Designs of Dual-Band Circularly-Polarized Filtering Antennas Using Multiple Resonant Modes

This work developed two cavity-backed slot antennas based on substrate integrated waveguide (SIW) technology. The designs offer the advantages of single-layer structure, dual-band operations, circular polarization, and filtering performance. Antenna I features a perturbed aperture on an SIW cavity and four cross slots distributed near its corners to excite seven resonant modes. Among them, two sets of orthogonal modes, namely TE120/TE210 and TE230/TE320, are used to create the first (3.7 GHz) and the second (5.6 GHz) operating bands, respectively, and to establish dual-band circularly-polarized properties for 5G NR n46 and n78 applications. The remaining three modes, TE110, TE220, and TE330, serve to generate radiation nulls to increase stopband rejection. Measured results indicated a peak gain value up to 7.1 dBic. The rejection levels at the lower, middle, and upper stopbands were 23, 10, and 17 dB, respectively. Antenna II further improves the rejection level at the middle stopband by introducing three complementary split-ring resonators (CSRRs) to the SIW feed line. It allows a broader stopband between the two passbands and enhances the corresponding rejection level. The middle CSRR is etched on the top of the SIW, whereas the other two CSRRs are on the bottom to avoid undesired resonance and to improve the rejection level effectively. The measured realized gain was as high as 7.78 dBic with excellent filtering characteristics. The rejection levels of 24, 24, and 20 dB were observed at the lower, middle, and upper stopbands, respectively.