Compact Folded Bandpass Filter in Empty Substrate Integrated Coaxial Line at $S$ -Band

This letter presents a novel compact folded bandpass filter in empty substrate integrated coaxial line (ESICL) technology. To obtain these compact filters, the inverters are modified to provide a 180° bend when inserted in an ESICL. As a result, the proposed filter has a more compact topology based on parallel resonators. A sixth-order Chebyshev bandpass filter operating at 3 GHz has been designed, manufactured, and measured to experimentally validate this new folded configuration. The length of the novel compact filter has been reduced by 82.2% with respect to an equivalent in-line filter, obtaining very similar results in terms of electrical response.