A compact second harmonic-suppressed bandpass filter using π-equivalent transmission lines

In this paper, a compact second harmonic-suppressed bandpass filter is developed using π-equivalent transmission lines. A conventional quarter-wavelength microstrip can be equivalent to a π-equivalent transmission line realized by a high-impedance line with bilateral shunt low-impedance stubs. The circuit configuration of the π-equivalent transmission line is beneficial to miniaturize the circuit size by merging open stubs into a single stub, and provide a good harmonic suppression. A compact bandpass filter can be implemented with a 45.3% circuit size of the conventional one with a 30 dB spurious suppression up to 3.4 f₀. The measured results of the developed bandpass filter are in a good agreement with the simulated results by the full-wave electromagnetic simulator.