Substrate integrated waveguides (SIW) maintain the advantages of planar circuits (low loss, low profile, easy manufacturing, integration in a planar circuit board), and improve quality factor of filter resonators. Empty substrate integrated waveguides substantially reduce the insertion losses because waves propagate through air instead of through a lossy dielectric. The first empty substrate integrated waveguide (ESIW) used a simple tapering transition that can not be used for thin substrates. A new transition has recently been proposed which includes a taper also in the microstrip line, not only inside the ESIW, and so it can be used for all substrates, although measured return losses are only 13 dB. In this work the cited transition is improved by placing via holes that prevent undesired radiation, as well as two holes that help to ensure good accuracy in the mechanization of the input iris, thus allowing very good return losses (over 20 dB) in the measured results. A design procedure, that allows the successful design of the proposed new transition, is also provided. A back to back configuration of the improved new transition has been successfully manufactured and measured.
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