Microwave Dielectric Spectrum of Vegetation-Part I: Experimental Observations

This is the first paper in a two-part sequence that evaluates the microwave dielectric behavior of vegetation material as a function of water content, microwave frequency, and temperature. Part I presents experimental measurements of the dielectric spectrum from 0.2 to 20 GHz for various types of vegetation material including leaves, stalks, and trunks at various moisture conditions. The measurements were acquired using a coaxial probe technique suitable for measuring the dielectric constant of both thick materials, such as tree trunks, and thin materials, such as leaves. In Part II, the experimental data are used to guide the development of a dual-dispersion dielectric model that incorporates the dielectric properties of water in both "free" and "bound" forms.     

Modeling and observation of the radar polarization signature offorested areas

To understand radar measurements of forested areas, the authors have developed a model of L-band (25-cm) microwave scattering from a forest. The forest floor is layer of nearly vertical dielectric cylinders representing tree trunks Above this layer is a second layer consisting of randomly oriented cylinders which represent branches. The authors identify several scattering mechanisms and calculate the corresponding Stokes matrices, which combine to give the total Stokes matrix and resulting polarization signature. It is found that this simple model permits accurate prediction of the polarization of the scattered waves and that additional mechanisms, including the effects of leaves and twigs, are not required for the 25-cm observation of the forests studied. The authors present measurements of the polarization signature acquired over a forested area and show comparisons with model calculations     

Automated instrumentation for continuous monitoring of thedielectric properties of woody vegetation: system design,implementation, and selected in situ measurements

The design and implementation of a system for the automated and continuous in situ monitoring of the dielectric constant of woody vegetation tissue are presented. The implementation of both single-channel and multichannel systems is discussed. These systems permit unsupervised continuous and long-term monitoring of vegetation canopy dielectric behavior in remote field sites. Utilizing open-ended coaxial lines, the real and imaginary parts of the microwave dielectric constant of woody plant tissue are inferred from direct measurement of the magnitude and phase of the microwave reflection coefficient. Samples of in situ data from forests in contrasting ecological environments are presented. Measurements obtained with the authors' systems allow new insight into the dielectric behavior of vegetation with respect to the physiological and hydraulic function of trees. The observations provide a significant advance in our ability to link canopy physiological and hydraulic behavior to radar remote-sensing observations     

Polarimetric channel characterization of foliage for performanceassessment of GPS receivers under tree canopies

The attenuation, depolarization, and fluctuation of a microwave signal going through a tree canopy are investigated by developing a Monte Carlo based coherent scattering model. In particular, the model is used to analyze the performance of Global Positioning System (GPS) receivers under tree canopies. Also the frequency and time-domain channel characteristics of a forest are investigated when a transmitter is outside and a receiver is inside a forest. A fractal algorithm (Lindenmayer system) is used to generate the structure of coniferous or deciduous trees whose basic building blocks are arbitrarily oriented finite cylinders, thin dielectric needles, and thin dielectric disks. Attenuation and phase change of the mean field through foliage is accounted for using Foldy's approximation. Scattering of the mean field from individual tree components and their images in the underlying ground plane are computed analytically and added coherently. Since tree trunks and some branches are large compared to the wavelength and may be in the close proximity of the receiver, a closed-form and uniform expression for the scattered near-field from dielectric cylinders is also developed. Monte Carlo simulation of field calculation is applied to a cluster of trees in order to estimate the statistics of the channel parameters, such as the probability density function (pdf) of the polarization state of the transmitted field, path loss, and the incoherent scattered power (the second moment of the scattered field), as a function of the observation point above the ground     

Radar modeling of a boreal forest

The authors report on the use of microwave modeling, ground truth, and synthetic aperture radar (SAR) data to investigate the characteristics of forest stands. A mixed coniferous forest stand has been modeled at SAR frequencies (P-, L-, and C-bands). The extensive measurements of ground truth and canopy geometry parameters were performed in a 200 m-square hemlock-dominated plot inside a forest. Hemlock trees in the forest are modeled by characterizing tree trunks, branches, and needles (leaves) with randomly oriented, lossy dielectric cylinders whose area and orientation distributions are prescribed. The distorted Born approximation is used to compute the backscatter at P-, L-, and C-SAR frequencies     

Electromagnetic scattering model for a tree trunk above a tiltedground plane

An efficient and realistic electromagnetic scattering model for a tree trunk above a ground plane is presented. The trunk is modeled as a finite-length stratified dielectric cylinder with a corrugated bark layer. The ground is considered to be a smooth homogeneous dielectric with an arbitrary slope. The bistatic scattering response of the cylinder is obtained by invoking two approximations. In the microwave region, the height of the tree trunks are usually much larger than the wavelength. Therefore the interior fields in a finite length cylinder representing a tree trunk can be approximated with those of an infinite cylinder with the same physical and electrical radial characteristics. Also an approximate image theory is used to account for the presence of the dielectric ground plane which simply introduces an image excitation wave and an image scattered field. An asymptotic solution based on the physical optics approximation is derived which provides a fast algorithm with excellent accuracy when the radii of the tree trunks are large compared to the wavelength. The effect of a bark layer is also taken into account by simply replacing the bark layer with an anisotropic layer. It is shown that the corrugated layer acts as an impedance transformer which may significantly decrease the backscattering radar cross section depending on the corrugation parameters. It is also shown that for a tilted ground plane a significant cross-polarized backscattered signal is generated while the co-polarized backscattered signal is reduced     

High frequency scattering from corrugated stratified cylinders

Interest in applying radar remote sensing for the study of forested areas led to the development of a model for scattering from corrugated stratified dielectric cylinders. The model is used to investigate the effect of bark and its roughness on scattering from tree trunks and branches. The outer layer of the cylinder (bark) is assumed to be a low-loss dielectric material and to have a regular (periodic) corrugation pattern. The inner layers are treated as lossy dielectrics with smooth boundaries. A hybrid solution based on the moment method and the physical optics approximation is obtained. New expressions for the equivalent physical-optics currents are used which are more convenient than the standard ones. It is shown that the bark layer and its roughness both reduce the radar cross-section. It is also demonstrated that the corrugations can be replaced by an equivalent anisotropic layer     

Modeling of geometric properties of loblolly pine tree and standcharacteristics for use in radar backscatter studies

An approach is presented to physically describe the tree and canopy components within loblolly pine forests in studies of microwave backscattering from forested canopies. The approach is based on a set of algorithms which describe the biomass characteristics of individual trees using measurements of tree diameter, height, and canopy depth. These algorithms predict bole, branch, and needle biomass; number, sizes, and orientations of primary and smaller branches; and number and density of needle clumps. Methods are discussed to aggregate the individual tree measurements into stand measurements (i.e., measurements per unit area). The results of the model clearly show how biomass distribution between tree components and canopy layers varies as a function of stand age for loblolly pines. The results also show the approach can be used to predict changes in the branch size and orientation within the canopy as a function of stand age, both on an individual tree basis as well as an entire stand basis     

BaO—TiO2—Sm2O3系陶瓷结构与介电性能研究

研究了BaO-TiO2-Sm2O3(BTS）系材料的结构和介电性能。实验结果证明：在富TiO2区的BTS系中,随着BaO/Sm2O摩尔比的增加,材料的介电常数ε和介电常数的温度系数αε发生变化,XRD分析证明这是因为材料的主晶相发生变化所致。当BaO/Sm2O3＝1．33时,可获得具有超低损耗的热稳定微波陶瓷材料,其主晶相为BaSm2Ti4O12。     

Electromagnetic spectroscopy of normal breast tissue specimens obtained from reduction surgeries: comparison of optical and microwave properties

Techniques utilizing electromagnetic energy at microwave and optical frequencies have been shown to be promising for breast cancer detection and diagnosis. Since different biophysical mechanisms are exploited at these frequencies to discriminate between healthy and diseased tissue, combining these two modalities may result in a more powerful approach for breast cancer detection and diagnosis. Toward this end, we performed microwave dielectric spectroscopy and optical diffuse reflectance spectroscopy measurements at the same sites on freshly excised normal breast tissues obtained from reduction surgeries at the University of Wisconsin Hospital, using microwave and optical probes with very similar sensing volumes. We found that the microwave dielectric constant and effective conductivity are correlated with tissue composition across the entire measurement frequency range ($vert rvert sim 0.5hbox{--}0.6, p ≪ 0.01$) and that the optical absorption coefficient at 460 nm and optical scattering coefficient are correlated with tissue composition ($vert rvert sim 0.4hbox{--}0.6, p≪ 0.02$). Finally, we found that the optical absorption coefficient at 460 nm is correlated with the microwave dielectric constant and effective conductivity ($r = hbox{--}0.55, p≪ 0.01$ ). Our results suggest that combining optical and microwave modalities for analyzing breast tissue samples may serve as a crosscheck and provide complementary information about tissue composition.      

Microwave Dielectric Properties of Plant Materials

Three waveguide transmission systems covering the 1-2, 3.5-6.5, and 7.5-8.5 GHz bands were used to measure the dielectric properties of vegetation material as a function of moisture content and microwave frequency. The materials measured included, primarily, the leaves and stalks of corn and wheat. Dielectric measurements also were made of the liquid included in the vegetation material after it was extracted from the vegetation by mechanical means. The extracted liquids were found to have an equivalent NaCl salinity of about 10 per mil, which can have a significant effect on the dielectric loss at frequencies below 5 GHz. The results of attempts to model the dielectric constant of the vegetation-water mixture in terms of the dielectric constants and volume fractions of its constituent parts (i. e., bulk vegetation, air, bound water, and free water) are discussed. Additionally, measurements of the temporal variations in the total attenuation at 10.2 GHz are presented for a corn canopy and a soybean canopy.     

（Pb,Nd）（Ti,Mn,In）O3陶瓷的微波介电性质

近年来，越来越多的具有高介电常数和低介质损耗的陶瓷介质用于微波领域。（Ｐｂ，Ｎｄ）（Ｔｉ．Ｍｎ，Ｉｎ）Ｏ＿３陶瓷是一种新型的微波介质材料。在本文中，作者致力于（Ｐｂ．Ｎｄ）（Ｔｉ，Ｍｎ，Ｉｎ）Ｏ＿３陶瓷的微波介电性质的研究，并且讨论了Ｍｎ含量对微波介电性质的影响。     

Dielectric constant, loss tangent, and surface resistance of PCB materials at K-band frequencies

This paper develops the theoretical approach and describes the design of a practical test rig for measuring the microwave parameters of unclad and laminated dielectric substrates. The test rig is based on a sapphire whispering-gallery resonator and allows the measurement of the following parameters: dielectric constant (epsiv) of the dielectric substrate in the range from 2 to 10, loss tangent (tandelta) of the dielectric substrate in the range from 10^-4 to 10^-2, and microwave losses of copper coating of the substrate in the range from 0.03 to 0.3 Omega. Measurements of numerous commonly used microwave printed-circuit-board materials were performed at frequencies between 30-40 GHz and over a temperature range of -50degC to +70degC     

微波介质陶瓷介电常数测量的简单方法

应用等效网络法分别测量了空气、聚四氟乙烯及微波介质陶瓷材料片状试样的介电常数。理论分析计算与实验结果表明,该方法在无损测量片状微波介质陶瓷材料试样的介电常数中是可行的,且有模型简单、计算量小、精度较高及操作容易等特点。是一种无损测量片状微波介质陶瓷材料介电常数的简单方法。     

A Microwave Technique for the Measurement of the Dielectric Properties of Soils (Correspondence)

During a recent investigation of antennas mounted flush with the earth, it was necessary to measure the dielectric properties of soil. Many different techniques for the measurement of dielectric constants and loss tangent have been developed. These techniques generally fall into two categories: those which utilize transmission through a sample and those which use the reflection from the sample. The particular technique employed is modified to suit the range of parameters being measured and the physical characteristics of the sample. In general, a measurement that provides an accurate determination of dielectric constant is relatively insensitive to variations in loss tangent and vice versa. The technique described here utilizes both types of measurements to find in a practical manner the dielectric constant and loss tangent of relatively moist loamy soil at microwave frequencies.     

Ba2Ti9O20/PTFE微波介电复合材料的制备及性能

采用粉末冶金工艺制备了以微波介质陶瓷Ba2Ti9O20微粉增强PTFE的微波基片复合材料。研究了陶瓷微粉含量对其介电性能的以及介电性能的频率和温度特性，并用介质混合法则对其介电常数进行拟合分析。发现某些组分的Ba2Ti9O20／PTFE复合材料的介电常数和介电损耗具有良好的频率和温度稳定性，拟合结果与实验测试结果一致性较好。     

钛酸锌微波介电陶瓷的改性研究现状

对钛酸锌介电陶瓷的改性研究进行了综述,分析了形成固溶体、生成新的化合物和添加助熔剂对钛酸锌陶瓷的烧结性、相结构和微波介电性能的影响。结果表明,形成无限固溶体的陶瓷主晶相为六方相,微波介电性能优良,但烧结温度偏高;添加助熔剂虽改善了烧结性,但主晶相与加入量及烧结温度有关,微波介电性能随主晶相变化。据此提出了复合改性的设想。     

PPS基纳米复合介质制备与性能研究

采用化学沉淀法制备了平均粒径约30 nm的TiO2粉料,以聚苯硫醚树脂为基复合纳米无机粉料,结合螺杆挤出与模压等工艺制备了复合介质基板,借助SEM、EDS、DTA-TGA分别对合成的纳米粉料及树脂基复合料进行了微观结构表征与分析,用带状线方法对PPS基无机复合介质基板微波复介电常数进行了测量。获得了相对介电常数为6.0～13.0,且损耗低的高性能新型微波复合介质材料。     

A modified cardioid model for Processing multiangular radiometric observations

L-band spaceborne microwave radiometers are becoming able to provide estimates of surface soil moisture, on both spatial and temporal scales compatible with applications to meteorology and hydrology. The basic rationale for retrieving soil moisture from radiometric measurements is the assumption that the surface layer can be modeled as a dielectric medium. Its dielectric constant then depends on several physical parameters, including soil moisture; emissivities for various incidence angles are computed using Fresnel's formulas. Many controlled field experiments have demonstrated the validity of this approach. Scenes exist still (e.g., ice-covered or frozen surfaces, barren areas) where surface soil moisture is not a relevant concept. For such scenes, information should, however, be available on the complex dielectric constant itself. This paper shows that the dielectric constant cannot be fully retrieved from single-frequency multiangular data; it describes, however, a methodology that aims at retrieving in an optimized way the available information.