均匀取向箔条云的RCS极值研究

箔条云的雷达回波为部分极化波,其散射波的极化度和RCS极值问题对于极化滤波、极化对比增强的效果具有重要的决定作用.首先用Mueller矩阵采表征了箔条云的变极化效应,基于Mueller矩阵对均匀取向箔条云散射波的极化程度、共极化和交叉极化的RCS极值进行了研究.研究发现,散射波的总能量不随发射极化改变,当发射波为线极化时,散射波的极化度最大,共极化RCS最大,交叉极化RCS最小,当发射波为圆极化时,散射波为完全未极化波,此时用任意的极化接收,平均功率是相同的.     

Analytical models for cross-polarization on earth-space radio paths for frequency range 9–30 GHz

Depolarization and attenuation of radiowaves along earth-space paths due to rain storms are characterised. Frequency-dependent expressions for specific attenuation and phase shift at (PC and 20°C and for Laws-Parsons raindrop size distribution are given. Using small amplitude and phase approximations, a simple relation for cross-polar discrimination due to rain in terms of co-polar attenuation, frequency, angle of elevation and polarization angle is derived. Expressions for depolarization due to ice crystals are given, treating them as Rayleigh scatterers of spheroidal shape. For both rain and ice the relationship between linear and circular crosspolar ratio can be shown to be simply sin 2 θ, where θ is the polarization angle.     

Pattern analysis of paraboloids fed by rectangular waveguides employing multipole expansion techniques

A multipole expansion technqiue is developed for calculating the co-polar and cross-polar radiation patterns of a prime-focus paraboloid fed by a dominant-mode rectangular waveguide feed. Initially, the feed radiation pattern is expressed in terms of electric and magnetic multipoles of different orders which are subsequently used to obtain the secondary radiation patterns (co-polar and cross-polar) in explicit closed form in the form of a rapid by a converging algebraic series. The validity of the analysis is effectively demonstrated by calculating the co-polar and cross-polar patterns of a typical paraboloid and comparing and results with calculations based on the direct numerical integration of the physical optics currents induced on the reflector surface.     

Cross polarization from dual mode horn antennas

The cross polarization from dual mode horn antennas supporting the TE11and TM11-modes has been analyzed both theoretically and experimentally. The cross polarization has been shown to be strongly dependent on the length and flare angle of the horn. When the flare angle for a given horn length becomes larger than a certain threshold angle, the cross-polar sidelobes increase rapidly. Simple formulas for this threshold angle are presented. This cross-polar degradation will not occur if a plane phase front is provided over the horn aperture, either by profiling the horn or by applying a lens in the aperture.     

A model for calculating the depolarization of microwave signals propagating through rain

A model for calculating the amount of depolarization for linearly polarized electromagnetic waves propagating through rain is described. The presented method provides mathematical expressions for the co-polar and the cross-polar components of the received signal emerging from a rain cell. Calculations are carried out in two ways: with a general distribution of canting angles as measured by Saunders and with an equivalent canting angle together with the associated percentages of the positively and the negatively canted raindrops in a rain cell. The predicted attenuation and the cross polarization as calculated by both methods are found to be in good agreement with the published data measured by various workers.     

Physical optics analysis of beam waveguides using auxiliary planes

A physical optics (PO) method is presented which in two steps calculates the scattered field of a reflector or a lens in a beam waveguide (BWG). The first step involves a calculation of a set of currents on an auxiliary plane and the second step a field calculation from this current set. The method is named A-PO that refers to the calculation of PO currents on an Auxiliary plane. A-PO is less accurate than PO for sidelobe analysis, but useful for first-pass analysis of the main beam region in a BWG where it is significantly faster than standard PO. Two examples show that the co-polar as well as cross-polar radiation in a submillimeter BWG can be efficiently determined 40 dB below peak.     

Scattering from ice crystals at 94 and 220 GHz millimeter wavefrequencies

Polarimetric scattering from cloud ice crystals modeled as hexagonal columns, hexagonal plates and stellar crystals are calculated at 94 and 220 GHz frequencies using the finite difference time domain (FDTD) method. Two orientation models are considered, In the first model the long axes of columns and broad surfaces of plates and stellar crystals are parallel to the horizontal plane and randomly oriented on this plane. The second model assumes uniform random orientation in three dimensions (3-D). The ratio of the backscattering cross sections at 220 and 94 GHz exhibits a steady decrease with increasing size up to about 1000 μm (2000 μm for columns) for horizontally aligned ice crystals at side incidence as well as for 3-D random orientation. This dual frequency ratio (DFR) may be useful in gauging the size of ice crystals, DFR may also be useful in discriminating between ice crystal types since it shows major differences between columns and planar crystals. The linear depolarization ratio (LDR), i.e. the ratio of the cross-polar to co-polar backscattering cross sections, at vertical incidence for the horizontal alignment model is much higher for columns (over 20 dB) compared to plates and stellar crystals. As a result LDR also has the potential for discriminating columns from planar crystals, Furthermore, the normalized Mueller matrices for horizontally aligned columns at vertical incidence are shown to be diagonal and a function of only LDR. This same result applies to the 3-D random orientation model for all three crystal types     

Broadband dual-polarised antenna with high isolation for wireless communication

A broadband dual-polarised antenna with four end-to-end folded dipoles and a U-shaped ground is presented. Stable and symmetric radiation patterns at slanted ±45° polarisation have been obtained within the band 1.71?2.17 GHz. Measurements show two linear polarisations with an isolation of more than 30 dB over the operating frequency band. A return loss less than 214 dB is achieved and the measured cross-polar level is 23.5 dB below the co-polar level within the above-mentioned band.     

Polarimetric Ka-band backscattering measurements at grazing-angle extremes

Polarimetric millimeter-wave backscattering data has been measured in true operational environments for perpendicular illumination and for grazing angles below one degree. Manmade objects, such as vehicles and light poles, were found to give a 5-10 dB lower response for vertical polarization, and the cross-polarized returns were about 10-20 dB below the reference level obtained with horizontal polarization. Forest and soil backscattering showed similar characteristics of -7 to -9 dB (vertical) and -14 to -17 dB (cross-polar). An anomaly was observed for horizontally transmitted/vertically received data, where gravel soil showed only about 2 dB attenuation compared to the co-polar reference level. The dynamic variations of mobile target signals at different polarizations indicated that millimeter-wave cross-polarized returns call for high transmitter power if true polarimetric target classification is attempted. Close-to-ground radars seem to benefit from multipath and thus from using horizontal polarization if only maximum range is defined. Variations in the cross-polarized echo amplitudes of moving manmade objects were found to be within 3-5 dB of their typical mean values.     

VHF频段隐身目标缩比模型的雷达散射截面测量

介绍了甚高频(VHF)频段、工作在水平(H)或垂直(V)极化发射-接收组合(即HH、VV、HV、VH)情况下隐身目标缩比模型的雷达散射截面积(RCS)测量方法;给出了低频段RCS测量与计算的详细过程,采用背景杂波对消和时域加窗处理的方法减少了低频段RCS测量的误差;并给出了两种隐身目标缩比模型的RCS测量结果。测量结果表明:由于谐振效应,在VHF下端的低频段,隐身目标的RCS在平方米的量级,远大于在微波段的测量值;在部分频点,交叉极化的RCS甚至比同极化还强。这为利用隐身目标在频率域的谐振效应和极化域的极化特征,设计具有探测隐身目标能力的现代雷达提供了理论依据。     

全极化合成孔径雷达多视图像的极化特征分析

该文基于乘性相干斑模型,对全极化合成孔径雷达多视图像的极化特征参数(同极化比、交叉极化比和同极化相位差)进行了分析,利用实际的模拟数据对特征参数的PDF进行了拟合测试,分析了多视极化特征参数的统计特性对极化相干斑抑制算法的影响。利用极化比和相位差的特点,对地物进行分类,所得分类精度与利用最优极化分类法的分类精度具有可比性。     

Dual polarised microstrip patch antenna with high port isolation

A probe fed stacked microstrip patch antenna for operation in dual polarisation mode is proposed. A slot loaded lower patch is presented to improve isolation between the two polarisation ports. Simulations and measurements show two linear polarisations with an isolation of more than 30 dB within the band 2.5-2.7 GHz. A return loss of -10 dB is achieved within the above-mentioned bandwidth for the proposed antenna. The radiation pattern for the antenna is similar to that of a conventional microstrip antenna and the measured cross-polar level is 16.5 dB below the peak co-polar level within the band 2.5-2.7 GHz     

Compound Fresnel Zone Lens Conformal to Truncated Cone: Antenna Application

A millimeter wave antenna consisting of two Fresnel zone plate lenses, plane and conical, is examined numerically by use of the vector diffraction theory. The lenses are of Wood-Wiltse (double-dielectric) or Soret (half-open) type, and are designed for the frequency of 117 GHz. The lenses are made conformal to a truncated circular cone with a base diameter of 500 mm and a plateau diameter of 250 mm. Designs for two opening semi-angles, 45° and 75° each of them with a particular lens thickness are presented. For the angle of 90° the cone lens becomes a plane ring lens, which in combination with the plateau zone lens forms a plane lens of size equal to the cone base diameter. Illuminated by directive feeds set at a focal distance of 525 mm from the cone apex, the double-dielectric and half-open compound and plane lenses, form three pairs of Fresnel zone lens antennas, the co-polar and cross-polar radiation characteristics of which have been compared numerically. The double-dielectric lens antennas examined are about 5 dB superior in gain to the half-open lens antennas, which has a gain of approximately 45 dBi. Because all lenses are of equal transverse aperture, the corresponding lens antennas exhibit the same ?3 dB beamwidth of about 0.33 degrees. The plane zone lens antenna is very thin and simple. Instead, the antenna comprising a 3-D compound Fresnel zone lens is thicker but can be made conformal to a specific surface shape and possesses more levels of design and optimization freedom.     

光折变晶体中非相干光前向耦合及其对噪音栅擦除的研究

对光折变晶体 Ce:KNSBN中两束 e偏振简并非相干光的前向耦合进行了理论分析和实验研究 ,并通过非相干光耦合作用在实验上获得了亮环和暗环的衍射图样 ,从而获得了一种利用非相干光擦除来克服光折变晶体中扇形效应的简便方法。     

Sidelobe level reduction by improvement of strut shape

From the viewpoint of effective utilization of both the geostationary orbit and the frequency spectrum in the satellite communication system, it is strongly required of an earth station antenna to have low sidelobe characteristics in order to minimize the interference from/to the adjacent satellite and the terrestrial microwave links. The reference diagram of the sidelobe gain for a large antenna recommnended by International Radio Consultative Committee (CCIR) is drawn as follows:G = 32-25 log thetadBi (1 deg leq theta leq 48deg);G = -10dBi (theta > 48deg), whereGis the sidelobe gain relative to an isotropic antenna andthetais an offset angle from the main lobe axis in degrees. In the case of an axisymmetrical Cassegrain antenna operating with circular polarization, the level of the cross-polar sidelobe due to diffraction by the subreflector support struts exceeds that shown in the above reference diagram. By introduction of a new type of strut, the sidelobe gain can be suppressed below that shown in the CCIR reference diagram. In this article, a design method based on geometrical theory of diffraction (GTD) for improving the strut shape is presented. The wide-angle sidelobe gain of the standard "A" earth station antenna, installed in Yamaguchi, Japan, being equipped with the improved strut, can meet that shown in the CCIR reference diagram in all directions for both co- and cross-polar polarizations in the 6 GHz band.     

A comparison between different polarimetric measurement schemes

The usefulness of polarization information in remote sensing applications has been clearly demonstrated in the last two decades. Land covers and meteorological targets are complex and their backscatter returns are time varying. Their polarimetric characteristics should be determined in terms of second order statistics of scattering matrix elements. These are contained. in the polarimetric covariance matrix. Estimation of polarimetric covariance matrix elements has been addressed by different authors based on different scattering matrix measurement methods. The most widely used and implemented method consists in the alternate transmission of two orthogonal polarizations while. receiving with both. Recently, a method that considers simultaneous transmission of two orthogonal polarizations, which are phase coded with orthogonal signals, has been proposed. Another method, specifically developed to obtain the polarimetric covariance matrix, is based on transmitting three different polarizations and receiving their corresponding co-polar and cross-polar counterparts. Different data sets result from each measurement technique and different estimators of polarimetric covariance matrix elements have been developed for the different data sets available. These various methods are investigated in here. Based on simulated data the performance of the different estimators under different working conditions are analyzed. Effects on the estimators of operation frequency, pulse repetition frequency (PRF) and temporal properties of targets are discussed     

Attenuation and Cross-Polarization Measurements at 11 GHz

Measurements of linear cross polarization and attenuation on a terrestrial link in the U.K. are reported and correlated with rainfall information. The cross polarization measured during rainfall is compared with theoretically predicted values and monthly cross-polar statistics are presented for a 16-month period.     

The influence of aberrations and aperture inclinations on the phase and intensity structure in the image region of a lens

Microwave-optical image formation by a lens focusing system is studied in the presence of aberrations. The theoretical approach is based on diffraction and aberration theories and a modified diffraction integral is presented, showing that previously unreported effects describable as additional primary order aberrations exist due to aperture inclination. Direct relationships are shown to exist between the geometry of Airy dark rings and the type and amount of aberration present, thus providing a means for determining the aberrations present in a physical system. The practical part of the study compares numerical evaluations of the modified diffraction integral with experimental measurements. Results presented are for the case of a solid dielectric microwave lens operated at 3.2-cm wavelength. Detailed maps of phase and intensity contours are given, showing the three-dimensional structure of the image region.     

Impedance and Polarization-Ratio Transformations by a Graphical Method Using the Isometric Circles

The isometric circles for the direct and inverse linear fractional transformations can be used for transformations of impedances and polarization ratios. In the Ioxodromic case an inversion is performed in the isometric circle of the direct transformation, followed by a reflection in the symmetry line of the two circles, and a rotation around the origin of the isometric circle of the inverse transformation. In the nonloxodromic case only the first two operations have to be applied. Three illustrative examples are given: the first shows the transformation of the right half of the complex impedance plane into the unit circle (Smith Chart); the second gives a circular proof of the Weissfloch transformer theorem; the third shows an example of cascading, Iossless, two terminal-pair networks.