Comment on sharp-rejection and ultra-wide bandstop filter with five transmission zeros frequencies

In this letter, explicit transmission zero conditions, for the ultra-wide bandstop filter (BSF) with five transmission zeros, are derived using even-odd mode, analysis. The BSF topology is obtained by folding the two connecting transmission lines in a conventional optimum BSF to compose a stub-loaded coupled-line section and using λ_g/2 end open-stubs. A necessary and sufficient condition is explored for the crucial coupled-line section to produce three transmission zeros. Further, it is shown that the same five zeros BSF stopband bandwidth and rejection level performance is obtained with compact three zeros BSF (without λ_g/2 open-stubs) with improved passband flatness and return loss.