Transient radiation from a linear antenna with nonreflecting resistive loading

The transient waveforms radiated by step voltage or Gaussian pulse excited resistively loaded linear antennas are investigated by numerical means. Current distributions on the harmonically excited antenna are obtained as functions of frequency and for different values of the loading. The transfer functions of the antenna and the spectral densities of the radiated waveforms are obtained as functions of frequency and the loading and for different directions in space. The time dependent radiated waveforms produced by the antenna are obtained by using the fast Fourier inversion technique. The effects of the various antenna parameters on the radiated waveforms are also investigated. Some of the results are compared with available approximate analytical results.     

Waveform optimizations for ultrawideband radio systems

Solutions are presented for various optimizations of transient waveforms and signals used in ultrawideband radio systems. These include the transmit antenna generator waveform required to maximize receive antenna voltage amplitude (with bounded input energy), the transmit antenna generator waveform that provides the "sharpest" received antenna voltage waveform, and the transmit antenna generator waveform that maximizes received energy with an inequality constraint on the radiated power spectral density. Using variational methods, general optimization results are derived for arbitrary antennas, including the effects of generator and load impedances, and numerical examples are provided for lossless dipoles and resistively loaded dipoles using moment method solutions. Closed-form results are provided for short dipole antennas for some special cases.     

Radiation Characteristics of Traveling-Wave Antennas Excited by Nonsinusoidal Currents

In this paper, the idea about traveling-wave antennas excited by nonsinusoidal currents for high-resolution exploration is advanced. First the analytic expressions of the radiation waveforms in the far zone produced by a single transient of current traveling along a straight wire are derived in time domain. Then the radiation characteristics of traveling-wave antennas excited by nonsinusoidal current, such as the mean-power pattern functions and the directivity, are given, and the space waveform control and radiation efficiency are discussed. Based on previous results, the radiation waveforms of dipole antennas excited by pulse current are obtained from time- domain analysis. In a practical example, comparing these waveforms with corresponding waveforms obtained from frequency-domain analysis and measured data in [4] and [13], good agreements are obtained in various radiation directions. Some errors in [14] are also pointed out and corrected.     

脉冲激励的长方形发射天线的分析与波形优化

在脉冲激励天线中，脉冲波形对于辐射功率有着重要的影响。对长方形行波天线的激励电流脉冲波形与其辐射功率的关系进行了分析。提出了激励电流的优化问题，使得天线在给定方向上的辐射能量最大化。并且给出了优化问题的解法和几种参数下的优化波形。通过具体的数值计算，与其他参考文献中使用的波形相比，优化波形在主辐射方向上可以辐射出更大的能量。     

Transient response of thick axially symmetric monopoles

The steady-state results for thick hemispherically capped monopoles with or without conical feed sections are used to construct the transient response of such monopoles. For a voltage pulse excitation, file transient waveforms for the transmitted-reflected feed currents, the radiated fields in different directions, and the instantaneous currents on the monopoles are calculated and presented. Steady-state results are obtained by solving the scalar potential integral equation with the axis extended boundary condition using the moment method.     

A simple FDTD model for transient excitation of antennas bytransmission lines

A simple FDTD model is developed for use with antennas that are fed from transmission lines. The model is especially designed for use with transient excitations, where the incident and reflected waveforms within the transmission line are of interest, and the latter is determined directly in the FDTD calculation. The model is verified for both transmission and reception of transient waveforms by comparison with measured results for a cylindrical monopole antenna with a plane reflector     

Transient response of a receiving dipole array: bounds andmaximization

The optimum solution for a receiving array of dipole antennas connected to a load through a feed network is obtained. It is given in terms of the incident electric field waveform that maximizes, at a specific time t₀, the received voltage across a specified load impedance. This result is used to set an upper limit on the peak voltage that the antenna can deliver to a load, thus providing guidelines for the design of pulse-protection circuits. The analysis uses the method of moments to approximate the currents induced in the wires by the incident field and S-parameter techniques to describe the feed incident electromagnetic plane waves. The Lagrange multiplier technique is then used to maximize the receive load voltage. The solutions presented are based on an idealized model for the feed network; however, the analysis can be extended easily to account for loss and asymmetry of physically realizable feed networks     

Scattering of electromagnetic waves by a slowly rotating rectangular metal plate

The amplitude modulation waveforms of linearly polarized electromagnetic waves due to the scattering by slowly rotating metal plates are investigated theoretically and experimentally. The nature of the modulation waveform depends on the orientation and dimensions of the object, its rotation speed, and very strongly on the incident and receiving (or scattering) directions. For a rectangular metal plate rotating in its own plane, the modulation is independent of time in the specular and forward directions. Near the specular and forward directions the modulation waveform is a sinusoidal function having a frequency equal to twice the rotation frequency of the plate. Further away from these directions, the waveform is a sinc-pulse type repeating at twice the rotation frequency. Similar results are also discussed for a rotating skewed metal plate. The observed waveforms are found to be in good agreement with theoretical results.     

On the transient radiation of energy from simple currentdistributions and linear antennas

Smith (1998) examined the radiation from two simple filamentary current distributions: traveling-wave and uniform. The radiated or far-zone electric field was computed for an excitation that was a Gaussian pulse in time. Two interpretations for the origin of the radiation were presented, based on the far-field results. The present article continues this investigation; however, the emphasis is on an examination of the near field and the related transport of energy away from the current filament. We examine traveling-wave and standing-wave current distributions, because these distributions are frequently used to model practical antennas. Exact analytical expressions are presented for the electric and magnetic fields of the assumed, filamentary current distributions when the excitation is a general function of time. For the filamentary distributions, the current and charge are confined to a line (a line source). There is no radius associated with the filament. The expressions for the fields apply in both the near and far zones, and are used to determine the Poynting vector. For an excitation that is a Gaussian pulse in time, exact analytical expressions are obtained for the energy leaving the filament per unit time per unit length, the total energy leaving the filament per unit length, and the total energy radiated. Graphical results based on these expressions are used to study the energy transport from the filamentary current distributions. The results for the standing-wave current distribution are compared with those from an accurate analysis of a pulse-excited, cylindrical monopole antenna, performed using the FDTD method     

Diverse waveform generation using semiconductor lasers for radar and microwave applications

The possibility of using semiconductor lasers to conveniently generate diverse microwave waveforms for radar and microwave applications is studied both numerically and experimentally. Such waveforms are generated from the dynamical states of semiconductor lasers in different perturbation schemes and varying operating conditions. Using an optical injection scheme, broad-band chaotic microwave waveforms and tunable narrow-band harmonic microwaves over a broad frequency range can be generated. Using an optoelectronic feedback scheme, chaotic pulsing, regular pulsing, frequency-locked pulsing, and quasi-periodic pulsing waveforms are generated. These optically generated microwave waveforms can be easily amplified and radiated out using microwave amplifiers and antennas. The power spectra, time series, and autocorrelation traces of such waveforms are studied. The peak-sidelobe level is calculated to quantitatively compare the correlation characteristics of these waveforms. A broad-band chaotic waveform with a clean single-spike /spl delta/-function-like correlation profile useful for radar and other applications that demand unambiguous correlation profile is demonstrated experimentally.     

Measurement of electromagnetic fields near a monopole antennaexcited by a pulse

To observe transient radiation fields, a technique for reconstructing electromagnetic (EM) waveforms using the complex antenna factor (CAF) have been developed. However, the CAF is originally defined assuming plane wave incidence, while the waveforms are measured in a vicinity of the radiating source. In order to examine the reconstruction technique using the CAF in the near-field measurements, the EM fields radiated from a monopole antenna excited by pulsed input voltage was reconstructed and compared with calculated results. For the geometry of the experiment, the waveforms of the reconstructed and calculated EM fields have shown good agreement. Therefore, the reconstruction technique using the CAF can be used for similar dimensions or at a greater distance when sufficient sensitivity is provided     

器件与电路参数对SET波形影响仿真研究

利用Sentaurus TCAD仿真软件,建立并校准了MOSFET仿真模型。分析了NMOS器件在重离子轰击下产生的SET波形。结果表明,轰击位置在漏极且入射角呈120°时,器件具有最大的峰值电流。通过建立MIX、TCAD、SPICE三种反相器模型并施加重离子轰击,研究了不同模拟方式下电路响应对SET波形的影响,指出了采用双指数电流源在SPICE电路中模拟的不准确性。采用MIX模型探究了器件结构及电路环境对SET波形的影响。结果表明,LET能量、栅极长度、轨电压和负载电容都会对SET波形脉宽及平台电流大小产生显著影响,说明了建立SET模拟波形时须综合考虑这些因素。     

A Study of the Single-Frequency Quenched-Domain Mode Gunn-Effect Oscillator

A large-signal analysis of the high-frequency quenched-domain mode (Q-mode) of Gunn-effect devices has been developed. This is a phenomenological model and includes such effects as distinct domain formation and quenching processes, domain behavior in the presence of an RF voltage, displacement current, voltage dependence of the domain width, and others. The basis of the analysis is to obtain an instantaneous current-voltage transfer characteristic for the device and use it to generate the current waveform corresponding to a given periodic voltage waveform. The basic results of the large-signal analysis are then obtained from these current waveforms. The analysis reveals the distinguishing features of Q-mode oscillators. Admittance measurements on the oscillating device have been made and support the theoretical results for single-frequency operation. Applications of this analysis for investigating other nonlinear properties of these devices are suggested.     

A study of pulse radiation from several broad-band loaded monopoles

Five previously proposed designs for broad-band monopole antennas are evaluated for pulse radiation. These designs use continuous resistive loading and/or discrete capacitive loading to increase the bandwidth over that of a simple, metallic monopole. The parameters for each of the designs are scaled so that the designs can be compared on a common basis (frequency range). Each of the antennas is analyzed numerically, and quantities characteristic of their pulsed performance are computed. These quantities include the reflected voltage in the feeding transmission line, radiated electric field, radiating efficiency, time-domain gain, fidelity, and symmetry when the monopoles are excited by a differentiated Gaussian pulse. In addition, the input reflection coefficient and gain at broadside for monochromatic excitation are shown. Explanations are provided for the differences in performance for these designs     

Evaluation of modulation schemes for three-phase to three-phase matrix converters

This paper presents a method for evaluating different modulation schemes employed with three-phase to three-phase matrix converters. The evaluation method addresses three important modulator characteristics: the output waveform quality, the input waveform quality and the switching losses associated with the modulation schemes. The method is used to evaluate four different modulation strategies, all based on the direct space-vector modulation approach. Further, regarding the switching losses, the paper proposes a new space-vector approach by which the switching losses can be reduced by 15%-35%, depending on the output load angle. This new modulation approach is applicable whenever the output voltage reference is below half the input voltage and the output voltage quality is then superior to that of the conventional space vector modulation scheme. The functionality of the new modulation scheme is validated by both simulations and experimental results and compared to waveforms obtained by using exiting space vector modulation schemes. The output voltage of the proposed scheme turns out to be comparable to the best of the conventional schemes while the input current is more distorted.     

Physical limitations on the measurement of transient fields in air and in dissipative media using electric and magnetic probes

The properties of electric and magnetic probes for the measurement of transient electromagnetic fields in air and in dissipative media are discussed briefly. It is shown that the effective height of an electrically small loop is independent of the ambient medium. This is also virtually true for a thin electrically short dipole (or monopole). If the open-circuit voltage of a magnetic probe can be measured accurately, it is possible (in principle) to reconstruct the time history of the incident magnetic field, even if the loop is immersed in dissipative media of unknown characteristics. The time function of the open-circuit voltage of an electric probe is essentially a replica of the time history of the incident electric field, even when the probe is immersed in a dissipative medium. For impedance-loaded probes the equivalent circuit of the receiving antennas involves the source impedances of the probes. These depend on the properties of the environment. In a dissipative medium a distortion of the response to transient fields results.     

TRAPATT oscillations in a p-i-n avalanche diode

The characteristics of TRAPATT oscillations in a p-in diode are discussed and an approximate semi-analytical solution for the diode voltage waveform is derived when the diode current is a square wave. It is shown that a traveling avalanche zone is not necessary to generate a dense "trapped" plasma and that the boundary conditions prevent the trapped plasma from completely filling the depletion layer. Typical voltage waveforms and corresponding diode power, efficiency, and impedance at the fundamental and higher harmonics are presented. When the diode current is a square wave the diode does not necessarily exhibit a negative resistance at all higher harmonics. A computer program for TRAPATT oscillations in a p-i-n diode is described. Its running time is two or three orders of magnitude less than more exact time domain computer analyses. Typical results of diode power, dc to RF conversion efficiency, and required circuit impedances are presented for several different current waveforms which are composed of up to the seventh harmonic of a square wave and the first two harmonics of a half-wave sine wave. It is shown that high-efficiency oscillations are possible with diode currents composed of only the fundamental and one harmonic.     

Transient response of the infinite cylindrical antenna and scatterer

The transient response of an infinite cylindrical antenna and scatterer is obtained by time gating a time-dependent electric-field integral equation for thin wires of finite-length. The driving point current and broadside radiated field approximate the driving point voltage in the antenna case, while the current excited in the scattering case approximates the time integral of the incident field. This approximate integral relation is used to estimate currents excited on cylindrical structures without employing the complete integral equation solution procedure.     

目前临床医用微波辐射器的缺陷与改进方向的研究

利用蛋清凝固热图实验和漏能功率仪测量，分析了不同的医用微波辐射器的能量分布，指出单极子天线根部辐射能量较大，顶部较弱，不适宜作为医用微波体内辐射器，而实验显示螺旋型天线在靠近顶部时是最强的，辐射能量集中而有效，实验结果和多例临床应用证明：螺旋型天线稳定、高效、安全，在临床上用有着广泛的应用前景，建议螺旋型天线代替单极子天线作为研究方向。     

F类功率放大器漏极电压研究

F类功率放大器实现高效率的基本原理,是利用输出滤波器控制漏极输出的电压或电流波形。基于这一点,文中首先理论分析了在不同谐波比例下的漏极电压波形,然后利用电磁仿真软件进行验证。结果表明,仅有三次谐波和基波组合在一起时,当三次谐波和基波电压比K=0.111时,漏极电压波形最平坦;当K接近0.4时,漏极电压波形趋于方波。最后选择合适的谐波比例,设计了一款功率附加效率最大值达到88.074%的F类功率放大器。