Bandpass Filter in Ridge Empty Substrate Integrated Waveguide With U-Shaped Impedance Inverters

Substrate integrated waveguides (SIWs) are a compromise in terms of performance, cost, and volume between classical planar technologies and high quality three-dimensional waveguides. Empty SIWs (ESIW) reduce the losses by removing the dielectric substrate. In order to increase the usable bandwidth of the ESIW, a ridge layer can be added to the ESIW, thus obtaining the so-called ridge ESIW (RESIW). A transition from microstrip to RESIW has already been presented, but no other passive component has yet been implemented in this new technology. In this letter, we present a novel bandpass filter in RESIW, based on a new U-shaped geometry acting as impedance inverter, that can be easily fabricated with planar circuit manufacturing techniques. A design procedure for this filter is presented. A four-pole Chebyshev filter centered at 15 GHz, with a relative bandwidth of 3%, has been manufactured. The measured values for insertion and return losses are, respectively, lower than 1.05 dB and greater than 18 dB in the whole passband.