On the wide-band CW illumination of large scale systems. I. TheEllipticus illuminator

The Phillips Laboratory has recently designed and constructed a continuous-wave illuminator for studying the responses of large-scale systems subject to electromagnetic stresses. The illumination is developed by an electrically large, ferrite-loaded elliptical loop configuration that is oriented over the system being studied. One half of the loop is completed by its image in the ground plane. With the loop being driven from its apex, the electric and magnetic fields are substantially horizontal, corresponding to illumination from an overhead source. Data are presented (over the frequency range 300 kHz to 1 GHz) for the illumination field and the response of an aircraft, recorded at the Phillips Laboratory large electromagnetic system level illuminator (LESLI) research facility     

Effect of antenna polarization on the capacity of a multiple element system in an indoor environment

We analyze the behavior of a multiple element antenna system in an indoor environment based on the measurements taken in the Lucent Bell Labs building in Crawford Hill, NJ, with a system of 12 transmitters and 15 receivers. In particular, we investigate the capacity behavior with respect to the polarization of the transmitting/receiving elements and the distance between the transmitting and the receiving arrays. The analysis of the power rolloff versus distance clearly demonstrates the different propagation characteristics of the horizontally versus the vertically polarized electric fields. Under strong line-of-sight (LOS) conditions (hallway environment), the power of the horizontally polarized waves falls off faster with distance than that of the vertically polarized fields. Also, the cross-polarization coupling is about -15 dB. Under nonline-of-sight (NLOS) conditions (labs), both polarizations display similar rolloff behavior with distance and the cross-polarization coupling is about 0 dB. There is a power loss of at least 15 dB under NLOS conditions relative to LOS conditions. The average received signal power affects the system capacity. In the hallway, horizontally polarized systems achieve lower capacities than their vertically polarized counterparts. Also the achievable capacity in the labs is much lower than that in the hallway, because of the lower average received power. The comparison of single polarization systems to hybrid polarization systems shows that combined polarization systems perform better in terms of achievable capacity. Therefore, there lies an advantage in using both electric field polarizations. However, under strong LOS conditions the channel itself inherently limits the capacity behavior of the system.     

水平极化CW辐照器辐射振子的仿真设计

核电磁脉冲武器对工作在战场中的电子设备危害巨大,为确保电子设备在复杂的战场电磁环境中可以正常工作,需要对其进行辐射测试。针对系统级电磁脉冲测试需求,文中提出了一种CW辐射装置的设计方法。通过对辐照器的辐射振子进行结构设计与加载设计并结合计算工具MATLAB,给出了一种工作频段为100 kHz~500 MHz的水平极化CW辐照器的设计方法。设计结果表明,在100 kHz~10 MHz的频段内,辐照器振子形状变化对于测试区域的场分布影响较小;当辐照器振子加载增加数倍时,测试区域空间波阻抗也增加相同倍数。     

Determination of the current distribution on an infinite periodic structure of thin-metallic wires

The singular integral equation of the diffraction by thin metallic wires is used to determine the current distribution on an infinite periodic structure in one dimension. The periodic pattern can be constituted by thin wires, joined or not between the cells. This method is specifically applied to a meander line. The induced current-distribution on this structure illuminated by a plane wave of any incidence, polarized either vertically or horizontally, is obtained by means of a matrix inversion algorithm deduced from a particular method of moments. This method does not need an estimation of the initial solution as in the iterative methods and finds its application in the domain of middle frequency waves and especially in the low frequency range.     

Circularly Polarized Transmitting Antenna For U. H. F.

This paper describes a combined panel antenna with an aperture of about eight wavelengths The combined antenna consists of a zigzag antenna and two collinear arrays. This antenna can be used to radiate vertically polarized, horizontally polarized waves or both, When properly phased it can be used to transmit or receive circularly polarized waves,     

Waveguide Modes in Inhomogeneous Media (Computer Program Descriptions)

To compute the horizontally polarized (TE) and the vertically polarized (TM) mode parameters in horizontally stratified dielectric waveguides with arbitrary permittivity profiles using generalized characteristic vectors. The dispersive properties of the wave-guides are also determined.     

Fully polarimetric bistatic radar scattering behavior of forestedhills

The bistatic radar scattering measurements of forested hills were performed at grazing incidence and at azimuth scattering angles from 28° to 66° from the forward scatter plane. Using pulse-to-pulse switching between orthogonal transmitted polarizations, the radar simultaneously measures two orthogonally polarized components of the scattered wave to obtain full polarimetric information about the scattering process. These are the first fully polarimetric terrain clutter measurements to be conducted at large bistatic angles. The complete Stokes matrix, computed by averaging successive realizations of the polarization scattering matrix, is used to examine the polarization sensitivity of the bistatic clutter. It is found that the polarization state of the EM wave scattered out of the plane of incidence strongly depends on the polarization orientation of the incident electric field. Unlike the monostatic case, these two incident wave polarization states are found to produce substantially different scattered wave behavior when trees are viewed at large bistatic angles. Scattered fields resulting from vertically oriented incident fields are found to be highly polarized and to produce bistatic clutter power levels that are strongly dependent on the polarization of the receiving antenna. In contrast, horizontally oriented incident fields are found to produce weakly polarized scattered waves with bistatic clutter power levels that are insensitive to the polarization of the receiving antenna     

Depolarization and scattering of electromagnetic waves by irregular boundaries for arbitrary incident and scatter angles full-wave solutions

Explicit expressions are presented for the radiation fields scattered by rough surfaces. Both electric and magnetic dipole sources are assumed, thus excitations of both vertically and horizontally polarized waves are considered. The solutions are based on a full-wave approach which employs complete field expansions and exact boundary conditions at the irregular boundary. The scattering and depolarization coefficients axe derived for arbitrary incident and scatter angles. When the observation point is at the source these scattering coefficients are related to the backscatter cross section per unit area. Solutions based on the approximate impedance boundary condition are also given, and the suitability of these approximations are examined. The solutions are presented in a form that is suitable for use by engineers who may not be familiar with the analytical techniques and they may be readily compared with earlier solutions to the problem. The full-wave solutions are shown to satisfy the reciprocity relationships in electromagnetic theory, and they can be applied directly to problems of scattering and depolarization by periodic and random rough surfaces.     

Electromagnetic fields of dipoles in stratified media

A general theory for the electromagnetic fields of dipoles in stratified isotropic media is outlined. The stratified model consists of a stack of layers sandwiched between two semi-infinite media. Either an electric or a magnetic dipole can be placed at any position in the stack, or in the upper or lower half-space. Dipoles can be electric or magnetic and can be oriented horizontally or vertically. The fields in the layer containing the source are given in terms of reflection coefficients, impedance and admittance terms, and wavenumber ratios. Recursion relations are developed to propagate the Hankel-domain field coefficients to other layers or to the half spaces. This allows the observation point to be placed anywhere except at the source. Numerical checks show that the derived algebra is at least self-consistent.     

On the excitation of VLF waves

The ratio between the excitation factors of vertically and horizontally polarized sources is equal to the squared ratio between the characteristic impedance of free space and the ground level coupling impedance of TE and TM field components. This relation provides an analytical check of numerically calculated excitation factors. Also, the need of directly calculating the two excitation factors of an anisotropic environment may be obviated if the excitation factor of an isotropic environment is used for estimating the dominant field components of a given quasi-TM or quasi-TE mode.     

Radar scattering behavior of estuarine outflow plumes

We present results of dual-polarized radar scattering measurements of the Chesapeake Bay outflow plume. Near-unity polarization ratios (ratios of horizontally polarized radar echoes over vertically polarized ones) are observed in large incidence angle (60/spl deg/ to 80/spl deg/) radar echoes from the outflow plume and its frontal boundary (normally referred to as a front) under strong surface current convergence (0.008-0.02 S/sup -1/), suggesting the existence of steepened and breaking waves in the regions. Cumulative distribution functions of the horizontally polarized radar returns from the front show approximately 90% of the radar echoes are from steepened and breaking waves. Vertically polarized echoes do not show this effect. These experimental results substantiate early modeling investigators' speculation of featured scattering contributing to horizontally polarized radar signatures of oceanic fronts. Our results also suggest that horizontal radar polarization can be used to remotely sense additional hydrodynamic processes such as wave trapping, blocking, and breaking near oceanic fronts better than what is possible with only vertical polarization.     

Analytical and experimental studies of coupling between antennas over a two-dimensional ground surface of known contour

A numerical procedure is presented for computing the transfer impedance between two antennas located above a two-dimensional ground surface of known contour. The antennas are either both horizontally or both vertically polarized, and may have arbitrary radiation patterns in the vertical plane. The antennas are assumed to be high enough above the ground so that surface wave coupling is negligible but close enough so that a significant portion of the received power arrives via ground scattering. For purposes of computation, the ground is divided into a series of connected (infinite) planar strips of identical projected width. The net scattered field is computed using Green's theorem applied to each strip individually but neglecting edge effects and multiple scattering. The practical utility of this technique has been tested in a limited way by measuring the variation in coupling between two identical horizontally polarized horn-aperture antennas over a single ground contour. Provided the baseline profile is surveyed carefully at the time the coupling measurements are made, the numerical predictions are seen to be in reasonable agreement with the test data.     

Depolarization of radio waves in jungle environment

While propagating through a jungle environment radiowaves get substantially depolarized due to their scattering by the vegetation. Due to its conductivity, the foliage supports induced currents that tend to be randomly oriented and, therefore, they produce a depolarization of the overall field. Results of cross-polarization measurements in tropical moist deciduous and tropical wet evergreen forests in the VHF/UHF band are reported in this communication. It was found that vertically polarized waves suffer 5-15-dB higher depolarization than horizontally polarized waves. The crosspolarization discrimination is also found to be dependent on frequency as well as the separation distance between the transmitter and the receiver.     

Microwave Propagation Constant for a Vegetation Canopy With Vertical Stalks

An equivalent-medium model is developed to relate the propagation constant y, associated with propagation of the mean field through a vegetation canopy, to the geometrical and dielectric parameters of the canopy constituents. The model is intended for media containing vertical cylinders, representing the stalks, and randomly oriented discs, representing the leaves. The formulation accounts for both absorption and scattering by the cylinders, but uses a quasi-static approximation with respect to the leaves. The model was found to be in good agreement with experimental results at 1.62 and 4.75 GHz, but underestimates the extinction loss at 10.2 GHz. The experimental component of the study included measurements of the attenuation loss for horizontally polarized and vertically polarized waves transmitted through a fully grown corn canopy, andof the phase difference between the two transmitted waves. The measurements were made at incidence angles of 20°, 40° , 60°, and 90° relative to normal incidence. The major conclusion of this study is that the proposed model is suitable for corn-like canopies, provided the leaves are smaller than ? in size.     

Tropical raindrop size distribution for the prediction of rainattenuation of microwaves in the 10-40 GHz band

This paper describes the measurement of microwave rain attenuation over a ten-year period (1988-1998) in Singapore. The rain attenuation of (vertically and horizontally polarized) microwaves at 15, 21, and 38 GHz propagating over a path length of approximately 1.1 km were recorded. A negative-exponential raindrop size distribution model is then developed by fitting of the experimental data with the total extinction cross sections of oblate spheroidal raindrops computed from the T-matrix approach. Graphs for attenuation versus rain rate (at selected frequencies) and for attenuation versus frequency (at selected rain rates) are also given     

大气激光通信中光PPM偏振调制方案及其关键技术

大气中的雨、雪、雾等对大气激光通信的质量和可靠性影响很大.提出了一种光PPM偏振调制方式,采用改变水平和垂直偏振光束的强度,检测接收到的激光的偏振角进行编码.由于水平偏振和垂直偏振光通过的是同一条光路,在求解光偏振角时大气中的衰减可以相互抵消,因此光PPM偏振调制可以减少甚至消除大气信道对大气激光通信的影响.仿真结果表明:这种方法不但可以消除大气信道对激光通信质量的影响,而且可以成倍提高大气激光通信的码速率.     

Application of Generalized Characteristic Vectors to Problems of Propagation in Clad Inhomogeneous Dielectric Waveguides

A transformation matrix that uses generalized characteristic vectors is used to convert Maxwell's equations into a set of loosely coupled equations for the wave amplitudes. This transformation is suitable for permittivity profiles with turning points. In earlier full-wave solutions to these equations, several special functions that account for the local features of the permittivity profile, especially near the turning points, were used to obtain appropriate expansions of the fields. The transverse field components, the propagation coefficients, as well as the phase and group velocities, are computed for both horizontally polarized (TE) and vertically polarized (TM) modes of the dielectric waveguides using the full-wave approach. These solutions are compared with analytic solutions for waveguides with special permittivity profiles. They are also compared with recently published results based on a perturbational approach.     

Induced foot-currents in humans exposed to VHF radio-frequency EMfields

In order to limit the specific absorption rate (SAR) in the ankles of a person exposed to an electric field at frequencies below 100 MHz, induced current limits are prescribed in the 1992 ANSI/IEEE safety standard. The authors have measured the induced currents passing through the feet of nine subjects exposed to vertically polarized electric fields from nearby antennas, transmitting at frequencies between 90 and 104 MHz (in the FM broadcast band). The experimental results are in excellent agreement with the results obtained analytically for frequencies up to 110 MHz. The analytical results were obtained by applying the finite difference time domain (FDTD) method to an anatomically-based model of an average height male (1.75 m) and to a model of the tallest subject in the study (1.91 m). For the mean height of the nine subjects (1.75 m), the measured induced foot current for a unit strength vertically polarized electric field varied with frequency from 4.46 to 3.45 mA/(V/m) for frequencies between 90-104 MHz. For the tallest subject (1.91 m), the corresponding values ranged from 5.42 to 4.45 mA/(V/m), Foot currents in excess of the induced current limits in RF safety guidelines for both the controlled and uncontrolled environments could result even when the vertical component of the incident electric fields comply with the corresponding field limits. It is important, therefore, to not only measure the E- and H-fields, but also the induced currents up to the recommended maximum frequency of 100 MHz, and perhaps up to the upper frequency of the FM broadcast band (108 MHz)     

利用辅助水平天线对Es层干扰的自适应抑制

基于一个高频双极化雷达系统,给出了一种利用辅助水平天线抑制Es层干扰的自适应系统.水平极化天线接收的数据与垂直极化天线接收的数据进行相关,自适应的估计出垂直天线接收中的干扰,然后对消掉它们.干扰对消在每个相参积累周期后进行.实际数据对消后的信噪比可以提高到25dB.     

Mobile radio signal correlation versus antenna height and spacing

In mobile radio communications, a given correlation value between two signals received at a base station can be achieved by adjusting the antenna spacing and height; this has been experimentally verified [1], [2]. From this fact, a parameter η, defined as η=antenna height/antenna spacing is proposed as a measure of correlation for design purposes. Data from field experiments using horizontally spaced, vertically polarized base-station antennas have been analyzed in terms of η and are presented here. These data provide an upper bound correlation coefficient for any antenna separation and height. The upper bound value can be directly applied in system design.