Inverse source problem with reactive power constraint

This paper reports a general Lagrangian formulation for constrained electromagnetic inverse source problems. The formulation is applicable to different forms of inverse source problems having different constraints. Two possibilities are emphasized in the paper. One yields the usual minimum energy solution. The other establishes a new minimum energy solution with the additional constraint that the source has a prescribed reactive power (which can be zero). The latter solution incorporates the important reactive energy issues of an antenna which had not been considered before in the context of the inverse source problem. The new solution is shown to obey a homogeneous wave equation in the interior of the source volume, and expressions for the associated interior field and interaction power are also derived. The derived theory is illustrated for a canonical dipolar source and an alternative approach where the reactive power is minimized for a prescribed maximum functional energy is also presented. Advantage is taken of some of the results to briefly discuss some questions about fundamental antenna limits.     

Inverse Source Problem in the Spheroidal Geometry: Vector Formulation

A formulation based on Lagrangian optimization and spheroidal vector wave functions is presented for the vector electromagnetic inverse source problem of deducing a time-harmonic current distribution that is confined within a spheroidal volume, that generates a prescribed radiation field, and that is subject to given constraints on the source functional energy, which characterizes antenna current level, and the source's reactive power, which models antenna resonance matching. The paper includes computer simulation results illustrating the derived inverse theory.     

Comments on “Propagation of EM pulses excited by electricdipole in a conducting medium”

The authors of the above cited paper dismiss previous work as being “either incomplete or approximate”. They deal with the “exact” solution of the electromagnetic fields of a Hertzian electric dipole or current element excited by an impulsive current. It should be pointed out that their solution, for the magnetic field waveform, is identical to that given by Wait and Spies(1970), who give the waveform of the derivative of the magnetic field for the excitation of the dipole by a step-function of current. Also, in another paper by Wait and Spies, the electric field waveforms, for step-function excitation, were given along with a discussion of the diffusion approximation, which seems to parallel that of Song and Chen. It is moot whether Song and Chen's solution is “complete”. Their solution and similar ones published in the past are valid only for an assumed frequency independent conductivity and permittivity. Such is really an approximation for any material media. I suggest that such adjectives as “exact” and “complete”, in describing formal solutions of unrealistically posed problems, should be discouraged     

阵列辐射瞬态电磁脉冲能量合成特性研究

根据单元天线辐射瞬态电磁脉冲轴线能量传输特性的解析解,提出了阵列辐射瞬态电磁脉冲的点源近似模型,给出阵列辐射瞬态电磁脉冲高效合成特性的物理实质。并采用阵列瞬态电磁脉冲产生系统,对高效合成特性进行实验验证。实验结果和采用点源近似模型计算结果都很好地证明了阵列瞬态电磁脉冲的高效合成特性。     

Diverging/focusing of electromagnetic waves by utilizing the curvedleakywave structure: application to broad-beam antenna for radiatingwithin specified wide-angle

A new “broad-beam antenna” for radiating electromagnetic energy in a specified wide angular region is proposed, utilizing the “curved leakywave structure”. This structure consists of the leakywave structure curved physically and longitudinally into the form of an equi-angular spiral. The expression of the radiation field from this structure is given, the effects of various geometrical parameters on the radiation properties is discussed numerically for the design of the broad-beam antennas, and a simple experiment is also performed to verify the beam broadening by the proposed method. The concept in this paper may be easily applied to any leakywave structure such as a periodic dielectric antenna and a leaky nonradiative dielectric (NRD)-guide antenna for millimeter-wave application     

Electromagnetic Source Azimuth Measurement Using Electrooptical Electromagnetic Field Probe

A new type electrooptical electromagnetic field probe was fabricated by combining three magnetic field sensors, each of which consists of a Mach–Zehnder modulator and an integrated annular antenna. The frequency response was flat from 50 MHz to 2.2 GHz. The average minimum detectable electromagnetic field intensity of the sensor is 2.22 mV/m. When the probe axis was pointed toward the electromagnetic radiation source, the highest sensitivity was obtained. The variation of the sensitivity is within $pm$0.5 dB with rotating the probe around its axis. This result indicates that the azimuth of the electromagnetic radiation source can be measured.      

Further modification of the radar range equation due to slow decayeffect in electromagnetic radiation

Conventional radar range equations based on the distance inverse-square spreading of energy should be further modified when taking into account a significant energy slow decay effect of the radiation. It is shown that the modification due to asymptotical energy slow decay effect in ultrawide-band (UWB) radiation can be neglected to certain extent since it is essentially equivalent to a continuous wave (CW) modification for a specified radiation frequency defined as the “characteristic frequency”. For a wide-spectrum short-pulse (SP) radar, notable modification due to significantly enhanced slow decay effect should be taken into account. Generally, a reduction of the radar performance compared to that calculated for the inverse-square spreading of energy is obtained. The prospect of improving the radar range performance with an electromagnetic missile is greatly reduced by the harsh excitation needed to excite it. Moreover, the modified radar equation contains time-space source parameters such as the signal waveform and the current distribution of the antenna. Thus, an SP radar has a specialized radar equation and due attention should be paid to source parameters in the design of such a radar     

A bound on EMP coupling

A bound on the energy coupled to a load by means of an antenna or aperture subjected to a step function electromagnetic pulse (EMP) is given by the polarizabilities of the antenna or aperture. Examples of coupling bounds for a slender electric dipole antenna, a circular aperture, and a narrow slot aperture with depth are given. It is demonstrated, by use of the bound on the integral of the effective area over all frequencies, that the maximum energy absorbed by an object subjected to a step function EMP is bounded in terms of the static polarizabilities of the object. (The energy spectrum of the step function EMP bounds the spectrum of EMP described by the double exponential over all frequencies.) It is demonstrated that this bound can be used on apertures in infinite ground planes for which Babinet's principle fails to apply (apertures having depth and cavity backed apertures), where the requirement is simply to use the polarizabilities on the incident side of the aperture.<>     

Ultimate signal-to-noise-ratio of surface and body antennas formagnetic resonance imaging

Approximating the human body by a homogeneous half-space or cylinder the electromagnetic fields of surface antennas or wholebody antennas for magnetic resonance imaging (MRI) can analytically be calculated. Using these expressions the signal-to-noise ratio (SNR) of a general magnetic resonance detecting antenna can be predicted. We show how the optimum magnetic resonance antenna must look like to achieve the maximum SNR, which is also presented. Finally, we apply the derived formulas to special antenna geometries like the single- and the double-loop coil, the magnetic dipole, and the “birdcage” resonator     

Characterization of an optoelectronic terahertz beam system

The performance of an optoelectronic terahertz (THz) beam system is described. The transmitter operation is based on the repetitive, subpicosecond laser excitation of a Hertzian dipole antenna embedded in a charged coplanar line. With this transmitter electromagnetic beams of 1/2 cycle THz pulses at a repetition rate of 100 MHz are produced. The associated optoelectronic receiver is gated in synchronism with the excitation of the transmitter by subpicosecond pulses from the same laser source. With this receiver, the 10-nW beams of THz pulses were observed with a signal-to-noise ratio greater than 10000:1. Several sources contributing to the noise of the receiver are discussed, together with ways to reduce them. With an integration time of 125 ms, a signal-to-noise ratio of 1 is obtained for a THz beam with an average power of 10^-16 W. The receiver operates in the sampling mode and has a time resolution of 0.5 ps     

微波传能发射系统电磁辐射安全性研究

无线传能系统工作时在周围空间会激发高频电磁场,因此对无线传能系统有较高的电磁兼容性指标要求.文中通过对无线微波传能系统发射天线周围环境的电磁辐射进行建模和分析,以及微波传能基站对周围环境的电磁影响,研究了无线微波传能发射端周围空间电磁辐射的安全性.发射功率为500 W时,通过计算人体头部的比吸收率值,根据国际射频暴露的安全导则,基于理论分析和仿真实验界定了人体电磁暴露的安全区域,划定基站天线主波束、旁瓣、后瓣的安全距离分别为25 m、3 m、2 m,并给出了无线微波传能基站部署的参考方案,对于无线微波传能的实际应用具有重要的参考意义.     

Accurate determination of antenna directivity

The derivation of a formula for accurate estimation of the total radiated power from a transmitting antenna for which the radiated power density is known in a finite number of points on the far-field sphere is presented. The main application of the formula is determination of directivity from power-pattern measurements. The derivation is based on the theory of spherical wave expansion of electromagnetic fields, which also establishes a simple criterion for the required number of samples of the power density. An array antenna consisting of Hertzian dipoles is used to test the accuracy and rate of convergence of the derived formula. Its performance is compared to that of numerical integration using Simpson's formula     

Design and experimental investigation of the asymptotic conicaldipole antenna

The paper describes the application of the “equivalent charge distribution method” for designing the geometry of a special type of dipole antenna used for the faithful reproduction of electromagnetic pulse type waveforms. Using this method, the design equation of the surfaces of an asymptotic conical dipole antenna (ACD) has been derived. The surface of the asymptotic conical dipole antenna, obtained by this method, is compared with those computed by the method of Baum (1969). The response of an ACD antenna, fabricated using design data computed from the design equations derived in the paper, is measured and presented     

A Transmission Line Method to Compute the Far-Field Radiation of Arbitrarily Directed Hertzian Dipoles in a Multilayer Dielectric Structure: Theory and Applications

A transmission line method is proposed to compute the far-field radiation patterns of arbitrarily directed Hertzian dipoles that are embedded in a multilayer dielectric structure. The evaluation of the field in the far-zone region is transformed into the evaluation of the field inside the multilayer structure by applying the reciprocity theorem. The horizontal field component inside the structure is derived by analyzing a transmission line circuit, and the vertical component is obtained from the horizontal component by separating the forward and backward waves. This method is implemented and verified by IE3D for the case of a three-layer structure excited by either electric Hertzian dipoles, magnetic Hertzian dipoles, or their combination. The radiation patterns of any antenna embedded in a multilayer dielectric structure can be computed with this method after replacing the physical antenna with a set of Hertzian dipoles. As examples, a quarter wavelength thin wire monopole antenna and a dielectric resonator antenna, both embedded in a multilayer structure, are investigated. Furthermore, the arrangement of the structure is optimized to maximize the antenna directivity. The results are also verified by the simulation of the entire structure with IE3D.     

表面波放电狭缝天线辐射电磁场的三维数值分析

针对平板型表面波放电等离子体源,建立了表面波放电狭缝天线辐射电磁波模型,对狭缝天线辐射电磁场分布进行了三维数值计算,并与表面波电磁场进行对比分析,讨论了平板型表面波放电机理。结果表明:整个狭缝天线阵激发的电磁场是每个狭缝天线激发电磁场的线性叠加;狭缝天线阵直接激发的电磁场强度在临近波导壁面处很大,并且随着空间距离的增大迅速衰减;狭缝天线阵直接激发和表面波的电场均远大于各自的磁场,分析电、磁场对带电粒子的力作用时可以忽略磁场力的作用;表面波电磁场远大于狭缝天线阵直接辐射的电磁场,强电磁场范围也远大于狭缝天线阵直接激发的强电磁场范围,等离子体有增强电磁场强度、扩大强电磁场范围的作用。     

Procedure for new draft of Defence Standard 59-41-calibratingscreened room antenna combinations against OATS

Describes a procedure developed to calibrate screened room/antenna combinations for measurement of radiated emissions in the frequency range of 10 kHz to 30 MHz. The work was performed in order to derive a new draft version of the U.K. Defence Standard 59-41 for electromagnetic compatibility (EMC). Results of measurements using specially designed test equipment are presented as are results of measurements made using “real” equipment. The procedure, which is suitable for automatic analysis using spreadsheets, requires the measurement of both electric and magnetic fields and predicts the fields, which would be measured on an open field test site from the results obtained in a screened room. The aim of the procedure is to provide more repeatable measurement results that are comparable between test houses. It also avoids the reliance on antenna factors, which may have significant errors in the screened room due to the proximity of the conducting walls     

On the Dispersive Nature of the Power Dissipated Into a Lossy Half-Space Close to a Radiating Source

We study how the power dissipated into a lossy medium excited by a nearby antenna is affected by drifts in the electrical parameters of the lossy medium. The statistical distribution of the sensitivity of the dissipated power is determined by means of a spectral analysis of the transmission of electromagnetic energy from air into the lossy half-space. A clear link is drawn between the reactive content of the field excited by the source and the dispersiveness of the sensitivity. The case of a stratified structure is also addressed, by defining a modification factor representing the alteration of the transmissivity and of its sensitivity when a buffer layer is introduced. All of the results provided point out that, in general, the sensitivity of the total amount of power dissipated into the half-space cannot be predicted independently from a precise knowledge of the source characteristics, unless under a paraxial propagation approximation or in a far-field configuration.     

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.     

Problem Solving in Electromagnetics, Microwave Circuit, and Antenna Design for Communications Engineering (Warnick, K. and Russer, P.; 2006) [Reviews and Abstracts]

Students and researchers have always seen classical electromagnetic theory as a hard-to-digest concept. In this book, attempts have been made to explain the facts in an approachable manner. The contents of this book have been divided into two sections. The first half is the theoretical part, and the second half contains applications and numerical methods. Topics covered include: the fundamentals of differential calculus, a brief review of basic electromagnetics; potentials and wave concepts; the concepts, methods, and theorems about the distribution of electromagnetic energy through space; static and quasistatic fields; the analysis of various waveguides' classical transmission-line theory; resonant circuits and resonators; microwave circuits; periodic structures and filters; radiation from dipoles; antenna basics; and numerical methods used in solving electromagnetic problems.     

Passive electro-optic antenna using polymer material

The experimental results of a polymer microwave electro-optic antenna are reported. The device amplitude-modulates an optical input beam with a free-space propagating electromagnetic wave. By using a new dipole printed antenna, the electromagnetic energy is concentrated inside the device. An antenna factor of 168 dB/m is achieved with only 1 mum of electro-optic polymer