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Öğe Beamforming Radiation Properties of Absorbing/Transparent Zones-Added Horn Antenna(Gazi Univ, 2020) Teber, AhmetIn this study, we present beamforming of radiation patterns on a transmitter horn antenna by mounting an electromagnetic wave absorber, in which electromagnetic wave absorber is fabricated as concentric zones. The concentric zones consist of an absorbing material of manganese soft spinel ferrite blended with multi-walled carbon nanotubes with the thickness of 1 mm installed on the aperture of the antenna working in Ku-band. We have experimentally measured scattering parameter of transmission, S-21, using a vector network analyser to perform radiation patterns in polar coordinates. As compared to the radiation properties of air horn antenna and absorbing/transparent zone-added horn antenna produces an appreciable effect to concentrate radiation beam in the main, side and back lobes with reducing side and back lobe levels for each frequency in H-plane. For the radiation pattern in E-plane, we have observed the suppressed side lobes at three frequencies of 15 GHz, 17 GHz and 18 GHz. The results indicate that the absorbing/transparent zone keeps an important role for further development of the applications of directional antennas so that the effective radiation is to be significantly concentrated.Öğe Cr-substituted nanocrystalline Ni-Co ferrites: tuning dielectric, magnetic, structural, X-band microwave absorption, and characteristic properties(Springer, 2023) Teber, Ahmet; Sarac, Mehmet F.The structural, characteristic, dielectric, and magnetic features of Co0.5Ni0.5CrxFe2 - xO4 (0.0 <= x <= 1.0 with a step of 0.2 x-concentration) nanoparticles are prepared via the hydrothermal method. The investigation of characteristics, structural, and magnetic features of the produced nanoparticles are examined through XRD, FT-IR, SEM-EDS, TEM-SAED, and VSM, respectively. AC conductivity and the dielectric investigations are also reported. According to the XRD measurements, all samples have the M-type hexagonal crystal structure nanocrystallites of between 27 and 97 nm. Refined structural parameters using Rietveld analysis are carried out using the TOPAS refinement program. A good match was observed between the diffraction patterns obtained and calculated by Rietveld analysis. Moreover, the morphological analyses indicate that the relatively small spherical structures of Ni-Co ferrites particles can be seen to change from spherical to relatively oval-shaped with increasing chromium content. The results of the FT-IR study show that the spinel structure has formed as predicted because the expected range of absorption bands is present. According to magnetic measurements, Cr-substituted Ni-Co ferrite nanostructures reveal a relatively soft ferromagnetic property. The coercive field (H-c) variation has a propensity to decrease in relation to the Cr-concentration. The electromagnetic constitutive parameters are calculated from measured S-parameters (S-11 and S-21) using network analyzer and the return losses of the samples are then obtained to determine microwave absorption properties. According to the dielectric and magnetic tangent losses or dissipation factors, both dielectric and magnetic features can mutually reinforce in terms of microwave absorption properties in the X-band. The Cr2O3 impurity peaks obtained by XRD analysis have an effect on the microwave absorption properties. The results of the microwave absorption based on each concentration reveal that there exist electromagnetic wave absorptions between approximately 11 GHz and 12.4 GHz in the X-band. The absorption trend is also towards the Ku-band.Öğe A Dual-Band Terahertz Metamaterial Absorber Using an All-Metal Aluminum Hexagonal Metasurface Structure for Sensing of Cancerous Cells(Springer, 2024) Teber, AhmetThis article proposes a dual-band terahertz metamaterial absorber (TMA) where all components of the designed structure are made of aluminum (optical) material. Unlike the metal-dielectric-metal structures typically found in the literature, the proposed structure has a lightweight material that does not contain a sandwich structure. Thus, the structure greatly simplifies the production process and reduces the cost. The results of electromagnetic and equivalent circuit simulations are compared based on the S11 parameters (in dB), resulting in good agreement of findings by CST and ADS, respectively. The proposed TMA operates in dual frequency bands at 1.902 THz (f1) and 1.976 THz (f2), with strong absorption of 98.17% and 99.05%, respectively. The quality factors (Q-factors) corresponding to these resonant frequencies are 240.15, (f1) and 181.61, (f2). When the refractive index (RI) of the medium surrounding the TMA varies, the absorption peaks shift accordingly. The detection properties of the proposed TMA are examined as the RI of the surrounding medium is varied primarily between 1.34 and 1.39 with an increase of 0.01 for biomedical applications. The corresponding sensitivities of two resonance peaks are 0.794 THz/RIU and 0.971 THz/RIU, respectively. Since many samples, such as human blood, basal/breast/cervical cells, and cancerous cells, are found in this RI range, the proposed TMA sensor can be used as a viable biosensor in the identification of biological samples for real-world applications.Öğe Electrical, Magnetic, Microstructural, and Characteristic Properties of Zirconium-Substituted Co0.5Ni0.5ZrxFe2-xO4Spinel Ferrite Nanostructures(Springer Heidelberg, 2023) Teber, Ahmet; Sarac, Mehmet F.The effect of zirconium (Zr) substitution in nickel-cobalt ferrites, Co0.5Ni0.5ZrxFe2-xO4 (0.0 <= x <= 1.0) produced by the hydrothermal method on the electrical, magnetic, microstructural and characteristic properties are reported. A secondary phase as tetragonal-ZrO2 phase in the crystal system is detected and have both M-type hexagonal and tetragonal nanocrystallites between 32 and 67 nm. Refined structural parameters using Rietveld analysis are carried out, resulting in lattice parameters, the unit cell volume, parameter ratios, and the crystallite size using the TOPAS refinement program. In addition, residuals for the R-Bragg factor and goodness of fit (chi(2)) value for each concentration are represented. The Rietveld analysis determines that the predicted and actual diffraction patterns are in accord. Furthermore, the Zr effect is presented in unit volume as seen by the drop in peak intensity that occurs (with the addition of Zr) due to the lattice tension. Moreover, the morphological analyses indicate that the shape transformed from agglomerated clusters to oval-shaped structures and a reduction in grain size. Fourier transform infrared (FT-IR) analysis in the range of 4000-450 cm(-1) correlates with the presence of the hexagonal spinel ferrite structure. The dielectric parameters, such as the relative permittivity, tangent loss, and ac conductivity, are experimentally measured with the oscillation voltage level of 0.05 V in the frequency range of 0.1-10 MHz. The dielectric permittivities and tangent losses of the specimens decreased with an increment in frequency. Magnetic measurements reveal that the values of coercivity (H-c) have the propensity to increase relating to the zirconium concentration.Öğe FR2/mmWave, N258 and N261 Bands, 5G Frequency-Compatible Ultra-Thin Metamaterial Absorber with Polarization Insensitivity for EMI shielding Applications(Wiley-V C H Verlag Gmbh, 2024) Teber, AhmetSince adopting 5G technology, a notable rise in the prevalence of electromagnetic (EM) waves in the environment has become a significant concern. To reduce EM interference, broadband metamaterial absorbers offer an alternative solution for protecting devices in defense, communication, and healthcare applications, unlike their narrowband counterparts. This article presents a numerical investigation of an ultra-thin, polarization-insensitive metamaterial absorber (MMA) that uses metasurfaces and provides electromagnetic interference (EMI) reduction, covering the FR2/mmWave licensed N258 and N261 bands with an oblique incidence angle of up to 15 degrees. Comprehensive simulations are performed within the CST Studio Suite S2 2023 to examine the absorber's performance. Under normal incidence, the proposed MMA exhibits a wideband absorbency response exceeding 90% within the frequency range of 24.30-28.12 GHz. Moreover, the physical absorption mechanism is explained with electric and magnetic field distributions, including surface current distributions. Confirmation of the lack of mutual interaction among neighboring unit cells is established through testing with 1 x 2 and 2 x 2 arrays. An equivalent circuit model is also developed to confirm the EM simulation findings. The notable features of the proposed MMA make it an exceptional candidate for EM shielding applications, providing EMI reduction and covering the N258 and N261 bands. The developed metamaterial absorber, resulting in an ultra-thin, polarization insensitivity, and partially incidence angle insensitivity, achieves over 90% absorption within the supplementary 5G spectrum, covering the licensed bands N258 and N261. This absorber also provides effective EM interference (EMI) shielding against radiation from 5G base stations, ensuring the protection of sensitive equipment across diverse sectors, such as industry, research, and medical, addressing concerns related to security/privacy. imageÖğe Investigation of Beam Width Shaping of a Ku-band Horn Antenna using a Diffractive Optic Element and an Electromagnetic Wave Absorber(2020) Teber, AhmetThe feasibility of beam-shaping of a Ku-Band horn as a transmitting (Tx) antenna by mounting two different versions of lenses which are kind of Fresnel Zone Plates (FZP) is studied. The designed and fabricated geometrical structures offer a simpler approach building Fresnel Zone Plates by using an electromagnetic wave absorber and diffractive optic material. The diffractive optic material, paraffin, forms a set of alternating open and opaque annular zones on a flat surface, based on the design principles of Fresnel Zone Plates. An electromagnetic wave absorber covers the top surface of the formed paraffin, but not including the grooves. Thereafter, the Fresnel Zone plates are suitably attached in front of the transmitter horn antenna, located in the far-field region of a receiving antenna. The half-power beam-widths for the horn antenna (unloaded) and with two types of lenses are investigated. The results indicate that Fresnel zone plate structures can play a role suppressing side lobes in H-plane so that the effective radiation is to be significantly concentrated.Keywords: Beam-width, diffractive optic elements, electromagnetic wave absorber, Fresnel zone plates, horn antennaÖğe Niobium modification on dielectric, magnetic and structural features of nanocrystalline Ni-Co ferrites(Indian Acad Sciences, 2024) Teber, Ahmet; Sarac, Mehmet FahriIn this study, we will investigate the structural, characteristic, dielectric and magnetic features of Co0.5Ni0.5NbxFe2-xO4 (0.0 <= x (with 0.2 increments) <= 1.0) nanoparticles, which are prepared via the hydrothermal approach. M-type hexagonal crystal structures are confirmed through XRD, FT-IR, SEM-EDS, TEM-SAED and VSM. Also, we have reported the investigations on dielectric properties and ac conductivity. Based on XRD measurements, it is concluded that all samples have M-type hexagonal crystal structure with nanocrystals ranging from 60 to 97 nm. The diffraction patterns obtained matched well with the calculated patterns from Rietveld analysis. Moreover, from the morphological analysis, it was observed that the small spherical structures of Ni-Co ferrite particles transformed into quadrilateral structures with increasing Nb concentration. The results of the FT-IR study show that the hexaferrite structure is formed as predicted because the expected range of absorption bands is present. According to VSM, Nb-substituted Ni-Co ferrite nanostructures reveal a relatively soft ferromagnetic property. Increasing the concentration of Nb tends to increase the coercive field (H-c) variation. The dielectric factors and ac conductivity were measured in the frequency range of 0.1-10 MHz. The reduction in dielectric factors and tangent losses at low frequencies can be elucidated by both deformation and relaxation polarization mechanisms. Another observation is that ac conductivity is amplified with the applied frequency. In this sense, the electron and hole hopping phenomenon can explain the conduction mechanism.Öğe Polarization Insensitive and Thin Metamaterial Absorber Performed in High-Frequency 5G Bands(2024) Madak, Seher Şeyma Arslan; Teber, Ahmet; Topkaya, RamazanA variety of fascinating applications, including 5G communication devices, high-speed data transfer, and large-scale Internet of Things (IoT), make life easier with 5G technology. Despite the 5G’s superior features, the percentage of electromagnetic (EM) waves in the environment execute a significant increase, unpleasantly. Broadband metamaterial absorbers are an appealing alternative to gather these unwanted signals. This study aims to numerically investigate a broadband metamaterial absorber (MMA) in the 5G high-frequency spectral range with the metasurface formed with coupled resistors. In addition, the 24.25-27.5GHz frequency range, one of the high-frequency 5G bands used by selected countries such as the European Union and China, was preferred. The minor aim of this study is that the usage of coupled elements as resistors may have the ability to increase the absorption bandwidth and magnitude. Comprehensive simulations were performed using the finite integration technique (FIT) utilized by the CST Microwave Studio program to investigate the absorber performance and other relevant parameters. The unit cell design is created metal-substrate-metal structures as asymmetric, single-layer, and easy to implement. The absorption responses are investigated according to the oblique incidence angle, polarization angle for TE &TM modes. The suggested MMA provided an absorbency response above 87.6% in the frequency range 24.20-27.30GHz under normal incidence. Moreover, to comprehend the physical mechanism on absorption, the top and bottom surfaces of the absorber's electric field and surface current distributions are assessed. The designed MMA resulting in relatively high performance and polarization insensitive is helpful for electromagnetic interference (EMI) shielding of 5G signals in the FR2/mmWave frequency regime.Öğe RANKING ALTERNATIVE ENERGY RESOURCES TECHNOLOGY PROGRAMSIN TURKEY BASED ON THE METHODS OF MULTI-CRITERIA DECISIONMAKING ANALYSIS(2020) Teber, Ahmet; Karakaş, AhmetThe diversity less among the Alternative Energy Resources Technology Programs (AERTPs) in Turkey makes giving the “proper” decision crucial by high school senior students. The literature review exhibits that there exists a need over the expectation for a ranking system of the two-year programs in vocational schools in Turkey so as identifying two-year programs compared to others. In this strategic decision context, it is especially important for the students to make a suitable decision associated with their higher education. Therefore, this study focuses on developing a specific ranking system of twoyear energy programs in Turkey. In order to classify the programs, the most important decision criteria should be required. In this aspect, a survey is applied to a hundred high school senior students to recognize which multi-criteria for selection of an energy program they pick. Thus, the survey makes this study is more applicable to build a foundation for a field-based ranking system in Turkey. In this regard, a framework for ranking of AERTPs during the academic year of 2019 in Turkey is accomplished using three methods of Multi-Criteria Decision Making Analysis including Analytic Hierarchy Process (AHP), Simple Multi-Attribute Rating Technique (SMART), and Technique for Order Preference by Similarity to Ideal Solution (TOPSIS).Öğe Reflective Polarization Conversion with Multi-Functional, Ultrathin Metasurface for Ku- and K-Band Applications(Gazi Univ, 2024) Teber, AhmetReflective polarization conversions with a simplistic design of an ultrathin, single-layered, and multi-functional anisotropic metasurface as a polarization converter is utilized for Ku- and Kband applications. The designs with two substrate thicknesses (0.095a,0 and 0.069a,0, respectively) are capable of a cross-polarization converter (CPC) and a linear-to-circular (LTC) polarization conversion. The design with 0.095a,0 thickness achieves a CPC between 17.96 and 26.90GHz with the efficiency of more than 90% and a relative bandwidth of 40% under normal incidence. It maintains angular stability by altering the oblique incidence angles up to 300 with greater than 80% of the PCR in the K -band. Meanwhile, an LTC in two frequency bands, 10.30-10.53GHz and 28.65-29.70GHz, is also numerically demonstrated. The second design with 0.069 a,0 thickness provides a CPC above the PCR value of 87% in the frequency range from 10.4623.05GHz (covering the entire Ku- and part of the K -band) with angular stability of 40 above the PCR value of 80%. In the meantime, an LTC with relative bandwidth of 75% in the frequency range from 9.53-9.79&24.74-25.27GHz is numerically revealed. These polarization converters exhibit relatively good performances of facile structure and multi-functional properties, which can be useful in Ku- and K -band applications.Öğe Refractive Index Sensor Based on Terahertz Metamaterial Absorber with High Sensitivity and Quality Factor for Sensing Applications(2024) Teber, AhmetThis research describes and assesses a terahertz metamaterial absorber (TMA) with a simple and easy-to-produce structure consisting of gold-silicon (optical)-gold design as a refractive index sensing. Due to the high field limitation in the detection regime, electromagnetic (EM) wave’s absorption reaches 99.40% at a frequency of 3.719 THz, including a significant quality factor (Q-Factor) of 65.77 and a figure of merit, FoM, of 21.49. The TMA has an remarkable sensitivity of 1.215 THz/RIU and can be used as a refractive index sensor (RIS). The proposed metamaterial absorber-based sensor is susceptible to refractive index changes (1.00 to 1.05) in the surrounding medium. For the physical absorption mechanism, the fundamental absorption peak is mainly due to the simultaneous occurrence of electric and magnetic dipole resonances. The proposed absorber has the feature of being an excellent RIS due to its sensing and detection applications.Öğe Tangential velocity and water vortex structure behaviour based on cylindrical-shaped basin's design parameters for gravitational vortex power plants(EDP Sciences, 2024) Teber, AhmetThe gravitational water vortex power plants (GWVPPs) have established in hydropower systems as a capable approach, generating micro-scale environment friendly electricity. In this system, energy is produced as a result of the interaction of water coming from an inlet canal with the turbine by creating a vortex in circular/conical-shaped structures. This interaction is based on an assumption that the water flow rate in a basin coincides with the tangential velocity on the turbine propeller. In this case, it is essential to investigate numerous geometric design parameters of the cylinder basin and their effects on vortex formation with the help of Computational Fluid Dynamics (CFD). Here, based on the CFD principles, the behaviour of various geometrical parameters on the vortex and tangential velocities in the basin are investigated separately with the help of Ansys Fluent, without using any turbine propeller. For a certain inlet flow velocity and head, different tunable geometric parameters for the gravitational water vortex plant with a cylindrical basin are inlet canal length, basin diameter, basin height and outlet diameter, respectively. The objective is to achieve an improved design configuration concerning factors such as tangential velocity, vortex arrangement, and the optimal placement of turbine propellers within this vortex. © The Authors, published by EDP Sciences.Öğe Ultra-thin Polarization Converter Using Single Layer Metasurface for X-, Ku-, and K-Band Applications(2024) Aslan Madak, Seher Şeyma; Teber, Ahmet; Topkaya, RamazanIn this article, an ultrathin (?0/14.9), single layer, reflective type polarization converter for either linear-to-circular (LP-to-CP) or linear-to-linear (LP-to-LP) polarization conversion is reported. It has been demonstrated to achieve the LP-to-CP conversion at two separate frequency bands, including 11.52-11.84GHz and 19.83-20.01GHz. The circular polarization type is specified as a right-hand circular polarization (RHCP) for the first band and a left-hand circular polarization (LHCP) for the second band. In both of frequency bands, the metasurface structure reveals highly efficient features. Besides that, the metasurface structure exhibits highly efficient features within 3-dB bandwidths of 10.29-12.58GHz and 19.32-20.50GHz. Crucial parameters of polarization conversion such as ellipticity, axial ratio, andÖğe Wideband Polarization Conversion Based on Elliptical-Shaped Metasurface for X-Band Applications(2023) Coşkun, Aykut; Teber, Ahmet; Ertugrul, MehmetHere, a single-layer, ultra-thin, multi-functional and high efficiency meta-surface is presented to perform wideband reflective linear (LP) and circular polarization (CP) transformations. The proposed meta-surface behaves as an excellent cross polarization converter for linear polarized wave over the relative absorption bandwidth (RAB) of 55.2% (7.89–13.91 GHz) with more than 97% efficiency. It successfully converts linearly polarized waves into circularly polarized waves in the 7.2–7.36 GHz frequency range. In addition, its polarization conversion rate (PCR) efficiency characteristics for TE and TM modes are retained across the entire X band, with a wide incidence angle up to $45^0$. The presented polarization converter has ultra-thin feature with 0.07?0 thickness. Due to its compact size, angular stability, high efficiency, simple structure and multi- functionality, this polarization converter is an important candidate for polarization manipulation and communication devices in many applications.Öğe Yttrium-substituted Co0.5Ni0.5YxFe2-xO4 ferrites as microwave absorbers by investigating structural, magnetic, dielectric, and absorption characteristics(Elsevier, 2023) Teber, Ahmet; Sarac, Mehmet F.The effect of Yttrium Substitution on Co0.5Ni0.5YxFe2-xO4 (0.0 <= x <= 1.0) Ferrites as Microwave Absorbers was investigated to include structural, magnetic, dielectric and X-band absorption properties. The main objective of this work is to enhance microwave absorption properties of materials with interesting dielectric/magnetic be-haviors by doping rare earth element of Yttrium based on transition metals. The hydrothermal method was used to create Yttrium (Y) substituted nanoparticles, and the effects of Y-ion substitution on the characteristics, structural, magnetic features were examined by XRD, FT-IR, SEM-EDS, TEM-SAED, and VSM, respectively. Electromagnetic properties were obtained from microwave scattering parameters measured via a metal-backed transmission line and Nicholson-Ross Weir techniques using a vector network analyzer (VNA) in conjunction with an X-band waveguide set. Return loss (RL) values of the samples were obtained from the electromagnetic constitutive parameters (permittivity and permeability). According to the XRD measurements, hexagonal crystal structure of ferrite and YFeO3 as secondary phase nanocrystal sizes are between 11 and 67 nm. Refined structural parameters using Rietveld analysis are carried out using the TOPAS refinement program. A good match was observed between the diffraction patterns obtained and calculated by Rietveld analysis. Moreover, the morphological analyses indicate that the relatively small spherical structures of Ni-Co ferrites particles can be seen to change from spherical to hexagonal-shaped with increasing yttrium concentration. The results of the FT-IR study show that the spinel structure has formed as predicted because the expected range of absorption bands is present. According to magnetic measurements, the coercivity (Hc) results show a modest increase in porosity as the Y-content rises. The results revealed that the minimum RL value and bandwidth vary significantly with the amount of Yttrium in the mixture, indicating that the obtained structures will be useful for broadband microwave applications.
