Phase center considerations for the monopole antenna

The quarter-wavelength monopole over an array of radial ground rods is analyzed using the method of moments to obtain the radiated field amplitude and phase characteristics. The phase center is calculated as a function of polar observation angle by finding the apparent center of curvature of the phase contour. It is found that the resulting phase center location is not confined to the monopole axis as has previously been suggested.     

A finite difference thermal model of a cylindrical microwave heating applicator using locally conformal overlapping grids: part II--numerical results and experimental evaluation.

In this paper, we present numerical results obtained from a robust, locally conformal 3-D Orthogonal Grid Finite Difference (OGFD) thermal algorithm introduced in Part I of our current investigation [Al-Rizzo et al., 2006] integrated with an Orthogonal Grid Finite-Difference Time Domain (OGFDTD) scheme [Al-Rizzo et al., 2000], which accurately models the volumetric electromagnetic (EM) power deposition pattern. A unified meshing scheme, which utilizes identical overlapping grids in Cartesian and cylindrical coordinates, is employed within the load zone in the OGFDTD and OGFD models. Local temperature profiles excited by the absorbed microwave energy were measured at seven locations within the sample as a function of heating time. In order to benchmark, or validate our model, an alternative analysis of the coupled EM and thermal simulations was performed using state-of-the-art, Finite Element Method-based Ansoft's High Frequency Structure Simulator (HFSS) and the coupled thermal/stress analysis tool ePHYSICS (http://www.ansoft.com). Additionally, we compare our numerical simulations against measured dynamic temperature profiles induced within a mineral ore sample maintained for exposure period of 28.5 minutes inside a cylindrical multimode heating furnace energized at 915 MHz with a microwave source power of 12.5 kW and accompanied with significant temperature elevation. A combination of convective and radiation thermal boundary conditions are considered at the interfaces between the cavity walls, air, and sample. There is a general agreement between simulated and measured spatial and temporal temperature profiles, which validates the proposed model. Results indicate that inevitable fluctuations in the frequency spectrum and output power of the magnetron, non-uniformity of sample packing, and heat released by uncontrolled exothermic chemical reactions have a significant effect on the comparisons between measured and computed temperature patterns.     

Analysis of a choke ring groundplane for multipath control inGlobal Positioning System (GPS) applications

In this paper a computational scheme is presented for accurately predicting the farfield amplitude and phase characteristics of Global Positioning System (GPS) antennas flush-mounted to a corrugated groundplane. The algorithm developed is particularly well-suited in beamshaping of (GPS) antennas in order to provide a high level of multipath rejection. The usefulness of the analytical model has been verified by the excellent agreement achieved between experimental data and predicted amplitude and phase patterns     

A finite difference thermal model of a cylindrical microwave heating applicator using locally conformal overlapping grids: part I--theoretical formulation.

In this paper, we present a versatile mathematical formulation of a newly developed 3-D locally conformal Finite Difference (FD) thermal algorithm developed specificallyfor coupled electromagnetic (EM) and heat diffusion simulations utilizing Overlapping Grids (OGFD) in the Cartesian and cylindrical coordinate systems. The motivation for this research arises from an attempt to characterize the dominant thermal transport phenomena typically encountered during the process cycle of a high-power, microwave-assisted material processing system employing a geometrically composite cylindrical multimode heating furnace. The cylindrical FD scheme is only applied to the outer shell of the housing cavity whereas the Cartesian FD scheme is used to advance the temperature elsewhere including top and bottom walls, and most of the inner region of the cavity volume. The temperature dependency of the EM constitutive and thermo-physical parameters of the material being processed is readily accommodated into the OGFD update equations. The time increment, which satisfies the stability constraint of the explicit OGFD time-marching scheme, is derived. In a departure from prior work, the salient features of the proposed algorithm are first, the locally conformal discretization scheme accurately describes the diffusion of heat and second, significant heat-loss mechanisms usually encountered in microwave heating problems at the interfacial boundary temperature nodes have been considered. These include convection and radiation between the surface of the workload and air inside the cavity, heat convection and radiation between the inner cavity walls and interior cavity volume, and free cooling of the outermost cavity walls.     

Range errors in global positioning system during ice cloud andsnowfall periods

Results from a global positioning system (GPS) measurement campaign conducted during March 1989 on a baseline of 13.1415 km length show a relatively large discrepancy in the adjusted baseline height component during ice cloud and snowfall periods. These experimental observations have stimulated the development of a propagation-prediction model to investigate the performance of GPS carrier beat phase measurements during ice cloud (predominantly present above the freezing level) and snowfall precipitation periods. Theoretical results revealed that even for extremely high ice cloud volume concentrations and for snowfall rates up to 20 mm/h the estimated phase delay, if identical at the two receiver sites, has negligible effect on the recovery of the vector baseline components. However, due to the localized nature of many snow storms and the nonuniform spatial cover of ice clouds, the differential effects between two distant receiver sites can become important for precise relative GPS applications. Finally, the model demonstrates that a differential phase delay of about 4.6° at zenith which is to be expected under adverse storm conditions may explain the range of experimentally observed discrepancies in the relative baseline height component     

Electromagnetic scattering from dielectric-coated axisymmetricobjects using the generalized point-matching technique (GPMT)

This paper describes the use of the generalized point-matching technique (GPMT) in analyzing plane electromagnetic (EM) scattering from 3-D bounded objects consisting of (or modeled by) an arbitrarily shaped axisymmetric perfect electrically conducting (PEC) or dielectric obstacle embedded in an arbitrarily shaped dielectric body of revolution and arbitrarily disposed with respect to the propagation direction of an arbitrarily polarized incident electric field vector. The treatment may be validly applied to scatterers whose boundary surfaces must have no sharp corners or edges which will introduce a discontinuity in the direction of the unit vector normal to the core and/or outer coat surfaces. It should be pointed out, however, that when applicable, the method is remarkably robust and capable of providing highly accurate numerical modelling predictions for the full-vector EM wave interactions with a large variety of arbitrarily shaped two-layered structures. Numerical results for a variety of scatterer configurations are provided and compared to exact or otherwise available results to demonstrate the potency and versatility of the suggested GPMT formulation     

Electromagnetic wave scattering by highly elongated and geometrically composite objects of large size parameters: the generalized multipole technique

The potency and versatility of a numerical procedure based on the generalized multipole technique (GMT) are demonstrated in the context of full-vector electromagnetic interactions for general incidence on arbitrarily shaped, geometrically composite, highly elongated, axisymmetric perfectly conducting or dielectric objects of large size parameters and arbitrary constitutive parameters. Representative computations that verify the accuracy of the technique are given for a large category of problems that have not been considered previously by the use of the GMT, to our knowledge. These problems involve spheroids of axial ratios as high as 20 and with the largest dimension of the dielectric object along the symmetry axis equal to 75 wavelengths; sphere-cone-sphere geometries; peanut-shaped scatterers; and finite-length cylinders with hemispherical, spherical, and flat end caps. Whenever possible, the extended boundary-condition method has been used in the process of examining the applicability of the suggested solution, with excellent agreement being achieved in all cases considered. It is believed that the numerical-scattering results presented here represent the largest detailed three-dimensional precise modeling ever verified as far as expansion functions that fulfill Maxwell's equations throughout the relevant domain of interest are concerned.     

A study of the quadrifilar helix antenna for Global PositioningSystem (GPS) applications

An analytic model for computing the radiation properties of the quadrifilar helix volute antenna is discussed and various design considerations for GPS applications are presented. The effects of modifying the antenna length and diameter on the antenna amplitude and phase performance are presented, and using the antenna for dual-frequency operation is discussed. The effects of phase imbalances are presented and compared with measured pattern anomalies