Compact reconfigurable bandpass filter using quarter wavelength stubs for ultra-wideband applications

In this article, the design of a compact reconfigurable bandpass filter using quarter-wavelength open/short-ended stubs for Ultra-Wideband applications is presented. The filter has been designed using three quarter wavelength stubs. Stubs are designed for specific frequencies. At the designed frequency, the quarter-wavelength stub exhibits either bandpass or bandstop characteristics according to its short or open-circuited configuration, respectively. The proposed filter generates five passbands and twelve transmission zeros through four reconfigurable states. The direct and higher-order responses of each stub are effectively utilized to achieve five passbands, which are at 3.8 GHz - 7.8 GHz (notch band from 5.9 GHz to 6.4 GHz), 2.8 GHz 5.9 GHz, 5.2 GHz - 8.0 GHz, 5.2 GHz - 5.88 GHz, and 1.7 GHz to 4.1 GHz. The filter is designed on FR-4 substrate and simulated in High Frequency Structure Simulator. To analytically validate the results, an equivalent transmission line circuit is extracted, and ABCD parameters have been derived. The S-parameters are derived from the ABCD parameters and plotted through MATLAB software. The designed filter is fabricated and tested. There is a good agreement between simulated and measured responses. The overall size of the filter is 24 x 15 x 0.8 mm(3).