This paper presents the synthesis and design of the multi-mode dual-band bandstop filter (MM-DBBSF). A highly selective multi-mode dual-band bandstop response is obtained using a quarter wavelength coupled line structure. It has been shown that by increasing the coupled line's order, the selectivity and the transmission zeros are increased in the desired stopband. Moreover, a step impedance resonator (SIR) is used between two coupled lines to achieve more transmission poles for better out of band selectivity. The paper show a detailed theoretical synthesis of the coupled line and SIR structure. In order to validate the theoretical model, ideal and microstrip topologies are designed and simulated. Furthermore, a high-frequency substrate is used to fabricate four prototypes. The simulated and measured results show good concurrence.

The design of single-and quad-band Bandpass Filter (BPF) topology has been presented in this paper for next-generation Internet of Things (IoT) devices. The main topology is constructed using the Split Ring Resonator (SRR), separated by the Anti-Parallel Coupled Line Structure (APCLS). A detailed analysis of APCLS has been presented, which is further used to construct the single-and quad-band BPF. The single-band BPF design consists of SRR loaded with APCLS. The developed single-band BPF displays a dual-mode response with a center frequency of 2.65 GHz and a measured fractional bandwidth of 17.17%. Moreover, a quad-band bandpass filter has been achieved using the same topology with minor modification in the SRR and APCLS electrical parameters. The developed quad-band BPF generates a dual-mode response having center frequencies of 1.2, 2.4, 3.5, and 4.7 GHz with the measured fractional bandwidth of 13%, 26%, 16%, and 5%, respectively. Two prototypes have been fabricated on the highfrequency substrate to validate the proposed topologies. Very high rejection in the stopbands region, little in-band insertion loss, and very selective passband have been measured for single-and quad-band BPFs. The measured and simulated results are well correlated.