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     

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     

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.     

Effects of finite ground plane on the radiation characteristics ofa circular patch antenna

An analytical technique to determine the effects of finite ground plane on the radiation characteristics of a microstrip antenna is presented. The induced currents on the ground plane and on the upper surface of the patch are determined from the discontinuity of the near field produced by the equivalent magnetic current source on the physical aperture of the patch. The radiated fields contributed by the induced current on the ground plane and the equivalent sources on the physical aperture yield the radiation pattern of the antenna. Radiation patterns of the circular patch with finite ground plane size are computed and compared with the experimental data, and the agreement is found to be good. The radiation pattern, directive gain and input impedance are found to vary widely with the ground plane size     

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     

Scattered and absorbed powers in receiving antennas

This paper discusses the amount of power, which is scattered and absorbed by a receiving antenna and in particular, whether an antenna can absorb the entire power incident upon it. The absorbed and scattered power from dipole arrays in either free space, or over ground plane is considered. By defining a suitable "aperture efficiency" for the receiving case, a dipole array without a ground plane can best absorb half of the incident power (scattering the rest), while an array over a ground plane can absorb all of the incident power. It is shown how aperture efficiency varies with load impedance, which is of practical interest for array designers.     

Reconnaissance with slant plane circular SAR imaging

This paper presents a method for imaging from the slant plane data collected by a synthetic aperture radar (SAR) over the full rotation or a partial segment of a circular flight path. A Fourier analysis for the Green's function of the imaging system is provided. This analysis is the basis of an inversion for slant plane circular SAR data. The reconstruction algorithm and resolution for this SAR system are outlined. It is shown that the slant plane circular SAR, unlike the slant plane linear SAR, has the capability to extract three-dimensional imaging information of a target scene. The merits of the algorithm are demonstrated via a simulated target whose ultra wideband foliage penetrating (FOPEN) or ground penetrating (GPEN) ultrahigh frequency (UHF) radar signature varies with the radar's aspect angle.     

A physical basis formulation for the electromagnetic backscatteringby a finite-length rectangular trough in a ground plane

The electromagnetic backscattering from a finite-length rectangular trough in an infinite ground plane is examined. A physical basis formulation is used to express the unknown currents in the trough aperture in terms of a forced wave resulting from the incident plane wave, and one or more pairs of oppositely directed traveling waves propagating along the trough. A Galerkin solution is employed to solve a very small system of equations to determine the weights of the postulated aperture currents, and then radiated to compute the radar cross section (RCS). These results are then compared with a measurement of a trough in a finite ground     

Low-profile ultra-wideband inverted-hat monopole antenna for 50 MHz?2 GHz operation

A compact low-profile UWB monopole antenna is proposed to operate from 50 MHz up to 2 GHz. The developed double-ellipse inverted-hat antenna (IHA) is 15 inches in aperture, 6 inches tall, and achieves 215 dBi gain at 75 MHz (when it is λ/10 in aperture size) in the presence of a finite 21 inch diameter ground plane, with the corresponding frequency being 50 MHz for an infinite ground plane. Calculated and measured gain data are given with reasonable agreement. The tripleellipse IHA is investigated to resolve impedance matching issues at higher frequencies. Gain performance against antenna height and aperture are also studied. The proposed antenna is especially suited for small unmanned aerial vehicles.     

Characterisation of aperture coupled microstrip patch antenna withthick ground plane

The feasibility of using a thick ground plane with an aperture coupled microstrip patch antenna while maintaining reasonable antenna performance is demonstrated using moment method and reciprocity analyses in the spectral domain. The thick ground plane, which may serve as a heatsink for active MMIC circuitry, or as a mechanical support for thin substrates, is particularly advantageous in millimetre wave phased array applications. It is found that the effect of ground plane thickness is to reduce the level of coupling from the feed line to the patch     

Cylindrical cavity-backed suspended stripline antenna-theory andexperiment

The problem of a cylindrical cavity-backed suspended stripline (SSL) antenna is viewed as a transition of the SSL to a circular cylindrical waveguide opening into an infinite ground plane. The fields in the waveguide are expanded in terms of TE and TM modes. The effect of the radiating aperture on the modal expansion of the fields is taken into account by introducing reflection coefficients for each mode. The current on the SSL probe is assumed to have sinusoidal distribution. These simplifications reduce the original problem to that of a known radially oriented current residing on a dielectric sheet inside a circular-cylindrical cavity whose top wall has known impedances corresponding to different modes. The Green's function for this modified structure is found and is used to obtain a general expression for the input impedance. This expression is specialized to the case where the SSL probe and the radiating aperture are coupled through the dominant TE ₁₁ mode only. This input impedance is translated to the measurement plane of the antenna. The computed and measured results are found to be in good agreement     

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     

多波束修正母线新型偏轴旋转反射面天线

以抛物线为母线的偏轴旋转抛面天线可以在锥面上或平面上实现宽角域扫描，或形成多束波天线，但沿性面和垂直之间的中间平面，子波束孔径上光程差很大，导致天线性能降低，本文以新型母线取代抛物线，创立新型偏轴旋转反射面天线，使天线性能得到显著改善。     

A note on the directivity of a uniformly excited circular aperture in an infinite ground plane

The directivity of a uniformly excited circular aperture in an infinite ground plane is reexamined in the light of recently acquired knowledge related to Q-type integrals. It is seen that depending on aperture size, numerical corrections to the directivity estimates for such apertures can be easily accommodated.     

Optimum distribution over a circular aperture for best mean-square approximation to a given radiation pattern

The distribution over a cicular aperture in a ground plane that produces the best mean-square approximation to a specified radiation pattern is derived subject to constraints on the mean-square aperture field strength. The result is derived in terms of the functions most natural for a least square fit: the eigenfunctions of a finite (circular) two-dimensional Fourier transform.     

Studying the properties of double-clad active cone optic fibers

A simplified model based on the weakly guiding waveguide approximation is proposed for estimating the effects occuring during propagation of radiation in a double-clad active cone optic fiber. This model is used for computer simulation, whose results agree with obtained experimental data. Practical recommendations on optimizing the shape of the cone fiber and the aperture of the pumping source for enhancement of the pump absorption efficiency are given.     

Fields in Planar Anisotropic Transmission-Line Metamaterials

An electromagnetic analysis of wave propagation in planar anisotropic transmission-line metamaterials is presented. It is shown that a planar square-celled grid, series-loaded with orthogonal inductors and capacitors and positioned over a ground plane, is magnetically anisotropic and may be described in terms of a diagonal permeability tensor. Resonance cone field concentrations form when two of the three diagonal elements of the permeability tensor are opposite in sign and the dispersion surface becomes hyperbolic. A theoretical treatment of an electric line current source excitation shows that the formation of resonance cones is a consequence of the singularity associated with the characteristic surface of a hyperbolic equation. The resonance cone angle, which also describes the direction of local power flow in the region between the grid and the ground plane, can be predicted to a good degree of accuracy. To the authors' best knowledge, the present work also verifies experimentally for the first time that current flow reverses direction across the resonance cone. Experiments, simulations, and analytical calculations of the cone angle are in good agreement.     

Theory of the directional pattern of a pyramidal horn

A method is presented for calculating the directional pattern of a pyramidal horn antenna from first principles by diffraction theory. It is based on first finding the field in the plane of the aperture, both inside and outside the physical aperture. This is then transformed to find the far-field pattern. The field in the aperture plane is made up of contributions from the primary wave reaching the aperture, whose phase surfaces are assumed to be spherical, edge waves propagating in the plane of the apertures and edge waves that have been reflected from inside the horn. The edge waves traveling in the plane of the aperture are the main object of interest. They propagate not only perpendicular to the edges, but also diffract parallel to the edges because of the finite length of the edges. By taking this fully into account, a directional pattern over the whole forward hemisphere can be predicted for any given frequency from the geometry of the horn. For a particular horn the principal cuts agree with direct measurements made at the National Physical Laboratory, Teddington, UK, to within about 4 dB (except near deep minima)     

Computations of the aperture admittance of an axial slot on a dielectric coated cylinder

An extensive set of curves for the aperture admittance of a waveguide-fed axial slot on a coated metal cylinder is presented for both an underdense plasma and a Teflon coating. Examination of these curves shows that a slot on a cylinder with aKaof 2 to 3 has about the same aperture admittance as the identical slot opening onto a flat ground plane.     

Integral equation analysis of a sheet impedance coated window slotantenna

Presents an integral equation and method of moments analysis of a window slot antenna. The antenna is modeled by a sheet admittance coated rectangular aperture in an infinite ground plane. It is shown that the sheet admittance coated aperture is complementary to a sheet impedance plate, which permits the window slot antenna to be analyzed with existing computer programs. Numerical results are compared with measurements for input impedance and radiation efficiency