A single-substrate microstrip/coplanar waveguide broadside-coupled structure is utilised to build a lowpass filter. The main features of the proposed filter are the extremely wide stopband, sharp cutoff response, compact size and closed-form design procedure. The theory of operation for the proposed filter is presented, its design procedure is derived and its performance is explained. The measured and simulated performance of a manufactured prototype filter agrees well with the theoretical analysis. The filter shows a negligible radiation, flat group delay and a sharp and wide stopband that is larger than 14 times the 3 dB cutoff frequency of the filter. The length of the coupled structure required to build the filter is less than 6 of the effective wavelength calculated at the cutoff frequency.
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