  The possibility of making compact stopband filters using coplanar-coupled EBG
resonators in inverted microstrip gap waveguide technology is studied in this
work. To do this, the filtering characteristics of different configurations of
mushroom-type elements are shown in which the short-circuit element is placed
on the edge of the resonator of the patch. The behavior of the structure as
well as its main advantages such as: low losses, self-packaging, low level of
complexity, flexibility and easy design are illustrated in the paper. To
evaluate the possibility of integrating these structures in gap waveguide
planar antennas feeding networks, a 5-cell EBG filter was designed and built at
the X band. The proposed filter reached a maximum rejection level of minus 35.4
dB, had a stopband centered at 9 GHz and a relative fractional bandwidth below
minus 20 dB of 10.6 percent. The new compact filter presented a flat passband
in which it was well matched and had low insertion losses that, including the
connectors, were close to 1.5 dB in most of the band. These results are enough
to improve low-complexity future antenna designs with filter functionalities in
this technology.
