• Korean Journal of Materials Research
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• v.8 no.8
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• pp.775-779
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• 1998

The resonant frequency fo and unloaded quality factor Qu of CoTi-substituted Barium ferrites($\textrm{BaCo}_{x}\textrm{Ti}_{x}\textrm{Fe}_{12-2x}\textrm{O}_{19}$, $1.0\leq\textrm{x}\leq5.0$) were measured at frequencies between 5 to 10GHz using the paralleled copper-plate wave guide method. The measurements showed that the permittivities of CoTi-substituted Barium ferrites(CoTi-BF) increased from 14.7 to 23.4 with the contents of CoTi, x, and the loss tangents had values of $\textrm{10}^{-3}$ order. From these results, CoTi-BF are considered to be very promising materials in microwave devices such as dielctric resonator antennas.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.23 no.2
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• pp.251-257
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• 2012

Human body communication means transmitting and receiving data through human body medium or through free space along with the human body skin. Electric field distribution around the human body between the transmitter and the receiver were calculated at five different frequencies with 5 MHz interval between 10 MHz and 30 MHz. Commercial electromagnetic simulation tool was used for the calculation of E-field distributions applying the Korean standard male model including 29 different kinds of human tissues. After calculating specific absorption rate(SAR) values on back of the hand, it was compared with International Commission on Non-Ionizing Radiation Protection(ICNIRP) human protection guideline. While conductivities(${\sigma}$) and relative permittivities(${\varepsilon}_r$) of the human tissues for each frequency were input as the analyzing parameters, electric field intensities near both hands were integrated along the integral line between the nearby electrodes for the calculation of the transmitting and receiving voltages whose ratio was defined as channel loss. The calculated channel losses were about ($75{\pm}1$) dB and showed nearly flat response all through the evaluated frequencies.

• The Journal of Engineering Geology
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• v.27 no.3
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• pp.217-231
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• 2017

The subsidence of ground in urban area can be caused by the occurrence of the cavity and the change in ground volumetric water content. The objective of this study is the detection of abnormal area of ground in urban area where the cavity or the change in ground volumetric water content is occurred by the ground penetrating radar signal. GPR survey is carried out on the test bed with a circular buried object. From the GPR survey, the signals filtered by the bandpass filtering are measured, and the methods consisting of gain function, time zero, background removal, deconvolution and display gain are applied to the filtered signals. As a result of application of the signal processing methods, the polarity of signal corresponds with the relation of electrical impedance of the cavity and the ground in test bed. In addition, the relative permittivity calculated by GPR signal is compared with that of predicted by volumetric water content of the test bed. The relative permittivities obtained from two different methods show similar values. Therefore, the abnormal area where the change in ground volumetric water content is occurred can be detected from the results of the GPR survey in case the depth of underground utilities is known. Signal processing methods and estimation of relative permittivity performed in this study may be effectively used to detect the abnormal area of ground in urban area.

• Journal of the Korean Geotechnical Society
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• v.21 no.5
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• pp.231-241
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• 2005

Laboratory experiments were performed to examine the effects of water content and to see if permittivity had sufficient sensitivity to identify subsurface contamination. Both real and imaginary permittivities of unsaturated sand were strongly governed by the volumetric water content. Especially, a linear relationship between real permittivity and volumetric water content was derived at high frequencies (MHz ranges). Heavy metals in pore fluid result in significant increases in the effective imaginary permittivity, due to ionic conduction, but decreases in the real permittivity arises due to the decreased orientational polarization of water molecules caused by hydration of ions. Clear increase in the effective imaginary permittivity with heavy metal concentration was found to be valuable in the application of electrical methods for detecting heavy metals in the subsurface. However, because the permittivity is primarily dependent on the volumetric water content of soil, pre-evaluation on the volumetric water content is required.

• Journal of Electrical Engineering and Technology
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• v.9 no.2
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• pp.643-648
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• 2014

The accelerated thermal aging of CSPE (chlorosulfonated polyethylene) was carried out for 40.41, 121.22, and 202.04 days at $100^{\circ}C$, which are equivalent to 20, 60, and 100years of aging at $50^{\circ}C$, respectively. The volume electrical resistivities of the accelerated thermally aged CSPE samples for 0, 40.41, 121.22, and 202.04 days were $1.107{\times}10^{14}-2.097{\times}10^{14}$, $7.752{\times}10^{13}-1.556{\times}10^{14}$, $7.693{\times}10^{13}-1.521{\times}10^{14}$, and $7.380{\times}10^{13}-1.304{\times}10^{14}{\Omega}{\cdot}cm$, respectively, at room temperature. The permittivities of the accelerated thermally aged CSPE samples for 0, 40.41, 121.22, and 202.04 days were $2.89{\times}10^{-11}-3.65{\times}10^{-11}$, $3.40{\times}10^{-11}-3.70{\times}10^{-11}$, $3.50{\times}10^{-11}-3.82{\times}10^{-11}$, and $3.76{\times}10^{-11}-4.13{\times}10^{-11}$ F/m, respectively, at room temperature. The EAB (elongation at break) of the accelerated thermally aged CSPE samples for 0, 40.41, 121.22, and 202.04 days were 98.8-101.3, 59.5-60.3, 37.8-39.2, and 41.8-44.3%, respectively, at room temperature. The apparent densities of the accelerated thermally aged CSPE samples for 0, 40.41, 121.22, and 202.04 days were 1.603-1.614, 1.611-1.613, 1.622-1.628, and $1.618-1.620g/cm^3$, respectively, at room temperature. The measured currents of the accelerated thermally aged CSPE and the standard sample were almost constant after 5 min of applying a 300-V/mm electric field to the CSPE. The V-I slope of the accelerated thermally aged CSPE sample was increased if the applied electric field was increased at room temperature, and the V-I slope of the accelerated thermally aged CSPE was higher than that of standard CSPE.

• Journal of Aerospace System Engineering
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• v.17 no.1
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• pp.42-50
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• 2023

In this study, hexagonal ferrite powder with chemical formula SrZn_2-xCo_xFe₁₆O₂₇ was synthesized by a solid-state reaction method and its electromagnetic (EM) wave absorption characteristics were evaluated in the frequency range of 0.1-18 GHz with absorber thickness range of 0 - 10 mm. Reflection loss (RL) affecting electromagnetic wave absorption performance was calculated based on the transmission line theory using measured complex permeabilities and permittivities. RL spectra were also directly measured for some samples. They were well matched with calculated results. High-frequency complex permeability characteristics were changed gradually according to the amount of Co substitution (x). The EM wave absorption frequency band could be tuned accordingly. Hexaferrite samples with x = 1.0, 1.25, and 1.5 exhibited remarkable maximum electromagnetic wave absorption performances with minimum RL (RL_min) lowered than -50 dB. They also showed a very broad frequency band (Δf > 10 GHz) in which more than 90% of the EM wave energy absorption occurred (RL ≤ -10 dB).

• Composites Research
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• v.21 no.5
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• pp.9-14
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• 2008

Carbon nanofibers (CNFs) were used as dielectric lossy materials and NiFe particles were used as magnetic lossy materials. Total twelve specimens for the three types such as dielectric, magnetic and mixed radar absorbing materials (RAMs) were fabricated. Their complex permittivities and permeabilities in the range of $2{\sim}18$ GHz were measured using the transmission line technique. The parametric studios for reflection loss characteristics of each specimen to design the single-layered RAMs were performed. The mixed RAMs generally showed the improved absorbing characteristics with thinner matching thickness. One of the mixed RAMs, MD3with the thickness of 2.00 mm had the 10 dB absorbing bandwidth of 4.0 GHz in the X-band ($8.2{\sim}12.4$ GHz). It also showed very broad 10 dB absorbing bandwidth as wide as 6.0 GHz in the Ku-band ($12.0{\sim}18.0$ GHz) with the thickness tuning to 1.49 mm. The experimental results for selected several specimens were in very good agreements with simulation ones in terms of the overall reflection loss characteristics and 10 dB absorbing bandwidth.