Henning Mextorf

Systematic design of reconfigurable quadrature directional couplers

This paper presents an approach for the systematic design of reconfigurable quadrature directional couplers. From even-odd-analysis of four-ports with two-fold symmetry four eigenreflections are derived. The scattering-parameters are linear combinations of them. The eigenreflections can be positioned deliberately with tuning reactances to create directional couplers which are reconfigurable in coupling ratio, operating frequency or the arrangement of the output ports. Two exemplary couplers, a LC-bridge and a modified branchline-coupler, are presented and measurement results are shown. Directional couplers with adjustable coupling ratio can also be created with cascaded couplers and additional phase shifters or discontinuities. A compact directional coupler using this principle is presented together with measurement results.

Design of Quadrature Directional Couplers with Continuously Variable Coupling Ratios

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 2 dB and 20 dB while maintaining excellent return loss and isolation.

UWB Free-Space Characterization and Shape Recognition of Dielectric Objects Using Statistical Methods

This paper presents a novel method for the free-space characterization and shape recognition of dielectric objects using multivariate calibration methods and linear discriminant analysis. The dimensions of the variously shaped objects are comparable with some of the transmitted wavelengths of the ultra-wideband (UWB) time-domain pulses used. A system illuminating the objects under test by a subnanosecond UWB pulse has been built. An array of receiving antennas receives the scattered pulses that contain information about the shape, the size, and the dielectric or related material properties of the objects. Multivariate analysis is applied to separate geometric effects from those due to the dielectric properties. Results are shown for two measurement series determining the amount of carbon and weight or the dielectric constant of the objects under test, independent of their shape, size, and orientation. Furthermore, a classification algorithm is applied, separating the objects into geometrical classes independent of all other varied parameters.

Numerical Multipole Analysis of Ultrawideband Antennas

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.

The Intrinsic Impedance and Its Application to Backward and Forward Coupled-Line Couplers

The even-odd-analysis of a reciprocal and twofold symmetrical four-port provides four eigenimpedances. A method for the derivation of the intrinsic impedance of the network from these eigenimpedances is shown. Once the conjugate complex of its intrinsic impedance is presented to the external ports of the network, it will be a quadrature coupler. The usefulness of the intrinsic impedance is shown exemplarily by two detailed examples: the improvement of the directivity of microstrip coupled-line couplers and the development of forward coupled-line couplers. Approaches for the broadening of the bandwidth are shown. All couplers presented achieve superior performance regarding directivity, return loss, and bandwidth.

Non-contacting moisture sensing using a dedicated UWB time domain instrument

Abstract This paper presents a method for the moisture determination of small and irregularly shaped objects independent from their shape and orientation. The objects under test are illuminated by UWB-pulses and the scattered pulses are received by an array of antennas. A dedicated measurement set-up and time domain transmission instrument is proposed. The scattered pulses contain information about the geometric as well as the dielectric properties. In order to separate this information multivariate calibration is applied. The time domain data is therefore subjected to principal component regression, partial least squares regression and an artificial neural network. All calibration methods deliver excellent performance. The moisture content of the objects can be determined with an accuracy of prediction of 0.69% while the range of moisture is within about 5% to 24%.

New UWB free-space method for the classification and characterization of dielectric objects

This paper presents a new approach for the accurate free-space characterization of irregularly shaped and arbitrarily oriented dielectric objects. The objects under test are illuminated by UWB-pulses and the scattered pulses are collected by an array of receiving antennas. The scattered pulses contain information about the geometrical properties as well as the dielectric properties; a multivariate calibration method is supposed to be able to separate these properties. A classification algorithm is applied prior to the multivariate calibration. It provides a classification into two classes, low or high permittivity. The statistical calibration method is then applied separately in each class. This leads to a significant improvement of the accuracy of the determination of the dielectric properties compared to processing without classification.

Free-space moisture prediction of small objects using m-sequences

This paper presents a method for the non-contacting moisture prediction of small objects using m-sequences. The objects under test are placed in a free-space transmission path. Their UWB impulse responses are captured using an m-sequence method which provides high signal quality by means of ultra low jitter and noise. The size of the objects is in the range of the coherence length and smaller than the footprints of the antennas. The impulse responses depend on the moisture content (dielectric properties) and the geometry. Multivariate calibration is applied in order separate these influences and to develop a statistical model for the moisture content. For the prediction of moisture content an accuracy of 0.55% was achieved while it was varied in a range between 4.3% and 23.4%.

Signal quality considerations for free-space UWB moisture measurements

This paper presents considerations regarding the signal quality for free-space UWB measurements. Scattered pulses from various small and irregularly shaped objects are subjected to multivariate calibration in order to develop a statistical model for the moisture content. The accuracy of this model strongly depends on both short term and long term variations of the received signals. Amplitude noise and jitter are referred to as short term variations. Low noise and low jitter are crucial for the accuracy of the multivariate calibration methods. The dependence of the signal quality on the accuracy is investigated. Furthermore, the influence of long term variations, e.g. temperature differences, is considered.

Simultaneous measurement of multiple impulse responses with a single receiver

This paper presents a method for the simultaneous measurement of multiple impulse responses with a single receiver. For UWB measurements multiple impulse responses are usually determined at different times. Here, an approach is investigated, which employs preferred pairs of maximum length sequences (m-sequences) having a low cross correlation function. Therefore, a measurement system is built capable of generating and sampling two m-sequences at 400Mb/s. Since the m-sequences are not decoupled perfectly, a method for the error correction is shown.