A Bandpass Filter Design Using Half-Wavelength Stepped Impedance Resonators With Internal Couplings

We propose a new type of microstrip half-wavelength (lambda/2) stepped impedance resonator (SIR) for use in bandpass filter (BPF) designs. This lambda/2 SIR has an internal coupling section that can be used to generate a coupling coefficient in the filter design in addition to couplings between SIRs. It also contains additional stepped impedance lines that shift and suppress the lowest even-mode resonance frequencies to more than what can be achieved by conventional lambda/2 SIRs. Moreover, a transmission zero is generated close to its fundamental resonance frequency (f₀) which can be used in definition of the filter response. A fourth-order microstrip BPF with 10% bandwidth was constructed using two of the proposed lambda/2 SIRs with a stepped impedance ratio (R) of 0.528. The experimental result shows that the filter can achieve low in-band loss and out-of-band attenuation of 52.6dB up to 3f₀. The lowest spurious resonance frequency is shifted to 3f₀ as opposed to 2.5f₀ in conventional lambda/2 SIRs with the same R value