Planar Bandpass Filters for Ultra-Wideband Applications

The design of planar bandpass filters (BPFs) with ultra-wideband (UWB) behavior is presented. The proposed filters utilize broadside coupling between elliptical-shaped microstrip patches at the top and bottom layer of the filter´s structure via an elliptical slot located at the mid layer, which contains the ground plane. A theoretical model is presented to explain performance of the suggested filters. Results of calculation show that the elliptical microstrip-slot-microstrip broadside-coupled structure can be used to build UWB BPFs with a flat group delay, which makes the presented configuration a good candidate for very narrow pulse transmission/reception. A design procedure for multisection broadside-coupled filters is explained. BPFs, which use different sections of the proposed structure, were designed and manufactured. Results of simulation and measurement show that the developed devices have a 3-dB insertion loss bandwidth from 3.1 to 10.6 GHz. They have less than 1-dB insertion loss at the center of the passband, a sharp cutoff stopband, especially at the low-frequency stopband, and a flat group delay within the passband.