Design of millimeter-wave microstrip BPF using dual-mode ring resonator and folded half-wavelength resonators

In this work, the design of microstrip bandpass filter (BPF) at the millimeter-wave band is investigated using a method that is based on dual-mode ring resonator and folded half-wavelength resonators. Earlier such structure has been reported to realize BPF in quasi-millimeter waveband. The filter design parameters are optimized using the EM simulation tool. A couple of filter structures are designed by deploying this method with a fractional bandwidth of 18% at a center frequency of 34 GHz and fractional bandwidth of 15% at a center frequency of 40 GHz and subsequently manufactured on Rogers 5880 10-mil substrate. The measured insertion losses of these two filters are
$2.27\pm 0.35\text{dB}$
and
$2.50\pm 0.4\ \text{dB}$
respectively. The filters exhibited flat response in the passband, sharp roll-off skirt, and good stopband suppression over a wide frequency range. The sizes of core structures of filters are 5.4×2.95mm and 4.7x2.66 mm, while connecting transmission lines were extended to fit the circuit in mechanical housing having a size of 13.8x7.6mm. Measured parameters of physical circuits are in close agreement with simulation results.