There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

The previous chapters have dealt mainly with scanning probe microscopies based upon the transport of electrons and upon static fields (forces).

Nano-optics and Scanning Near-Field Optical Microscopy

This thesis describes a novel approach to microwave imaging. The proposed methods are based on the spectral representation of the object. Regularization schemes for the solution of electromagnetic inverse problems are addressed. An iterative algorithm for the solution of the nonlinear least-squares problem arising in iterative image reconstruction is developed. The application of this algorithm to one-dimensional and three-dimensional imaging of high-contrast lossy objects is examined. The properties of the imaging systems designed to implement the algorithms are verified by simulation and actual measurements. The algorithms proposed in this work are computationally more expensive but accuracy and stability are superior compared to other microwave imaging methods.

Microwave imaging of high-contrast objects.

Superluminal Photonic Tunneling in Inhomogeneous Waveguides

Wie schnell ist schnell? Geschwindigkeitsdefinitionen bei der Übertragung von Signalen

Abstract Considering the narrow bandwidth of microstrip antennas, but also their applicability in upcoming technologies, this paper addresses the problem of wide-band matching, the theoretical bounds on the matching bandwidth and low-cost and low-complexity matching strategies. In this context the Bode-Fano bounds of single mode, linearly polarized aperture-coupled microstrip antennas is evaluated, optimized and compared to the theoretical bounds on matching bandwidth of other common feeding technologies. A detailed study of the input impedance of aperture-coupled patch antennas shows how to widen the Fano bounds. Based on this, a straight-forward and effective method to optimize the Fano bound is given. After optimization of the antennas input impedance, basic matching techniques can be applied, to exploit the enlarged bandwidth potential. As an example a λ/4 $\\lambda/4$-transformer as matching element is proposed. Design equations and simulation and measurement results of X-band prototypes are given as verification.

A Broadband Impedance-Matching Method for Microstrip Patch Antennas Based on the Bode-Fano Theory

The radiation mechanism of biconical antennas is investigated by means of field-line images. To that end, a new theory for graphical representation of antenna radiation fields in spherical coordinates is developed. All plots are presented in such a manner, that there is always carried the same electric flux between two neighbouring field lines. Thus, the field-line density is a criterion for the local strength of the electric field

Ein Beitrag zum Abstrahlungsmechanismus von Doppelkonusantennen

The mode-matching method is an analytically rigorous and numerically efficient technique for solving canonical boundary value problems. Finite series expansions lead to numerical inaccuracies, which in addition increase whenever edge singularities occur because of non-physical overshoot due to the Gibbs phenomenon. A method of using weighted series will be shown to overcome these difficulties and get physically meaningful results.

Konvergenz und Wichtung von Orthogonalreihen bei Beugungsproblemen

A double conical horn which consists of two coaxially arranged conical horns is analyzed by means of a mode-matching procedure. It can be dimensioned, to produce nearly identical radiation characteristics at two different frequencies. Therefore the double conical horn can serve as a feed of reflector antennas in order to double the transmission capacity of the antenna system. A design procedure for this feed is given

Klaus Kark

Papers

Wie schnell ist schnell? Geschwindigkeitsdefinitionen bei der Übertragung von Signalen

A Broadband Impedance-Matching Method for Microstrip Patch Antennas Based on the Bode-Fano Theory

Ein Beitrag zum Abstrahlungsmechanismus von Doppelkonusantennen

Konvergenz und Wichtung von Orthogonalreihen bei Beugungsproblemen

Das doppel-kegelhorn, eine antenne fur zwei frequenzbereiche