Input admittance and complete near-field distribution of an annular aperture antenna driven by a coaxial line

An annular aperture antenna driven by a coaxial transmission line is studied. An analytical and closed-form expression for input admittance is obtained. The effect of all higher order TMonto the admittance is found to be quite significant. Aperture field distribution is expressed in terms of the TEM and the first five TMonmodes. Higher order modes at the aperture can be estimated from the edge behavior. Near-field distribution based upon these modes is then cast into a form suitable for numerical evaluation.     

Admittance correction factor for coaxially excited cylindricalantennas

A long standing problem in antenna theory has been the accurate determination of the input admittance of coaxially excited antennas. While reasonably accurate results have been obtained for a range of problems using the so called “TEM approximation” for the coaxial aperture electric field, these results nonetheless contain an error of small but unknown extent. By examining the infinite coaxially excited antenna problem this approximation error has been quantified and a simple approximation factor has been deduced. This correction factor is applicable to a range of antenna problems having a coaxial source geometry     

The annular aperture antenna with a hemispherical center conductor extension

An annular aperture antenna mounted on an infinite ground plane and containing a hemispherical center conductor extension above the ground plane is investigated. A Green's function for the region above the ground plane is derived so as to be compatible with numerical solution techniques. A magnetic field integral equation is obtained in terms of the unknown tangential aperture electric field and is solved by the method of moments. A comparison between flush mounted and hemispherically extended annular aperture antennas is presented for the tangential aperture electric field, the coaxial line apparent input admittance, and the far radiated field.     

The analysis of general axially symmetric antennas with a coaxialfeed line by the method of lines

The use of the method of lines in the analysis of various circular antennas-circular patch antennas and various forms of monopoles is proposed and substantiated. The antennas considered are fed by coaxial lines. Impedance/admittance transfer procedures are developed, which allow to calculate the antenna input impedance by a successive transfer from the aperture through the different sections. The described relations are also useful for other applications     

The Annular Slot Antenna in a Lossy Biological Medium (Short Papers)

An integral equation is formulated for a coaxially fed annular aperture antenna. The integral equation in terms of the unknown tangential aperture electric field is solved numerically by the Method of Moments. The coaxial feed line is air filled while the exterior region consists of i) air, ii) fat or bone, and iii) muscle. Results are given for the aperture electric field, apparent input admittance, and contours of constant power absorption when the excitation frequency is 2.45 GHz.     

Analysis of a tubular monopole loaded with a shielded helical coil

An analysis of a shielded helical coil in a cylindrical tubular antenna is presented, based on a combination of aperture theory and a network model containing lumped RLC components, which is motivated by the desire to use this analysis in antenna-optimization routines. Measured input admittance data for an antenna loaded with a shielded helical coil are presented for comparison with computed data. Also, an experimental procedure by means of which one may measure the transverse electro-magnetic (TEM) input impedance of a coaxial guide loaded with helical coil is described.     

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.     

Electromagnetic scattering from three-dimensional cavities via aconnection scheme

The electromagnetic scattering from arbitrary three-dimensional cavities is presented. To alleviate computational constraints for three-dimensional problems, a connection scheme is developed based on microwave network theory. This scheme allows the cavity to be divided into sections and each section to be analyzed independently of the rest of the cavity. Each section of the cavity is represented by a generalized admittance matrix which if formulated via a boundary-integral equation approach. Using the concept of input and load admittance, the aperture admittance matrix of the cavity can be derived by cascading the admittance matrices of individual sections. Once the cavity aperture admittance matrix is obtained, the aperture electric field and the backscattered field are found by the standard generalized network formulation. Numerical results are compared against modal solutions of regularly shaped cavities with good agreement. This connection scheme leads to a reduction in computational resources, especially for cavities with one dimension much larger than the other two     

Input admittance of a multilayer insulated monopole antenna

An efficient numerical technique based on the modal-expansion method in conjunction with a recursive algorithm is developed for a multilayer insulated monopole antenna fed by a coaxial transmission line. The modal-expansion analysis is facilitated by introducing a perfectly matched boundary (PMB) at a variable height over the ground plane of the monopole. The current distribution and input admittance are computed by finding the expansion coefficients of the electromagnetic field expressions. Numerical results for the input admittance of a dielectric-coated monopole antenna and an air-insulated monopole are compared with experimental ones available in the literature. Good agreement is achieved. Calculated results for the effects of various parameters on the input admittance of an air-insulated monopole antenna are presented and discussed     

A theoretical study of the input impedance of a circular microstrip disk antenna

A theoretical study is presented of the input impedance of a circular microstrip disk antenna excited by a coaxial line. The theory is based on Green's function technique applied to the disk cavity with the boundary admittance at the edge. Both the feed pin size and the boundary admittance are shown to be important in deriving the analytical expression for the input impedance. The boundary admittance is obtained by considering the radiated power and the electric and magnetic stored energies in the fringe capacitance. The analytical expression for the input impedance includes the feed location, the feed pin size, the disk size and thickness, and the dielectric constant of the material, and is useful for optimizing various parameters. The calculations are compared with experimental data showing a good agreement.     

Linear antennas in a warm plasma driven from a coaxial air-filled transmission line

The Green's function is formed for the warm-plasma region surrounding an antenna driven from a coaxial air-filled transmission line, and hence the current distribution and input admittance of three types of antenna having this feed system are computed. Comparison is made between the antenna characteristics when driven in this fashion and by an idealised delta gap.     

An Equivalent Feed Model for the FDTD Analysis of Antennas Driven Through a Ground Plane by Coaxial Lines

For the finite-difference time-domain (FDTD) analysis of antennas excited by coaxial lines, an equivalent model of the antenna feed is presented here. Under the quasi-static approximation, the coaxial aperture is simply represented as the equivalent magnetic-frill current and the equivalent load circuit including the effects of the coaxial line. It leads us to a simple modification of the standard FDTD update equation at the aperture without any additional cell modeling of the line. The validity of the proposed model is confirmed by comparing of the FDTD results and the measured data.      

Note Regarding the Propagation of Electromagnetic Fields Through Slots in Cylinders

A coaxial line is terminated at each end in an impedance. Excitation is provided by an incident plane polarized field that propagates through a small aperture in the sheath of the cable. The writers begin with the theory of diffraction by small holes presented by other investigators, and develop an exact theory for load current calculation.     

边界积分法及连接算法分析任意腔体的散射

本文利用边界积分法分析二维任意腔体的散射,给出一种基于微波网络原理的连接算法,将腔体分为几段,分别用积分方程法计算每段的广义导纳矩阵,然后利用连接算法将各段连接,得到整个腔体的口径导纳矩阵,最后由广义网络原理求解腔体的等效磁流及后向散射场。本文方法可作为一种机辅设计算法。     

An experimental study of the insulated dipole antenna immersed in a conducting medium

The driving-point admittance and the amplitude and phase distributions of the current referred to the driving-point were measured for an insulated cylindrical antenna immersed in a conducting medium. The ratiosigma/omegaepsilon_{r}epsilon_{0}of the conducting medium was varied fromsigma/omegaepsilon_{r}epsilon_{0}= 0.036to 8.8, a range which includes a variety of media such as poor insulators, the ionosphere, plasmas, dry earth, wet earth, lake water and sea water. The antenna heightbeta hin radians was varied frombeta h=0.1throughbeta h=2piat intervals of 0.1. The thickness of the insulator was varied fromb/a = 1.25tob/a = 12.0whereais the radius of the antenna andbthe radius of the insulator. Measurements have also been made of the admittance, current and phase distributions along an insulated antenna with a conductive top load, that is, one whose tip is in direct contact with the conducting medium. It is found that when the tip of the antenna is in direct contact with the conducting medium, the current increases almost linearly as the end of the antenna is approached. This is quite unlike the decaying sinusoidal distribution on the completely insulated antenna. In a general sense, the experimental results are in fair agreement with an approximate theoretical expression for the admittance of an insulated antenna immersed in a highly conducting solution. The approximate theory is based on the driving-point admittance of a coaxial line whose outer conductor is imperfectly conducting and infinite in extent.     

Analysis of linear coaxial antennas

Two types of linear coaxial antennas, coaxial-colinear antennas, and slotted coaxial antennas are studied to check the possibility of using them as the base-station antenna in personal communication systems. The slot voltages and input impedance of linear coaxial antennas are obtained by using a transmission-line analysis where the radiation effect is accounted by a shunt and a serial admittance, respectively. The current distribution is obtained by solving an integral equation using the method of moments. The radiation pattern and directivity are then obtained from the current distribution and the reflection coefficient inside the coaxial cable. Factors analyzed include frequency, coaxial filling permittivity and segment number     

空间激光通信离轴两镜反射望远镜自由曲面光学天线设计

同轴两反结构作为空间激光通信光学天线时常存在接收视场小、发射效率低等缺点。同时,因中心遮拦造成的能量损失,也会大大降低通信系统的瞄准和捕获概率。针对此缺陷,提出采用离轴两反天线结构并引入自由曲面设计以提高通信系统的性能。由初级像差理论推导得出离轴两反系统的初始结构,并采用Zernike多项式自由曲面面形,增加系统设计自由度,提高系统平衡像差的能力。给出入瞳口径与焦距相同的同轴两反和离轴两反系统的设计实例,并比较二者的光学传递函数MTF、点列图、衍射包围圆能量、波前误差等。结果表明:离轴两反自由曲面天线的像质明显优于同轴两反天线,因而该种天线结构的使用可提升空间激光通信系统的整体性能。     

Cylindrical antenna in a rectangular waveguide driven from a coaxial line

The problem of a cylindrical antenna in a rectangular waveguide driven from a coaxial line, where the antenna spans the entire guide height, is analysed, and the admittance evaluated. An experimental investigation shows the theory to be very accurate.     

Discussion of the relevance of transfer admittance and some throughelastance measurement results

Transfer admittance for electromagnetic compatibility (EMC) prediction is reviewed. A method for measuring the transfer admittance of cables is presented. Some experimental results are given for coaxial cables. Although the through elastance is not measurable, the authors show that it is at least one order of magnitude smaller than what current theory predicts     

A numerical solution for the near and far fields of an annular ring of magnetic current

A simple method for calculating the near and far zone fields from an annular ring of circumferentially directed magnetic current which may be used to represent coaxial apertures is presented. Near-field contours are given for two ring sizes. The utility of the method has been illustrated by its application in several practical antenna problems where the magnetic ring current serves as the primary source. Among these are the analysis of dipole antennas mounted on a conducting sphere or cylinder, the impedance of a coaxially fed Yagi-Uda antenna, a coaxially driven wire loop, and the radiation from a coaxial aperture at the base of a cone.