Incident Microwave Frequency Research Articles

The Correction Method of Water and Fresnel Reflection Coefficient for Soil Moisture Retrieved by CYGNSS

Spaceborne GNSS-R technology is a new remote sensing method for soil moisture monitoring. Focusing on the significant influence of water on the surface reflectivity of CYGNSS, this paper improved the removal method of water influence according to the spatial resolution of CYGNSS data. Due to the disturbance effect of the incident angle, microwave frequency and soil type on the Fresnel reflection coefficient in surface reflectivity, a normalization method of Fresnel reflection coefficient was proposed after analyzing the data characteristics of variables in the Fresnel reflection coefficient. Finally, combined with the soil moisture retrieval method of linear equation, the accuracy was compared and verified by using measured data, SMAP products and official CYGNSS products. The results indicate that the normalization method of the Fresnel reflection coefficient could effectively reduce the influence of relevant parameters on the Fresnel reflection coefficient, but the normalization effect became worse at large incident angles (greater than 65°). Compared with the official CYGNSS product, the retrieval accuracy of optimized soil moisture was improved by 10%. The method proposed in this paper will play an important reference role in the study of soil moisture retrieval using spaceborne GNSS-R data.

Effect of incident microwave frequency on curing process of polymer matrix composites

Microwave assisted curing of aerospace and structural polymer matrix composites depends on several factors such as dielectric properties of the curing substrate, type of tooling used, thickness of the composite part, kind of applicator chosen and so on but one of the most important factors is the frequency of radiation used for the purpose of curing. Majority of the work reported on microwave curing of polymer composites is restricted to the commonly utilized frequency of 2450 MHz, whereas 915 MHz frequency is extensively employed chiefly for food processing and wood drying applications. This paper evaluates the potential of 915 MHz frequency for curing polymer matrix composites.2 mm thick glass-epoxy composite laminates were prepared using bi-directional glass fiber reinforced epoxy resin system. These composites were then cured in three different ways: microwave curing with 915 MHz as incident microwave frequency; microwave curing with 2450 MHz as incident microwave frequency and conventional thermal (oven) curing. Comparison between microwave and thermal cure process times as well as energy consumption revealed the significant time and energy savings realized by microwave curing. Further, mutual comparison of the cured laminates was drawn in terms of attainment of lamination quality, glass transition temperature and mechanical properties. The results obtained showed that the 915 MHz microwave cured laminate had superior inter-laminar shear strength (ILSS) and flexural strength as compared to 2450 MHz microwave cured and thermally cured laminates. This, in essence, implies that incident microwave frequency plays a crucial role in designing cure process parameters for polymer matrix composites.

Characterization of frequency-chirped dynamic nuclear polarization in rotating solids

Continuous wave (CW) dynamic nuclear polarization (DNP) is used with magic angle spinning (MAS) to enhance the typically poor sensitivity of nuclear magnetic resonance (NMR) by orders of magnitude. In a recent publication we show that further enhancement is obtained by using a frequency-agile gyrotron to chirp incident microwave frequency through the electron resonance frequency during DNP transfer. Here we characterize the effect of chirped MAS DNP by investigating the sweep time, sweep width, center-frequency, and electron Rabi frequency of the chirps. We show the advantages of chirped DNP with a trityl-nitroxide biradical, and a lack of improvement with chirped DNP using AMUPol, a nitroxide biradical. Frequency-chirped DNP on a model system of urea in a cryoprotecting matrix yields an enhancement of 142, 21% greater than that obtained with CW DNP. We then go beyond this model system and apply chirped DNP to intact human cells. In human Jurkat cells, frequency-chirped DNP improves enhancement by 24% over CW DNP. The characterization of the chirped DNP effect reveals instrument limitations on sweep time and sweep width, promising even greater increases in sensitivity with further technology development. These improvements in gyrotron technology, frequency-agile methods, and in-cell applications are expected to play a significant role in the advancement of MAS DNP.

Measurement of Instantaneous Microwave Frequency by Optical Power Monitoring Based on Polarization Interference

A photonic-assisted approach for instantaneous frequency measurement based on optical power monitoring is proposed and demonstrated. Instead of using optical filter, a dual-polarization Mach–Zehnder modulator is used to modulate the unknown RF signal with a designed electrical time delay. Due to the property of electrical time delay, the frequency information is firstly converted to phase shift and then to the optical power. By monitoring the optical power, a monotonic relationship between the incident microwave frequency and optical power ratio can be easily established. A detailed theoretical analysis is carried out to illustrate the mechanism of the proposed instantaneous frequency measurement setup. Simulation has been performed to investigate the tuning of measurement range and the impact of imperfection devices. A proof-of-concept experiment has been carried out to verify the mechanism.

Plasma filamentation and shock wave enhancement in microwave rockets by combining low-frequency microwaves with external magnetic field

A filamentary plasma is reproduced based on a fully kinetic model of electron and ion transports coupled with electromagnetic wave propagation. The discharge plasma transits from discrete to diffusive patterns at a 110-GHz breakdown, with decrease in the ambient pressure, because of the rapid electron diffusion that occurs during an increase in the propagation speed of the ionization front. A discrete plasma is obtained at low pressures when a low-frequency microwave is irradiated because the ionization process becomes more dominant than the electron diffusion, when the electrons are effectively heated by the low-frequency microwave. The propagation speed of the plasma increases with decrease in the incident microwave frequency because of the higher ionization frequency and faster plasma diffusion resulting from the increase in the energy-absorption rate. An external magnetic field is applied to the breakdown volume, which induces plasma filamentation at lower pressures because the electron diffusion is suppressed by the magnetic field. The thrust performance of a microwave rocket is improved by the magnetic fields corresponding to the electron cyclotron resonance (ECR) and its higher-harmonic heating, because slower propagation of the ionization front and larger energy-absorption rates are obtained at lower pressures. It would be advantageous if the fundamental mode of ECR heating is coupled with a lower frequency microwave instead of combining the higher-harmonic ECR heating with the higher frequency microwave. This can improve the thrust performance with smaller magnetic fields even if the propagation speed increases because of the decrease in the incident microwave frequency.

The origin of the strong microwave absorption in black TiO2

In this study, the mechanism of the strong microwave absorption in black TiO2 nanoparticles has been investigated both experimentally and theoretically. In experiment, the amorphous TiO2 nanoparticles/paraffin wax composites show the reflection loss (RL) of −4.0 dB, which is much smaller compared with the RL of −49.0 dB in those core/shell structure ones. Theoretically, the calculation illustrates that the accumulated charge of 1013 cm−3 at the core/shell interface results in the plasmon resonance with the incident microwave frequency at 9.3 GHz and 27.0 GHz. The microwave absorption enhancement of the black TiO2 nanoparticles is proposed to originate from the synergy mechanism between their crystalline-cores and amorphous-shells, rather than the defects and impurities in amorphous phase.

The Interaction of C-Band Microwaves with Large Plasma Sheets

A large plasma sheet 60 cm×60 cm×2 cm in size was generated using a hollow cathode, and measurements were conducted for interactions including transmission, reflection and absorption. With different discharge parameters, plasma sheets can vary and influence microwave strength. Microwave reflection decreases when the discharge current rises, and the opposite occurs in transmission. The C-band microwave is absorbed when it is propagated through large plasma sheets at higher pressure. When plasma density and collision frequency are fitted with incident microwave frequency, a large amount of microwave energy is consumed. Reflection, transmission and absorption all exist simultaneously. Plasma sheets are an attractive alternative to microwave steering at low pressure, and the microwave reflection used in receiving radar can be altered by changing the discharge parameters.

均一・高密度プラズマ生成アンテナを用いたマイクロ波放電型イオンスラスタの高性能化

The dependency of thrust performance on thruster configurations such as antenna length, antenna height, number of antenna, magnetic field configuration, and microwave frequency, was investigated with the objective of improving the thrust performance of microwave discharge ion thruster using antennas for uniform and dense plasma production. The experimental results showed that there was an optimum length of the antennas, and it was 3/4 times the wave length of incident microwaves. The ion beam current reaches its maximum value when the antenna was set at 2mm downstream of an electron cyclotron resonance layer. There was an optimum number of the antennas. This is due to the tradeoff between the coupling of plasma with microwave and the surface recombination on the antenna. The expansion of ionization zone was made successfully by changing magnetic field configuration. In addition, the thrust performance was slightly improved with increase in incident microwave frequency from 2.45GHz to 4.2GHz. A value for the ion beam current with 2.45GHz is compensated by high electron number density and less magnetized ions for the disadvantage of small plasma number density. Overall, the propellant utilization efficiency, ion production cost, and estimated thrust were found to be 0.62, 300W/A and 6.2mN, respectively at mass flow rate of 0.22mg/s for xenon, ion beam voltage of 1,500V and 2.45GHz microwave incident power at 32W.

New Microwave Diagnostic Theory for Measurement of ElectronDensity in Atmospheric Plasmas

We present the phase shift of an electromagnetic wave passing throughan atmospheric plasma layer. In this kind of plasma, the phase shiftdepends not only on the line average electron density and layer width,but also on the electron-neutral collision frequency. Since thecollision frequency is close to or even larger than the incidentmicrowave frequency, a one-dimensional code for the numerical solutionof the wave equation with full time and space variables is establishedto give the phase shift. When the width and the pressure (hence theelectron-neutral collision frequency) are known, the measurement ofphase shift will uniquely determine the line average electrons densityin an atmospheric plasma.

Production of low electron temperature ECR plasma

The experiments and simulations on the control of the plasma parameters are performed using large diameter electron cyclotron resonance (ECR) plasmas. It is found that the plasma parameters are controlled by diluting with N 2 gas in the mirror magnetic field. The electron temperature ( T e) decreases down to 40% and the electron density ( n e) increases up to 80%, respectively, that is, a low T e ECR plasma with high n e is realized. Furthermore, the T e control is attempted by changing the spatial profile of the microwave power absorption. It is observed that T e decreases with changing the power absorption profile that is achieved by changing the external conditions such as incident microwave power, magnetic field configuration, and incident microwave frequency. The production mechanism of an ECR plasma with plasma parameter variable is also discussed.

Linear and nonlinear spin wave resonances in a very thin amorphous wire

Linear and nonlinear FMR spectra of a very thin amorphous wire are investigated. A phenomena called “spin wave ripple” is reported here. It is observed on the low field side of the linear resonance curve and is attributed to the excitation of radial standing spin waves with the energy corresponding to the incident microwave frequency. The fine structure of the nonlinear resonance curves, recently observed on thin amorphous wires, is then explained by the dipole-exchange models of a cylinder parametrically excited by Suhl's first-order process. This explanation is consistent with the value of the exchange stiffness constant A determined from the spin wave ripple.

Effects of microwave radiation on two strongly coupled indium microbridges

Effects of incident microwave radiation on two closely spaced indium micro-bridges are investigated by measuring the current-voltage(I–V) characteristics of the weak links, using microprobes prepared by an electron beam lithography technique. Pronounced distortions as well as negative dynamic resistance are found in theI–V curves where the sum of the two Josephson frequencies is equal to the incident microwave frequency or its harmonics. These results are interpreted in terms of interactions of the weak links with the microwave radiation and with each other via exchange of quasiparticles.

Theory of Cyclotron Resonance of Piezoelectric Polarons

The cyclotron resonance of piezoelectric materials with special reference to cadmium sulfide is discussed on the basis of the recently developed general quantum-mechanical expression for conductivity. The previous analysis by Mahan and Hopfield is shown to be valid only for very weak magnetic fields, viz., under the semiclassical conditions. In the actual experimental conditions, however, where the Landau level separation is larger than k B T (quantum conditions), the cyclotron mass m c differs from the result of semiclassical analysis: (a) in most regions of temperature, m c is an increasing function of temperature, and (b) m c depends on the magnetic field and thus on the incident microwave frequency. The line width is finite and differs appreciably from the inverse of the lifetime of the electronic state, which is shown by simple treatments to be infinite. Numerical calculation of the line shape shows that m c is, though smaller, very close to m , in contrast to Mahan and Hopfield. It is proved on the basis of the general expression that m c reduces to the tube mass and the line width tends to the inverse of the static transport relaxation time in the weak magnetic field limit.