A Hybrid 3-D Frequency Selective Structure Featuring Enhanced Stopband Suppression and Reduced Insertion Loss

This article introduces a hybrid 3-D frequency selective structure (FSS) that offers high stopband suppression and low insertion loss to mitigate pattern degradation resulting from the mutual coupling of co-aperture array antennas. Traditional 2-D FSSs exhibit good passband performance but lack cross-coupling between resonators, making it difficult to achieve strong stopband suppression at a lower frequency. In contrast, traditional 3-D FSSs provide strong stopband suppression, but the vertical structures pose challenges for impedance matching, leading to high passband loss and poor angular stability. The FSS proposed in this article combines the advantages of both 2-D and 3-D FSSs. It introduces transmission zeros by combining a vertical surface with a cross-shaped metal strip on the foundational 2-D passband FSS. This design significantly enhances the stopband suppression capability of the FSS at the frequencies of interest without compromising the low insertion loss performance in the passband. Simulation results indicate that the proposed 3-D FSS achieves a −10 dB stopband within the frequency range of 1.2 to 2.93 GHz, with a stopband suppression of −16 dB at 2.7 GHz. Furthermore, the average insertion loss within the passband frequency range of 3.3–3.8 GHz remains low, staying below 0.23 dB. The FSS's transmission response remains stable under oblique incidences ranging from 0° to 60°. The operating principle of the FSS is elucidated using an equivalent circuit model. A prototype FSS is fabricated and experimentally characterized for validation. The measured results align with the simulated ones, confirming the FSS's high stopband suppression, low insertion loss, and excellent angular stability, making it a viable solution for mitigating pattern degradation in co-aperture array antennas.