In this study, almost monochromatic tuning of absorption strength, i.e. tuning the absorption peak level at almost same frequency, is presented by using angle dependent closed ring resonator (AD-CRR) type narrow band absorber structure in S-band. On this aim, firstly an ordinary planar closed ring resonator (CRR) structure having ground plane is designed and then the structure is folded by a rotation angle α from α = 0° (i.e. planar CRR) to α = 90° (i.e. completely folded CRR) along the center line which is parallel to the incident electric field. Both simulation and experimental results agree that the rotation angle α provides the freedom of tuning the absorption strength of the absorber structure almost monochromatically.

In this study, antenna performance improvement has been investigated without changing the antenna size at 2.45 GHz. For the proof of concept, three types of microstrip antenna is designed and simulated at 2.4 GHz. Type I is the conventional microstrip patch antenna which is taken as a reference antenna, Type II is the split-ring resonator based antenna, Type III is the split ring resonator based antenna with defected ground structure. After the numerical analysis and simulation, Type III antenna with the highest gain and bandwidth has satisfied our expectations.

In this study, the effects of dielectric substrate thickness and the dielectric loss tangent on the absorption spectrum are investigated parametrically in S-band. The study has been conducted on two different absorber topologies, one is closed ring resonator (CRR) and the other is composed of a split ring resonator (SRR), to observe the effects on both LC - and dipole-type resonances. The studies on the substrate thickness have been performed both numerically and experimentally, whereas the studies on the dielectric loss tangent have been performed numerically. The results agree with the literature such that the substrate thickness has significant effects on the resonant frequency and the absorption peak level which is explained by the impedance matching phenomenon. Besides, we show that the frequency shift behavior (i.e. redshift or blueshift) in response to substrate thickness change highly depends on the coupling between the resonator structure and the metallic ground plane. Moreover, the responses of absorption spectra to the changes of substrate thickness and dielectric loss are very similar whether it is due to an LC or a dipole-type resonances. We believe that the comprehensive and systematic parametric analyses in whole contributes to the literature especially considering the experimental studies in microwaves.

In this study, a hybrid sensor based on a defective square-truncated patch antenna (STPA) and a frequency-selective surface (FSS) was analyzed numerically and experimentally for different glucose–distilled water solutions. Here, an FSS was employed to enhance the sensitivity of the hybrid sensor. The sensing principle relies on monitoring variations in the loss tangent ($tan\delta$) and relative permittivity (${\epsilon }_r$) caused by different glucose concentrations applied to the sample under test (SUT). An open-ended coaxial probe was used to measure the complex permittivity of the solutions, which was then fitted to the Debye relaxation model. The simulated and experimental results of the novel sensor showed good agreement in a glucose concentration monitoring application. The sensor spanned the glucose range from 0 mg/dL to 5000 mg/dL, exhibiting a sensitivity of 55.44 kHz/mgdL⁻¹ and a figure of merit (FOM) of 6.23 × ${10}^{-4}$ (1/mgdL⁻¹) in the experiments and 53.60 kHz/mgdL⁻¹ and 1.71 × ${10}^{-4}$ (1/mgdL⁻¹) FOM in the simulations. When solutions with different concentrations were tested in the SUT, the resonance frequency of the antenna ($f_0$, in GHz) changed. To further characterize the sensor response, the relationship between the glucose concentration (C, in mg/dL) and $f_0$ was examined. A regression-based prediction model was constructed to map the measured scattering parameters to the glucose concentration, yielding a coefficient of determination ($R^2$) of 0.976. The high sensitivity, compact size, and compatibility with planar fabrication suggest that the proposed hybrid sensor has the potential to contribute to the development of non-invasive glucose-monitoring systems.

In this study the effects of using defected ground structures (DGS) composed of a complementary split ring resonator (CSRR) and CSRR with dumbbell (CSRR-D) for rectangular microstrip antennas are investigated. On this aim, two different antennas, which are Antenna B having CSRR etched DGS and Antenna C having CSRR-D etched DGS are designed and fabricated in comparison with the ordinary rectangular patch antenna, which is Antenna A. In both Antennas B and C, CSRR structures are etched in the same position of the ground planes. On the other hand, another ordinary microstrip antenna, called Antenna D, is designed at resonance frequency of Antenna C. For the characterization; resonance frequencies, voltage standing wave ratios, percentage bandwidths, gains, ka values and gain radiation patterns are investigated both in simulations and experiments. The numerical analyses show that 29.39% and 44.49% electrical size reduction (ESR) ratios are obtained for Antenna B and Antenna C, respectively in comparison to Antenna A. The experimental results verify the ESR ratios with 29.15% and 44.94%. Supporting, Antenna C promises 68.12% physical size reduction (PSR) as it is compared with Antenna D. These results reveal that Antenna C is a good alternative for DGS based microstrip electrically small antennas.