Manohar D. Deshpande

Input impedance of microstrip antennas

Using Richmonds reaction integral equation, an expression is derived for the input impedance of microstrip patch antennas excited by either a microstrip line or a coaxial probe. The effects of the finite substrate thickness, a dielectric protective cover, and associated surface waves are properly included by the use of the exact dyadic Greens function. Using the present formulation the input impedance of a rectangular microstrip antenna is determined and compared with experimental and earlier calculated results.

Fast RCS computation over a frequency band using method of moments in conjunction with asymptotic waveform evaluation technique

The method of moments (MoM) in conjunction with the asymptotic waveform evaluation (AWE) technique is applied to obtain the radar cross section (RCS) of an arbitrarily shaped three-dimensional (3-D) perfect electric conductor (PEC) body over a frequency band. The electric field integral equation (EFIE) is solved using the MoM to obtain the equivalent surface current on the PEC body. In the AWE technique, the equivalent surface current is expanded in a Taylors series around a frequency in the desired frequency band. The Taylor series coefficients are then matched via the Pade approximation to a rational function. Using the rational function, the surface current is obtained at any frequency within the frequency range, which is in turn used to calculate the RCS of the 3-D PEC body. A rational function approximation is also obtained using the model-based parameter estimation (MBPE) method and compared with the Pade approximation. Numerical results for a square plate, a cube, and a sphere are presented over a frequency bandwidth. Good agreement between the AWE and the exact solution over the bandwidth is observed.

A new approach to estimate complex permittivity of dielectric materials at microwave frequencies using waveguide measurements

In this paper, a simple waveguide measurement technique is presented to determine the complex dielectric constant of a dielectric material. The dielectric sample is loaded in a short-circuited rectangular waveguide. Using a network analyzer, the reflection coefficient of the waveguide is measured. Using the finite-element method (FEM) the exact reflection coefficient of this configuration is determined as a function of the dielectric constant. The measured and calculated values of the reflection coefficient are then matched using the Newton-Raphson method to estimate the dielectric constant of a material. A comparison of estimated values of the dielectric constant obtained from simple waveguide modal theory and the FEM approach is presented. Numerical results for dielectric constants of Teflon and Plexiglas measured at the X- and Ku-bands are presented. Numerical inaccuracies in the estimate of the dielectric constant due to: 1) the presence of airgaps between sample and sample holder waveguide surfaces and 2) inaccuracy in the sample dimensions are also discussed.

Radiation characteristics of cavity backed aperture antennas in finite ground plane using the hybrid FEM/MoM technique and geometrical theory 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.

Comparison of two optimization techniques for the estimation of complex permittivities of multilayered structures using waveguide measurements

In this paper, two separate techniques, i.e., sequential quadratic programming (SQP) and a genetic algorithm (GA), were used to estimate the complex permittivity of each layer in a multilayer composite structure. The relative performance of the algorithms was characterized by applying each algorithm to one of three different error functions. Computer generated S-parameter data sets were initially used in order to establish the achievable accuracy of each algorithm. Based on these data sets and S-parameter measurements of single and multilayer samples obtained using a standard X-band waveguide procedure, the GA was determined to be the more robust algorithm in terms of minimizing rms error of measured/generated and formulated S-parameters. The GA was found to perform exceptionally well for all cases considered, whereas SQP, although a more computationally efficient method, was somewhat limited for two error function choices due to local minima trapping.

Analysis of an End Launcher for an X-Band Rectangular Waveguide

The analysis of an end-launcher type, coaxial-to-rectangular waveguide transition, exciting dominant TE/sub 01/ mode in X-band rectangular waveguide is presented. Expressions for the real and imaginary parts of the input impedance seen by the coaxial line are derived for the general case of an offset launcher using self-reaction of an assumed current over the loop. The dimensions of the combined electric and magnetic loops having low input VSWR in the coaxial line are determined. There is satisfactory agreement between theoretical and experimental results.

Analysis of finite phased arrays of circular microstrip patches

A method is presented for analyzing a finite planar array of circular microstrip patches fed by coaxial probes. The self- and mutual impedances between array elements are calculated using the method of moments with the dyadic Greens function for a dielectric layer on a ground plane. The patch circuits are determined by using the reaction integral equation. The active input impedance as well as the active element pattern of the array are computed from a knowledge of the resultant patch currents. The calculated results for two-element and eight-element linear arrays are in good agreement with experimental data. The active reflection coefficient and element pattern for the center and edge elements of a two-dimensional array as a function of scan angle are also presented. >

Scattering from Microstrip Patch Antennas Using Subdomain Basis Functions

ABSTRACT The scattering properties of microstrip patch antennas are examined with a spectral domain moment method approach. The Galerkin solution presented uses subdomain basis functions to model the current distribution on the patch so that a space varying surface resistance on the patch may be included. The subdomain approach is also used to model circular and triangular microstrip patches by approximating the patch boundary with a rectangular grid. Both calculated and measured results are presented for a few representative cases.

An investigation of EME as a potential cause of fuel tank ignition

NASA researchers were tasked to study the potential for radio signals to excite an aircraft fuel quantity indication system (FQIS) enough to cause arcing, sparking or excessive heating within a fuel tank. Computational techniques were used to determine the threat from external high intensity radiated field transmitters nearby, such as shipboard and airborne radar systems. Experimental methods were used to determine the threat from portable electronic devices (PEDs) carried aboard by passengers. To support this work, unique electromagnetic coupling measurements were performed on a retired Boeing 747 aircraft, and new test and analysis methods were developed that may be applied to other FQIS designs as well as other aircraft electronic systems.

Estimation of electromagnetic energy coupling to a wire residing inside a transport aircraft due to external radiating sources

An analytical method is presented to estimate EM energy coupled to a wire residing in the fuselage of an aircraft. A fuselage with a wire inside is modeled by a metallic rectangular cavity with a wiring inside and rectangular windows on its side walls. Using the equivalence principles and the rectangular cavity Greens functions, the EM fields inside the cavity due to the equivalent current sources on windows and wires are determined. By matching the tangential magnetic fields across the apertures and equating the total tangential electric field over the cylindrical surface of wires to zero, coupled integro-differential equations are obtained. The integro-differential equations in conjunction with the Method of Moments (MoM) are then solved for the unknown magnetic and electric current amplitudes. One of the advantages of the present approach is that it gives an accurate and efficient analytical model for the EM coupling studies which is computationally less intensive compared to other numerical methods. Numerical data on EM energy coupled to a cylindrical wire of various lengths and oriented along the two principle axes of rectangular cavity is presented. From the numerical data it is concluded that a cylindrical wire of length n/spl lambda//4 where n is an odd integer, would pick up maximum energy.