Variational principles compete with numerical iterative methods for analyzing distributed electromagnetic structures

An explicit variational principle (EVP) for the propagation constant of EM waves is compared with four numerical tools: the Newton-Raphson algorithm solving a transcendental equation, the spectral domain approach (SDA) applied to rite Galerkin method, the 3-D simulator HFSS from XP, and rite finite element method (FEM). Each tool analyses a different planar topology : a lossy dielectric slab supporting surface waves, a planar slotline modelled by transmission line parameters (TLP), a multilayered high-loss co-planar waveguide, and a shielded microstrip line. For these various structures, rite EVP is more efficient than previous tools yielding the propagation constant, its explicit form and variational nature yield a drastic reduction of the number of iterations

Electromagnetic modes in conical transmission lines with application to the linearly tapered slot antenna

A transmission line analysis of bow-tie antenna and linearly tapered slot antenna (LTSA) is presented. These structures belong to the class of conical transmission lines defined here in terms of conical coordinates. A complete set of solutions of Helmholtz equation is obtained exhibiting TE and TM modes. Modal fields are expressed by Lame and Bessel-Schelkunoff functions. TE and TM eigenmode analysis is particularized to the bow-tie structure. Bow-tie antenna and LTSA are shown to be dual conical transmission lines by the image method and Babinet's principle, The modes of LTSA are calculated on the basis of the results obtained for the bow-tie structure. The radiation pattern of LTSA is computed as the integral of a closed-form expression of the dyadic Green's function weighted by the modal electric field distribution over the slot aperture. The obtained dominant mode radiation patterns are validated by measurements from the literature. The radiation patterns of the first two-order modes are calculated and compared

Copolar and cross-polar radiation of Vivaldi antenna on dielectric substrate

The copolar and cross-polar radiation patterns of the Vivaldi antenna on a dielectric substrate are calculated anal validated by measurements. The method involves a two-step procedure, The electric field distribution across the antenna slot aperture is calculated first. The radiated fields are then calculated, using Green's functions. The continuous exponential tapered shape is approximated by annular linearly tapered sections, The conical transmission-line theory and a variational method yield the electric field in each section. The radiation calculation is based on closed-form expressions for the dyadic Green's function of an elementary electric field source in a conducting half sheet. Both copolar and cross-polar radiation patterns of the Vivaldi antenna are calculated by integrating the Green's functions weighted by the electric field distribution over the antenna aperture. The effect of lateral truncation is taken into account by defining weighting patterns. The method is validated by original measurements and limitations of the model are discussed, Antenna directivity and sensitivity are calculated

Microwave antennas at UCL: a design point of view

Antenna activities at UCL have led to validated design methods at microwave frequencies for planar antennas and base stations: computation of the patch antenna resonant frequency, calculation of co-polar and cross polar radiation patterns radiated by microstrip antennas coupled to a director, lens design for narrowing patch beamwidth, space wave and surface wave radiation prediction in slot structures, efficient computation of co polar and cross polar radiation patterns radiated by tapered slot antennas (including the Vivaldi antenna), site shielding efficiency evaluation, cellular antenna coverage computation in urban environments and evaluation of people's exposure to microwave fields by a simplified method.Anglai

Asymptotic Green's function of a surface magnetic current element on a perfect electric conductor plane covered by a lossy dielectric substrate

Published analyses of radiation modeling for slot structures on dielectric substrate are empirical or numerical, This paper proposes exact analytical asymptotic expressions of the far-field Green's functions of a surface magnetic current element on a perfect electric conductor plane covered by a lossy dielectric substrate of finite thickness. From these expressions, the radiation pattern of both the space wave and surface wave far away from an arbitrary shaped-slot antenna structure can be calculated, provided the source distribution across the slot is known. The potentials used in the analysis are defined and their boundary conditions are expressed. Helmholtz equation is solved in Laplace domain and the solutions are transformed into space domain using inverse Hankel transform and steepest descent method. The influences of the substrate thickness and dielectric constant are analyzed using the calculated expressions. The model is validated by comparison with surface wave and space wave measurements and with numerical results obtained from a commercial electromagnetic simulator

An efficient energetic variational principle for modeling one-port lossy gyrotropic YIG straight-edge resonators

This paper presents a new variational principle for the design of one-port gyrotropic magnetostatic-wave (MSW) resonators. We first prove the stationary character of the magnetic energy in case of a resonator containing lossy gyrotropic media and supporting microwave MSW's. We then show that the variational expression may be successfully used for calculating the input reflection coefficient of a planar multilayered MSW straight-edge resonator (SER), Results obtained using the variational formulation are validated by experiment carried out at X-band. Hence, the resulting model is an efficient tool for designing low-noise wide-hand yttrium-iron-garnet (YIG) tuned oscillators

A simple detection method of buried cylindrical targets applicable to landmines

This paper presents a contribution to the problem of detecting cylindrical targets. We first show that cylindrical targets embedded into lossy dielectric media, such as soil have natural resonant frequencies which can be calculated using simplified models. Next, we demonstrate that those resonant frequencies can be observed experimentally using an adequate field detector and could be used as the signature of the presence of landmines into lossy soils. (C) 2003 Wiley Periodicals, Inc

Variational principles are efficient CAD tools for planar tunable MICs involving lossy gyrotropic layers

The paper presents an efficient design method for frequency-tunable planar devices using gyrotropic ferrite YIG films. The behaviour of the tunable YIG film is modelled by an equivalent lossy transmission line. Its parameters are the function of the permeability tensor components, which depend on the frequency and magnitude of the DC biasing magnetic field. We propose two analytical variational models for computing these transmission line one- or two-port parameters, depending on their coupling to the planar MIC accesses. Compared with most numerical methods, the two resulting models offer the advantage to be fully analytical and remain valid when losses of the anisotropic material are taken into account. These two models are able to predict the expected and also the unwanted effects, such as non-reciprocity and losses, with a very limited numerical complexity. The efficiency of the proposed approach is illustrated by comparing theoretical designs using these variational models with measurements carried out on the various topologies of the tunable one- and two-port planar resonators in the microstrip and slotline technology. Copyright (C) 1999 John Wiley & Sons, Ltd