Enhancing the selectivity of frequency selective surfaces for terahertz sensing applications

This paper introduces a new technique for enhancing the selectivity (or the quality factor, Q-factor) of frequency selective surfaces (FSS) for sensing applications. The proposed FSS functions as a free-space bandpass resonator, designed to sense the changing dielectric properties of minute amount of materials loaded on the FSS. The Q-enhancement technique is mainly based on two concepts; enhancing the field concentration in a given area and introducing transmission zeros in the FSS response. Two designs based on a modified complementary split-ring resonator (CSRR) at 300 GHz have been proposed. The first one is composed of complementary triple-split ring resonators. The splits divided the structure into arcs of different lengths. As a result, the transmission zero is obtained in the passband due to a destructive coupling. This produces a resonance Q-factor of 41. By controlling the orientation of the three splits, higher Q-factor of 84 is attainable. The second structure is designed using concentric triple-split rings. The added electromagnetic coupling between the concentric rings makes the transmission response steeper as compared with the single triple-split ring, and the quality factor increases from 41 to 90. By reducing the inter-spacing distance by three times, the Q-factor can be further increased to 256. The parameter studies of the FSS structures based on full-wave simulations have been presented

Highly Selective Circularly Polarized Filtering Slot Antenna Based on Stub-Loaded Resonators

This letter presents a novel approach to designing a highly selective circularly polarized (CP) filtering antenna. It is based on rectangular waveguide cavity resonators that are directly coupled to each other. Capacitive stubs are loaded onto the cavity ends without increasing the circuit size to create adjustable transmission nulls. Two orthogonal radiating slots are opened onto the side-wall of the last coupled resonator to achieve end-fire CP radiation. An equivalent circuit model is presented to analyze the numerical results. The approach is eventually demonstrated by designing, simulating, and fabricating a fourth-order CP filtering antenna operating at X-band. The measured and simulated results are in excellent agreement, showing a quasi-elliptic response with a gain value of 7.0 ± 0.1 dBic over the passband. The 10-dB reflection coefficient (S11) and 3-dB axial ratio (AR) bandwidths are 7.5% and 5.5%, respectively. The competitive performance and compactness of the proposed device make it a promising candidate for stringent frequency selectivity applications

An x-band waveguide orthomode transducer with integrated filters

This letter presents a novel waveguide orthomode transducer (OMT) with integrated filters. Resonator-based bandpass filters are used for the two channels forming a duplexing OMT. As an example, a two-channel duplexing OMT is designed with channels at 9.9-10.1 GHz and 10.9-11.1 GHz. A good filter response with 20 dB return loss is achieved for both the channels. The upper and lower bands are isolated to −80 dB due to the filtering elements and a transmission zero. This new multifunction compact filter-OMT design leads to the size reduction and improved inter-band isolation. A CNC machined waveguide OMT is demonstrated and its performance is verified with excellent agreement between the electromagnetic wave (EM) simulation and measured results.</p