In this paper, a circular microstrip antenna for stress evaluation is studied. This kind of microstrip sensor can be utilized in structural health monitoring systems. Reflection coefficient S₁₁ is measured to determine deformation/strain value. The proposed sensor is adhesively connected to the studied sample. Applied strain causes a change in patch geometry and influences current distribution both in patch and ground plane. Changing the current flow in patch influences the value of resonant frequency. In this paper, two different resonant frequencies were analysed because in each case, different current distributions in patch were obtained. The sensor was designed for operating frequency of 2.5 GHz (at fundamental mode), which results in a diameter less than 55 mm. Obtained sensitivity was up to 1 MHz/100 MPa, resolution depends on utilized vector network analyser. Moreover, the directional characteristics for both resonant frequencies were defined, studied using numerical model and verified by measurements. Thus far, microstrip antennas have been used in deformation measurement only if the direction of external force was well known. Obtained directional characteristics of the sensor allow the determination of direction and value of stress by one sensor. This method of measurement can be an alternative to the rosette strain gauge.

In this paper miniaturization of a microstrip patch strain sensor (MPSS) using fractal geometry was proposed and analyzed. For this purpose, the transducer of Sierpinski curve geometry was utilized and compared with the most commonly utilized rectangular resonator-based one. Both sensors were designed for the same resonant frequency value (2.725 GHz). This fact allows analysis of the influence of the patch (resonator) shape and size on the resonant frequency shift. This is very important as the sensors with the same resonator shape but designed on various operating frequencies have various resonant frequency shifts. Simulation and experimental analysis for all sensors were carried out. A good convergence between results of simulation and measurements was achieved. The obtained results proved the possibility of microstrip strain sensor dimensions reduction using Sierpinski curve fractal geometry. Additionally, an influence of microstrip line deformation for proposed sensors was studied.

Until now, strain gauges, fiber optic sensors, magnetic sensors, and piezoelectric sensors have been used for strain measurements. In recent years, the idea of using microstrip antennas in strain measurement applications has emerged. One of the key requirements for deformation sensors is the ability to mount them on small-size elements (therefore the size of these sensors is crucial). The possible way to miniaturize microstrip sensors is the application of appropriate patch geometry, e.g. fractal geometry. In this article application of various fractal patch geometries in microstrip strain sensors were investigated. All sensors were designed for the same operating frequency ${f}_{\textbf {r}} =2.725$ GHz and on the same laminate. This approach showed how the influence of a specific geometry affects the sensitivity and the transducer size. The Sierpinski carpet, the Sierpinski triangle, the Sierpinski curve, and the combination of the Koch curve and the Sierpinski carpet were used to obtain the fractal geometry and miniaturize the sensor dimensions. They have been compared with the widely used rectangular patch. Numerical and experimental analyzes for the proposed sensors were carried out. The best solution to this problem was to use a combination of the Koch curve and the Sierpinski carpet. This fractal geometry, compared to a rectangular patch, enables the reduction of the patch size by 70% while reducing the sensitivity by 26%.

In the last decade, the application of metamaterials has become a very interesting way of implementing passive devices in microwave, terahertz, and optical frequency ranges. Up until now, selective filters, absorbers, polarizers, and lenses have been designed and constructed using these artificial materials, simultaneously showing the possibility for many other potential applications. Because of the simplified fabrication process, in particular, planar structures called metasurfaces (MS), are developing very fast. In the literature, there are many studies on the properties of various metasurfaces, but there are a lack of papers related to the analysis of the impact of structure deformations on their properties. In this paper, three commonly utilized structures of metasurfaces were designed for the same resonant frequency and on the same substrate. The numerical models were built and verified using the measurements of fabricated structures. During the experiment, the geometrical parameters of the metasurface cells were swept and a mechanical in-plane deformation in orthogonal directions was applied to the examined structures. Finally, sensitivity to the geometry distortions of the analyzed structures was evaluated and discussed.

The purpose of this paper was to investigate the influence of fabrication process uncertainty on terahertz metasurface quality. The focus was on the effect of metasurface fabrication inaccuracy on resonances. To the best of our knowledge, this is the first paper to study the effect of the metasurface fabrication process on its resonant frequency. The terahertz split ring resonator-based metasurface is under consideration. Using a numerical model, the influence of the uncertainty of various geometrical parameters obtained during the fabrication process (mainly layer deposition, photolithography, and etching processes) is analyzed according to the resonance of the designed metasurface. The influence of the following parameters causes a shift of resonant frequencies of the considered metasurface: etching deviation e, metallization thickness t_Al and SiO₂ layer thickness t_SiO2. The quality of the metasurface affected by the variations of obtained geometrical parameters was determined by the deviation of resonant frequency Δf_r. The developed numerical model was verified by THz-TDS (terahertz time-domain spectroscopy) measurements of the fabricated structure.