Free-space Method Research Articles

Research Of Carbon Fibre Absorbing Material Based On Impedance Matching

The carbon fibre has a good characteristic of electrical conductivity. When cut short and dispersed, it reacts as electric dipole or harmonic oscillator to radar wave. This paper blends the short carbon fibre with ethoxyline resin, and makes films by spraying the blend on prepreg which made by glass fibre and resin, then measures the EM parameters with the free-space method, and designs the low reflectivity absorbing material from 8GHz to 18GHz by matching impedance and calculating the reflectivity of multilayer films. Adjusting the kinds and quantities of matching layers can shift the absorbing frequency bands, and become better absorbing respective in X-band and Ku-band. The absorbing material samples are made by vacuumizing and heating method, and their reflectivity correspond to the designed result. The designing method and absorbing material made in this paper can be referred in engineering application.

Dielectric properties of coals in the low-terahertz frequency region

The dielectric properties of Shanxi anthracite and Shandong bituminous coals in China are investigated in the low-terahertz (THz), W-band of frequency from 75GHz to 110GHz for the first time. In this frequency range, the complex dielectric constant of coal samples is obtained using the free space method. It is found that both the real parts of the dielectric constant for bituminous and anthracite decrease considerably with increasing frequency from 75GHz to 110GHz. The anthracite coals exhibit higher real and imaginary part values than bituminous coals. The imaginary part of the coal samples exhibits a more significantly decreasing trend in the frequency range from 90GHz to 110GHz compared with frequencies below 90GHz. The dielectric properties of all the coal samples are strongly dependent on the moisture content of the coals. Increasing moisture content leads to higher complex dielectric constant values. The effect of moisture on the dielectric properties of coals depends substantially on the influence of moisture content on the transmission and reflection of THz wave in the coals. The results show that the transmission coefficient of anthracite and bituminous exhibits an exponentially decreasing trend with increasing moisture content (from 0% to 10%). However, the reflection coefficient seems to follow a Gaussian-like changing trend with increasing moisture content, reaching a maximum around 4.5%.

Evaluation of profiles of an inhomogeneous chiral slab using state transition matrix method

A free-space method for the reconstruction of longitudinally inhomogeneous chiral material is proposed in this paper. The developed technique is based on the use of state transition matrix method. The problem is first formulated as an inverse scattering one based on a state transition matrix in terms of the unknown constitutive parameters. By using knowledge of scattering parameters, state transition matrix and its properties are employed to solve the inverse problem. The idea of the state transition matrix method for the characterization is to exploit the state transition matrix and then go back to the inhomogeneous chiral slab parameters. The advantage of using this analytic reconstruction technique is its ability to remove complexity and nonlinearity of the inverse problem. To test the performance of the proposed method and to show its effectiveness, it is tested against a typical example. The results show that the inverse algorithm can be used for the characterization of inhomogeneous chiral material without needing a priori information.

Reflecting and Polarizing Properties of Conductive Fabrics in Ultra-High Frequency Range

The system based on ultra-wide band (UWB) signals was employed for qualitative estimation of attenuating, reflecting and polarizing properties of conductive fabrics, capable to prevent local static charge accumulation. Pulsed excitation of triangle monopole antenna of 6.5 cm height by rectangular electric pulses induced radiation of UWB signals with spectral density of power having maximum in ultra-high frequency (UHF) range. The same antenna was used for the radiated signal receiving. Filters and amplifiers of different passband were employed to divide UHF range into subranges of 0.3-0.55 GHz, 0.55-1 GHz, 1-2 GHz and 2-4 GHz bands. The free space method, when conductive fabric samples of 50x50 cm 2 were placed between transmitting and receiving antennas, was used to imitate a practical application. Received wideband signals corresponding to the defined range were detected by unbiased detectors. The fabrics made of two types of warps, containing different threads with conductive yarns, were investigated. It was estimated attenuation and reflective properties of the fabrics when electric field is collinear or perpendicular to thread direction. In the UHF range it was revealed good reflecting properties of the fabrics containing metallic component in the threads. The system has advantages but not without a certain shortcoming. Adapting it for specific tasks should lead to more effective usage, including yet unused properties of the UWB signals.

Free-Space Nanometer Wiring via Nanotip Manipulation.

Relentless efforts in semiconductor technology have driven nanometer-scale miniaturization of transistors, diodes, and interconnections in electronic chips. Free-space writing enables interconnections of stacked modules separated by an arbitrary distance, leading to ultimate integration of electronics. We have developed a free-space method for nanometer-scale wiring on the basis of manipulating a metallic nanotip while applying a bias voltage without radiative heating, lithography, etching, or electrodeposition. The method is capable of fabricating wires with widths as low as 1–6 nm and lengths exceeding 200 nm with a breakdown current density of 8 TA/m2. Structural evolution and conduction during wire formation were analyzed by direct atomistic visualization using in situ high-resolution transmission electron microscopy.

Effects of Curved Wavefronts on Conductor-Backed Reflection-Only Free-Space Material Characterization Techniques.

A true plane wave is often not physically realizable in a laboratory environment. Therefore, wavefront curvature introduces a form of systematic error into Free-space material characterization methods. Free-space material characterization is important to the determination of the electric permittivity and magnetic permeability of conductor-backed and in situ materials. This paper performs an error analysis of the impact on wavefront curvature on a Free-space method called the two-thickness method. This paper compares the extracted electric and magnetic permeability computed with a plane wave versus a line source for a low-loss dielectric and magnetic radar absorbing material. These steps are conducted for TE and TM plane waves and electric and magnetic line sources.

Reconfigurable planar metamaterial unit cell

A new reconfigurable metamaterial unit cell is introduced. DC-biased controlled PIN diodes are embedded in the structure of this unit cell so that its configuration can be controlled by switching the diodes. The presented unit cell operates in two different modes. In the first mode, which is called the horizontal mode, the unit cell shows an effective permittivity and permeability close to one which leads to a refractive index around 1, whereas at the other mode, which is called vertical configuration, it possesses a double positive behaviour for the constitutive parameters and shows a refractive index of 2. Both modes are simulated using a lumped-element model for the embedded PIN diodes and the constitutive parameters are extracted using a well-known algorithm. To validate the results, the unit cell is fabricated and the free-space method is used to measure the S-parameters of the unit cell. This unit cell will be used in advanced antenna applications.

Free-space reflection method for measuring moisture content and bulk density of particulate materials at microwave frequency.

A measurement system based on free-space reflection method is designed for simultaneous and independent determination of moisture content and bulk density of particulate materials. The proposed system consists of microwave cavity oscillator, horn antenna, slide rail, sample holder, mixer, and digital meter. Sand and rice with different moisture contents and bulk densities are chosen as samples. Calibration models for moisture content and bulk density are proposed according to the measurement of the position of the minimum of the traveling-standing wave and the ratio of the maximum-to-minimum field strength of the traveling-standing wave at different temperatures. The moisture constant, ranging from 0% to 24.6%, is obtained with a coefficient of determination (R(2)) greater than 0.982 and a standard error of prediction (SEP) value of less than 0.695%. The bulk density, ranging from 0.501 g/cm(3) to 1.822 g/cm(3), is determined with a R(2) ≥ 0.961 and a SEP value ranging from 0.0144 g/cm(3) to 0.0382 g/cm(3) for different samples.

Numerical Verification of the Applicability of the Effective Medium Theory With Respect to Dielectric Properties of Biological Tissue

Especially in the THz region, little data is available regarding material properties based on measurements. It has been argued that the effective medium theory could provide a useful tool to estimate material data needed for electromagnetic field computations. In this paper, two numerical approaches are presented to test the applicability of the effective medium theory (EMT) with special regard to mm- and sub-mm-wavelengths. One approach is based on the well-known free-space method and the other one on a power loss evaluation scheme. Within the scope of application of the free-space method, the usability of the EMT is proven for two sets of dielectric tissue parameters on a longitudinally homogeneous and transversely structured sample. Moreover, power loss evaluation is a suitable method to show the applicability of the EMT. Analysis of layered models at 1-10 THz confirms that the EMT is a suitable tool to develop equivalent homogenized models with maximum errors for the dissipated powers of about 1%. In a more realistic example for a physiological fluid with spherical inclusions, the EMT yields an error of less than 1%.

Effect of stepped-impedance resonators on rectangular metamaterial unit cells

In this article, effect of the stepped-impedance resonator SIR miniaturization technique on rectangular metamaterial unit cells is investigated and the influence of the method's structural parameters on the characteristic impedance and the distance between higher resonance modes is discussed. According to the results, the ideal unit cell for this method should be thick enough and have an impedance ratio greater than one. Furthermore, the SIR technique is applied on a conventional two-turn spiral metamaterial unit cell and a new compact spiral unit cell is introduced. The effect of this method on the unit cell parameters is investigated and the new unit cell is compared to the conventional spiral. According to the results, a miniaturization factor of 0.75 can be achieved with the new unit cell. To validate the results, a two dimensional array of the unit cell is fabricated and its S-parameters are measured using the free space method. © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:582-590, 2015.

High-efficiency ultra-wideband polarization conversion metasurfaces based on split elliptical ring resonators

Polarization state of electromagnetic waves plays a significant role in the fields of signal transmission and sensitive measurements. High-efficiently manipulating and controlling polarization state by two-dimensional flat metamaterials over a wider bandwidth has been turned into hot issues in recent years. A polarization conversion metasurface based on the split elliptical ring resonator is designed, simulated, and experimentally validated in the microwave regime. The proposed metasurface can convert a linear polarization state into its orthogonal one with a high efficiency for an ultra-wide band. Theoretically, the mechanism of polarization conversion is explained by the theoretical models of high-impedance surface and multi-plasmonic resonances. The metasurface has a strong anisotropy, which behaves as a high-impedance surface, and serves as a metal sheet in orthogonal orientation in the vicinity of the resonant frequencies. The reflection phase has a delay of π for one of the two electric field components and remains unchanged for the other. As a result, the polarization angle of the synthesized reflection electric field rotates by π/2. The fourth-order plasmonic resonances are generated by the electric and magnetic resonances, which contribute to the bandwidth expansion of cross-polarization reflection. Numerically, by means of simulation and analysis on the axial ratio and flare angle of the split elliptical ring resonators, the influences of these structure parameters on the bandwidth and efficiency of the polarization conversion are clarified. And then the design method of multi-peaks and wideband polarization conversion metasurfaces with split elliptic ring resonators is proposed for different kinds of applications. Experimentally, the geometry is implemented within the currently available printing circuit techniques, and a free space method is adopted to measure the scattering coefficients. A polarization conversion ratio of the fabricated sample is larger than 85% at a relative bandwidth of 104.5%, and approximately 100% of the polarization conversion ratio can be achieved around the resonant frequencies. Experimental results are in good consistency with the simulation results. Compared with the anterior polarization conversion metasurfaces, the proposed metasurface broadens the cross-polarization bandwidth greatly with little efficiency expenses. These works provide beneficial guidance for manipulating and controlling polarization states of electromagnetic waves, and have potential applications in modern radar and communication systems, signal detection systems, and sensitivity measurement systems, etc.

Greek-Key Pattern as a Miniaturized Multiband Metamaterial Unit-Cell

In this letter, a new metamaterial unit-cell is designed, fabricated, and measured. The unit-cell is based on a Greek-key pattern and operates at the frequency range from 1 to 5 GHz. The proposed structure offers a multiband metamaterial behavior and at the same time provides a miniaturization factor compared to conventional multiband metamaterial unit-cells. The Greek-key unit-cell is first simulated and its S-parameters are calculated and then its effective constitutive parameters are extracted using a well-known algorithm. To validate the simulation results, a $10 \times 10$ array of the unit-cell is fabricated and measured using the free-space measurement method.

The Preparation of Slag Fiber and Its Application in Microwave Absorption

AbstractIn this research, the slag fibers coming from water‐quenched slags by using thermal plasma technology were successfully prepared and introduced into epoxy resin to be microwave absorber. The fiber‐blowing equipment for manufacturing slag fibers through controlling the nozzle angle and vertical distance from the nozzle to the melt outlet was also studied. The spectroscopic characterization of the formation processes of slag fiber was studied by using X‐ray diffraction (XRD), inductively‐coupled plasma (ICP), differential thermal analysis (DTA) and scanning electron microscopy (SEM). Microwave absorbing properties of the slag fibers and thermal plastic resin were investigated by measuring reflection loss in the 2–18 and 18–40 GHz microwave frequency range using the free space method. It was found that the composite specimens of slag fiber and thermal plastic resin had the best microwave absorption due to the reflection losses between from −4 to −8 dB and from −11 to −17 dB at frequencies between 2–18 and 18–40 GHz.

Geometrical Optics-Based Model for Dielectric Constant and Loss Tangent Free-Space Measurement

A microwave free-space reflection method for determining the complex permittivity of planar dielectric materials is demonstrated. The method makes use of the measurement of the near-field microwave reflection coefficient of a metal-backed sample. The modeling of the structure and its calibration are based on geometrical optics considering spherical electromagnetic waves propagating through the material. The technique that presents a number of features such as low-cost, compactness, robustness, and reliability is a good candidate for industrial applications. As a demonstration, dielectric parameters extraction of building materials is experimentally demonstrated for wireless local area network operations in the 2.45- and 5-GHz bands.

13-28GHz 대역에서 다양한 매질의 유전율 측정

Abstract This paper presents the results of permittivities of various materials by measurements of the dielectric properties for the materials in millimeter-wave band. Since the advantages of millimeter wave band include the miniaturization, weight reduction of component and a lot of information transfer, its study has received increasing attention of researches. Also, because of the characteristics of the band have the superior straightness and the small diffractive, in the band, the analysis for the reflection and transmission characteristics of the materials will be very important. Moreover, a low interference is one of characteristics of the band due to the fact that the millimeter wave is absorbed in the materials. Therefore, in this paper, the reflection and transmission characteristics of materials are measured in the band by using the free space method. Based on the presented result, the permittivities of the materials are derived by the calculation of Nicolson-Ross Weir method.

Development of Complex Relative Permittivity Measurement System Based on Free-Space in 220–330-GHz Range

For measuring complex relative permittivity of more than 300 GHz in the THz region, time-domain spectroscopy (TDS) is usually used. On the other hand, a free-space method using a vector network analyzer (VNA) is used below 300 GHz. However, these methods have not been compared and continuity of complex relative permittivity measurement around 300 GHz has not been evaluated. We developed a system for measuring complex relative permittivity that can be operated at 220–330 GHz. This system is based on the free-space method. We compared the complex relative permittivity using a VNA and by TDS to evaluate our system and the continuity of complex relative permittivity measurement in the frequency domain. We first compared relative permittivity and dielectric loss between both methods. We then evaluated the measurement uncertainty in consideration of the thickness of materials under test (MUT), time span of the time-domain gating, linearity, stability, aperture alignment, and measurement repeatability. The dispersion in MUT thickness measurement was found to be the dominant source of uncertainty in measuring complex relative permittivity measurement. The maximum difference in relative permittivity and dielectric loss between both methods was less than 0.22 and 0.17 when MUT E was measured. For the measurement result with expanded uncertainty, the relative permittivity of VNA was $3.92\pm 0.44$ and that of TDS was $3.70\pm 0.16$ , the dielectric loss of VNA was $0.30\pm 0.52$ and that of TDS was $0.13\pm 0.02$ , respectively. The measured complex relative permittivity by using the VNA and by TDS were observed within the expanded uncertainty. We verified the availability of the measured complex relative permittivity by using our measurement system and the continuity of complex relative permittivity measurement at 300-GHz band.

基于十字单元的可调谐互补屏频率选择表面

To tune the operating frequency of a Frequency Selective Surface (FSS), the load impedance of the loop slot FSS forms was separated to an inductive surface and a capacitive surface, and the coupling mechanism between the two surfaces was used to develop a Complementary FSS(CFSS). An equivalent circuit model of the CFSS was established and its tunable resonance mechanism was analyzed quantitatively. The influences of the rotating angle of the load patch, coupled dielectric thickness and the relative dielectric constant on the transmission characteristics of the CFSS were calculated accurately by using the coupled integral equation method, then the inductive surface and the capacitive surface were fabricated on both sides of the coupled dielectric by the coating and lithography methods. The transmission response of a 250 mm 250 mm CFSS prototype was measured by using the free space method. The calculated and measured results show that the CFSS resonance will be tuned from 18.2 GHz to 14.8 GHz by rotating the cross patch from 0 degree to 10 degree. The coupling effect between the capacitive surface and the inductive surface will disappear by increasing the physical thickness of the coupling dielectric from 0.1 mm to 1 mm. With increasing the relative dielectric constant of the coupling medium, the coupling between the CFSSs will be strengthened and the operating frequency will shift to lower frequencies. It concludes that the CFSS can realize the active resonance conversion function as the rotating angle changes, which provides references for the design and manufacture of active FSSs.

A novel frequency selective surface of hybrid-element type with sharply decreased stop-band

Frequency selective radome is one of the most important applications of frequency selective surface (FSS). In order to obtain better stealth performance, a novel element FSS, based on a regular slot element FSS, is presented in this paper. The novel element consists of a slot element in the center and at least two slot strips placed on the periodic boundary. We call such FSS the “hybrid-element type FSS” because it exhibits characteristics of both slot type and patch type FSS. Simulation and optimization work is carried out by using a period moment method and a discrete particle swarm optimization method based on the application requirements of a missile radome. Simulation results show that the hybrid-element type FSS has much steeper transition section between pass-band and stop-band, and much lower transmittance in stop-band when compared with the corresponding slot type FSS. The new FSS also has much lower insertion loss in pass-band, much thinner thickness, much simple structure and fabrication process when compared with the ordinary two-layer FSS. Equivalent sample plate is fabricated using printed circuit method and tested using the free space method. Good fit between simulation and testing results verify the accuracy and feasibility of this novel FSS design. The hybrid-element type FSS is especially suitable for the stealth radome when woking frequencies of both sides are very close. It provides a simple and feasible approach for developing frequency selective radome.

A Polarizability Measurement Method for Electrically Small Particles

A simple and precise free-space polarizability measurement method for electrically small particles is presented. The method is based on measuring the scattering parameters of a waveguide loaded by a particle and on knowledge of the dipolar polarizabilities of two calibration standards. The method is verified on a metallic disc and a resonant conducting ring terminated by a capacitor .

Optical Implementation of Triple DES Algorithm Based on Dual XOR Logic Operations

In this paper, we propose a novel optical implementation of a 3DES algorithm based on dual XOR logic operations for a cryptographic system. In the schematic architecture, the optical 3DES system consists of dual XOR logic operations, where XOR logic operation is implemented by using a free-space interconnected optical logic gate method. The main point in the proposed 3DES method is to make a higher secure cryptosystem, which is acquired by encrypting an individual private key separately, and this encrypted private key is used to decrypt the plain text from the cipher text. Schematically, the proposed optical configuration of this cryptosystem can be used for the decryption process as well. The major advantage of this optical method is that vast 2-D data can be processed in parallel very quickly regardless of data size. The proposed scheme can be applied to watermark authentication and can also be applied to the OTP encryption if every different private key is created and used for encryption only once. When a security key has data of <TEX>$512{\times}256$</TEX> pixels in size, our proposed method performs 2,048 DES blocks or 1,024 3DES blocks cipher in this paper. Besides, because the key length is equal to <TEX>$512{\times}256$</TEX> bits, <TEX>$2^{512{\times}256}$</TEX> attempts are required to find the correct key. Numerical simulations show the results to be carried out encryption and decryption successfully with the proposed 3DES algorithm.