Optimization of the S-parameter response of a coplanar waveguide series short stub for broadband applications

A novel approach for the optimization of the S-parameter response of a coplanar-waveguide (CPW) series short stub has been proposed in this paper. This stub acts as a bandstop structure and the usual method for controlling the centre frequency is to vary the length of the short-end stub whereas the fractional bandwidth (FBW) of the stopband is controlled by varying the width of the stub. In this work, the CPW short stub is analyzed fully to show the dependency of the S-parameters on all of the proposed new dimensions. Mainly, the effect of variation of the new dimensions on the fractional bandwidth, centre frequency of resonance and the cutoff frequencies is found and then the possible applications are stated. Moreover, for low frequency structures employing the CPW short stub, the length of the short-end of the stub needs to be increased if length is used as an optimizing parameter which results in an overall increase in the size of the structure. However, in the current work, such dimensions have been proposed which need to be reduced rather than increased for low frequency applications and hence result in an overall compact and smaller size. Such dimensions have also been introduced which would reduce the FBW considerably and hence can find applications in notched filter designs. Moreover a dimension has also been proposed to increase the FBW significantly which can be useful in broadband applications. Finally, dimensions for altering only one of the cutoff frequencies; either the lower or the higher without affecting the other one have also been proposed. Most CPW structures employ a series short stub and when the proposed optimization approach is used to achieve the desired S-parameter response for a CPW broadband structure, a more compact and smaller sized design results.