Harmonic Suppression in a Meander-Loop Dual-Mode Bandpass Filter by using Spurline

The present article proposes a compact microstrip dual-mode meander-loop resonator bandpass filter for the wireless communication system. The proposed filter has a bandwidth of 0.1 GHz (fractional bandwidth of 4%) and operates at a center frequency of 2.5 GHz. A size reduction of 30% has been achieved for the meander-loop bandpass filter over the conventional square-loop dual-mode bandpass filter. Despite having improved skirt properties in the passband, the meander-loop dual-mode filter suffers from the presence of spurious harmonics. Subsequently, symmetrically folded spurlines have been incorporated into the feed ports of the meander-loop filter to generate multiple transmission zeros in the stopband by utilizing the bandstop nature of the spurline. As a result, an extended stopband up to 3.08f<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> (where f<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> is the center frequency) with a rejection level greater than 30 dB has been obtained in the simulation. Furthermore, the skirt properties of the filter have been enhanced by the creation of two sharp transmission zeros at the passband edges with attenuation levels greater than 42 dB.