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This paper presents a systematic approach for the design of directional couplers with tunable coupling strength, which maintain the properties of perfect impedance match and isolation over the whole tuning range. The devised method is based on eigenreflections and requires the synthesis of one-ports. This can be carried out with the help of graphical presentations in the Smith chart, thus fostering intuition in finding new circuit structures. The method is theoretically demonstrated by the development of a novel tunable LC-coupler or Maxwell bridge and a tunable proximity coupler. An experimental realization of the Maxwell bridge yields a tunable coupling ratio between 2dB and 20dB while maintaining excellent return loss and isolation.

A recently introduced systematic approach to efficiently obtain the time-domain electromagnetic field of an arbitrary antenna radiating into the free space is applied to numerically analyze the frequency-domain features of an ultrawideband antenna. In a first stage, the finite-difference time-domain method is employed to obtain the time-domain spherical-multipole amplitudes for the antenna which is driven by a wide-band signal. This time-domain representation is valid in the far field only, but a single numerically performed Fourier transform leads to the frequency-domain spherical-multipole amplitudes valid for the entire spectrum of the input impulse and at any point outside a minimum sphere containing all radiating elements. The approach is applied to an antipodal Vivaldi antenna to obtain three dimensional radiation patterns which are compared to experimental results as well as to the outcomes of other numerical schemes. Moreover, a total scattering error is defined and numerically evaluated to estimate the overall accuracy of the proposed method.Index Terms-Antenna radiation patterns, error analysis, finite-difference time-domain (FDTD) methods, near-field far-field transformations, ultrawideband antennas, Vivaldi antennas.

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