Semi-Analytical Design Approach to Mitigate Radiation Nulls and Cross-Polarization in HOM-Based Wideband SIW Antennas

Higher-order modes (HOMs) impede the homogeneity of the radiation patterns over wide bandwidths by introducing radiation nulls, higher side lobes, and increased cross-polarization compromising polarization purity. This article proposes a semi-analytical approach to reduce the radiation nulls and cross-polarization by positioning the multiple radiating slots at unequal inter-element spacing and enhancing the current path without increasing the design complexity and size. Using the proposed approach, a substrate-integrated waveguide (SIW)-based dual-polarized antenna is designed that couples TE<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">hybrid</sup><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">130/310</sub> and TE<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">hybrid</sup><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">230/320</sub> modes to exhibit wide bandwidth that ranges from 26.0 GHz to 30.5 GHz (16.0%) with consistent radiation performance and a considerable reduction in H-plane cross-polarization. To further show the effectiveness of the formulation in dual-band antennas, a SIW based self-diplexing antenna is also designed with reduced cross-polarization. Both antennas are fabricated and validated with a good agreement between the measured and simulated results. The dual-polarized antenna shows a reduction in radiation null (E-plane) at 335° from –23 dB to –10.3 dB at port-2 and at 320° from –17.5 dB to –2.5 dB at port-1 at 27.5 GHz. The radiation null is completely eliminated at the broadside from –40.7 dB to 0 dB at 29.0 GHz (TE<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">hybrid</sup><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">230/320</sub> mode). A reduction of 21.7 dB in the H-plane cross-polarization is achieved at 29.0 GHz with the cross-polarization discrimination of 14.3 dB. In the self-diplexing antenna, a reduction in H-plane cross-polarization at 25.0 GHz is 4.5 dB, whereas it is 1.5 dB at 28.1 GHz with a slight gain enhancement of 0.24 dBi in both ports. Consequently, the proposed approach underscores the potential for antenna designs with improved polarization purity and reduced radiation nulls suitable in urban environments, smart cities, backhaul links, and next-generation systems.