In this paper, the patch-type frequency selective surfaces (FSS) based on substrate-integrated waveguide (SIW) technology is proposed to improve the bandwidth (BW) and angular performance. The proposed FSS configuration overcomes the limitations of both conventional 2D and 3D FSS structures. A closely coupled cascaded mechanism is employed to combine two identical FSS elements separated by thin dielectric substrate results in incorporation of SIW technology; hence, named as 2.5D FSS. A derived equivalent circuit model is used to estimate the basic performance of proposed FSS–SIW elements, and the response of analytical expressions has been validated and final design is obtained using full-wave simulations. Two basic FSS elements viz. single square loop and a Jerusalem cross have been investigated to prove the enhancement in their BW and angular stability. The proposed technique evidently improves the BW and angular stability of FSS structures than in its established form. Besides, various important parameters that influence the performance characteristics of reported 2.5D FSSs are also studied. The important observations made on the thickness, as the thickness increases the bandstop FSS, can change to bandpass FSS. Finally, the proposed FSS structure has been fabricated and measured using free space measurement setup, to show the effectiveness of theoretical results. The measured results show good agreement with simulated results at normal and oblique incidence angle.

The airborne radomes have to cater superior electromagnetic (EM) performance with bandpass characteristics of stealth application. In this regard, a hybrid A-sandwich radome is proposed in this paper. The proposed radome consists of a novel strongly coupled frequency selective surface (FSS) core sandwiched between two dielectric layers (acts as skin) to form an A-sandwich structure. The dielectric layers are cascaded in such a way that the middle layer has less dielectric parameters than the skin dielectric. The core layer comprises a modified FSS array using strongly coupled FSS layers through a series of metallic vias. This strongly-coupled FSS element will have the advantage of eliminating inter-element interference and improves the EM performance characteristics of the structure. The structure exhibits very good band-pass characteristics (>90%) at a normal impinging angle with sharp roll-off characteristics. To show the efficacy of the proposed structure, the transmission loss has been compared with that of conventional A-sandwich radomes at 0°, 50° incidence angle for both TE and TM polarization. Conformal analysis of the unit cell has been carried out, and sector-wise thickness optimization was performed to analyze the structure for the conformal shaped radome application. Finally, a physical prototype has been fabricated and measured its scattering parameters, radiation characteristics in a fully shielded anechoic chamber. The results are encouraging and prove its suitability for radome application.

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The design of novel ultra‐thin polarisation rotating frequency selective surface based on substrate integrated waveguide (SIW) technology has been presented. The primary function of the structure is to select the linear polarisation from the impinging electromagnetic wave on it and to rotate the wave into the 90° counter‐clockwise direction in the given frequency band. The proposed array consists of periodic Y‐shaped slot elements surrounded by SIW cavities. The structure shows a relative bandwidth of 8% with impedance matching better than −10 dB and a very good insertion loss of 0.3 dB. It also offers co‐polarisation below −20 dB in the passband. The proposed structure is very thin (0.055λ0) compared to the existing SIW‐based frequency selective polarising rotators. Also, the conformal analysis of the proposed structure has been carried to study its behaviour for real‐time applications when used on curved surfaces. Finally, to prove the efficacy of proposed structure, a prototype has fabricated and measured its performance. A very good agreement is observed between the experimental and simulated results.

This study describes substrate integrated waveguide (SIW) cavity models to control the bandwidth of frequency selective surfaces (FSSs). Two different SIW cavities were presented: one model to reduce (cavity‐I) and another model to improve (cavity‐II) bandwidth of FSS, respectively. The periodic structures were made of SIW cavies and an FSS slot on either side of the dielectric substrate at the centre of the cavity. To prove the concept, a well‐known cross‐slot FSS element has been studied in two different cases and compared their performance. The conformal behaviour of the unit cells has been estimated by considering the practical requirements. The addressed SIW cavities will improve the electromagnetic (EM) performance of conventional FSS without altering its basic shape. The high Q‐factor of SIW cavity greatly improves the performance of FSS. A full‐wave simulation is used to analyse the response of FSS. The cavity‐I shows 12.61% lesser bandwidth and cavity‐II shows 25.0% higher bandwidth compared to its conventional shape in planar form and 9.1, 26.17% in the conformal form at a bent radius of 50 mm. Besides, performance parameters were studied to evaluate the structural dependency on EM performance. Finally, two designed cavity models are fabricated and proved their performance experimentally.