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We report several types of coplanar waveguide tri-section stepped-impedance resonator (SIR) structures that offer further size reduction compared to the conventional two-section SIRs. In addition, the tri-section SIRs provide flexibility of introducing effective capacitive coupling to the ground lines for realizing slow-wave structures. The slow-wave effect can be used to implement more compact bandpass filters. The principles of achieving size reductions using tri-section SIRs are described and confirmed by simulation results. To demonstrate the effectiveness of the proposed tri-section SIRs in size reduction, the tri-section SIRs with slow-wave effects were implemented in a two-pole directly coupled bandpass filter and a fourth-order quasi-elliptic bandpass filter with reduced size. The measured results matched well with the theoretical prediction.