Abstract: A split-ring resonator (SRR)-based sensor for the detection of solid thickness and relative permittivity characterization of solid and liquid materials is proposed. The structure is composed of two SRRs hosted in a microstrip transmission line. The sensing principle is based on the detection of the notch introduced by the resonators in the transmission coefficient. Hence, a frequency shift of the notch is related to a change in the effective permittivity of the structure when the sensor is covered with any solid or liquid material. A complete characterization of the sensor, for the three proposed applications, is performed through simulations. Finally, all simulated results are corroborated with measurements. The proposed sensor is implemented in single-layer printed technology, resulting in a low-cost and low-complexity solution. It presents real-time response and high sensitivity. Moreover, it is fully submersible and reusable.

Abstract: This paper focuses on the analysis of splitter/ combiner microstrip sections where each branch is loaded with a complementary split ring resonator (CSRR). The distance between CSRRs is high, and hence, their coupling can be neglected. If the structure exhibits perfect symmetry with regard to the axial plane, a single transmission zero (notch) at the fundamental resonance of the CSRR, arises. Conversely, two notches (i.e., frequency splitting) appear if symmetry is disrupted, and their positions are determined not only by the characteristics of the CSRRs but also by the length of the splitter/combiner sections. A model that includes lumped elements (accounting for the CSRR-loaded line sections) and distributed components (corresponding to the transmission lines) is proposed and used to infer the position of the transmission zeros. Frequency splitting is useful for the implementation of differential sensors and comparators based on symmetry disruption. Using the model, the length of the splitter/combiner sections necessary to optimize the sensitivity of the structures as sensing elements is determined. Parameter extraction and comparison with electromagnetic simulations and measurements in several symmetric and asymmetric structures is used to validate the model. Finally, a prototype device sensor/comparator based on the proposed CSRR-loaded splitter/combiner microstrip sections is presented.

Abstract: The symmetric spoof surface plasmon (SSP)-based slow-wave transmission line (SW-TL) with compact transition, low ohmic loss, and low crosstalk between SW-TLs is proposed and investigated. First, the SSP cells are modeled by equivalent circuit elements. The proposed equivalent circuit model is appealing for the integration of SW-TLs with other microwave circuits. With the symmetricity, the SW-TLs are readily realized by a compact mode converter providing gradual impedance, momentum, and polarization matching from guided waves to spoof microwave plasmons. Then, simulation studies and experiments verify that the proposed SSP SW-TL features as low as half the ohmic loss of the traditional counterparts. After that, the low mutual coupling between the proposed SSP SW-TLs is numerically and experimentally substantiated and showed to be up to 10 dB lower than that between conventional microstrip TLs. The proposed low loss, highly isolated and compact SW-TL along with the reliable circuit model enables the further exploitation of promising spoof plasmon modes in microwave technology.

Abstract: This communication presents a small, low-profile planar triple-band microstrip antenna for WLAN/WiMAX applications. The goal of this communication is to combine WLAN and WiMAX communication standards simultaneously into a single device by designing a single antenna that can excite triple-band operation. The designed antenna has a compact size of $19 \times 25\;\text{mm}^{2}$ ( $0.152 \lambda_{0}\;\times 0.2 \lambda_{0}$ ). The proposed antenna consists of F-shaped slot radiators and a defected ground plane. Since only two F-shaped slots are etched on either sides of the radiator for triple-band operation, the radiator is very compact in size and simple in structure. The antenna shows three distinct bands I from 2.0 to 2.76, II from 3.04 to 4.0, and III from 5.2 to 6.0 GHz, which covers entire WLAN (2.4/5.2/5.8 GHz) and WiMAX (2.5/3.5/5.5) bands. To validate the proposed design, an experimental prototype has been fabricated and tested. Thus, the simulation results along with the measurements show that the antenna can simultaneously operate over WLAN (2.4/5.2/5.8 GHz) and WiMAX (2.5/3.5/5.5 GHz) frequency bands.

Abstract: A frequency selective surface with absorptive/ transmissive property is represented. It allows waves at high frequency around 10 GHz to transmit with very low insertion loss by using the resonance between a parallel microstrip LC structure. It also possesses a wide absorption over lower band by inserting lumped resistors into elements. The absorption band is over 3–9 GHz. A prototype is fabricated and its absorptive/ transmissive performance is measured.

Abstract: This paper reports on an effective mutual coupling suppression technique in which a metamaterial superstrate is placed in between the elements of densely packed microstrip phased array. Modified complementary split ring resonators are printed on the decoupling superstrate slab which caters for both surface and space wave effects. A detailed analysis of this proposed scheme is carried out on a low as well as on a high-permittivity substrate. Coupling suppression of 27 and 11 dB is achieved experimentally on the low- and high-permittivity substrates, respectively, with an element separation of $\lambda_{o}/8$ . The design is compact and easy to realize and it removes drawback of poor front-to-back ratio previously reported in other decoupling techniques. In addition to high-coupling suppression, the decoupling slab can be added or removed in real time which makes this technique versatile for various applications having stringent performance requirements.

Abstract: This paper presents a novel, easy-to-fabricate and operate, single-layer leaky-wave antenna (LWA) that is capable of digitally steering its beam at fixed frequency using only two values of bias voltages, with very small gain variation and good impedance matching while scanning. Steering the beam of LWAs in steps at a fixed frequency, using binary switches, is investigated, and a new half-width microstrip LWA (HW-MLWA) is presented. The basic building block of the antenna is a reconfigurable unit cell, switchable between two states. A macrocell is created by combining several reconfigurable unit cells, and the periodic LWA is formed by cascading identical macrocells. A prototype HW-MLWA was designed, fabricated, and tested to validate the concept. To achieve fixed-frequency beam scanning, a gap capacitor in each unit cell is independently connected or disconnected using a binary switch. By changing the macrocell states, the reactance profile at the free edge of the microstrip and hence the main beam direction is changed. The prototyped antenna can scan the main beam between 31° and 60° at 6 GHz. The measured peak gain of the antenna is 12.9 dBi at 6 GHz and gain variation is only 1.2 dB.

Abstract: Differential techniques are widely used in communication and sensor systems, as these techniques have been shown to improve the performance. This paper shows how differential sensing of permittivity can be conducted in a simple way. For that purpose, a microstrip line loaded with a pair of stepped-impedance resonators is used in two different resonator connections: parallel and cascade. Each resonator is individually perturbed dielectrically so that: 1) when the two individual permittivities are identical, the structure exhibits a single resonance frequency and 2) when the permittivities are different, resonance frequency splitting occurs, giving rise to two resonances (all these resonances are seen in the form of transmission zeroes). The two sensing approaches are successfully validated through electromagnetic simulations and experiments. By virtue of a differential measurement, robustness against changing ambient factors that may produce sensor miscalibration is expected.

Abstract: In this communication, a high gain circularly polarized (CP) dielectric resonator antenna (DRA) is proposed. The radiating DR is coupled to a $50~\Omega $ microstrip line through an X-shaped slot etched off the common ground plane. Using a frequency selective surface superstrate layer, a gain enhancement of 8.5 dB is achieved. A detailed theoretical analysis is given and used to optimize the superstrate size and the air gap height between the antenna and superstrate layer. The proposed DRA is designed, simulated, implemented, and tested. The high gain CP DRA has the potential to be used for millimeter wave wireless communication and imaging systems.

Abstract: In this letter, a novel printed frequency-reconfigurable microstrip square slot antenna for switchable Bluetooth, WiMAX, and WLAN applications is presented. The proposed antenna has a small size of $20 \times 20~\hbox{mm}^2$ in order to be able to cover lower frequencies; as for Bluetooth applications, miniaturization techniques such as modification of the ground plane and inserting an adjustable backplane cross-shaped sleeve have been employed. Moreover, by implementation of p-i-n diodes within the antenna structure, switchable frequency responses are achieved. The presented antenna has a small size while providing suitable switchable radiations at 2.3–2.51 GHz $({\rm BW} = 8.7\%)$ Bluetooth, 3.35–3.75 GHz $({\rm BW} = 11.2\%)$ WiMAX, and 4.95–5.53 GHz $({\rm BW}=11\%)$ WLAN.

Abstract: Microwave broadband wide-scan antenna arrays are typically implemented resorting to vertical arrangements of printed circuit boards (PCBs). Here, we propose a planar solution realized with a single multi-layer PCB, with consequent reduction in cost and complexity of the array. It consists of an array of connected slots backed by a metallic reflector and loaded with superstrates. Artificial dielectric layers (ADLs) are used in place of real dielectrics to realize the superstrates, as they are characterized by very low surface-wave losses. For the unit-cell design, we developed an analysis tool based on closed-form expressions and thus requiring minimal computational resources. Finite-array simulations are also performed by generalizing the analysis method to account for the truncation effects. The presence of the ADL superstrate allows reducing the distance between the array plane and the backing reflector while maintaining good matching performance. A realistic feed structure is also proposed, which consists of a microstrip line connected to a coaxial feed. Such a solution does not require balanced-to-unbalanced transitions, which often limit the achievable bandwidth. The proposed structure achieves in simulations more than an octave bandwidth (6.5–14.5 GHz), within a scanning range of $\pm 50$ ° in all azimuth planes.

Abstract: This letter is focused on the modeling, analysis, and applications of microstrip lines loaded with pairs of electrically coupled complementary split-ring resonators (CSRRs). Typically, these epsilon-negative (ENG) metamaterial transmission lines are implemented by loading the line with a single CSRR (etched beneath the conductor strip) in the unit cell. This provides a stopband in the vicinity of the CSRR resonance. However, by loading the line with a pair of CSRRs per unit cell, it is possible to either implement a dual-band ENG transmission line (useful, for instance, as a dual-band notch filter), provided the CSRRs are tuned at different frequencies, or to design microwave sensors and comparators based on symmetry disruption (in this case by using identical CSRRs and by truncating symmetry by different means, e.g., asymmetric dielectric loading). The design of these CSRR-based structures requires an accurate circuit model able to describe the line, the resonators, and the different coupling mechanisms (i.e., line-to-resonator and inter-resonator coupling). Thus, a lumped element equivalent circuit is proposed and analyzed in detail. The model is validated by comparison to electromagnetic simulations and measurements. A proof-of-concept of a differential sensor for dielectric characterization is proposed. Finally, the similarities of these structures with coplanar waveguide transmission lines loaded with pairs of SRRs are pointed out.

Abstract: Microstrip patches and dielectric resonators (DRs) are two low-profile variants of modern microwave and wireless antennas. However, the DR antenna (DRA) is relatively new and still passing through the stages of development. Both variants are quite similar in terms of performance and characteristics. This article focuses on a meaningful comparative study where we have considered all commonly used feed mechanisms such as coaxial probe, microstrip line, and rectangular aperture for both antennas operating near the same frequency. Circular geometry, i.e., cylindrical DRA (CDRA) and circular microstrip patch antenna (CMPA), have been chosen, and a systematic investigation based on thorough experiments has been executed. Multiple sets of prototypes have been fabricated and measured at 4 GHz. All available data have been furnished and compared, indicating relative advantages and disadvantages. This comparative study should provide qualitative and quantitative instructions to a designer for choosing the right element and corresponding feed based on design requirement and feasibility.

Abstract: A novel compact low-profile circularly polarized Fabry-Perot resonator (CP-FPR) antenna fed by a linearly polarized microstrip patch tilting by 45 ° with respect to the axes is presented. The FP cavity consists of a partially reflective surface formed by cross-slot frequency selective surface (FSS) and a nonstandard artificial magnetic conductor (AMC) acting as reflective ground plane. Its profile is reduced to a quarter of a wavelength. For verification, a prototype antenna is designed and simulated by HFSS. Reasonable agreement between the simulated and measured results is observed. The prototype has a common frequency bandwidth of 6.4% for S 11 ≤ - 10 dB, gain-drop ≤ 3 dB, and axial ratio ≤ 3 dB.

Abstract: A dual-band circularly polarized (CP) microstrip line fed slot antenna using a set of split ring resonators (SRRs) is proposed in this communication. Outer ring connected SRRs are placed on the back side of the slot to create the CP upper band. Diagonally opposite corners of the slot are truncated, which together with the SRRs give CP response in the lower band. The proposed dual-band CP antenna provides design flexibility to control the resonance frequency and sense of polarization at the dual bands, independently. The proposed antenna is designed to operate at 3.1 and 4.7 GHz, which is fabricated and tested. The experimental results show the −10-dB impedance bandwidths of 400 MHz in both the bands and the 3-dB axial ratio bandwidth of 3.1% (from 3.05 to 3.15 GHz) and 4.2% (from 4.65 to 4.85 GHz) in lower and upper bands, respectively. Finally, a metallic cavity is used with the antenna to achieve a unidirectional radiation pattern with front-to-back ratio of more than 15 dB without affecting the impedance bandwidth and CP performance of the antenna.

Abstract: Wideband microstrip leaky-wave antennas (LWAs) that radiate two symmetrical side beams are described. The two beams are steered simultaneously by sweeping the operating frequency. To achieve this, the second higher order mode of the microstrip is excited. Two electric field nulls are created between the microstrip and the ground plane using via arrays to suppress lower order modes. To test the concept, one of the antenna designs was prototyped. The prototyped antenna is capable of steering two symmetrical beams within a range of 37° when frequency is swept between 6.92 and 8.75 GHz. The measured peak gain of the antenna is 12.7 dBi and the variation of gain from 6.92 to 8.75 GHz is 3.1 dB. The measured 10-dB return loss bandwidth is 23%, which is very large for a dual-beam microstrip LWA. Such a wide impedance bandwidth is essential to achieve beam scanning over a wide angular range by sweeping frequency. Another advantage is that this single-layer antenna is easy to fabricate.

Abstract: This work presents new developments in the microwave transduction and its application to gas sensors. Microwave measurements were extended to reflection/transmission coefficients through the use of a microstrip interdigital capacitor design. A sensitive layer composed of commercial TiO2 nanoparticles was deposited on the sensor surface, in order to detect the ammonia target gas. A complete analysis methodology is proposed. It allows to identify without ambiguity all the sensor frequencies of interest, as well as a full characterization of usual gas sensor parameters such as reproducibility, stability, response time, etc. Furthermore, it involves a new method of representation which significantly limits the amount of unexploited data collected during microwave measurements. The proposed sensor exhibits a strong correlation between response values and injected ammonia concentration between 100 and 500 ppm, with a good reversibility and stability of the measure.

Abstract: In this paper, characteristic mode analysis (CMA) of three empirical design techniques for the probe-fed, symmetrically located, U-slot microstrip patch antenna, on a single-layer grounded substrate, is presented with supporting experimental data. The first method, resonant frequency (ResF), utilizes the existence of the four distinct ResFs, while the second one, dimensional invariance (DI), relies on the property of DI, for the design of the U-slot microstrip patch. In both these methods, the optimization of the probe location is necessary for further enhancement of the 10-dB return loss bandwidth. The third method, dimensionally invariant ResF, that optimally combines the features of the previous two is developed here and shown to yield better bandwidth performance with minimal or no probe location optimization, and hence is superior to the other two for rapid prototyping. CMA is carried out for critical parameters, such as substrate electrical thickness, slot width, probe radius, and feed location variations, to assess their dominant influence on the characteristics of the U-slot microstrip patch antenna.

Abstract: This paper presents a low profile microstrip patch antenna for next generation 5G devices. The proposed patch antenna has a compact structure of 20mm × 20mm × 1.6mm including the ground plane, which is suitable to be used in handheld devices. The antenna resonates at 10.15 GHz covering 5G frequency band. The proposed design provides a gain of 4.46dBi and the radiation pattern is omni-directional. In this paper geometry of the antenna and various parameters such as return loss plot, gain plot, radiation pattern plot and VSWR plot are presented & discussed. Measured results are also presented and they are in good match with simulated results.

Abstract: A shaped defected ground structure (DGS) that is asymmetric in a specific radiating plane of a microstrip element has been explored with a view to addressing the cross-polarization (XP) issues. The asymmetric configuration has been conceived from an insight of the inherent asymmetry of the modal fields underneath a probe-fed microstrip patch and has been experimentally demonstrated as the best possible design compared to its predecessors in terms of the reduction in XP fields, angular span of suppression around boresight, and the space occupied by the defects. Different orientations of the defects with reference to the patches of different aspect ratio values have been thoroughly examined. More than 28-dB isolation between co-pol and cross-pol has been achieved over 190° angular range, which is indeed 140° more if compared with a conventional ground plane. Its superior characteristics with respect to the earlier designs have also been documented.

Abstract: In order to obtain a wide-angle scanning and low sidelobe level (SLL) microstrip phased array with a finite metal ground, a novel microstrip phased array based on microstrip magnetic dipole is presented in this communication. Microstrip magnetic dipoles are employed as the driven elements in the phased array. Meanwhile, coupling patches are embedded between the adjacent driven elements, and coupling energy is transferred between driven elements by the coupling patches. Strong coupling has been constructed between elements as the driven element spacing is only about ${0}.\text{35}\; {\lambda}$ . With the influence of adjacent elements, the 3-dB beamwidth (BW) of each active element can reach over $\pm \text{80}^\circ $ in the elevation plane. From simulation and measurement, the main lobe scanning ranges of an 8-element and a 16-element phased arrays can both extend over $\pm \text{80}^\circ $ in the elevation plane with a gain fluctuation less than 3 dB. Furthermore, in order to keep the SLL low in the scanning, especially at the low elevation angles, genetic algorithm (GA) has been used, and the SLL has been decreased to the value of $- \text{9}\;\text{dB}$ in the full scanning range of $\pm \text{77}^\circ $ for the eight-element array.

Abstract: An improved concept is introduced to facilitate the design of low-loss planar circuits using printed gap waveguide technology. The method is based on using separate layers to realize the EBG cells and the lines. As a result, the operating bandwidth of the waveguide is increased compared to the printed ridge gap waveguide. Also, the design process is simplified regarding the placement of the cells around the resonators and lines. Measured results of a line with two 90° bends and a quadruplet bandpass filter are presented that prove the concept. A transition to microstrip line is used in order to acquire the ability to use test probe fixture.

Abstract: A 3-dB planar wideband power divider with an ultra-wide stopband and isolated frequency band is proposed. In order to extend the passband, the quasi-coupled lines are adopted. Moreover, the open and shorted stubs added at the input and output ports, respectively, can increase the bandwidths of the passband and stopband. Furthermore, in order to broaden the isolated frequency band, the impedance $Z _{\mathrm {iso}}$ with the series connected resister and capacitor on the right side of the middle coupled line is adopted. The detailed derivation is proposed as well. In addition, this power divider is fabricated on the substrate Rogers RO4003C with a compact size of 15.19 mm $\,\times \,11.4$ mm.

Abstract: A new single-fed wide dual-band circularly polarized (CP) dielectric resonator antenna (DRA) is presented in this letter. The DRA is excited by a microstrip line through the narrow underneath rectangular aperture, and its HE 111 and HE 11δ (2 <; δ <; 3 ) modes are utilized for the dual-band design. To achieve CP fields, two notches are truncated from the cylindrical DRA at φ = 45 ° and 225 ° . A pair of equal arc-shaped slots are used to improve the axial-ratio (AR) and impedance bandwidths. The antenna features impedance bandwidths (|S 11 | <; -10 dB ) of 26.25% and 11.17%, and 3-dB AR bandwidths of 15.8% and 5.02% in the lower and upper bands, respectively. To verify the simulation, the antenna is designed and measured. Reasonable agreement between measured and simulated results is obtained.

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Abstract: This communication presents a microstrip patch array, using sequentially rotated feed networks to achieve dual circular polarization in the same band. By interlacing the microstrip patches of adjacent $2 \times 2$ subarrays, the element spacing is minimized to avoid the challenging grating lobe issue. In this manner, the number of radiating elements is reduced from the conventional 4N to $2N + 2$ , where $N$ is the number of $2 \times 2$ subarrays. The array is implemented on a two-layer substrate, with the upper layer hosting the microstrip patch array and the sequentially rotated networks, and the lower layer hosting the power dividers, sharing a common ground plane in the middle. To validate this novel configuration, a prototype is fabricated and measured. It achieves both $\vert S11\vert dB and axial ratio < 3 dB bandwidth of 12.5% (4.95–5.61 GHz) for right-hand circular polarization and 14.7% (5.05–5.85 GHz) for left-hand circular polarization. The isolation between the orthogonal circularly polarized ports is about 12 dB in band.

Abstract: This letter presents a dual-band branch-line coupler with compact size and planar structure. The $\pi$ -shaped dual transmission lines structure is used to realize 90 $^{\circ}$ phase shift at two arbitrary frequencies. Explicit design equations are derived using the ABCD and admittance matrices. For verification, a prototype is designed, fabricated, and tested. The coupler has a compact size with 76% size reduction compared with the conventional circuit at the first band. The measured results are in good agreement with the simulated results.

Abstract: A class of slow-wave substrate integrated waveguide (SIW) structures patterned with microstrip polyline is presented, theoretically studied, and experimentally validated, which demonstrates some interesting slow-wave propagation effects. The slow-wave SIW (SW-SIW) enables the size reduction of a physically large circuit without sacrificing its performance. A size reduction of 40% of the lateral dimension is achieved with reference to that of the conventional SIW counterpart at the same cutoff frequency. Meanwhile, the phase velocity of the waveguide is also reduced by 40%, resulting in a smaller longitudinal dimension for a given electrical length. Both lateral and longitudinal effects give rise to a total size reduction, largely extending the operation range of SIW structures in the low-frequency region, which has often been restrained by a physical dimension-related cutoff frequency. Also, a transmission line-based two-dimensional (2-D) equivalent-circuit model is proposed and deployed for the modeling and analysis of the slow-wave mechanism. The results from the equivalent-circuit model agrees very well with that from the full-wave simulations. Furthermore, a broadband microstrip to SW-SIW taper with good return loss is designed for measurement verification. Using the proposed SW-SIW structure, the size of conventional SIW-based microwave circuits such as power splitters, couplers, and filters can be further reduced in addition to the existing size-reduction techniques.