A noninvasive contactless binary gas sensing system is enabled using coupled passive flexible tag integrated with porous adsorbent beads tower. The platform is based on two flexible passive tags coupled with a dual-reader resonator operating at 4.86 GHz, while the distance between passive reader and tags can be extended to 10 mm. The sensor demonstrates the capability of detecting and monitoring more than one gas simultaneously and independently from each other. Various concentrations of methyl ethyl ketone (MEK) and humidity have been detected distinctively by using a hyper cross-linked polymer as volatile organic compounds (VOCs) and silica gel as humidity adsorbent beads. Selective accumulation of the gas molecules in the beads' pores is monitored with less than 5% error and by separate coupled tags and their associated resonant frequencies. The proposed structure offers a low-cost conformal solution and has the potential for real-time contactless multiple gas sensing in environmental applications. The structure demonstrates the sensitivity of 0.18 MHz/ppm in VOC sensing and 0.19 MHz/ppm while performing selective binary sensing.

This research study reports on a new coupling structure to demonstrate a constant coupling strength for longer distances in planar microwave sensors. It is shown that there is a positive correlation between the gap size in a pair of gap-coupled transmission lines (GCTLs) and the effective distance of reader and sensing resonators in a planar microwave sensor. On the other hand, the coupling strength in a pair of GCTLs is negatively correlated to the gap size between the two lines, where the larger gap sizes result in weaker coupling strength or vice versa. A technique is introduced to make the coupling strength partially independent of the gap size in GCTLs. The technique enables a sensor designer to increase the gap size, and subsequently, the distance between the reader and sensing resonators of a sensor. Strong electromagnetic coupling is locally demonstrated between a pair of GCTLs using a new artificial perfect electromagnetic conductor (PEMC) boundary, which does not allow electromagnetic power reflect toward the source over larger gap sizes in GCTLs. The concept is experimentally verified with a gap size up to 10 mm between GCTLs with a constantly measured insertion loss of 0.9 dB. Moreover, the structure is employed to design distant microwave resonator for humidity sensing applications. The design has demonstrated the ability of sensing, while the sensing resonator is placed at 35-mm distance from the readout circuitry. The structure is reported to increase the sensing distance with a ratio of about 12 (36:3) compared to the conventional microwave sensors.

Glycerol concentration measurement is one of the most important biomarkers for many diagnostic applications, such as indication of hyperglyceridaemia and coronary heart disease. A new distant microwave sensing platform to monitor the concentration of glycerol in a water and serum solution inside a microfluidic channel is presented. The sensor is based on a microstrip line reader with a defected ground plane coupled to a chipless microwave resonator tag. Due to the strong coupling between the reader and the tag, the distance between them can be increased up to 45 mm. The high level of sensitivity of the structure makes it suitable for nanolitre volume scale chemical sensing inside the microfluidic channel. The sensor demonstrates a frequency shift of 6 MHz when the volumetric percentage (V%) of glycerol in DI water changes from 70% to 0%. The proposed solution provides an opportunity to integrate the chipless passive tag with the microfluidic channel while the reader is distant from the microfluidic system.

This paper investigates L-shaped iris (LSI) embedded in substrate integrated waveguide (SIW) structures. A lumped element equivalent circuit is utilized to thoroughly discuss the iris behavior in a wide frequency band. This structure has one more degree of freedom and design parameter compared with the conventional iris structures; therefore, it enables design flexibility with enhanced performance. The LSI is utilized to realize a two-pole evanescent-mode filter with an enhanced stopband and a dual-band filter combining evanescent and ordinary modes excitation. Moreover, a prescribed filtering function is demonstrated using the lumped element analysis not only including evanescent-mode pole, but also close-in transmission zero. The proposed LSI promises to substitute the conventional posts in (SIW) filter design.

New ultra‐wideband (UWB) band‐pass filters (BPFs) with multiple (triple and quadruple) notches are proposed. A systematic approach is demonstrated to establish one to four or more notch‐bands within the passband of a UWB BPF on a single‐layer substrate using µ‐strip technology, while avoiding the use of any via‐holes. The proposed filters are based on parallel integration of gap‐coupled microstrip resonators (GCMR) and two tri‐section stepped‐impedance resonators (TSSIR) to generate multiple notches. Two notches are produced by the use of TSSIR fundamental and first spurious resonance frequencies and the third and fourth notches are introduced by developing transmission zeros using additional integrated GCMR. Two samples of triple and quadruple notch‐band UWB filters are fabricated and tested. In comparison to the previous triple single‐layer notch‐band BPFs with via‐holes, the proposed single‐layer triple notch BPF offers a size reduction of around 52%. The authors have also expanded the concept and designed a new quadruple notch‐band UWB BPF. The proposed filter uses only a single layer and can be easily mass‐produced with low cost.

Single-layer capacitively coupled structure integrated with substrate-integrated waveguide (SIW) is developed to design evanescent-mode bandpass filters. The planar gap-coupled structure is proposed as an alternative for the conventional inductive aperture-coupled one for filter excitation below the cutoff frequency of the SIW. The proposed planar capacitive excitation enables considerable miniaturization and resonates at a small fraction of a 3-D structure's cutoff frequency. The proposed integration is analytically discussed using lumped-element equivalent circuit accompanied with experimental verification. Measured scattering parameters are compared to those of simulations, which demonstrate good agreement.