compact size, sharp and flat response with low insertion loss (IL), low return loss (RL), and high band-to-band rejection. The filter is designed for UWB communication in wireless body area networks and fabricated on Arlon substrate with relative permittivity ${\varepsilon_{\textrm{r}}} = 3.25$, thickness $0.8\;{\textrm{mm}}$. The resulted dual-bands are centered at $4{\textrm{ GHz}}$ and $8.3{\textrm{ GHz}}$ with fractional bandwidths $37{\textrm{% }}$ and $48{\textrm{%}}$. The simulation was carried out using CST Microwave Studio. The filter provides good passband performances, with IL 0.49 dB and 0.31 dB at the center frequency of lower and higher bands, respectively. The band-to-band 40 dB rejection is realized by adding circular spiral at the input/output of the filter.