Design of a High-Order 2 × 2 SIW Cavity-Backed Patch Subarray and Its Application in Millimeter-Wave CP Filtenna Array

A novel millimeter-wave 2 × 2 substrate integrated waveguide (SIW) cavity-backed patch subarray with a simplified feeding structure is presented. Two higher order SIW cavity modes (quasi-TE
₄₁₀
and quasi-TE
₃₃₀
modes) are designed using eigenmode analysis to support 2 × 2 patches with in-phase radiation. The proposed 2 × 2 patch subarray has multiple resonant modes and a simplified feeding network. Moreover, this subarray can create a radiation null at the upper band edge, which helps to synthesize the filtering response. To verify the potential application in large-scale array, four 2 × 2 subarrays are combined into a 4 × 4 circularly polarized filtenna array with a filtering sequentially rotated feeding network. The measurements of the fabricated filtenna array show an operating bandwidth of 8.9% from 26.6 to 29.1 GHz (|
S
₁₁
| < −10 dB and AR < 3 dB), a peak gain of 15.9 dBic, and stable boresight radiation patterns. Excellent bandpass filtering response with high out-of-band rejection levels of 32 and 48 dB in the lower and upper stopbands, respectively, is observed.