Improved Stopband Rejection in Compact Microstrip Filters by Transmission Zero Reallocation

The design of compact microstrip bandpass filters with a center frequency of 5.25 GHz applicable for WLAN with the standard of IEEE 802.11a is presented in this article. The conventional coupled-line filter has been folded at the symmetry plane, which results in the improvement of upper stopband skirt characteristics. However, the harmonic rejection level remains almost unaffected due to the phase velocity imbalance. Subsequently, the coupled-lines have been modified by inserting quarter-elliptical grooves and the transmission zero reallocation has been employed. Fabrication of the optimized third-order folded filters has been carried out to justify the effectiveness of periodic grooves in the improvement of harmonic suppression. A wide stopband below 35 dB of attenuation level up to 2.4f0 has been obtained experimentally. Moreover, the filter size has been reduced to 58% compared to the conventional filter.