Conventional planar lowpass filters often suffer from the problem of spurious passbands.For spurious response suppression

integration of bandstop filters in a conventional open-stub lowpass filter is presented.By using equivalent T-shaped transmission lines to replace series connecting transmission lines of the conventional lowpass filter

the T-shaped lines work as bandstop filters at the spurious passbands.Furthermore

general design equations are given to transform arbitrary series connecting lines of the filter into the equivalent T-shaped lines.The proposed filter improves the out-of-band rejection over very wide operation band width and reduces the overall size with low insertion loss.Very good agreement has been observed between the experimental results and simulated data.The insertion loss is less than 0.5 dB from DC to 3GHz and the rejection is greater than 60 dB from 3.6GHz to 12GHz.