Dj Dave Bekers

Eigencurrent analysis of resonant behavior in finite antenna arrays

Resonant behavior in a finite array that appears as (modulated) impedance or current-amplitude oscillations may limit the array bandwidth substantially. Therefore, simulations should predict such behavior. Recently, a new approach has been developed, called the eigencurrent approach, which can predict resonant behavior in finite arrays. A study of line arrays of either E- or H-plane-oriented strips and rings in free space and in half-spaces confirms our conclusion in earlier research that resonant behavior is caused by the excitation of one of the eigencurrents. The eigenvalue (or characteristic impedance) of this eigencurrent becomes small in comparison to the eigenvalues of the other eigencurrents that can exist on the array geometry. We demonstrate that the excitation of this eigencurrent results in an edge-diffracted wave propagating along the surface of the array, which may turn into a standing wave. In that case, the amplitudes and phases of the element impedances show the same standing-wave pattern as those of the excited eigencurrent. We demonstrate that the phase velocity of this wave is approximately equal to or slightly larger than the free-space velocity of light. Finally, we throw light on the relation between the excitation of eigencurrents with a small eigenvalue and the behavior of super directive arrays

An Eigencurrent Approach for the Analysis of Finite Antenna Arrays

An accurate description of typical finite-array behavior such as edge effects and array resonances is essential in the design of various types of antennas. The analysis approach proposed in this paper is essentially based on the concept of eigencurrents and is capable of describing finite-array behavior. In the approach numerical simulation is carried out, first, by computing element eigencurrents from chosen expansion functions and, second, by expanding a limited set of array eigencurrents in terms of element eigencurrents that contribute to the mutual coupling in the array. Both types of eigencurrents are eigenfunctions of an impedance operator that relates the current to the excitation field. Highlighting both mathematical and physical features we describe the basic concepts of the approach, in particular the relation between eigenvalues and mutual coupling. We illustrate these features for uniform linear arrays of loops and dipoles, and demonstrate that the approach provides significant improvements in terms of computation time and memory use.

EBG enhanced dielectric lens antennas for the imaging at sub-mm waves

This paper presents a truly integrated antennas in mm and sub-mm wave regimes can be realized by printing planar radiating elements in the focal plane of elliptical or extended hemispherical lens antennas. The elliptical shape of the lens gives high focusing properties provided that its eccentricity is properly related to its dielectric constant (e=1/epsiv1/2 r). In the frame of cooperation activity between TNO and SRON, the main purpose of this contribution is to present a new way to increase the directivity of the focal plane feeds typically used to excite such lenses. The driving reason for this effort is the desire to diminish the impact of the reflections at the dielectric air interface when these structures are proposed as focal plane imagers.

A current‐matrix model for metallic and dielectric postwall waveguides

Waveguide structure integration in planar substrates for use in microwave components has received considerable attention in recent years. Waveguides with side walls consisting of cylindrical posts (postwall waveguides or PWWGs) are of interest, since they are compatible with standard PCB fabrication technology and exhibit low loss. In this paper we present an electromagnetic model for PWWG building blocks, whose characteristics are described entirely in terms of equivalent electric and magnetic surface currents at predefined port interfaces consistent with Lorentz’s reciprocity theorem. Introducing input and output surface currents, we determine the response of a block for a given port excitation. The expansion of the currents in terms of suitable bases results in a matrix that relates input and output currents. The scattering parameters of a building block are determined by expressing waveguide modes in terms of these bases. This facilitates the future integration of PWWG components in a microwave circuit simulator. We validate our model by comparing the results for simulated and measured uniform PWWGs implemented with metallic and dielectric posts.

Design and measurement of metallic post-wall waveguide components

In this paper we discuss the design and measurement of a set of metallic post-wall waveguide components for antenna feed structures. The components are manufactured on a single layer printed circuit board and excited by a grounded coplanar waveguide. For a straight transmission line, a 90° H-plane bend, and a H-plane T-junction, we compare measurements with simulations obtained from commercial software. In a next phase, the set of test components will be used to validate a software tool for component analysis. This tool is based on our own numerical method and will be integrated with a circuit simulator.

An eigencurrent description of finite arrays of electromagnetically characterized elements

To describe the behavior of a finite array, the current distribution on each of its elements is represented by a finite number of eigencurrents or modes. Subsequently, the eigencurrents of the array are expanded in terms of these element eigencurrents. For uniform linear arrays of loops and dipoles, the array-eigencurrent expansions and their associated eigenvalues are investigated. We focus on their parameter independent and diagonalizing features, and on their interpretation in terms of far-field characteristics and (standing) wave behavior

An Analysis Technique for Post-Wall Waveguides

In the last decade, post-wall waveguides have emerged as an interesting building block for (antenna) feed networks because of their potential low losses, low costs, ease of manufacturing and integrability with existing printed circuit board techniques. In this paper we present a general formulation to analyze the characteristics of post-wall waveguides with metal or dielectric posts. This formulation is based on a field expansion technique, applied to infinite arrays of posts. The modal behavior of post-wall waveguides is analyzed by considering the behavior of the field expansion coefficients as function of frequency and we compare our results with Ansoft HFSS simulations and measurements.

Analysis of resonant behavior in planar line arrays of rings by the eigencurrent approach

Resonant behavior in a finite array that appears as (modulated) impedance or current-amplitude oscillations may limit its bandwidth substantially. Therefore, simulations should predict such behavior. Recently, a new approach has been developed, called the eigencurrent approach, which can predict resonant behavior in finite arrays. Analysis of line arrays of II-plane oriented microstrip rings by the eigencurrent approach reveals that resonant behavior is caused by excitation of one of the eigencurrents. The characteristic impedance of this eigencurrent becomes small in comparison to the characteristic impedances of the other eigencurrents that can exist on the array geometry.

Modeling and analysis of a long thin conducting stripline

A long thin conducting stripline embedded in a dielectric and centered between two large conducting plates, i.e., the stripline environment, is considered. The stripline is modeled as infinitely long, infinitely thin, and perfectly conducting by first considering a stripline of finite length, thickness, and conductivity in a dielectric layer. Starting from Maxwells equations and assuming that the current on the stripline is a propagating wave in length direction, asymptotic expressions for the fields inside and in the neighbourhood of the stripline are deduced. These expressions are used to model the stripline in the stripline environment, which leads to a boundary-value problem for the electric potential. This problem is solved by two different approaches, leading to integral equations for the current and for an auxiliary function describing the electric potential. A relation between the current and the auxiliary function is deduced, which is used to obtain asymptotic expressions for current and impedance. Results are compared with a numerical solution of the integral equation for the current and with results in literature.

Mutual-coupling based phased-array calibration: A robust and versatile approach

The transmit and receive modules of a large phased array are often calibrated for amplitude and phase variations by an internal calibration network and an offline characterization of the complete array in an anechoic chamber. Such a solution is less obvious in view of current trends towards integration and modularity. An alternative to surpass network and offline characterization is mutual-coupling based calibration. However, existing stepwise procedures for resolving the module transfer characteristics from (active) coupling measurements can be subject to propagating errors originating from e.g. measurement noise, manufacturing differences, and defective modules. In this paper we propose an alternative approach, which is more versatile and robust than a stepwise procedure, and we test this approach for two different array architectures with simulated coupling data.