High-Gain Compact SIW-Based Quadruplex Filtering Antenna With Small Frequency Ratio

This letter presents a high-gain compact quadruplex filtering antenna with a small frequency ratio (FR) based on a two-layer substrate integrated waveguide (SIW) structure. The lower-layer feeding section consists of two isosceles right-triangle SIW cavities sharing a common hypotenuse, where independent excitation of the TE102 and TE201 modes in each cavity enables four isolated channels. The upper-layer radiation section is composed of a hexagonal SIW cavity with a centered square aperture, incorporating a square via-wall and two nested triangular patches. The SIW slot supports two orthogonally polarized modes for the lower bands, while the triangular patches dominate the two higher bands. Perturbation posts and coupling posts enable independent quad-band frequency tuning through modal decoupling. A prototype operating at 3.33 GHz, 3.59 GHz, 3.84 GHz, and 4.03 GHz demonstrates high gains of 6.91 dBi, 6.72 dBi, 6.22 dBi, and 5.3 dBi with channel isolation exceeding 18 dB, validating its suitability for compact multiband wireless front ends.