Impedance Matching Degree Research Articles

Enhancing the microwave absorbing properties of molybdenum dioxide by designing a double-layered structure

MoO2, which has a high electrical conductivity and chemical stability, is suitable for use as a dielectric absorbing material. In this research, MoO2 was synthesized by reducing MoO3 at a high temperature, and the electromagnetic and absorption properties of MoO2 were tailored by the reduction time. According to the testing results, MoO3 was transformed into MoO2 by prolonging the reduction time, which was accompanied by a simultaneous increase of the complex permittivity; however, the absorption bandwidth was narrow because of the low impedance matching degree or attenuation constant. With the aim of broadening the absorption bandwidth, a double-layered absorbing structure was designed based on the tested electromagnetic parameters. When the thicknesses of the impedance matching layer and the absorbing layer were both 1.2 mm, the absorption bandwidth was 5.1 GHz, representing a highly enhanced absorption bandwidth.

A Novel Voltage Stabilization and Power Sharing Control Method Based on Virtual Complex Impedance for an Off-Grid Microgrid

Microgrid (MG) usually operates in medium/low-voltage systems, where the line impedance parameters are mainly resistive, and traditional P-f/Q-U droop control is no longer applicable. When the virtual complex impedance method is adopted, the resistance component of line impedance can be counteracted by a virtual negative resistance. Unfortunately, the improper design of the virtual negative resistance will result in an unstable system due to the problem of line impedance parameter drift and estimation error. According to the line parameters characteristics of the off-grid MG with medium/low voltage, the P-U/Q-f droop control is adopted in this study, where the virtual complex impedance composed of a virtual negative inductance and a virtual resistance is introduced in the control loop. The virtual negative inductance is used to reduce the power coupling caused by the inductive component of the system impedance. The virtual resistance is implemented to enhance the resistive component and adjust the impedance matching degree for raising the accuracy of power sharing. However, the power sharing is still affected by the system hardware parameters; meanwhile, the voltage deviation caused by the droop control and the virtual impedance exists. In this study, a novel voltage stabilization and power sharing control method based on the virtual complex impedance is investigated to achieve accurate power sharing without the impact of hardware parameters variations and to improve the voltage quality. Moreover, the small-signal model of the inverter-based off-grid MG with the proposed controller is established, which can be utilized to analyze the stability and dynamic performance of the system. Meanwhile, the control parameters can be sequentially determined. Analysis shows that the strategy is robust against the line-impedance parameter drift and the estimation error and has a large stability margin and fast dynamic-response speed. Finally, numerical simulations and experimental results are provided to verify the effectiveness of the proposed control method in comparison with traditional frameworks.

Arc-discharge synthesis of nitrogen-doped C embedded TiCN nanocubes with tunable dielectric/magnetic properties for electromagnetic absorbing applications.

The development of novel composites consisting of ceramic and C materials to alleviate increasingly serious electromagnetic radiation is of great significance in the microwave absorption (MA) field, considering their superior anti-oxidation/corrosion performances and good mechanical strength as well as adjustable dielectric loss capabilities. However, it is still a great challenge to broaden their effective absorption bandwidth (reflection loss value ≤-10 dB) and strengthen the absorption intensity simultaneously, which is mostly attributed to the unreliable impedance matching degree at the absorber/air interface. Herein, a feasible strategy is adopted to synthesize TiCN@N-doped C nanocubes, whose low graphitization degree provides desirable impedance matching conditions. In the meantime, masses of core/shell hetero interfaces ensure strong microwave absorption capability. Experimental results reveal that the optimal effective absorption bandwidth of the prepared TiCN@N-doped C nanocubes can reach up to 5.44 GHz with a thickness of 1.88 mm. Our work demonstrates that the TiCN@N-doped C nanocubes have potential for electromagnetic absorbing applications.

微电网逆变器的改进型下垂控制策略

本文通过对传统下垂控制算法的有功、无功分析，引入虚拟电感的电压电流双环控制策略，虚拟电感使逆变器等效输出阻抗成感性，从而通过调节感抗匹配程度提供功率均衡效果；提出一种多逆变器并联运行的改进型下垂控制算法，通过改进型下垂控制参数设置，减弱了阻抗对环流的影响。实验仿真结果表明，改进型多逆变器并联的微电网改进型下垂控制算法提高了多逆变器的并联运行性能，有效减小了多逆变器并联运行的环流问题，大大提高了多逆变器并联运行的无功均衡效果。 In this paper, through the analysis of active and reactive power of traditional droop control algorithm, the voltage and current double loop control strategy is introduced. The virtual inductor causes the equivalent output impedance of the inverter into emotional, to provide a power equalization effect by adjusting the impedance matching degree; an improved droop control algorithm is proposed for multi-inverter parallel operation, and the influence of impedance on the circulation is weakened by the droop control parameters setting. The simulation results show that the improved droop control algorithm of multi-inverters parallel operation in micro-grid improves the performance of multi-inverters parallel operation. The circulating current problem of multi-inverters parallel operation is effectively reduced, and the reactive power balance effect of multi inverters parallel operation is greatly improved.

Effects of particle size on the magnetic and microwave absorption properties of carbon-coated nickel nanocapsules

To study the effects of particle size on magnetic and microwave absorption properties, carbon-coated nickel (Ni/C) nanocapsules were prepared by an arc discharge method, where their particles size was adjusted by varying the arc current value from 40 to 100 A. The average particle size of the Ni/C nanocapsules increases from 25 to 53 nm while the thickness of carbon shells keeps independence with increasing the arc current value. The saturation magnetization and coercivity increase with increasing the particle size, due to the small size effect. The complex permittivity, dielectric loss and attenuation constant of paraffin-Ni/C composites are in inverse proportion to the particle size of Ni/C nanocapsules, ascribed to the larger interface area and more defects in the smaller nanocapsules. With the reduction of particle size of Ni/C nanocapsules, the peak of reflection loss (RL) at fixed absorber shifts to lower frequency and the bandwidth becomes broader, while the maximum RL of the composites can be achieved at thinner absorber layer. The phenomenon results from the bigger dielectric loss, larger attenuation constant and better impedance matching degree. The control of particle size is an effective way for adjusting the microwave absorption properties.

A New Method to Calculate the Degree of Electromagnetic Impedance Matching in One-Layer Microwave Absorbers

A delta-function method is proposed to quantitatively evaluate the electromagnetic impedance matching degree. Measured electromagnetic parameters of α-Fe/Fe3B/Y2O3 nanocomposites are applied to calculate the matching degree by the method. Compared with reflection loss and quarter-wave principle theory, the method accurately reveals the intrinsic mechanism of microwave transmission and reflection properties. A possible honeycomb structure with promising high-performance microwave absorption, devised according to the method, is also proposed.

Dependence of microwave absorption properties on ferrite volume fraction in MnZn ferrite/rubber radar absorbing materials

We report the analysis of measurements of the complex magnetic permeability ( μ r ) and dielectric permittivity ( ε r ) spectra of a rubber radar absorbing material (RAM) with various MnZn ferrite volume fractions. The transmission/reflection measurements were carried out in a vector network analyzer. Optimum conditions for the maximum microwave absorption were determined by substituting the complex permeability and permittivity in the impedance matching equation. Both the MnZn ferrite content and the RAM thickness effects on the microwave absorption properties, in the frequency range of 2–18 GHz, were evaluated. The results show that the complex permeability and permittivity spectra of the RAM increase directly with the ferrite volume fraction. Reflection loss calculations by the impedance matching degree (reflection coefficient) show the dependence of this parameter on both thickness and composition of RAM.

Impedance Matching and Emission Properties of Nanoantennas in an Optical Nanocircuit

An experimentally realizable prototype optical nanocircuit consisting of a receiving and an emitting nanoantenna connected by a two-wire optical transmission line is studied using finite-difference time- and frequency-domain simulations. To optimize the coupling between optical nanocircuit elements we apply impedance matching concepts in analogy to radio frequency technology. We show that the degree of impedance matching, and in particular the impedance of the emitting nanoantenna, can be inferred from the experimentally accessible standing wave pattern on the transmission line. We demonstrate the possibility of matching the nanoantenna impedance to the transmission line by variations of the antenna length and width realizable by modern microfabrication techniques. The radiation efficiency of the emitting antenna also depends on its geometry but is independent of the degree of impedance matching. The case study presented here provides the basis for experimental realizations of general optical nanocircuits based on readily available gold nanostructures and a large variety of derived novel devices.

Electromagnetic and absorption properties of carbonyl iron/rubber radar absorbing materials

We measured the effective complex magnetic permeability /spl mu//sub eff//sup */ and dielectric permittivity /spl epsiv//sub eff//sup */ spectra in rubber radar absorbing material (RAM) with various carbonyl iron volume fractions by using the transmission/reflection method with a vector network analyzer. We studied the effects of carbonyl iron content and rubber thickness on the microwave absorption properties in the frequency range of 2.6 to 18 GHz. Our mathematical analysis is based on electromagnetic theory. The results indicate that the effective complex magnetic permeability and dielectric permittivity values of the RAM increase as the carbonyl iron volume fraction increases. For sample thickness of 3.0 mm, an increase in carbonyl iron content reduces the minimum reflection loss from -1.3 to -23.9 dB and shifts the frequency of the minimum reflection loss from 15.5 to 3.5 GHz. For an equal volume fraction of carbonyl iron, the frequency of the minimum reflection loss decreases as the thickness is increased. However, the dip in the reflection loss plot (in decibels) initially decreases to a minimum value before it increases with a further increase in thickness. We determined the value of the reflection loss for the samples by the impedance matching degree (reflection coefficient), which depends on the thickness and composition of the RAM.