An Efficient FDTD Model for Resistively Loaded Cylindrical Antennas With Coaxial Lines

A full coarse-grid based finite-difference time-domain (FDTD) model is proposed for an efficient analysis of resistively loaded cylindrical antennas driven by coaxial feed lines. In the case of the electrically thin resistive antenna, the thin-wire approximation is applied to the near fields around the antenna. The resistive antenna is equivalently represented by a series connection of piecewisely lumped resistors along the antenna axis. And the coaxial line is replaced by an equivalent source over the feed aperture of the line. Then the corresponding FDTD update equations make it possible to implement the full coarse-grid model without additional grid refinements for the antenna and the feed line. The transient reflected feed voltage and the input impedance of resistive antennas are calculated numerically and compared with those of a full fine-grid.      

Optimization of transient feeding to parallel-plate transmission lines from coaxial line

The transient feeding to parallel-plate transmission lines from coaxial line is optimized by using the Finite-Difference Time-Domain (FDTD) method and a simple FDTD feed model. Observing the reflected voltages, this letter presents the optimal feeding position and ratio of width to height for a given input impedance of the coaxial line.     

On the Excitation of a Coaxial Line by an Incident Field Propagating Through a Small Aperture in the Sheath

A coaxial line having a small aperture in the sheath and terminating in arbitrary impedances is considered to be illuminated by an incident plane wave. The excitation of currents in the termination impedances is shown to depend upon three independent factors. Simple analytical forms are given for equivalent voltage and current sources of the aperture excitation.     

An accurate theoretical model for the thin-wire circular half-loopantenna

The conventional Fourier series analysis for the thin-wire circular transmitting loop, or its image equivalent to the half-loop, uses a delta-function generator for excitation. This method of excitation introduces two problems: it does not correspond to any realizable method of feeding the antenna, so an accurate comparison with measurement is not possible, and it produces a divergent series for the input susceptance. To overcome these problems, a new theoretical model is used for the antenna: a half-loop driven through an image plane by a coaxial transmission line, with a transverse electromagnetic mode assumed in the aperture of the coaxial line. This model is solved in a manner that preserves the simplicity of the original Fourier series analysis. All coefficients are obtained as closed-form expressions. Input admittances calculated from this new theoretical model are in excellent agreement with accurate measurements     

Excitation of a Coaxial Line Through a Transverse Slot

A rocket with removed access plate is simulated by a section of coaxial transmission line with a transverse elliptical slot cut in its sheath. The internal circuit consists of two arbitrary impedances in series with the inner conductor at its ends. The object is to find the currents in these impedances when the cylinder is illuminated from the outside by an electromagneticfield that enters the aperture and excites the internal circuit. The problem is solved by application of the reciprocal theorem. The current in a dipole antenna is determined when this is inthe far field maintained by the slotted coaxial line when driven by a generator in series with one of the load impedances. The field in the aperture is replaced by equivalent electric and magnetic dipoles. The reciprocal theorem gives the current in the load impedance when the distantdipole is driven. A numerical example is given.     

Leaky coaxial cable with adjustable coupling loss for mobilecommunications in complex environments

A new kind of leaky coaxial cable composed of an axially-slotted line covered with periodic metallic patches is proposed. The basic cable sets up a surface wave and does not radiate itself, but excites the patch apertures into radiation. The direction of the radiation field can be changed by adjusting the size and period of the patches according to the environmental application. A combined method that involves FDTD iteration and integration of the equivalent surface magnetic current is introduced in order to determine the radiation field accurately     

Modelling of coaxial-fed microstrip patch antenna by finite difference time domain method

A direct three-dimensional finite-difference time-domain (FDTD) method is applied to coaxial-fed microstrip antennas. The model is shown to be an efficient and accurate tool for modelling coaxial-fed structures. The reflection coefficient can be determined from the simulated time-domain wave that is reflected down the coaxial line. Excellent agreement over a wide frequency range is shown in two cases between the measured and FDTD derived results.<>     

Radiation from a dielectric coated hemispherical conductor fed by acoaxial transmission line

A dielectric-coated hemispherical conductor mounted on an infinite perfectly conducting ground plane and fed by a coaxial transmission line is investigated. Green's functions for the region above the ground line are derived with separated homogeneous and particular solution parts so as to be compatible with numerical analysis techniques. A magnetic field integral equation is constructed in terms of the unknown annular aperture tangential electric field and is solved by the method of moments. A comparison of the characteristics of the dielectric-coated hemispherical conductor and a flush-mounted coaxial line to an infinite homogeneous region above the ground plane is presented with respected to the tangential aperture electric field, with respect to the tangential aperture electric field, the coaxial line apparent input impedance, and the far radiated field     

On the Pulse Response of a Flush-Mounted Coaxial Aperture

The radiation field pattern of a flush-mounted coaxial aperture is derived in the frequency domain. By using the Lorentz reciprocity theorem, the equivalent current generator is obtained which drives the apparent annular antenna admittance in shunt with the load admittance. The current source, aperture admittance, and load admittance determine the transfer function. The transfer function multiplied by the expression for the spectrum of the incident pulse forms the integrand of the Fourier transform from which the time history of the current waveform in the load admittance may be obtained using a computer. There are several applications of the theory. The one of dominant interest at present may be described as follows. A coaxial transmission line terminated in a known load admittance is located in the interior of a missile. The free end of the cable forms an aperture antenna at the skin of the missile, i.e., at the normal junction of coaxial connectors. The problem is to determine the time history of the current in the load admittance when an intense transient electromagnetic field impinges on the receiving antenna. Very accurate numerical results may be obtained from this theory provided the ground plane is sufficiently large and certain dimensional limitations on the sizes of the inner conductor and sheath of the coaxial aperture (expressed in terms of the wavelength) are met.     

Accurate characterization of planar printed antennas usingfinite-difference time-domain method

The finite-difference-time-domain method (FD-TD) is used to characterize complex planar printed antennas with various feed structures, which include coaxial probe feed, microstrip line feed, and aperture coupled feed structures. A coaxial probe model is developed by using a three-dimensional FD-TD technique. This model is shown to be an efficient and accurate tool for modeling coaxial line fed structures. A novel use of a dispersive absorbing boundary condition is presented for a printed antenna with a high dielectric constant. All the numerical results obtained by the FD-TD method are compared with experimental results, and the comparison shows excellent agreement over a wide frequency band     

Considerations on performance evaluation of cavity-backed slotantenna using the FDTD technique

The finite-difference time-domain (FDTD) method is considered a versatile and efficient tool for the solution of Maxwell's equations in complex structures for any time dependence. We show an antenna feed model suitable for performance evaluation of a cavity-backed slot antenna using the FDTD technique. The gap voltage and the coaxial feed models are examined, and their input characteristics and absolute gains are compared. Analytical results show that the input characteristics are estimated with fewer time steps for the coaxial model than for the conventional gap voltage model. Furthermore, we show how to calculate absolute gains and radiation patterns using the coaxial model and a sinusoidal voltage source at the desired frequency. The computed results of the absolute gain converge after the fifteenth period of the voltage source for the coaxial model and are in good agreement with the experimental results. On the other hand, the absolute gain is observed to fluctuate when the gap voltage model is used. The performance evaluation and comparison reveals that the coaxial model is an appropriate feed model for use in the analysis of the performance of the cavity-backed slot antenna using the FDTD technique. The good agreement of the FDTD results with the experimental measurements demonstrates the effectiveness of the model and the method proposed     

Semi-empirical modeling of apertures for shielding effectiveness simulations

A novel method to include models of complex apertures into the finite-difference time-domain (FDTD) method is presented. Instead of resolving the geometrical details of the aperture, the aperture is treated as a magnetic dipole. The properties of the magnetic dipole moment are determined using the measured transmission cross section of the aperture. This semi-empirical model permits inclusion of complex apertures in FDTD simulations where the smallest dimension of the aperture is only a fraction of the FDTD cell size. Two types of complex apertures positioned in the top panel of a realistic enclosure have been modeled using the semi-empirical model in FDTD. Comparisons between simulated and measured shielding effectiveness of the enclosure are presented, and the results show that complex apertures can indeed be represented by simple magnetic dipoles in shielding effectiveness simulations.     

Effective permittivity at the interface of dispersive dielectrics in FDTD

A simple treatment of E-field component tangential to dispersive media interfaces in FDTD is introduced. The method uses concepts from the auxiliary differential equations method to average the constitutive parameters. The cases of a wave propagating in a coaxial line and of an open-ended coaxial line radiating into the dispersive media are investigated. Results show that the simulations could be significantly erroneous if the interface is not handled properly.     

一种含同轴电缆的线束内部串扰高效分析方法

为了高效分析含同轴电缆的线束内部串扰,提出了一种新的简化电缆束模型的方法. 该方法把简化电缆束的过程分为两步:首先,为了解决同轴电缆编织屏蔽层难以仿真的问题,利用内外传输线转移矩阵简化同轴电缆,将含同轴电缆的电缆束转化为全由单芯线组成的三维电缆束模型;然后,使用等效线束法将全单芯线的电缆束进一步简化,减少需要分析的单芯线数量. 简化后的模型可用于任何三维电磁仿真软件中的电缆束内部串扰仿真分析,而无需考虑基于经典传输线理论的等效电路法的局限性. 将提出的方法与等效电路法分别应用于电缆束内部串扰的分析,两者的仿真结果一致性良好,说明文中提出的方法是准确可行的,能够实现含同轴电缆的线束内部串扰的高效分析.     

基于FDTD方法的同轴电缆孔缝辐射效应研究

利用时域有限差分法(FDTD)研究了同轴电缆传输数字脉冲信号时的孔缝辐射效应。文中首先推导了三维柱坐标系下完全匹配层(PML)的差分表达式，并给出保证其解稳定性的更为严格的时间步长公式。然后利用FDTD法分析了数字信号经同轴电缆孔缝的辐射效应，对孔缝内外的电磁场及其远场进行了数值分析和计算。处理孔缝边界时，用级数展开和积分方程得到金属拐角处电磁场的近似解。最后对结果进行了分析。     

单极天线和偶极子天线辐射特性数值计算*

采用时域有限差分法(FDTD)在柱坐标系下对同轴线馈电单极天线辐射特性进行数值计算,推导了二维扩展柱坐标下修正的Maxwell方程,应用辅助微分方程FDTD法对修正的Maxwell方程进行了差分离散,以TM波在自由空间中的传播为例,仿真了电磁波在自由空间中传播过程并分析了坐标伸缩完全匹配层的性能,建立了单极同轴线馈电天线仿真模型,仿真分析了同轴线馈电单极天线的近场、远场及阻抗特性。在三维直角坐标系下建立了偶极子天线仿真空间模型,数值计算了偶极子天线不同切面的远场方向图,仿真结果与客观实际相同,验证了空间模型及相关理论的正确性。     

Theory on small radiating apertures in the outer sheath of a coaxial cable

An analytical expression for the aperture field is obtained from an integral equation formulation and a subsequent use of a quasistatic approach in which the dominant part of the solution can be extracted from canonical problems. Unlike the conventional approach of replacing the aperture field by equivalent dipoles, our method retains the basic features of the propagating mode in the coaxial region, and the radiation characteristic of the coaxial cylinder. An equivalent network representation of a small aperture is then derived. The network equivalence of a symmetrical aperture with its axes oriented in the direction of the incident field is shown to consist of a lumped shunt capacitanceC_{b}and a series impedanceZcomposed of an inductive elementL_{b}, a capacitive elementC_{T}and the input admittanceY_{binfty}of the external antenna, all in parallel.     

An efficient numerical approach for modeling microstrip-typeantennas

An efficient numerical approach to model antennas that include a microstrip element radiating in the presence of material layers is developed. The class of antennas considered is fed through the ground plane by a coaxial transmission line. The reaction integral equation is formulated by treating the coaxial aperture as part of the antenna. The substrate thickness can be arbitrary, making this numerical technique suitable for high-frequency applications. The effects of the substrate are also included in the analysis. Numerical results are obtained for the current distribution and input impedance. The algorithm is validated with experimental results     

利用FDTD 法分析同轴馈源性能

本文利用FDTD 方法分析了同轴馈源的性能。文章首先介绍了共形FDTD 的建立方法,利用该方法分析了不 同激励形式时同轴馈源的性能,并给出同时形式和模方向图和差模方向图的工程实现方法。同轴馈源对实现反射面天 线的双频段馈源设计具有重要意义。     

Optimization of bow-tie antennas for pulse radiation

Resistively loaded bow-tie antennas are considered as radiators for temporally short, broad-bandwidth pulses. Analysis is by the finite-difference time-domain (FDTD) method. The geometrical details of the antenna and the resistive loading along the antenna are chosen to optimize this antenna for pulse radiation. Theoretical results for the reflected voltage in the coaxial feed line and the time-varying radiated electric field are compared throughout with experimental measurements. The optimized, resistive bow-tie antenna is shown to radiate a pulse that more closely resembles that of the excitation than is radiated by a metallic bow tie of comparable size. Issues involving the use of the FDTD method for modeling fully three-dimensional antennas are also discussed. These issues include the use of a simple feed model and the staircasing of the edges of the antenna