Accurate determination of gain and radiation patterns by radar cross-section measurements

Using a two-port network and geometrical interpretation of equations involved in antenna scattering, it can be derived that antenna characteristics may be determined in properly designed scattering measurements. As an alternative to this approach it is shown that measurement procedures for gain and radiation pattern can be developed from simple considerations of the receiving, transmitting, and scattering properties of antennas. The main advantages of the technique are that no gain standard is required and a disturbing feedline to the antenna can be avoided. In addition to this the technique seems to be highly accurate. These general conclusions are well corroborated by experimental data on a standard gain horn. Sources of errors are outlined and compared with sources of errors in conventional techniques.     

3D autocorrelation coefficients of dipole antenna

An analytical expression is given for the 3D autocorrelation coefficient of a half-wavelength dipole antenna as a function of spatial separation in an isotropic scattering environment. Results are verified experimentally through measurements conducted in antenna anechoic chambers. The results closely match previously published work based upon measurements collected in a typical indoor mobile communication environment. It is recommended that the 3D autocorrelation coefficient model be used to gauge diversity antenna measurement systems.     

Stimulated Brillouin scattering as a parametric interaction

In the stimulated Brillouin scattering of an intense laser beam, a forward-traveling coherent acoustic wave and a backward-traveling electromagnetic wave are produced. This interaction is studied from the coupled mode approach and shown to be describable as a parametric interaction. An inherent instability of this particular interaction is discussed. Frequencies of the interacting waves are calculated for quartz and cadmium sulfide. The non-linearity is shown to be due to the electrostrictive and photoelastic properties of the medium. By expressing the coupling coefficient as a tensor of rank four, selection rules for the interaction in isotropic and certain anisotropic media are derived. Finally the experimental approach for detection of the acoustic wave is discussed.     

平坦地条件下移动通信基站天线远场分析

介绍了一种平坦地条件下基站天线远区场分析的理论方法。文章利用射线跟踪法,通过建立适当坐标系进行三维建模,结合电磁散射矩阵关于散射场入射场关系进行理论分析。文中采用最常见的对称天线模拟基站天线,对某900 MHz GSM落地式基站进行电磁环境仿真,对仿真结果进行分析并与实测数据比较,得到结论:在距离基站30 m范围之内,要同等考虑直射信号与地面反射信号对场观察点的场强贡献,在距离＞30 m时,只需考虑天线直射信号对场观察点的场强贡献。     

基于最小二乘法的平面波综合

将最小二乘法引入近场天线测量中,提出了一种新的平面波综合方法。以半波对称振子组成的矩形平面阵作为平面波综合的理论模型,用这一新方法寻求阵列单元的最佳幅相分布,以使所综合出的准平面波质量最好。计算结果表明在待测区域大小一定的条件下,利用本文方法可以实现较传统方法更小的探头阵列要求。良好的仿真结果证实了该方法在天线测量及散射测量应用中的有效性。     

Plane wave spectrum scattering analysis of near-field obstacle effects on directive antenna patterns

A plane wave spectrum scattering analysis of the effects of a near-zone obstacle on the pattern performance of a directive antenna is discussed. The free-space azimuth monopulse antenna is characterized by its sum and difference mode plane wave spectra, and a computed plane wave scattering dyad is used to characterize the scattering by a metallic right circular cylinder when it is excited by the incident antenna spectra. An efficient computer algorithm has been developed to compute the far-zone scattered and total fields for the antenna/cylinder system. Extensive experimental data have been obtained, and the patterns calculated using the present analysis are in agreement with the measured data.     

Radar reflectivity of the ocean surface for circular polarization

If one transmits an electromagnetic wave, an electric field is set up in space. The earth's surface modifies the field and an interference pattern results. To calculate such a pattern one must know the reflection characteristics of the surface for the polarization of the incident wave. These are known for linear polarizations. Since a circularly polarized wave is a combination of the two linear polarizations, we can compute the reflection coefficient for circular polarizations by proper combination of the two linear reflection coefficients. This reflection coefficient is known for smooth sea conditions. In this paper the vector model of reflectivity for a rough sea is extended to include circular polarization. Reflection coefficients for the rough sea case are derived. Analytical expressions are given for the total received signal including the coherent and incoherent components. Receiver antenna polarizations of the same and opposite senses to that of the transmitter are considered.     

On the far-field approximation for scattering from randomly rough surfaces

Various authors have justified the far-fields approximation for rough surface scattering using one of the classical approximations for the scattered fields, usually considering either the coherent scattered field or the incoherent scattered intensity. An exact expression for the field scattered from a perfectly conducting rough surface is considered. The expression for the incoherent scattered intensity is formally derived, and a condition under which the far-field approximation is valid is found, independent of specific approximations for the surface or scattered fields or for the surface height statistics. The condition so derived is, under many circumstances, substantially less restrictive than that derived before in the general case. Furthermore, the previous results may be easily recovered by further specialization of our result.     

Application of plane waves for accurate measurement of microwavescattering from geophysical surfaces

The authors utilized the concept of a compact antenna range to obtain plane-wave illumination to accurately measure scattering properties of simulated sea ice. They also made simultaneous measurements using conventional antennas. Measured scattering coefficients obtained with the plane-wave system at 10 GHz decreased by about 35 dB when the incidence angle increased from 0° to 10°. Scattering coefficients derived from data collected with the radar system at 13.5 GHz using conventional far-field antennas decreased by about 20 dB over the same angular region. This demonstrates that the far-field properties of a widebeam antenna are inadequate for measuring the angular scattering response of smooth surfaces. They believe that application of the compact antenna range concept for scattering measurements has a wide range of applications and is the solution to the long-standing problem of how to directly measure scattering consisting of coherent and incoherent components     

Network modeling of an aperture coupling between microstrip lineand patch antenna for active array applications

An analytical method based on the aperture coupling theory and the derivation of the S-parameter matrix has been developed for modeling a microstrip line coupled to a microstrip patch antenna using a circular coupling aperture. Closed-form solutions were derived for scattering parameters of the coupling circuit. Input impedance and matching condition can be calculated from the equivalent six-port network. The theoretical results agree well with the measurements. The analysis should have many applications in active array and spatial power combining systems     

An enhanced millimeter-wave foliage propagation model

In this paper, the behavior of wave propagation through coniferous forest stands at millimeter-wave frequencies is characterized both theoretically and experimentally. A coherent wave propagation model is used to simulate the propagation through foliage. The coherent model is composed of two components: a forest stand generator that makes use of a stochastic fractal model, and an electromagnetic model that makes use of Foldy's approximation and single scattering. An outdoor measurement system is designed and used for characterizing the channel behavior for a pine tree stand at Ka-band (35 GHz). In this experiment, 84 independent spatial samples of transmitted signal through the pine stand were collected to obtain the path-loss statistics. The comparison between measurement and simulation results showed that single scattering theory overestimates the wave attenuation through foliage. To improve the accuracy of the coherent model, partial multiple scattering occurred among the needles of highly dense leaf clusters must be included for the estimation of the coherent attenuation. Distorted Born approximation is used to macromodel the scattering pattern from needle clusters. This technique has comparable accuracy and requires much less computational resources than a full-wave solution, such as method of moment. By including multiple scattering effects of needle clusters in the simulation model, much better agreement is obtained for both mean and standard deviation of the path-loss.     

生物组织光学特性参量的图像测量研究

为了测量生物组织的光学特性参量,采用CCD漫反射法和透射法,通过分析CCD摄取的待测样品表面的漫反射光分布图像,利用漫射近似理论,实现了漫反射法对生物组织模拟液(intralipid-20%稀释液)、牛肌肉、猪肌肉和鸡胸肉光学特性参量的测量,获得了样品的吸收系数和有效散射系数;测量了不同浓度生物组织模拟液的透射光强度,根据Beer-Lambert定律,实现了透射法对光学特性参量的测量,获得了样品的散射系数,进而得到了有效散射系数;并将漫反射法和透射法对同种样品光学特性参量的测量结果以及他人的测量结果进行了比较。结果表明,漫反射法和透射法的测量结果有很好的吻合性,测量生物组织光学特性参量采用的CCD测量装置和处理方法具有较高的精度。     

Introducing Space into MIMO Capacity Calculations

The large spectral efficiencies promised for multiple-input multiple-output (MIMO) wireless fading channels are derived under certain conditions which do not fully take into account the spatial aspects of the channel. Spatial correlation, due to limited angular spread or insufficient antenna spacing, significantly reduces the performance of MIMO systems. In this paper we explore the effects of spatially selective channels on the capacity of MIMO systems via a new capacity expression which is more general and realistic than previous expressions. By including spatial information we derive a closed-form expression for ergodic capacity which uses the physics of signal propagation combined with the statistics of the scattering environment. This expression gives the capacity of a MIMO system in terms of antenna placement and scattering environment and leads to valuable insights into the factors determining capacity for a wide range of scattering models.     

Computer-aided analysis of a finite arbitrarily shaped dielectric antenna

A numerical method of analysis is developed to determine the characteristics of a homogeneous dielectric antenna of arbitrary shape based on a general expression of electromagnetic wave scattering by a homogeneous dielectric body derived by Barrar and Dolph. This method is tested on a dielectric-disk antenna.     

Isotropic transmission-line antenna and its toroid-pattern modification

The far-field strength of thelambda/4transmission-line antenna is derived as a complex vector containing amplitude, phase, and direction of theEvector. The product of this phasor-vector and its conjugate is constant and proves that the transmission-line antenna is an isotropic radiator with respect to the powerflow density of coherent electromagnetic waves. Folding of the line conductor parallel to the symmetry plane reduces the pattern to toroid shape analogous to radiation of a short dipole. While the unbalanced straight transmission-line antenna is used on rockets and vehicles, the unbalanced folded type is suitable as efficient transmit antenna at VLF, LF, or MF.     

Probe compensated near-field measurements on a cylinder

A new method is developed for determining the farfield pattern of an antenna from probe compensated near-field measurements over the surface of a right circular cylinder enclosing the antenna. The method is derived by first expanding both the field radiated by the antenna and the field radiated by the measurement probe, when it is used as a transmitter, into cylindrical wave expansions. The Lorentz reciprocity theorem is then used to solve for the field radiated by the antenna from the probe output voltage. It is shown rigorously that the antenna pattern can be determined independently of the characteristics of the measurement probe provided that certain calibration data are known. A method for determining these data from the measured far field radiated by the probe is described. It is shown that the necessary numerical integration can be performed with the fast Fourier transform algorithm. Experimental results are presented to validate the theory and to demonstrate its practicality from a measurement and computational viewpoint.     

Exact derivation of aperture errors in antenna measurements

Antenna-pattern measurements are generally undertaken by placing the antenna under test (usually the receiving antenna) sufficiently far from the transmitting antenna that the receiving antenna is illuminated with a plane wave. Gain measurements, whether by substitution, paired horns, or integration of the area under the beam pattern, depend upon the same criterion. A true plane wave is only achieved by separating the two antennas by an infinite distance, but it has been shown by Milne (1950) that small departures from a plane wave do not usually seriously degrade the measurement. The authors consider three aspects of the fields across a linear aperture. The finite spacing between the antennas produces a phase deviation and an amplitude taper. If we are only interested in the apparent gain loss, we need to combine both tapers into an illumination efficiency. It is surprising how little the illumination loss is when the phase deviation and amplitude taper are large. The formulation is given in terms of linear apertures, and can be easily extended to circular or rectangular apertures by adjusting the limits of the integrals     

Threshold condition of DFB semiconductor lasers by thelocal-normal-mode transfer-matrix method: correspondence to thecoupled-wave method

An analytical expression for the threshold condition of DFB semiconductor lasers is derived based on the local-normal-mode transfer-matrix method (TMM). The threshold condition is expressed in terms of an equivalent reflection coefficient of a DFB laser structure and a Bloch-wave propagation constant. It is shown that the TMM threshold condition can reproduce the one which is derived by the coupled-wave method (CWM) under certain conditions. It is also shown that the CWM can predict very accurately the spatially averaged values of the threshold modal gain and the detuning frequency which are obtained by the TMM if the coupling coefficient in the CWM is replaced by the TMM coupling coefficient     

An inductive antenna mismatch recoverable RF power amplifier

An inductive antenna mismatch recovery method is presented. The proposed method uses series LC matching network to maintain a power amplifier (PA) performance under an inductive antenna mismatch condition. This method requires only the voltage amplitude information, which reduces the complexity in implementation. This paper also presents design considerations of integrated recovery system in a complementary metal-oxide semiconductor (CMOS) process. Simulation results show that the proposed CMOS PA maintains its output power as well as efficiency very close to the well-matched 50 Ω condition with inductively mismatched antenna impedance up to +j·70 which is corresponding to the voltage standing wave ratio of 2.5.