Presented here is a method for computing near- and far-field patterns of an antenna from its near-field measurements taken over an arbitrarily shaped geometry. This method utilizes near-field data to determine an equivalent electric current source over a fictitious surface which encompasses the antenna. This electric current, once determined, can be used to ascertain the near and the far field. This method demonstrates the concept of analytic continuity, i.e., once the value of the electric field is known for one region in space, from a theoretical perspective, its value for any other region can be extrapolated. It is shown that the equivalent electric current produces the correct fields in the regions in front of the antenna regardless of the geometry over which the near-field measurements are made. In this approach, the measured data need not satisfy the Nyquist sampling criteria. An electric field integral equation is developed to relate the near field to the equivalent electric current. A moment method procedure is employed to solve the integral equation by transforming it into a matrix equation. A least-squares solution via singular value decomposition is used to solve the matrix equation. Computations with both synthetic and experimental data, where the near field of several antenna configurations are measured over various geometrical surfaces, illustrate the accuracy of this method.

Near-field to near/far-field transformation for arbitrary near-field geometry utilizing an equivalent electric current and MoM

A new method for predicting the far-field radiated emissions and for finding the radiation sources of a device from near-field measurements is presented. It is based on the substitution of the original device by an equivalent set of elemental dipoles, placed over the main radiating sources, which radiate the same near-field (and therefore, far-field). This equivalent set of elemental dipoles is generated using a genetic algorithm. From the position and type of the equivalent elemental dipoles, the position of the actual radiating sources is determined. Since the field produced by an elemental dipole is known, the far-field radiation of the actual radiating source can be calculated. The new method has been tested using synthetic data and real measurements from the radiation generated by a modem PCB demonstrating its viability and usefulness.

A genetic algorithm based method for source identification and far-field radiated emissions prediction from near-field measurements for PCB characterization

The binary Genetic Algorithm (GA) optimization method is used to simulate antennas from their near-field distribution by a set of infinitesimal dipoles. The infinitesimal dipoles could be of electric and/or magnetic types that produce the near field of the actual antenna and thus the same far field. The method is verified using near fields from known infinitesimal electric and/or magnetic dipoles. Some simple antennas have been simulated by infinitesimal dipoles such as dipole, loop, waveguide, and dielectric resonator antenna. The obtained equivalent dipoles from single frequency measurements are found to be valid for certain frequency band.

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Antenna Modeling by Infinitesimal Dipoles Using Genetic Algorithms

A nonredundant sampling representation on arbitrary surfaces in near-field antenna measurements together with a direct near-field far-field transformation is described The utilized sampling criterion depends on the electrical size of the antenna under test (AUT) along with the separation between the AUT and the measurement surface The minimum number of samples required is directly linked with the number of unknowns in the linear system of equations derived considering spherical expansion of radiated AUT fields Equivalent plane wave sources are used to represent the AUT fields Plane wave translations can be directly performed for any arbitrary measurement grid with the same efficiency Thus, nonredundant sampling representations can be utilized on any scan surface In contrast, other fast field transformation techniques would lose their efficiency The proposed sampling is applied to near-field measurements and synthetic data using a variety of antennas and the results are compared to the case of standard sampling Remarkable decrease in the number of measurement points is observed with a negligible change in the accuracy of the transformed pattern, thereby greatly reducing the measurement time and the computational effort

Efficient Near-Field Far-Field Transformation for Nonredundant Sampling Representation on Arbitrary Surfaces in Near-Field Antenna Measurements

This paper presents a simple method to analyze and design a desired frequency band rejection in microstrip transmission lines with defected signal strip structure. Also some new structures called ADMS have been introduced and compared. The proposed circuits can be applied to various microwave and millimeter wave components. Finally this paper introduces the RCMS method, a very fast and efficient solution that determines current distribution on the cross section of the signal strip with arbitrary defection pattern. One microstrip line with defected patterns is discussed and then modeled using RCMS method. The results of the current and voltage distribution along an ADMS obtained using RCMS method are in good agreement with those obtained using FEKO (a full wave simulator).

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Simulation, Analysis, Design and Applications of Array Defected Microstrip Structure (ADMS) Filters Using Rigorously Coupled Multi-Strip (RCMS) Method

In a printed circuit board environment, the solution domain is highly inhomogeneous, and analytic expressions for design parameters are very difficult to obtain, even for the simplest configuration. The finite element method (FEM) offers an attractive alternative for solving the problem in all its aspects including the determination of parasitic effects. A problem of practical interest, related to crosstalk, is formulated to be solved using the FEM. The technique is first applied to obtain the field distribution, and then the field is used to calculate transmission line parameters of conducting tracks on printed circuit boards. FEM is used to solve a radiation problem in three dimensions. The solution is used to predict far-field radiation levels of electronic equipment from near-field measurements. >

Finite-element method applied to EMC problems (PCB environment)

This paper presents a new approach to derive far-field data needed in antenna and EMI/EMC testing from near-field measurements. An exact integral equation solution to the wave propagation problem is used to transform the near-field data to the far field. The method requires near-field measurements on two closed surfaces enclosing all sources and inhomogeneities. The approach is validated with numerical simulation of measurements of fields radiated from a known antenna. >

R. Laroussi

Papers

Finite-element method applied to EMC problems (PCB environment)

Far-field predictions from near-field measurements using an exact integral equation solution