Thermal Microwave Emission from Vegetated Fields: A Comparison between Theory And Experiment

The radiometric measurements over bare field and fields covered with grass, soybean, corn, and alfalfa were made with 1.4-and 5-GHz microwave radiometers during August-October 1978. The measured results are compared with radiative transfer theory treating the vegetated fields as a two-layer random medium. It is found that the presence of a vegetation cover generally gives a higher brightness temperature TB than that expected from a bare soil. The amount of this TB excess increases with increase in the vegetation biomass and in the frequency of the observed radiation. The results of radiative transfer calculations, which include a parameter characterizing ground surface roughness, generally match well with the experimental data.     

Microwave Emission and Scattering From Deserts: Theory Compared With Satellite Measurements

The emission and scattering from desert surfaces are analyzed using simulations and measurements from the Special Sensor Microwave/Imager (SSM/I) and the Advanced Microwave Sounding Unit (AMSU) microwave satellite instruments. Deserts are virtually free of vegetation, so the satellite radiometers are able to observe the emissivities of different minerals, such as limestone and quartz. Moreover, since deserts contain little moisture, the thermal emission originates below the surface at a depth of many wavelengths. At high frequencies, where the penetration depth of radiation is smallest, the radiometric measurements display the large diurnal variation in surface temperature, which reaches its maximum at around 1 P.M. Conversely, at low frequencies, where the penetration depth is largest, the radiation measurements display the small diurnal variation of subsurface temperature, which reaches a minimum at around 6 A.M. In addition to these emission signals, sand particles also scatter microwave radiation. Volume scattering causes the measurements to decrease as the frequency increases; although compared to other scattering media (snow cover and precipitation), the larger absorption and fractional volume (i.e., solidity) of sand reduce the scattering. Although the scattering effect is small, SSM/I measurements between 19 and 85 GHz show that deserts scatter the upwelling microwave radiation in a manner similar to light precipitation, which makes it difficult to uniquely identify precipitation over arid regions. Interestingly, the higher frequency AMSU measurement at 150 GHz is nearly the same as at 89 GHz for deserts, whereas the 150-GHz measurement is much lower than at 89 GHz for precipitation. These different spectral features at high frequencies can provide a means of separating the scattering from desert surfaces from that of precipitation.     

Snow Property Measurements Correlative to Microwave Emission at 35 GHz

Snow microstructure, measured by plane section analysis, and snow wetness, measured by the dilution method, are used to calculate input parameters for a microwave emission model that uses the radiative transfer method. The scattering and absorbing properties are calculated by Mie theory. The effects of different equivalent sphere conversions, adjustments for near-field interference, and different snow wetness characterizations are compared for various snow conditions.     

Historical Perspectives on Microwave Field Theory

Microwave Field Theory furnishes the foundation for all of the microwave circuitry that made possible the great advances achieved by the microwave field. It is important to understand that microwave field theory is only a portion of electromagnetic field theory in general, and to appreciate how it is distinguished from it. In addition, the network formulation of microwave field theory has been fundamental to the rapid progress made by the microwave community; the formulation in network terms has been so intertwined with the development and understanding of the field concepts and behavior that the network formulation should be viewed as an integral part of microwave field theory. The first part of this introduction explains the point of view indicated in the above paragraph. The second part of the introduction outlines the scope of the historical review presented here.     

Comparison Decrement Method for Microwave Resonator Q Measurements

The sensitivity of resonator quality-factor measurements using the decrement method can be improved by the comparison of the decay curve of a measured resonator with that of a reference standard resonator. The decay curves are compared using the integral of the function given as difference between decay curves. The comparison method, to be proposed, also has the advantage that the calibration of neither time base nor power scale is needed. Some results of the experimental verification are presented.     

A Microwave Method for Measuring Changes in Lung Water Content: Numerical Simulation

A numerical procedure to simulate several physiological and geometrical changes occurring during development of edema and to study their effects on the sensitivity of the microwave detection method is presented. The method of solution utilizes a model which consists of a two-dimensional cross section of a thorax. The model is based on an X-ray CAT scan taken with the microwave applicators in place. The electromagnetic boundary value problem is then solved numerically using the method of moments. Numerical results are presented to show the effects of several parameters, such as the uneven water distribution in the lung, the field distribution in the aperture of the microwave transmitter, and the location of the receiver on the sensitivity of the microwave detection method. Based on these results, several suggestions are made to help optimize the sensitivity of the microwave method of measuring changes in lung water content.     

A Shunt Technique for Microwave Measurements

A new method of measuring low-loss quantities at microwave frequencies, which employs a Iossy shunt structure, is described. The additional loss introduced thereby reduces the excessively high VSWR's to values lying in the measurable range. The relevant information is abstracted from the data in a precision manner independent of the parameters of the shunt structure itself. Applications are made to the measurement of low-loss dielectric constants, structures with shunt representations such as certain bolometer elements, attenuation constants, and VSWR's of slightly Iossy variable short circuits. Physical realizations of such shunt structures are discussed.     

楔形发射体的电子场发射理论

本文提出楔形发射体的电子场发射理论并给出计算实例；该理论以单原子发射点假设为基础，用经典电动力学方法计算楔形发射体的隧道势垒，然后用Ｎｕｍｅｒｏｖ方法计算其透射系数，最后用到场发射体伏安特性。     

Microwave measurements Part I: Linear Measurements

By convention, radio frequency (RF) and microwave frequencies range between 30 MHz and 300 GHz. Conversely, this means their wavelengths range between 10 m and 1 mm. Intense research in radar development during World War II extended the RF spectrum beyond the usual applications in radio communications. The use of shorter wavelengths resulted in laboratory equipment with proportionally smaller dimensions to generate, convey, transmit, and detect higher-frequency signals. Wavelengths shorter than 1 mm require equipment too small to be realized. Voltage, current, and impedance concepts lose their conventional meanings when the operating wavelength is approximately equal to the dimensions of the structures under test. The behavior of propagating electromagnetic waves must then be analyzed in terms of electric and magnetic field. Unfortunately, no simple and direct way exists to measure these quantities, so we must resort to indirect methods. In the following sections, we briefly review the concept and need for measurements of scattering parameters. We also describe the two key instruments for microwave measurements: VNAs and spectrum analyzers.     

A Simple Relation between Active And Passive Microwave Remote Sensing Measurements of Earth Terrain

A simple approximate relation between backscattering coefficient and emissivity measurements is derived. The relation is applicable to active and passive microwave remote sensing of earth terrain where volume scattering plays a dominant role. From the relation, one can obtain a rough estimate of the backscattering coefficient from the emissivity and vice versa. Such estimation is useful in checking experimental measurements and also helps to ascertain the validity of theoretical models. It also safeguards against obtaining arbitrarily high values for both backscattering coefficients and emissivities.     

电磁场理论与微波工程的本科教学探讨

“新工科”背景下,电子科学与技术专业的重要性越来越高。其下属学科电磁场与微波技术属于传统的学科,但随着无线通信、5G、物联网的发展,其学科又有“新”的特点。作为其学科的两大必修课,“电磁场理论”及“微波工程”在本科教学中素以“难”著称。本文首先对电磁场理论与微波工程方面的教学改革论文进行研究提出了要解决的问题,紧接着分别对电磁场理论的教学内容顺序的安排、教学内容的范围进行了讨论;同时,针对教学关键问题：1、电磁场基础数学知识教学的必要性,2、微波工程与电磁场理论的结合必要性,3、教学过程中仿真分析教学的必要性进行了详细探讨。     

Outdoor Measurements of Leaf Water Content Using THz Quasi Time-Domain Spectroscopy

We report the first outdoor measurements for continuous in vivo leaf-water monitoring using THz spectroscopy. For this, we have developed a compact and portable THz quasi time-domain spectrometer which can be powered by a battery. We monitor the water status of a corn plant (Zea mays) and discuss the influence of the day-night variations of the outdoor temperature.     

场发射微型三极管研究

我们已成功地制造并测试了模拟集成场发射微型三极管结构。这种微型三极管采用了Spindt型钼(Mo)场发射阴极和三种不同结构的阳极。阳极到栅极和发射极锥尖的距离d分别是>5mm、≈1.25mm和≈2～20μm。本文讨论了器件工作状态对紧靠阳极的钼场发射锥尖工作特性造成的影响。测得的跨导值为0.5mS(5μS/锥尖或12S/cm~2)、截止频率是0.8MHz。照此结果推算,有望得到250mS/mm的跨导和100GHz的截止频率。     

GR-1089-CORE的传导骚扰和辐射骚扰测试

介绍了GR-1089-CORE Issue4中包含的电磁兼容部分的传导骚扰和辐射骚扰测试要求,阐述了电磁兼容测试设备、场地的要求及被测设备的工作状态和测试布置。     

A Comparison of Hyperspectral Chlorophyll Indices for Wheat Crop Chlorophyll Content Estimation Using Laboratory Reflectance Measurements

The objective of this paper is to investigate the relationship between a wide range of hyperspectral chlorophyll indices and wheat crop chlorophyll content using laboratory measurements. These measurements included the GER-3700 spectroradiometric data, leaf chlorophyll content using the Soil-Plant Analyses Development (SPAD)-502 meter and leaf chlorophyll content estimated from chemical laboratory analysis. The SPAD-502 readings were correlated with leaf chlorophyll content extracted in the laboratory to establish calibration equations for the computation of chlorophyll-ab (Chl-ab) and chlorophyll-a (Chl-a) content. This resulted in a coefficient of determination (R2) of 0.72 for the Chl-ab content and 0.69 for the Chl-a content and a root mean square error (RMSE) of 3.53 and 1.94 mug/cm², respectively. These estimates were used to establish relationships against hyperspectral chlorophyll indices calculated from the GER-3700 data. From the investigated indices, the NPCI showed the best results with R2 of 0.84 and RMSE of 11.0. The other indices, such as GNDVI, OSAVI, PSSR_a, PSND_a, CAI, HNDVI, and MTCI did not perform satisfactorily. The better ones, but still showing a relatively week relationship with leaf chlorophyll content, are the indices NDPI, SIPI, PRI and SRPI with R^2's of 0.56, 0.62, 0.54, and 0.57, respectively and RMSEs of 11.06, 10.27, 11.32, and 10.96 mug/cm², respectively.     

Exposure of Man in the Near-Field of a Resonant Dipole: Comparison Between Theory and Measurements

The rate of the radio frequency energy deposition in a block model of the human body exposed in the near-field of a resonant dipole at 350 MHz was calculated using the moment method. Detailed maps of the electric field strength in a homogeneous model of a realistic shape under the same exposure conditions were obtained using a computer-controlled scanning system and an implantable electric field probe. A comparison of the measurement data with the calculations shows a relatively good agreement when average values over relatively large volumes are concerned; however, the calculations do not show large spatial gradients and tend to underestimate the magnitude of "hot spots" observed experimentally.     

可印制碳纳米管冷阴极中碳纳米管含量的优化

采用丝网印刷法制备了不同碳纳米管含量的冷阴极.研究了不同碳纳米管含量的冷阴极的场发射特性与其微结构和电学特性的关系.实验发现,碳纳米管冷阴极的导电性和场发射特性受碳纳米管含量影响,具有适当的碳纳米管含量的冷阴极的场发射特性最佳.     

微波测湿原理及其应用

论述了微波测湿原理以及当前微波技术在物质含水量测量中的应用研究现状。希望微波在测湿领域中的应用得到关注。