Antenna Effective Aperture Measurement With Backscattering Modulation

A scattering measurement method for antenna characterization is described. The antenna backscattering is modulated by an oscillator circuit. The modulation begins, when a known RF power is transferred to the oscillator circuit from the antenna. This enables the measurement of the effective aperture of the antenna, from which the antenna bandwidth and radiation pattern are obtained. A theory for antenna aperture measurement is developed using a simple circuit model for the antenna-oscillator system. A dipole and a PIFA with a reactive input impedance at the application frequency were measured. The antenna aperture was measured to an accuracy of 9%, and the measurements complied with simulated and measured references. The method provides simple and accurate bandwidth and radiation pattern measurements with the reactive load the antenna is designed to work with.     

Passive Microwave Remote Sensing of Soil Moisture: The Effect of Tilled Row Structure

The tilled row structure is known to be one of the important factors affecting the observations of the microwave emission from a natural surface. Measurements of this effect were carried out with both L-and X-band radiometers mounted on a mobile truck on a bare 40 m × 45 m row tilled field. The soil moisture content during the measurements ranged from ~10 to ~30 percent by dry weight. The results of these measurements showed that the variations of the antenna temperatures with incident angle ? changed with the azimuthal angle ? measured from the row direction. In particular, at ? = 0° and ? ? 45°, the observed horizontally and vertically polarized antenna temperatures, TBH(?, ?) and TBV(?, ?), were not equal. In general, TBH(?°, ?) > TBV(?°, ?) when 0° ? ? < 45° and TBH(?°, ?) < TBV(0°, ?) when 45° < ? ? 90°. The difference between TBH(0°, ?) and TBV(0°, ?) was observed to decrease with ? approaching 45° and/or with soil moisture content. A numerical calculation based on a composite surface roughness-a small-scale RMS height variations superimposed on a large periodic row structure-was made and found to predict the observed features within the model's limit of accuracy. It was concluded that the difference between TBV(0°, ?) and TBH(0°, ?) was due to the change in the local angle of field emission within the antenna field of view caused by the large-scale row structure.     

Quantitative retrieval of soil moisture content and surface roughness from multipolarized radar observations of bare soil surfaces

A semiempirical polarimetric backscattering model for bare soil surfaces is inverted directly to retrieve both the volumetric soil moisture content M/sub v/ and the rms surface height s from multipolarized radar observations. The rms surface height s and the moisture content M/sub v/ can be read from inversion diagrams using the measurements of the cross-polarized backscattering coefficient /spl sigma//sub vh//sup 0/ and the copolarized ratio p(=/spl sigma//sub hh//sup 0///spl sigma//sub vv//sup 0/). Otherwise, the surface parameters can be estimated simply by solving two equations (/spl sigma//sub vh//sup 0/ and p) in two unknowns (M/sub v/ and s). The inversion technique has been applied to the polarimetric backscattering coefficients measured by ground-based polarimetric scatterometers and the Jet Propulsion Laboratory airborne synthetic aperture radar. A good agreement was observed between the values of surface parameters (the rms height s, roughness parameter ks, and the volumetric soil moisture content M/sub v/) estimated by the inversion technique and those measured in situ.     

Snowcover Influence on Backscattering from Terrain

The effects of snowcover on the microwave backscattering from terrain in the 8-35 GHz region are examined through the analysis of experimental data and by application of a semiempirical model. The model accounts for surface backscattering contributions by the snow-air and snow-soil interfaces, and for volume backscattering contributions by the snow layer. Through comparisons of backscattering data for different terrain surfaces measured both with and without snowcover, the masking effects of snow are evaluated as a function of snow water equivalent and liquid water content. The results indicate that with dry snowcover it is not possible to discriminate between different types of ground surface (concrete, asphalt, grass, and bare ground) if the snow water equivalent is greater than about 20 cm (or a depth greater than 60 cm for a snow density of 0.3 g · cm-3). For the same density, however, if the snow is wet, a depth of 10 cm is sufficient to mask the underlying surface.     

Non-Lambertian Effects on Remote Sensing of Surface Reflectance and Vegetation Index

This paper discusses the effects of non-Lambertian reflection from a homogeneous surface on remote sensing of the surface reflectance and vegetation index from a satellite. Remote measurement of the surface characteristics is perturbed by atmospheric scattering of sun light. This scattering tends to smooth the angular dependence of non-Lambertian surface reflectances, an effect that is not present in the case of Lambertian surfaces. This effect is calculated to test the validity of a Lambertian assumption used in remote sensing. For the three types of vegetations considered in this study, the assumption of Lambertian surface can be used satisfactorily in the derivation of surface reflectance from remotely measured radiance for a view angle outside the backscattering region. Within the backscattering region, however, the use of the assumption can result in a considerable error in the derived surface reflectance. Accuracy also deteriorates with increasing solar zenith angle. The angular distribution of the surface reflectance derived from remote measurements is smoother than that at the surface. The effect of surface non-Lambertianity on remote sensing of vegetation index is very weak. Since the effect is similar in the visible and near infrared part of the solar spectrum for the vegetations treated in this study, it is canceled in deriving the vegetation index. The effect of the diffuse skylight on surface reflectance measurements at ground level is also discussed.     

Miniaturised reconfigurable window slot antenna using mechanical tuning

A novel design approach of varying the slot width on radiating patch of a microstrip antenna for achieving reconfigurable dual-band operation is proposed. The design of this miniaturised reconfigurable antenna is done without the use of active switches or MEMS. The dual band is obtained by rotating the strips on the patch creating slots and varying the slot width using mechanical tuning. The strip is rotated in angular position from 0° to 90° varying the slot width that alters the resonant frequency of the antenna making it resonate at different frequencies covering the wireless bands for global system for mobile communication, global positioning system, wireless local area network and Bluetooth. This antenna provides a dual-band operation that shifts over the bands depending upon the slot width. This antenna achieves the bandwidth requirement and gain in the bands of operation. The effect of change in slot width on the radiation pattern is also discussed. The prototype antenna is fabricated and tested to prove the proposed concept of design. The measured results are in agreement with the simulated results.     

Coherent scattering of a spherical wave from an irregular surface

The scattering of a spherical wave from a rough surface using the Kirchhoff approximation is considered. An expression representing the measured coherent scattering coefficient is derived. It is shown that the sphericity of the wavefront and the antenna pattern can become an important factor in the interpretation of ground-based measurements. The condition under which the coherent scattering-coefficient expression reduces to that corresponding to a plane wave incidence is given. The condition under which the result reduces to the standard image solution is also derived. In general, the consideration of antenna pattern and sphericity is unimportant unless the surface-height standard deviation is small, i.e., unless the coherent scattering component is significant. An application of the derived coherent backscattering coefficient together with the existing incoherent scattering coefficient to interpret measurements from concrete and asphalt surfaces is shown.     

Petaloid antenna for passive UWB-RFID tags

An ultra-wideband (UWB) antenna designed with a petal-shaped structure is presented. Its measured impedance bandwidth of 10 dB return loss is from 6.6 to 17.5 GHz whereas the size is just 18times16 mm. The radar cross-section and scattered waveforms of the proposed petaloid antenna are theoretically studied using the finite-difference time-domain (FDTD) method. Its backscattering of antenna mode is comparable to that of structural mode, and the scattering pattern is almost omnidirectional. This antenna is suitable for passive ultra-wideband radio-frequency identification (UWB-RFID) tag applications.     

Experimental analysis of metal coated dielectric waveguides

This paper concerns the experimental characteristics of metal coated dielectric waveguides with a rectangular surface corrugation. Waveguide are designed to operate at a second Bragg frequency of 90 GHz. The period, height and the duty cycle of a rectangular grating were calculated using the chosen frequency. A metallic layer of aluminum is sputtered on one side of the slab waveguide. The purpose of the metallic layer is to simulate a layer of high density plasma on the surface of the waveguide similar to that obtained by optical excitation of semiconductor structures. Experiments were performed to examine the far field radiation pattern, attenuation constant and the dispersion relation. Due to the presence of the plasma layer there will be an angular shift in the far field radiation pattern. We have observed angular shifts of about 20? in the radiation pattern of the waveguide before and after coating. Measurements are made in the frequency range of 88–95 GHz. This waveguide structure can be used to design an electronically steerable antenna and an electronic phase shifter operating in the millimeter-wave frequency band.     

Multilevel Surface Decomposition Algorithm for Rapid Evaluation of Transient Near-Field to Far-Field Transforms

A fast multilevel algorithm with reduced memory requirements for the evaluation of transient near-field to far-field transforms is presented. The computational scheme is based on a hierarchical decomposition of an arbitrary shaped enclosing surface over which the near-fields of an antenna or a scatterer are given. For surface subdomains at the highest decomposition level, the angular-temporal far-field patterns are calculated directly from the known near fields over a sparse angular grid of directions and a short temporal duration. The multilevel computation comprises angular and temporal interpolations thus increasing angular resolution and temporal duration of radiation patterns while aggregating the subdomain contributions between successive decomposition levels. These steps are repeated until obtaining the transient far-field response of the whole enclosing surface. The computational complexity of the proposed algorithm is substantially lower than that of the direct evaluation. Reduction in memory requirements is obtained by formulating the algorithm as a marching-on-in-time windowed scheme. This approach allows for embedding of the accelerated transforms within existing near-field modeling tools.      

95-GHz scattering by terrain at near-grazing incidence

This study, consisting of three complimentary topics, examines the millimeter-wave backscattering behavior of terrain at incidence angles extending between 70 and 90°, corresponding to grazing angles of 20° to 0°. The first topic addresses the character of the statistical variability of the radar backscattering cross section per unit area σ_A. Based on an evaluation of an extensive data set acquired at 95 GHz, it was determined that the Rayleigh fading model (which predicts that σ_A is exponentially distributed) provides an excellent fit to the measured data for various types of terrain covers, including bare surfaces, grasses, trees, dry snow, and wet snow. The second topic relates to the angular variability and dynamic range of the backscattering coefficient σ⁰, particularly near grazing incidence. We provide a summary of data reported to date for each of several types of terrain covers. The last topic focuses on bare surfaces. A semi-empirical model for σ⁰ is presented for vertical (VV), horizontal (HH), and cross (HV) polarizations. The model parameters include the incidence angle &thetas;, the surface relative dielectric constant ϵ, and the surface roughness ks, where k=2π/λ and s is the surface root mean square (RMS) height     

Application of ERS-1 wind scatterometer data to soil frost and soilmoisture monitoring in boreal forest zone

The feasibility of the ERS-1 Wind Scatterometer (WS) for monitoring the boreal forest zone is investigated, concentrating on soil frost and soil moisture monitoring. The ERS-1 WS measures the target area with coarse spatial resolution (about 50 km) using three separate antenna beams and a wide angular range. The investigations are concerned with the boreal forest zone using data (1) from test areas located in Finland and (2) covering the whole northern European boreal forest zone. The seasonal behavior of WS data is studied and a semiempirical forest backscattering model-based inversion method for the retrieval of soil moisture and for soil frost monitoring from WS data is developed. The developed inversion method employs nearly simultaneous three-beam measurements and a varying incidence angle. Promising results were obtained in the monitoring of soil frost, and the retrieval of soil moisture also appears to be a feasible field of application. The applicability of the instrument for forest biomass retrieval using single images was found to be limited to long-term change detection     

Resistive tapers that place nulls in the scattering patterns ofstrips

Tapering the resistivity on the surface of an object modifies the scattering patterns of that object. For instance, gradually tapering the resistivity on a strip, half-plane, or an antenna ground plane reduces the edge effects of that surface. Greater control over the sidelobe response of the scattering pattern of a strip is possible by relating the resistive taper to a low sidelobe taper via physical optics. The idea of relating antenna aperture tapers to scattering patterns is extended to placing nulls in the sidelobes of scattering patterns of strips. A resistive taper for placing nulls is found by solving the scattering integral equations of a resistive strip for the resistivity, then substituting the desired nulling current density taper into the appropriate equation and solving for the resistive taper. This method is capable of placing multiple nulls in the bistatic scattering pattern or a single null in the backscattering pattern     

Effect of Antenna Directivity on Angular Power Distribution at Mobile Terminal in Urban Macrocells: A Geometric Channel Modeling Approach

We present a geometric channel model to study the effect of antenna directivity on angular power distribution at the mobile terminal in urban macrocells. The methodology reviewed in this paper integrates the antenna effect into the model geometry, thereby facilitating a system-dependent channel characterization. As each device is limited in terms of measurement sensitivity, the effective scatterer distribution is essentially dependent on the antenna beam pattern. Subsequently, two heuristic rules are proposed to establish the underlying relationship between the model geometry and the corresponding wave-propagation processes. It is shown that the influence of directional antenna is twofold. First, it alters the spatial distribution of scatterers by providing a different sample space for the random field, and secondly, it distributes signal components into the angles-of-departure or collects them from the angles-of-arrival by weighted combination. Important channel parameters measured at the mobile terminal such as the angular power distribution, Doppler spectrum, and multipath shape factors are also investigated to further exemplify the usefulness of the proposed model.     

带限周期函数的Fourier插值法在天线测量中的应用

天线远场测量是获得其辐射特性的一种常用方法,为得到方向图的细节,测量方向图的角度间隔一般应小于待测天线半功率波束宽度的1/10,否则会导致方向图相关参数如半功率波束宽度、副瓣电平等的不准确. 针对高增益、窄波束天线的远场测量,为了得到精确测量结果,测量间隔要求非常小,导致测试时间长,测试效率低下. 文中提出了一种基于带限周期函数的Fourier插值法的高效率方向图测试方法,此方法将采样间隔增加到待测天线半功率波束宽度量级,仍然可以准确重构出角度间隔任意小的方向图,从而能够显著提高测试效率. 仿真和实测结果证明了本方法的可行性.     

Backscattering coefficient of a perturbed sinusoidal surface

A coherent model for scattering from a randomly perturbed periodic surface is developed using the Kirchhoff approximation. It is intended to model wave scattering from ground surfaces with row directions. Results are illustrated and compared with an incoherent scatter model and measured backscattering angular curves from ploughed fields.     

Measurement of soil backscattering with a 60-GHz scatterometer

Experiments designed to measure the backscattering coefficient of soil surfaces were performed indoors using a 60-GHz scatterometer. The roughness of the soil surface was measured with a laser profile meter to determine the roughness dependence of the scattering coefficient. The incidence angle dependence of the measured backscattering coefficient shows distinctive features according to the roughness of the soil surface. These results are compared with estimates from a small-perturbation theory     

利用后向散射回波相对相位计算角闪烁的条件

在测量点散射单元为球体的两点目标的后向散射时发现，它的后向角闪烁可以通过直接求后向散射回波相对相位测量数据关于入射角的导数而得到。为了揭示这一实验现象所隐含的仙在规律，本文提出了一个具有典型意义的各赂异性矢量Ｎ点目标模型用来进行相关的研究。理论结果表明，在一定的条件下，角闪烁的后向值与后向散射回波相对相位对入射角的导数之间存在的等效性。     

Near axial backscattering from finite cones

The near-axial backscattering from a finite cone is studied using the equivalent current concept based on the uniform geometrical theory of diffraction (UTD). The creeping waves associated with the conical surface are also incorporated into the equivalent current technique. The contributions from the creeping waves are significant for the oblique-incidence case. There is evidence to speculate that the poorer agreement between the previously calculated results and the measured data for the vertically polarized backscattering is probably a result of the omission of the creeping wave contribution     

地表后向散射测量实验及其等效介电常数和粗糙度参数的反演

由于地物类型繁多且分布不均匀,地表粗糙度的测量存在不准确性,且地表分层、体散射机理较难准确界定.针对上述问题,将裸土和水泥路面在入射角小于66°的散射模型等效为面散射模型,联合中国电波传播研究所X波段裸土和Ku波段水泥路面的后向散射实测数据,采用遗传算法同时反演了地表的等效介电常数和等效粗糙度参数,并采用剩余实测数据对反演结果进行了验证,吻合良好.该等效面散射模型在保证裸土和水泥路面后向散射预测精度的同时,降低了模型复杂度,仅需反演面散射模型的3个等效参数(等效均方根高度、等效相关长度和等效介电常数)即可实现对复杂地表后向散射特性的快速、准确预测,具有重要的工程应用价值.