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.     

Radiation characteristics of cavity backed aperture antennas infinite ground plane using the hybrid FEM/MoM technique and geometricaltheory of diffraction

A technique using the hybrid finite element method (FEM)/method of moments (MoM) and geometrical theory of diffraction (GTD) is presented to analyze the radiation characteristics of cavity fed aperture antennas in a finite ground plane. The cavity which excites the aperture is assumed to be fed by a cylindrical transmission line. The electromagnetic (EM) fields inside the cavity are obtained using finite element method (FEM). The EM fields and their normal derivatives required for FEM solution are obtained using: (1) the modal expansion in the feed region and (2) the MoM for the radiating aperture region (assuming an infinite ground plane). The finiteness of the ground plane is taken into account using GTD. The input admittance of open-ended circular, rectangular, and coaxial line radiating into free space through an infinite ground plane are computed and compared with earlier published results. Radiation characteristics of a coaxial cavity-fed circular aperture in a finite rectangular ground plane are verified with experimental results     

Rigorous evaluation of the input impedance of a sleeve monopole bymodal-expansion method

A rigorous modal-expansion analysis is developed for obtaining the current distribution and input impedance of a sleeve-monopole antenna fed through an infinite ground plane by a coaxial line. A “perfectly matched boundary,” which is simply the combination of an electric wall and a magnetic wall, is initially proposed to facilitate the modal-expansion analysis. The walls are parallel to the ground plane at a variable height above it. Expansion coefficients of the electromagnetic fields are then determined by enforcing the boundary conditions at conducting surfaces and regional interfaces. Numerical results for the current distribution and input impedance of the sleeve monopole are presented and compared with available data in the literature. Very good agreement is observed. Also examined are the radiation pattern for different sleeve lengths and the effects of the length and finite thickness of the sleeve conductor on the antenna's input impedance     

The transient response of the coaxial cone antenna

A coaxial cone antenna whose aperture is in the plane of a ground plane has been described theoretically and experimentally in the time domain. Its response to an incident step along the ground plane is essentially a pulse whose width is equal to the diameter of the outer conductor at the aperture. Limitations on the cone dimensions are also discussed.     

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.     

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.     

Hemispherically capped thick cylindrical monopole with a conical feed section

The axis boundary condition scalar potential integral equation is used to study the hemispherically capped thick cylindrical monopole with a conical feed section. The monopole is perfectly conducting and is fed by a coaxial line whose outer radius is connected to an infinite perfectly conducting ground plane. One approximation in the equation is that the field across the coaxial aperture is approximated by the fundamental TEM coaxial line mode. The integral equation is numerically solved by the moments method using entire domain basis functions with delta weights. A simple way for checking the convergence of the solution that needs no new integrations is given. The theoretical current distributions, input admittances, and radiation patterns are given for a thin, moderately thick, and thick monopole with and without a conical feed section. This feeding section simplifies the matching of the antenna to conventional signal generators and improves the high frequency characteristics of the monopole.     

Transmission from a Rectangular Waveguide into Half Space through a Rectangular Aperture (Computer Program Descriptions)

The program calculates the admittance seen by the waveguide, the tangential electric field in the aperture, and the radiation gain patterns for a rectangular waveguide feeding a rectangular aperture in a perfectly conducting plane of infinite extent.     

Simulation models of a dissipative transmission line above a lossyground for a wide-frequency range. I. Single conductorconfiguration

The simulation model of a single conductor dissipative line above a lossy ground, based on the exact formulation of the Maxwell equations, is proposed for a wide frequency range. The transmission-line (TL) and fast-wave (FW) propagation constants of the line are computed by solving the modal equation coming from the continuity of the tangential component of the electric field at the air-wire interface. Three different expressions of the distributed line impedance and admittance are suggested with reference to different definitions of the line voltage. Moreover, logarithmic approximations of the Sommerfeld integrals are proposed in order to obtain an easy-to-implement formulation of the simulation models for use in computer codes. Comparisons between the proposed models and the Carson (1926) approach are carried out with reference to a single conductor line above a lossy earth, considering different values of the line geometrical parameters and ground conductivity and permittivity     

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.     

A generalized network formulation for aperture problems

A general formulation for aperture problems is given in terms of the method of moments. It applies to any two regions isolated except for coupling through the aperture. The aperture characteristics are expressed in terms of two aperture admittance matrices, one for each region. The admittance matrix for one region is independent of the other region, and hence can be used for any problem involving that region and aperture. The solution can be represented by two generalizedn-port networks connected in parallel with current sources. The current sources are related to the tangential magnetic field which exists over the aperture region when the aperture is closed by an electric conductor. Formulas for fields (linear functionals) and power (quadratic functionals) are given in terms of the admittance matrices.     

Scattering from apertures in infinite ground planes using FDTD

A scattered field version of FDTD for scattering from an aperture in an infinite ground plane is presented. In this formulation the fields reflected from the infinite ground plane are computed analytically, not as FDTD scattered fields. This is necessary to eliminate scattering from the edges of the ground plane, where it is terminated at the FDTD outer boundary. Also, the fields scattered by the ground plane are usually of much higher amplitude than the desired aperture-scattered fields. In this formulation these fields need not be absorbed by the FDTD outer boundary. This provides more accurate calculation of low amplitude scattering from the aperture. The formulation can include materials in the aperture and on both sides of the infinite ground plane. For example, scattering from an aperture antenna with a dielectric cover backed by an aperture filled with lossy dielectric can be computed with this formulation     

A Numerical Calculation of the Capacitance for the Rectangular Coaxial Line with Offset Inner Conductor Having an Anisotropic Dielectric

The capacitances of the rectangular coaxial lines with an offset zero-thickness inner conductor having a sapphire dielectric are presented by using an expanded charge simulation method, In order to apply the method to an anisotropic region, we propose an electric potential formula for a two-dimensional system consisting of a line charge and an infinite plate conductor which are arbitrarily situated in the region. The potential formula is anatyticafly derived by means of an affine transformation, a confomal mapping technique, and the method of images. The capacitance calculated using this method is in good agreement with those of other available methods.     

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     

TE11mode excitation of flanged circular coaxial waveguides with an extended center conductor

Properties of circular coaxial Waveguide antennas terminated in a plane conductor are analyzed where in the most general case the center conductor is of infinite extent. Solutions for mutual admittance and radiated fields are detailed for all asymmetric modes. An asymptotic solution is also derived for the far-zone fields. We describe results forTE_{11}mode excitation of coaxial waveguides where 1) outside the aperture the conductor is absent, and 2) the center conductor extends to infinity. These antennas are of practical interest as feeds for reflectors. Experimental results are described for several examples and excellent agreement with theory is shown. Comparison of the two cases shows that in 1) there is always an optimum frequency for good pattern symmetry and low cross polarization. The mismatch in case 2) is less than in 1) but pattern symmetry is poor if the ratio of inner to outer conductor radius is greater than about 0.2. Further, in 2) mutual coupling to adjacent annular rings is stronger and the principal plane radiation patterns has widely spaced phase centers.     

Quasi-TEM Analysis of Microwave Transmission Lines by the Finite-Element Method

This paper describes a finite-element approach to the quasi-TEM analysis of several different types of isolated and coupled microwave transmission lines. Both the first- and higher order ordinary elements, as well as singular and infinite elements, are used to solve for the potential and field distributions in the cross section of the line. Next, the cross-sectional field distribution is inserted in a variational expression to compute the capacitance per unit length of the line, and the effective permittivity and characteristic impedance of the line are obtained from the capacitance value. A perturbational approach is developed for estimating the losses due to conductor and dielectric dissipation and computing the attenuation constant. Both the upper and lower bounds for the capacitance and the characteristic impedance are found by solving the original and the corresponding dual problem. Lines treatable by this method may contain an arbitrary number of arbitrarily shaped conductors, including a system of conductors placed either above a single ground plane or between two parallel ground planes, and inhomogeneous dielectric regions that can be approximated Iocally by a number of homogeneous subregions. The results obtained using the finite-element procedure have been compared for various types of microwave transmission lines and have been found to agree well with available theoretical and measured data.     

Bounds on the Load Currents of Exposed One-and Two-Conductor Transmission Lines Electromagnetically Coupled to a Rocket

Three circuits are analyzed. One consists of an isolated two-wire transmission line with terminating impedances; another of a single conductor with terminating impedances grounded to an infinite perfectly conducting plane; and, finally, a terminated two-wire transmission line in the vicinity of an infinite perfectly conducting plane. In all cases a plane monochromatic electromagnetic wave is incident on the wires with the electric vector parallel to their axes. The wires are oriented with respect to the incident field and the ground plane, if present, for maximum response. The objective is to derive formulas for the currents in the load impedances of the three circuit configurations described in the preceding. The writer then presents a heuristic argument to the effect that solutions of these problems bracket the response of exposed unshielded one-and two-wire transmission lines arranged parallel to the axis of a rocket and close to its surface. The established upper and lower bounds for the load currents are sufficiently close together to be of considerable practical value in the study of the electromagnetic compatibility of rockets.     

Equivalent currents for an aperture in an impedance surface

Expressions of equivalent magnetic and electric currents for an aperture in an impedance surface is derived in a self-consistent manner. Each equivalent current consists of the combination of the tangential electric and magnetic fields in the aperture, and is placed in front of the original aperture where the surface is extended to close the aperture. The result is particularly useful for problems involving apertures in an impedance ground plane     

Measured admittances of electrically thick monopoles

Extensive measurements of admittances are reported for electrically thick cylindrical monopoles driven from coaxial lines over a large ground plane. The monopoles have circular cross sections with radii in the range froma/lambda = 0.039to 0.140 or circumferences from approximately0.25lambdato0.88lambdawherelambdais the wavelength. Results are given for tubes with open ends, flat metal ends, and ends with hemispherical caps. Graphical representations of the measured values are given over a range of lengths of the monopoles extending fromh/lambda = 0.05toh/lambda = 1.0. The ratios of the radii of the outer to the inner conductor of the coaxial lines used are near 1.2. Comparisons with available theory are considered briefly together with the related problem of end correction for the coaxial line.     

Coaxial-Slot Surface Wave Launchers

The radiation characteristics of 360/spl deg/ circumferential slots in rigid dielectric-coated coaxial cable are studied both theoretically and experimentally. The ultimate aim of this work is to use an array of such slots to launch a surface wave on the Goubau line formed by the outer conductor of the coaxial cable and the concentric dielectric coating. The possible advantage of this method over that of the flared horn is that the launching efficiency is controlled by the length of the launchlng section and is relatively independent its cross-sectional area.