Design and Fabrication of the Miniaturized Ultra-Wide Band Pass Filter Using the Microstrip Rectangular Ring Loaded by Open Stubs

In this article a miniaturized ultra-wideband (UWB) bandpass filter (BPF) using the microstrip rectangular ring loaded by two open stubs is presented. The proposed structure consists of two doublet parallel coupling gaps at each side of a microstrip ring. In comparison with some other filters, this structure shows a significantly wider passband due to the introduction of a cross-coupling between the feed lines (input and output) which generates four pairs of transmission poles in the passband. In order to control the transmission poles in the pass band and adjust the transmission poles near the lower and upper edges of the filter, two open stubs have been loaded on the microstrip rectangular ring. Consequently, a compact four poles ultra-wide bandpass filter is designed which exhibits extremely sharp rejection skirts around the target passband. The proposed filter has a passband that covers 3.2 to 10.6 GHz and its measured 3 dB fractional bandwidth is about 107.5%. In order to validate the performance of the proposed technique, the designed UWB band-pass filter is fabricated and experimental verification is provided. A good agreement has been found between the results obtained by simulation and the measurements. Furthermore, rejection level better than 20 dB in upper stopband is extended to around 15 GHz. To our knowledge, the size of the proposed ultra-wideband filter is more compact in comparison with known similar filters.