Ultracompact microstrip dual‐ and triple‐band bandpass filters with a common resonator feeding structure

Microstrip dual- and triple-band bandpass filters (BPFs) exhibiting an ultracompact size and high frequency selectivity are proposed in this study. A common resonator feeding technique is proposed and was applied to design the multiband BPFs. In the design, the first and last resonators are realised using stepped-impedance resonators and are used to form a passband that has lower operating frequencies than the passband(s) of other resonator(s). Moreover, the first and last resonators serve as the input-output feeding structure and provide a source-load coupling for the passband(s) operating at higher frequencies. Thus, finite transmission zeros can be formed over the stopband. Hence, size miniaturisation and high frequency selectivity can be achieved simultaneously. The higher operating passband(s) is (are) formed by the dual-mode stub-loaded resonator(s) to reduce the number of resonators and the overall size of a filter. To verify the design approach, a dual-band BPF centred at 1.3 and 2.58 GHz with an ultracompact circuit size of 0.048 λ g × 0.088 λ g ( λ g : guided wavelength at the central frequency of the lower passband) and a triple-band BPF centred at 1.24, 2.5, and 3.5 GHz with an ultracompact circuit size of 0.15 λ g × 0.12 λ g were designed and fabricated. The measurement and electromagnetic simulation results were in good agreement.