Probing Thick Vegetation Canopies with a Field Microwave Scatterometer

With a narrow-beam field microwave scatterometer having a variable slant range and a small slant-range resolution, investigators can estimate the distribution of backscattering sources in a medium as a function of slant range. From this, they can calculate the backscattering coefficient. With less confidence, they can estimate the volume extinction coefficient, the volume density of backscattering cross sections, the transmittance, and the backscattering coefficient of a semiinfinite medium with the same volumetric microwave properties. These parameters allow one to understand the interactions of microwaves with media such as vegetation canopies with their substrates.     

Effects of Varying Soil Moisture Contents And Vegetation Canopies on Microwave Emissions

This paper presents an analysis of radiometric data taken at 21, 2.8, and 1.67 cm during a NASA sponsored flight over agricultural fields in Phoenix, AZ. The objective of the mission was to provide comprehensive information concerning microwave responses due to a broad range of soil moisture contents. Generally, data taken over bare fields agree well with theoretical estimates from a combined multilayer radiative transfer model with simple roughness correction. With the surface moisture content ranging between <5 and >35 percent, the emissivity ranges between >0.9 and ~0.7. The response to soil moisture content at 21 cm is more senstive than that at either 2.8 or 1.67 cm. The vegetation model takes into account both the effect of dielectric coefficient and the volume scattering characteristics of the vegetation layer. At the longer wavelengths (e.g., 21 cm) radiation from soil penetrates through vegetation layers of wheat and alfalfa and provides surface moisture information. However, short wavelength radiation from soil cannot penetrate through vegetation canopies; the volume scattering characteristics of vegetation controls the overall microwave signatures.     

Modeling the Radiant Transfers of Sparse Vegetation Canopies

The three-dimensional radiative transfer model of Kimes [2] was used to extend our understanding of the physical principles causing the scattering dynamics in sparse vegetation canopies (? 50-percent ground cover). The model was upgraded by including an aniotropic scattering algorithm for soil developed by Walthall et al. [7]. The model was validated using measured directional reflectance data that covered the entire exitance hemisphere. Two canopies were chosen to present in this study-an orchard grass canopy (50-percent ground cover) and a hard wheat canopy (11-percent ground cover). These canopies showed the typical scattering behavior of canopies with low and intermediate vegetation density. A red wavelength (0.58-0.68 pm) band was used throughout the study. A number of phenomena contributed to the directional reflectance distributions observed in the field. These include: 1) the strong anisotropic scattering properties of the soil, 2) the geometric effect of the vegetation probability of gap function on the soil anisotropy and solar irradiance, and 3) the anisotropic scattering of vegetation which is controlled by the phase function (for an infinitely small volume of representative leaves) and geometric Effect 1 (cause by layering of leaves). These phenomena as identified in this paper account for the major scattering behavior of observed data sets of directional reflectance distributions. Such knowledge provides an intelligent basis for defining specifications of earth-observing sensor systems and for inferring important aspects of physical and biological processes of the plant system.     

Sources of Scattering from Vegetation Canopies at 10 GHz

High-resolution backscatter measurements were made at X-band, with vertical polarization, and incidence angles of 30° and 50°, for winter wheat, soybeans, and corn during the summer of 1984. Plants were observed both under natural conditions and with partial-to-complete defoliation. The resolution volume of the measurement system is better than 20 × 20 × 11.25 cm3 within the entire range of measurements. Results are presented as a radar image, which also provides information on the spatial distribution of the (attenuated) scattering inside the canopy observed under natural conditions. Measured results indicate that: 1) for the full-grown wheat plants, heads are the dominant scatterers at the early heading stage and the ground is the dominant scatterer at the later growth stages; 2) for the soybeans, returns are mostly from the upper 30-cm portion of the plants, and leaves and stems dominate the radar signal at 30° and 50° incidence angles, respectively; 3) for the corn plants, leaves are the single dominan scatterers at both angles of incidence; and 4) ground returns are relatively unimportant as they are highly attenuated by the vegetation cover.     

AlN-C(石墨)复相材料微波衰减性能的研究

采用热压烧结工艺,通过合理的烧结温度及保温时间的控制,制备了性能优异的AlN-C复相微波衰减材料。通过网络分析仪、SEM等测试手段,研究了衰减剂C含量对AlN-C复相材料微波衰减性能的影响,结果表明,当衰减剂C含量极小时(w≤1.0 %),AlN-C复相材料不具备衰减电磁波的特性;当衰减剂C含量为3.0 %～5.0 %时,AlN-C复相材料呈现良好的宽频衰减特性;当衰减剂C含量大于7.0 %时,AlN-C复相材料呈现明显的选频衰减特性,且随着C含量的增加,材料的衰减量增大,有效衰减带宽减小,中心谐振频率f0有向高频漂移的趋势。对AlN-C复相材料的频谱特性进行了初步的分析。探讨了AlN-C复相材料的微波衰减机理,电导损耗、界面极化损耗是其主要的微波衰减机理。     

Modeling of Vegetation Permittivity at Microwave Frequencies

When materials are to be treated with microwaves for different purposes such as drying, online moisture measurements, disinfestation, and remote sensing, thorough knowledge of the material dielectric properties becomes extremely important. Unlike for other materials, measurement of vegetation dielectric properties is very complicated due to the nature of the materials themselves. Hence, vegetation dielectric models that require fewer accurately and easily measurable physical quantities are of great importance. Therefore, dielectric models that only require the measurement of moisture content (MC) have been investigated for alfalfa, a plant material that has high commercial value in national and international markets. The models were developed by measuring the dielectric properties by using an open-ended coaxial probe at frequencies ranging from over 300 MHz to 18 GHz at 22 degC and at an MC ranging from 12% to 73% in wet basis     

Remote Sensing of Temperature Profiles in Vegetation Canopies Using Multiple View Angles And Inversion Techniques

Most remote sensing applications in natural resource disciplines deal with vegetation or with substrate materials covered by a vegetation canopy. Remote sensing of vertical temperature profiles of the vegetation canopy (including the substrate) provide valuable information to resource managers. A mathematical approach is presented which theoretically allows the determination of a temperature profile from multiple, sensor view angles and a priori knowledge of vegetative geometric structure. The technique was evaluated on data from several wheat canopies at different stages of development. The technique was shown to be most applicable for the separation of vegetation and substrate temperatures. One can expect relatively accurate inferences of mean vegetation surface temperatures for intermediate and dense vegetation canopies, and relatively poor inferences of vegetation temperatures for sparse canopies. The converse is true for inferring substrate temperatures. The root-mean-square prediction accuracy of vegetation temperatures for intermediate-dense wheat canopies was 1.8 and 1.4°C for an exact and overdeterminate system, respectively. These findings have significant implications for any discipline concerned with the vegetation itself or the underlying substrate. For specific vegetation geometries this technique may provide a means for uniquely separating mean vegetation and substrate temperatures when a priori knowledge of vegetation geometry and two or more sensor view angle measurements can be obtained.     

微波宽带衰减自动检定和校准系统

衰减量是微波宽带电子系统或其零部件（同轴及波导）性能的一项重要参数。该文介绍一种基于HP8902A测量接收机、毫米波混频器及计算机的衰减自动测量和校准系统，该系统可以实现频率达到40GHz衰减量快速、准确的测量。文中对硬件配置与软件设计作了详细介绍，对误差的分析作了较详细的说明。     

纳米碳管复合陶瓷微波衰减材料的研究

采用无压烧结工艺,以AlN和镁橄榄石（M2S）粉作为基体制备了纳米碳管（CNT）复合陶瓷。制备了热导率高、衰减量大及频率匹配特性良好的AlN—CNT复合微波衰减陶瓷。制备出的致密的M2S-CNT复合微波衰减材料有希望替代用在真空电子器件中的氧化铝多孔渗碳微波吸收材料。     

MgO-SiC系列复合微波衰减材料

介绍了衰减材料的基本原理,国内外MgO-SiC系列复合材料研究和应用现状。     

阻带具有衰减极点的双通带滤波器

在单通带滤波器的基础上引入谐振元件上,提出了一种滤波网络,通过分析微波网络的传递函数,得到阻带具有衰减极点的双通带滤波器。通过比较单通带滤波器与双通带滤波的传输特性,可以得出衰减极点来源于谐振器的共振。设计了具有2个滤波单元的双通带滤波器,通带的品质因数分别为Q1=3.7316和Q2=9.7885.并且分析了滤波单元的个数对传输系数的影响.随着滤波单元个数的增加,传输系数的幅度在通带内震荡加剧,过渡带变得更加陡峭。这种滤波网络在实际的应用很容易实现。     

阻带有衰减极点的微波带通滤波器

研究了阻带有衰减极点的微波带通滤波器的设计方法.从理论上证明了中间谐振器上加载电容器能在阻带低端形成衰减极点,并推导出计算极点位置的关系式.设计了中心频率为945 MHz的5级带通滤波器,它是由5个介质同轴谐振器和1块介质基片构成,第3个谐振器上加载电容,在915MHz处形成衰减极点.用εr=38的微波介质材料制成了实物,滤波器插入损耗小于2.5dB,915MHz处衰减大于62dBc,测量结果与设计相吻合.该滤波器已用于通信系统中.     

特种泡沫云干扰微波的实验研究

简要介绍了特种泡沫云多波段干扰新技术。利用某导弹技术阵地专用的综合测试系统，测定了它对军事上常用的3mm、8mm和3cm频段微波的干扰效果。结果表明泡沫云只需几到几十厘米厚即可使微波具有很强的衰减作用，可以满足不同温度下不同目标战时实施干扰的要求。所得结论为这一新技术的战术运用设计提供了必要的理论基础和实验依据。     

Calculation of Microwave Attenuation Effect Due to Charged Sand Particles

Based on the forward scattering amplitude function for charged sand particles under the Rayleigh approximation and the effective permittivity method, a calculation model for microwave attenuation due to charged sand particles is given in terms of equal sized distribution and lognormal size distribution, and the attenuation is calculated and analyzed. The results show that the attenuation with charged sand is greater than the case of no charge, and the more concentrative the surface charges on sand particles are, the greater are the influences on microwave attenuation. When the frequency is not too high, natural sand and vehicular sand have little effect on microwave attenuation, whereas the attenuation of explosive sand need be considered.     

微波功率模块微波特性的研究

集成固态/真空电子技术的微波功率模块(MPM)是新一代电子武器装备的"超级微波元器件".文中对MPM微波特性进行了研究,由于行波管(TWT)良好的色散特性,使MPM具有宽频带和功率、带宽乘积的优点;又由于固态放大器(SSA)低噪声性能,使MPM具有低噪声特性;高稳定的集成电源(IPC)决定了MPM具有低相位噪声与低杂散的频谱特性,合理的增益分配也使得MPM具有良好噪声特性.文中具体分析了集成电源(IPC)性能对频谱特性的影响,SSA与TWT噪声特性、增益分配对MPM噪声特性的影响,MPM幅相一致性和功率合成效率问题,并给出了仿真曲线,通过对某X波段MPM微波特性测试,验证了MPM优越的性能,也充分表明了MPM具有广泛的应用前景.     

Measurement of Equivalent Dielectric Parameters of Sandstorms from the Microwave Attenuation

The theory is presented for one method of determining the equivalent dielectric parameters of sand storms from microwave propagation measurement over a line of sight link. The method required the transmission of linear polarized or circle-polarized signal and attenuation measurement. The equivalent dielectric parameters of sandstorms determined at frequencies of 33.5 GHz and 93 GHz over a terrestrial link.     

TiO2添加剂对AlN-SiC材料微波衰减性能的影响

在AlN-SiC复相材料中加入TiO2,采用热压工艺,制备了性能优异的AlN-SiC-TiO2复相微波衰减材料。通过矢量网络分析仪、SEM等测试手段,研究了AlN-SiC复相材料微波衰减性能与SiC含量的之间的关系,以及添加剂TiO2对AlN-SiC复相材料微波衰减特性和显微结构的影响。结果表明,SiC是良好的宽频微波衰减剂,对衰减频谱曲线特征起决定作用;TiO2的添加大大促进了AlN-SiC复相材料的烧结性能和微波衰减性能,添加了TiO2后6 GHz下衰减量由0.6 dB增加到0.85 dB。初步探讨了AlN-SiC-TiO2复相材料的微波衰减机理,电导损耗、介质损耗是其主要的微波衰减机理。     

Computation of Microwave Attenuation on Coplanar Waveguide with Conformal ADI-FDTD Method

A novel Conformal Finite-Difference Time-Domain based on Alternating-Direction Implicit Method(ADI-CFDTD) used to calculate the microwave attenuation on coplanar waveguide(CPW) with complicated cross-section up to 200GHz is presented. This ADI-CFDTD algorithm is the combination of conformal algorithm dealing with the deformed cell with ADI-FDTD, and has unconditionally stable. The difference equations are derived from the Faraday's and Ampere's law. To validate its accuracy and efficiency, as an example, a CPW fabricated on lithium niobate is proposed. Numerical results demonstrate that this new ADI-CFDTD algorithm has more accuracy than that of quasi-static and CFDTD and shows good agreement with experiment data. When frequency is up to millimeter wave band the radiation loss is not negligible in total power loss.     

高阻带微波带通滤波器的研制

为了提高通信设备的性能,研制了高阻带的微波滤波器。该滤波器由5个高Q值的同轴介电陶瓷谐振器构成的带通滤波器。谐振器之间通过介电基片上的电容相互耦合。在装入外壳前,调整好每个谐振器的谐振频率和耦合电容,之后就不需要再调整,外壳上也没有可调零件,从而保证滤波器结构稳定,性能可靠。用εr=74的介电陶瓷制作出滤波器的性能为f0=982 MHz,Δf=10.9 MHzI,L=3.8 dB,阻带在f0±30 MHz处大于69 dB,远阻带衰减优于70 dB。     

SDH微波建设中克服多径衰落的措施

1概述微波是一种视距传播的无线通信手段，受气候和传播环境的影响很大。所谓多径衰落，是指在微波信号的传播过程中，由于受地面反射和大气折射的影响，会产生多个经过不同路径到达接收机的信号，通过矢量叠加后合成的信号有强有弱。多径衰落可分为平衰落和频率选择性衰落。平衰落是指在信号射频带宽内各个频率点的幅度都降低，从而使得接收端的接收电平降低；频率选择性衰落是指在信号射频带宽内不同频率点的幅度降低不同，从而导致接收端的接收电平降低且信号产生失真。2江苏省二级干线SDH微波系统的建设2.1SDH微波系统的概况为了确保…