电离层人工调制的极低频辐射和在潜艇通信中的应用探讨

本文分析被调制的高频波加热电离层所产生的极低频辐射过程和极低频在地球-电离层波导中的传播模式,以及应用于潜艇通信中的可能性。数值结果表明,如果高频有效发射功率为100MW量级,频率为5MHz,调制频率为100Hz,则准TEM模式在地球-电离层波导中传播的衰减率一般小于1dB/Mm,距加热地点1000km处海面(地面)上磁场强度的量级为10-8A/m。对于潜艇通信,在辐射源强度确定的情况下,对不同的潜艇深度和通信距离,存在一个最佳选择频率。     

电离层电急流的人工调制

由基本的磁离子理论出发,构造了大功率电波调制电离层电急流的自洽理论模型.利用此模型,计算了电离层D/E区高度上电子温度随加热时间的变化,以及不同高度上的加热时间和冷却时间.基于方波脉冲的入射方式,详细地研究了加热区电离层电导率和赤道电急流的振荡过程.计算结果表明,地面入射的大功率电波能有效地调制电离层中存在的大尺度直流电流,而且该调制电流可以作为ELF/VLF电波的辐射源.最后简单地分析了在未来实际工程应用中所需要解决的问题.     

Some notes on ELF Earth-ionosphere waveguide daytime propagation parameters

The transverse-electromagnetic (TEM) propagation constants for extremely low frequency (ELF) daytime propagation in the earth-ionosphere waveguide have been calculated for frequencies of 5-2000 Hz. The recently developed theory of Greifinger and Greifinger and the Wait very low frequency (VLF) exponential ionospheric-conductivity profile have been used in the analysis. It is shown that the resulting values of ELF attenuation rate, phase velocity, and ionospheric-reflection height are in excellent agreement with the measured data.     

The Radiation of Extremely Low Frequency (ELF) Waves

The excitation of extremely low frequency (ELF) waves by a horizontal electric dipole in the earth-ionosphere waveguide can be obtained by a heuristic approach based upon the observation of a relationship between a horizontal electric dipole and a horizontal magnetic dipole. Then multiple imaging takes into account the effect of the ionosphere. This approximate derivation preserves the physical picture which is obscured by complex mathematics in rigorous analysis.     

极低频垂直偶极子在地-电离层中场的数值积分算法

针对地-电离层空腔中极低频(ELF)场强问题,提出一种应用于任意复数角v的勒让德函数数值积分算法,并与已有渐近计算法、级数展开法进行比对,结果表明:该方法使用频率范围更广,计算精度、效率更高。利用此数值积分算法,计算了垂直偶极子激励的30Hz以下典型频点的场强分布,所得场强分布符合物理规律,无损耗条件下舒曼谐振频率与前人计算结果完全一致,印证了ELF垂直偶极子在地-电离层腔体中场的数值积分算法的正确性。     

电离层预加热幅度调制模式的理论与模拟

从基本的电离层能量方程和连续性方程出发,构造了预加热模式下低电离层幅度调制加热理论模型,详细研究了预加热模式对电离层幅度调制加热产生极低频/甚低频(Extremely Low Frequency/Very Low Frequency,ELF/VLF)强度的影响.以HAARP加热站点为例,对比分析了正常幅度调制模式和预加热幅度调制模式两种模式下产生的ELF/VLF偶极矩的大小.分析结果表明,当加热系统有效辐射功率较大、调制频率较小时,预加热幅度调制模式更有效;在一定加热条件下,相对正常幅度调制模式,预加热幅度调制模式辐射ELF/VLF强度可提高约5.3 dB.     

地下ELF线天线在地-电离层壳体中产生的场

为了研究地震产生的机理,需要分析和研究地下任意ELF线天线在地-电离层壳体中产生的电磁场。地下任意ELF线天线产生的电磁场可由地下ELF点源产生的电磁场进行叠加。首先导出了地下ELF点源在地-电离层壳体中产生的电磁场表达式,然后得到了地下任意ELF线天线在地-电离层壳体中产生的电磁场的表达式。分别讨论了地下单条和两条ELF线天线在地面上产生电磁场的场强空间分布图,用色标表示了场强值在空间分布的强弱。电磁场的水平分量在地面上产生电磁场的场强值要比垂直分量产生的场强值大,单条线天线在地面上产生电磁场的场强值要比两条线天线产生的场强值小。     

Very-low-frequency propagation below the bottom of the sea

Radio-wave propagation at very low frequencies (VLF) in the stratified rock below the bottom of the sea is studied. A reasonable assumption of extremely low electrical conductivity in the stratified rock is based upon available geological data. The surface wave traveling along the interface between this region of low conductivity and the highly conducting sea is compared with the vertically polarized ground wave found in VLF radio-wave propagation at the surface of the earth. When extremely low frequencies (ELF) are transmitted, the highly conducting layer found at greater depths below the bottom of the sea forms the lower surface of a spherical waveguide. This waveguide at ELF supports a propagation mode similar to the mode existing at VLF between the surface of the earth and the lower boundary of the ionosphere. The similarity in propagation mechanisms leads to the name "inverted ionosphere" (described by Wheeler [1]) for the underground region. The sea or relatively highly conducting soil at the surface of the earth is an almost impregnable shield against atmospheric noise and effects from sudden ionospheric disturbances or solar flares. In addition to providing a noise-free medium, the sea has the advantage that construction costs are much less than those of a VLF transmitter at the earth's surface. Presumably communication between shore installations and submarines on the floor of the ocean could be achieved with the inverse ionosphere. The power requirement for such communication with existing VLF transmitters at the earth's surface renders such transmission unattainable.     

电离层中ELF辐射源向下传播衰减理论分析

利用大功率高频（HF）电波调制加热电离层可在电离层中有效形成辐射源,并用于辐射ELF电磁波.本文基于磁流体力学的基本方程通过对电离层中极低频（ELF）辐射源的辐射场分析,获得ELF电磁波在电离层中传播的色散关系式,建立电离层中的ELF辐射源向下传播衰减模型.并依据建立的传播衰减模型,分析不同纬度地区传播衰减的差异,以及传输频率和背景电离层参数对传播衰减的影响.     

地面甚低频辐射渗透进电离层的数值模拟分析

利用全波解模型进行数值模拟实验, 研究了具有不同辐射源参数(辐射频率和功率)的地面甚低频(Very Low Frequency, VLF)辐射源在不同地磁场参数(地磁场强度和倾角)和电离层参数(电子密度和碰撞频率)条件下激发的地球-电离层波导和电离层中的电磁场能量空间分布, 并重点研究了电离层D/E区对电磁辐射能量的吸收.模拟结果发现:VLF辐射在波导中衰减只受辐射源频率的影响, 不受辐射功率、地磁场参数和电离层参数变化影响, 波导中的衰减随频率的增大而减小, 而电离层D/E区吸收随频率增大而增大, 两者总衰减量随频率增大而增大.辐射源功率对电离层D/E区的吸收也无影响.地磁强度和地磁倾角越大, 电离层D/E区吸收越小; 电离层碰撞频率和电子密度越大, 电离层中能量衰减越大.     

地基ELF线天线在地-电离层壳体中产生的场

为了研究地震产生的机理, 需要分析和研究地基极低频(Extreme-Low Frequency, ELF)电磁波的传播特性.地基任意ELF线天线产生的电磁场可由ELF点源产生的电磁场进行叠加.通过地基ELF点源电磁场的表达式导出了ELF线天线在地-电离层壳体中电磁场的表达式.利用加速收敛算法得到了单条和两条ELF线天线产生电磁场的场强空间分布图, 用色标表示场强值在空间分布的强弱.电场水平分量的场强值要比垂直分量的场强值小, 磁场水平分量的场强值要比垂直分量的场强值大.而且两条线天线产生的场强值要比单条线天线产生的场强值大.     

地-电离层中水平激发极低频场的勒让德展开算法

地-电离层空腔中水平激励极低频(Extremely Low Frequency,ELF)场强可表示为勒让德函数及其导数的级数和,利用勒让德函数迭代公式,提出了极低频水平偶极子在地电离层空腔中激励场的计算新算法.将该方法近区场的计算结果与半空间水平偶极子激励场计算结果对比,远场区结果与Bannister(dir.+ind.)公式计算结果对比,验证了新算法的正确性和精度.将本算法应用至几Hz以下,可获得特殊规律：远区电场、磁场水平分量与方位角无关,电场水平分量方向与偶极矩方向平行,磁场水平分量方向与之垂直.     

地层各向异性对极低频水平低架天线的影响

讨论了当极低频水平低架天线场地周围的地层导电率呈现各向异性时对水平低架天线方向性和发射效率的影响.分析了导电率各向异性条件下的地层表面阻抗,导出了极低频电波在地-电离层波导中传播的模方程,给出了典型条件下各向异性的导电率参数对TM0波模在各个方向上的相速和衰减率影响的规律及导电率各向异性和各向同性情况下天线方向图的比较,得出了各向异性的导电率对天线方向图和发射效率有较大影响.对设计极低频天线及其工程建设具有理论指导意义.     

Superdirective linear array antennas (optimization according to an integral criterion)

Phased array antennas with the element currents ensuring superdirectivity are analyzed. Three types of elements are studied: an isotropic dipole, a dipole oriented along the array axis (a collinear dipole), and a dipole oriented across the array axis (a perpendicular dipole). The currents on these elements are calculated using an integral criterion, according to which the total power radiated within some angular sector is minimized, whereas the power radiated in the main direction is kept at a prescribed level.     

Circularly polarized dielectric-loaded planar antenna excited bythe parallel feeding waveguide network

A new receiving planar array antenna for DBS (direct broadcasting satellite) is proposed. The element antenna is a short waveguide aperture mounted in the ground plane, loaded with a dielectric and polarizers, and excited through its side wall by another feeding rectangular waveguide. The gain of the element antenna loaded with a dielectric is so high that the grating lobes can be reduced sufficiently even if the element spacing in the array is wider than the wavelength in free space. Therefore we can reduce the number of the array elements, and parallel feeding by the low loss waveguide network can be feasible to provide a planar array antenna. This paper describes the experimental results of several kinds of the circularly polarized dielectric-loaded element antennas and the planar antennas fed by the waveguide network. In the 12 GHz band the planar antenna with 64-element radiators has a maximum gain of 31.9 dBi with an aperture efficiency of 94.7%, the 1 dB-down frequency bandwidth of the gain is about 800 MHz (6.7% for a center frequency of 11.85 GHz), and the frequency bandwidth of the axial ratio is less than 1 dB of 850 MHz (7.2%)     

Analysis of the time-reversal operator for a small spherical scatterer in an electromagnetic field

The time-reversal operator (TRO) for a planar array of crossed dipole elements illuminating a small conducting and/or dielectric sphere is investigated in order to determine the general properties of an electromagnetic time-reversing array system. The behavior of such a system for a given frequency is analyzed by studying the eigenvalues and eigenvectors of the TRO. Each eigenvector specifies a set of complex driving currents for the array elements that produce received voltages that are proportional to the conjugates of the drive currents. The proportionality constant is equal to the square root of the associated eigenvalue and is the same for all elements. The eigenvalues and eigenvectors can be determined by performing a singular value decomposition (SVD) on the multistatic data matrix of the array. The eigenvalues of the TRO are the squares of the singular values, and the eigenvectors are identical to the singular vectors. We have shown that the maximum number of singular vectors associated with the sphere is equal to the number of orthogonal orientations of the dipole moments induced in the sphere when irradiated by the array, so there is a maximum of six for a conducting sphere but only three are significant when the conductivity is small and the sphere may be considered being just a dielectric. Numerical results are presented for linear and circular arrays to show the general behavior of the system.     

Radio Emission of the High-Latitude Ionosphere as a Result of the Cutoff Mode of the Auroral Ionospheric Duct

Issues of radio emission of the auroral ionosphere are considered with the use of the classical electrodynamics approach, according to which a natural waveguide is formed in the Earth’s ionosphere owing to formation of a trough in the high-latitude ionosphere. Parameters and characteristics of the auroral ionospheric dielectric duct have been estimated on the basis of the obtained experimental data on the properties of the ionospheric trough. The hypothesis that the LF and VLF electromagnetic auroral radio emission in the high-latitude ionosphere is due to the approach of the auroral waveguide to the cutoff mode has been considered. It has been found that the radio emission bands correspond to the eigenmodes of the ionospheric duct under consideration. It has been shown that, within the proposed approach, not only the radio emission frequency band of the high-latitude ionosphere but also the polarization of the radio emission observed in the experiments can be explained.     

张衡一号卫星观测的地基VLF波电离层加热扰动特征

电离层扰动主要由太阳活动和地球表面活动引起,通过对已知地基甚低频（very low frequency, VLF）波发射源引发的等离子体扰动进行特征研究,对于认识电离层环境变化具有重要的参考意义. 文中主要介绍了张衡一号01卫星原位探测科学载荷等离子体分析仪（plasma analyzer package, PAP）观测系统和数据产出,对该载荷在507 km轨道原位观测的、由地基NWC站大功率VLF人工源辐射引起的电离层加热扰动现象及特征进行了总结和分析. 观测结果表明:氧离子密度、离子温度、离子漂移速度等多个原位观测量在夜侧轨道均同时记录到NWC站上空北侧约50～500 km区域出现的电离层加热扰动现象,扰动持续范围约为300～400 km;通过对相同轨道电场数据进行功率谱计算可知,该区域电场增强主要频率与NWC站发射频率相同. 上述观测结果验证了张衡一号01卫星PAP对电离层离子参量扰动现象具有一定的探测分辨能力,地基大功率VLF电波能够对电离层等离子体环境产生明显的扰动影响,且扰动区域相对于发射源位置向磁赤道方向发生偏移.     

Analysis of waveguide antenna arrays with protruding dielectricelements

The two-dimensional problem of radiation of TE and TM waves from a waveguide array with protruding smooth dielectric elements of arbitrary shape is considered, and solution algorithms are suggested. The algorithms are based on applying the method of auxiliary sources for the representation of electromagnetic fields outside and inside the protrusions in combination with the method of integral equations for the electric field at the waveguide aperture. The point matching of the field tangential components on the protrusion-to-free-space boundary and at the waveguide aperture is used to reduce the problem to a system of linear algebraic equations for the amplitudes of the auxiliary filamentary currents and of the waveguide aperture electric field, which is assumed to be piecewise constant. The amplitudes obtained from the solution of the system are used for computing the array reflection coefficient and element pattern, which are shown in some cases to be significantly dependent on the protrusion shape. Examples of arrays with flat-topped element patterns resulting from array geometry numerical optimization are also presented     

Asymmetric ridge waveguide radiating element for a scanned planararray

A novel linear array radiating element for use in a large low-sidelobe, planar slot array electronically scanned in azimuth is described. The planar array consists of many vertical resonant linear array elements; i.e. single ridge waveguides with broadwall shunt slots, closely spaced to avoid array grating lobes when the array is scanned in azimuth. Traditional linear broadwall slot arrays require alternating slot displacement from the waveguide centerline, which generates undesirable secondary beams in skewed directions during electronic scan. To eliminate the secondary beams, a design has been devised in which all slots are exactly collinear and lie on the waveguide centerline. The methods considered to achieve the required waveguide asymmetry, the design procedure used in the successful approach, and the experimental results obtained with a demonstration model are presented