Compact Single‐Cavity SIW Fifth‐Order Filtering Antenna With Dual Radiation Nulls

This letter presents a compact fifth‐order substrate integrated waveguide (SIW) filtering antenna integrating bandpass filtering and radiation in a single cavity. A five‐arm star‐shaped slot network excites multiple hybrid modes to realize a fifth‐order passband. An upper‐stopband radiation null (RN U ) is generated by a nonadjacent cross‐coupling path at the slot intersection, while a narrow U‐slot perturbation on the feeding side forms a localized open‐ended half‐wavelength resonant path to create a lower‐stopband radiation null (RN L ). The dual nulls enhance out‐of‐band suppression and frequency selectivity. A direct‐current (DC)‐isolated interlayer coupling feed is implemented using a bottom‐layer grounded coplanar waveguide (GCPW), a vertical via, and an aperture in the intermediate ground plane, avoiding an extra feeding cavity and improving manufacturability and excitation efficiency.