Pulse radiation from an insulated antenna: an analog of Cherenkovradiation from a moving charged particle

Cherenkov radiation arises when a charged particle moves with a constant velocity that is greater than the speed of light in the surrounding medium. This radiation has distinctive characteristics. Near the charge, the electric field is most intense along a conical surface with apex at the charge-the Mach cone. In the far field, the radiation occurs predominantly in one direction-at the Cherenkov angle. An insulated antenna consists of a metallic cylindrical conductor covered by a concentric sheath of dielectric. In use, this antenna is embedded in a medium whose permittivity is often much greater than the permittivity of the insulation. When the antenna is excited by a pulse of voltage, a pulse of charge appears to travel along its length. The apparent velocity of this charge is close to the speed of light in the insulation, which, because of the difference in the permittivities, is greater than the speed of light in the surrounding medium. Thus, the radiation from the pulse excited, insulated antenna should be analogous to Cherenkov radiation from the moving charged particle. In this paper, the pulse-excited, traveling-wave insulated linear antenna is accurately analyzed using the finite-difference time-domain (FDTD) method. Results are obtained for the charge on the conductor, the near field, and the far field. These results show the striking similarity of the radiation from this antenna to Cherenkov radiation from the moving charge     

Aperture field of a leaky-wave antenna of finite length

An anomaly in the radiation field of a leaky-wave slot antenna is apparent when experimental and theoretical results are compared. An investigation of the field along the aperture of the antenna, in which experiment is supported by theory, indicates that this is probably due to the continuous excitation of a TEM wave along the slot itself.     

单馈电双模方形微带天线及圆极化实现收稿

基于展开理论,给出了沿贴片对角线馈电时方形微带天线的内场分布和输入阻抗公式.通过在贴片四周切割缝隙,有效地降低了天线双主模的谐振频率并实现了圆极化辐射,讨论了缝隙长度与天线极化之间的关系.为实现微带天线的小型化提供了一种新途径,给出了UHF波段天线设计实例.     

A Talk-In Procedure for Critical A.M. Directional Antenna Adjustment

A new technique has been developed for easier and more accurate analysis of field intensity scattering effects along the critical radials of a directional antenna radiation pattern. Easily performed, it requires only a two-man team with base to mobile radio communication, little if any use of antenna monitor data and results in a simple determination of which measuring location along a given radial is the best choice of a truly indicative monitoring point. A good monitoring point is one at which field intensity variations produce the most linear resulting variations along an entire radial or a major portion of it.     

圆环天线的时域有限差分分析

采用时域有限差分算法（FDTD）计算了圆环天线在高斯脉冲激励下的瞬时电流分布，通过付里叶变换，获得天线在不同频率下的稳态电流分布，经过进一步计算，得出天线在这组频率下的辐射方向图和输入阻抗。     

Optimization of the geometry of a TEM-horn for radiation of ultrashort pulses used as an element of an antenna array with controlled position of the main lobe

The finite-difference time-domain method is used to build the model of a TEM-horn with exponential longitudinal distribution of the characteristic impedance along the axis. This model is used to calculate the antenna energy pattern and the shape of the ultrashort pulse of the electromagnetic field in the far zone. Geometrical parameters of the antenna aperture are selected so as to increase the width and the uniformity of the energy pattern in the direction of radiation as well as minimize the level of the backward radiation.     

一种有效减缩微带天线RCS的新方法

根据经典理论,对于线极化矩形贴片天线,其主模电场强度在平行于非辐射边的中心线上为零。如果沿此中心线对微带天线加载电阻,对其辐射性能影响不大,然而对散射性能则有很大改善。通过引入一种阻抗加载技术和接地板开槽技术使微带天线的RCS大幅减缩,结合小型化技术,微带天线的RCS得到进一步减缩,而且天线外形尺寸也比原天线有所减小。这样,就同时兼顾了微带天线的辐射性能和散射性能。     

Theoretical design and predictions of volute antenna performance

To provide generic design guidelines for volute antenna elements over a wide range of parameters, a theoretical formulation and algorithm for computing the antenna radiation field have been developed. The procedure relies on the vector combination of the radiation fields of the helical wires and the end connecting wires with a continuous sinusoidal current distribution along each wire of the volute. The antenna patterns of circularly polarized fields, directivity, 3-dB beamwidth, axial ratio, front-to-back power ratio, etc. can all be calculated from a given set of volute parameters. A practical design example utilizing this procedure is presented     

不同参数超宽带反射面天线系统辐射特性

在超宽带反射面天线设计中,首要问题是馈电天线的方向图与反射器匹配。以往在频域上对反射面天线的研究发现,当馈源天线方向图给出约-11 dB的边缘照射时,反射面天线可实现其最佳性能。在参数化模拟了作为馈源的透射电子显微镜(TEM)喇叭天线方向图特性的基础上,根据反射面天线的频域设计准则,相应地加入合适的超宽带反射面天线,对其远场辐射特性进行模拟计算。为了获得尽可能大的远场辐射场,采用计算机仿真技术(CST)数值模拟软件,在0 GHz~1 GHz频率范围内模拟了以不同参数TEM喇叭天线作馈源的超宽带反射面天线的远场辐射特性,并在时域上对其结果进行分析。模拟结果表明,由于其相位中心的不确定性,作为馈源的TEM喇叭天线无法与反射面天线完全匹配,其沿着反射面天线主轴移动时产生的反射面口径场相位和幅度的变化影响着远场辐射场的变化,TEM喇叭天线的遮挡效应也不容忽视。     

自由空间导线天线辐射模式分析

采用分段正弦基矩量法计算线天线电流,借助分段正弦电流辐射场表达式,将各分段电流产生的辐射场进行线性叠加,进而获得天线总的辐射场。该辐射场利用球面波展开法进行展开,同时采用数值积分方法求取辐射场球面波展开式中各模式的系数值。以3种基本单元天线:偶极子天线、圆环天线以及螺旋天线为例,定量分析了3种基本单元天线的辐射模式,验证了这一方法的正确性。     

Electrostatic resonance-cone waves emitted by a dipole in theionosphere

The theory of whistler-mode radiation from a dipole antenna has been tested in the ionospheric sounding-rocket experiment OEDIPUS-C (OC). In this bistatic investigation, 25-kHz waves were received at a distance of 1200 m from a synchronized transmitter. Electric fields near the group resonance cone were significantly stronger than in other directions, as predicted by theory. The transmitting antenna RF current was obtained from an equivalent circuit including an antenna impedance predicted by quasistatic theory. This current was applied to resonance-cone radiation theory. Observed radiated levels are about a hundred times greater than theoretical predictions based on some assumptions about the electromagnetics of the transmitting and receiving antennas. The modulation of the received signal caused by the spins of the transmitting and receiving dipoles was consistent with the electric field being polarized predominantly along the electrostatic wave-vector direction     

Terahertz radiation of a butterfly-shaped photoconductive antenna (invited)

The terahertz(THz) far-field radiation properties of a butterfly-shaped photoconductive antenna (PCA) were experimentally studied using a home-built THz time-domain spectroscopy(THz-TDS) setup. To distinguish the contribution of in-gap photocurrent and antenna structure to far-field radiation, polarization-dependent THz field was measured and quantified as the illuminating laser beam moved along the bias field within the gap region of electrodes. The result suggests that, although the far-field THz radiation originates from the in-gap photocurrent, the antenna structure of butterfly-shaped PCA dominates the overall THz radiation. In addition, to explore the impact of photoconductive material, radiation properties of butterfly-shaped PCAs fabricated on both low-temperature-grown GaAs(LT-GaAs) and semi-insulating GaAs(Si-GaAs) were characterized and compared. Consistent with previous experiments, it is observed that while Si-GaAs-based PCA can emit higher THz field than LT-GaAs-based PCA at low laser power, it would saturate more severely as laser power increased and eventually be surpassed by LT-GaAs-based PCA. Beyond that, it is found the severe saturation effect of Si-GaAs was due to the longer carrier lifetime and higher carrier mobility, which was confirmed by the numerical simulation.     

The Radiation Properties of a Novel Wire Antenna for the Security Fence Radar

The radiation properties of a novel wire antenna are investigated. The main part of this antenna is a sinusoidal wire undulator, which radiates by coupling electromagnetic energy from a Goubau line located near the antenna. Far-field patterns and S-parameters composed of three sets of antenna are measured. The measured patterns are compared with the calculated ones and the phase and attenuation constants of all the antennas are calculated. Frequency behavior and the dependency on the antenna dimensions of those wave parameters are investigated. Measured and calculated field patterns are also compared with the MoM patterns and some properties of the wave propagating along structure are explained by using MoM current distributions. It is shown that a broadside transversal radiation occurs in a narrow frequency band. The radiation intensity strongly depends on the coupling distance between the Goubau line and the sinusoidal undulator. This antenna is used as the basic element of the security fence radar antenna array working at 1.25 GHz in the L-band, which detects intruders approaching the fence. The performance of the antenna in the array is investigated and the near field distribution of the array is measured. The received signals caused by an intruder are given. The effects of rain and wind are also considered. The results suggests that the security fence radar introduced in this study can be used for the perimeter control of closed areas such as airports, malls, etc.      

A singular integral representation of the electromagnetic field: Physical regularization of the ill-posedness of the near-field zones of antennas

A classification of ill-posedness in the antenna theory is presented in the case when the basic principles of electromagnetics are not fulfilled, which is observed in the near-field radiation zones. The Tikhonov regularization of Fredholm integral equations of the first kind is compared with the physical regularization (a self-consistent method). The ill-posedness arising in the analysis of the electromagnetic field (EMF) of an electric dipole, an equidistant array on its basis, frame antennas, and a cylindrical helical antenna is considered. In particular, for the EMF of an electric dipole, the gap between the field and surface current density is removed and the limits of applicability of the method of induced electromotive forces of coupled dipoles are established. It is shown that unidirectional radiation in the azimuthal plane is typical of a ring frame antenna with a certain dimension of the ring. For a cylindrical helical antenna, the calculation of the surface current density is reduced to a hypersingular integral equation with a quadratic Cauchy singularity. The analysis of the behavior of the current flowing along the helix made it possible to explain the peculiarities of the antenna pattern for various helical turns.     

Performance of a small loop antenna in the 3-10 MHz band

The performance of a typical small loop antenna of 1 m diameter is presented at frequencies in the range 3.6-10.1 MHz. It is argued that the antenna may reasonably describe as electrically small in this frequency range. Measurements of the antenna's bandwidth and, hence, Q factor are presented, along with direct measurements of the radiated field at various distances with a carefully measured RF input power. The radiation efficiency of the loop is derived from the measurements, and is compared with theoretical predictions of the efficiency based on classical electromagnetics. Close agreement is demonstrated between measurements and predictions. Consistency with the Chu bandwidth limit is also demonstrated for this antenna.     

Radiation resistance of a cylindrical antenna immersed in a weakly ionized magnetoplasma. II†

The field components for electromagnetic and eletromagnetic waves from a cylindrical antenna radiating in a weakly ionized magnetoplasma and expressions for radiation resistances assuming sinusoidal current distribution in the antenna have been obtained. The effect of a steady magnetic field on the radiation resistances has been discussed. it has been concluded that the magnetic field changes considerably the radiation characteristics of an antenna.     

机械天线通信技术研究现状

与电调天线相较,机械天线具有全新的电气参数定义,其信号的频率只与携带辐射场的物质机械运动的频率相关,因而克服了电调天线信号频率对天线尺寸的依赖,可以用小型设备产生低频信号,应用于水下和地下便携式低频通信设备的设计中。从辐射场特性、辐射场发生装置以及信息加载三个方面介绍了机械天线通信系统的研究现状,总结了机械天线发展需要重点解决的技术难点。     

舰载天线辐射场的计算及分布特性研究

对舰载天线辐射场分布特性进行分析,是研究舰船电磁兼容性及工程建设的关键环节。为此,采用FEKO软件建立了舰船及天线的全尺寸仿真模型;基于多层快速多极子理论,给出了舰载天线辐射场的计算公式;以上述研究为基础,分别仿真计算了天线载体、天线频率及天线倾角对辐射场分布的影响规律。研究成果对提高舰载天线电磁兼容适应性具有一定的参考价值。     

A new ultrawide-bandwidth dielectric-rod antenna for ground-penetrating radar applications

A new ultrawide-bandwidth dielectric-rod antenna is presented with its application in detecting shallow targets, such as antipersonnel (AP) mines. The lowest hybrid mode is launched and guided along a circular-dielectric waveguide. The end of the waveguide is tapered to a point where electromagnetic waves are radiated out with field behavior similar to that radiated from a Hertzian dipole in the forward direction. The low antenna clutter and weak antenna-ground interaction are two unique features. Its near-field radiation properties are investigated by directly probing the fields and by numerical simulation with a three-dimensional finite-difference time-domain technique. Both measurement and numerical simulation results are presented for the detection of buried AP mines using a prototype dielectric-rod antenna operated at a frequency range from 1 to 6 GHz.     

Ground and flight test results for standard VOR and double parasitic loop counterpoise antennas

Results of ground and flight tests carried out at 109 MHz to study the vertical plane radiation patterns of standard VOR and double parasitic loop counterpoise antennas above ground are discussed. The ground tests consist of the measurement of the fields produced in the quasi-radiation zone of the test antenna The flight tests consist of measuring the far field with the help of an aircraft flying at a constant altitude along chosen radial paths to and from the test antenna. The reduced test results are compared with available theoretical values.