A Lossy Filter Using Quarter-mode Substrate Integrated Waveguide and Coplanar Waveguide

A novel lossy bandpass filter is designed by using quarter-mode substrate integrated waveguide and coplanar waveguide in this paper. Compared with the traditional substrate integrated waveguide cavity resonator, a quarter-mode substrate integrated waveguide (QMSIW)is utilized to decrease the filter dimension by 75%. This structure can further reduce the overall volume, allowing for better design freedom in practical applications. Two coplanar waveguide (CPW) transmission lines across the cavity are added to the filter to improve the upper stopband of the filter. Firstly, a second-order lossless filter is designed using a hybrid structure of QMSIW and CPW. Then a second-order lossy filter in the form of insertion resistance coupling is investigated. The center frequency of both filters is 5GHz. For the lossless filter, its insertion loss is 0.4dB, with the relative fractional bandwidth of 35% from 4.6 to 5.3GHz, and its return loss is greater than 27dB. After loading a resistor coupling for the lossy filter leading to 1dB insertion loss, the relative fractional bandwidth increases to 40% with passband between 4.5 and 5.39GHz, and the return loss is better than 33dB. It is obviously found that the bandwidth for the lossy filter increases nearly 200MHz, and a 6dB return loss within the passband is improved at the expense of 0.6dB insertion loss in the passband by introducing resistor coupling, which gives rise to an improved passband and sideband performance.