Covering compact metric spaces greedily

A general greedy approach to construct coverings of compact metric spaces by metric balls is given and analyzed. The analysis is a continuous version of Chvatal's analysis of the greedy algorithm for the weighted set cover problem. The approach is demonstrated in an exemplary manner to construct efficient coverings of the n-dimensional sphere and n-dimensional Euclidean space to give short and transparent proofs of several best known bounds obtained from deterministic constructions in the literature on sphere coverings.Comment: (v2) 10 pages, minor revision, accepted in Acta Math. Hunga

Reduction of Skylab 2 ground winds

A dampometer was designed and fabricated for measuring the amount of damping in the wind tunnel test model. The ground winds data reduction system (GWDRS) was taken to Langley Research Center in support of wind tunnel tests on the Skylab 2 program. Final data reduction of the test results was accomplished by the GWDRS

Routing of railway carriages: A case study

In the context of organizing timetables for railway companies the following railway carriage routing problem occurs. Given a timetable containing rail links with departure and destination times/stations and the composition of the trains, find a routing of railway carriages such that the required carriages are always available when a train departs. We will present a local search approach for this routing problem for the railway carriages. The approach uses structural properties of an integer multi-commodity network flow formulation of the problem. Computational results for a real world instance are given

Methods for the calibrated measurement of the scattering parameters of planar multi-port devices

In this article, the error-corrected determination of complex scattering parameters of multi-port devices by means of a 2-port vector network analyzer is presented. As only two ports of the device under test can be connected to the analyzer ports at a time, the remaining device ports have to be terminated by external reflections. In order to measure the scattering parameters of the DUT without the influence of systematic errors and of the external terminations, an error correction has to be performed besides the calibration. For this purpose, the application of the multi-port procedure is presented. This method has the advantage, that the external reflective terminations can be chosen arbitrarily. Furthermore, these terminations can be unknown except for one. An automatized measurement system based on a switching network is shown, which is optimized for the measurement of planar microwave circuits. An error model for the description of the measurement setup as well as a calibration procedure for the elimination of the systematic errors are presented

Considerations on the frequency resource of professional wireless microphone systems

This Paper presents the results of spectral observations in the UHF TV Bands IV and V from 470 MHz up to 862 MHz with focus on the TV-Channels 61 to 63 and 67 to 69. Concerning the discussions on citet{WRC} this frequency range is in great demand of several applications and is usually treated as a "white space" in the TV-Bands. According to typical scenarios, two different spectral loads will be presented considering the requirements of professional wireless microphone receivers with respect to in-band intermodulation

Sievenpiper HIS and its influence on antenna correlation

This paper deals with the influence of artificial magnetic conductors (AMC), so-called Sievenpiper High Impedance Surfaces (HIS), on the MIMO and Diversity performance of a planar linear-polarized 2×2 dipole array in the ISM-band at 2.45 GHz. The characteristic performance criteria such as envelope correlation coefficient, spectral efficiency, Mean Effective Gain (MEG) and Diversity gain of a coupled 2×2 dipole array are investigated. By means of full-wave electromagnetic analysis as well as Monte-Carlo simulations applying statistical channel models the characteristic antenna pattern just as the MIMO and Diversity analysis is performed, respectively. The obtained results show that the application of Sievenpiper High Impedance Surfaces to planar antenna arrays enables good MIMO and Diversity performance compared to ideal configurations in free-space while offering the design of low profile antennas with simultaneously enhanced characteristics. © Author(s) 2008

Calibration methods for microwave free space measurements

In this article calibration methods for the precise, contact-less measurement of the permittivity, permeability or humidity of materials are presented. The free space measurement system principally consists of a pair of focusing horn-lens antennas connected to the ports of a vector network analyzer. Based on the measured scattering parameters, the dielectric material parameters are calculable. Due to systematic errors as e.g. transmission losses of the cables or mismatches of the antennas, a calibration of the measurement setup is necessary. For this purpose calibration methods with calibration standards of equal mechanical lengths are presented. They have the advantage, that the measurement setup can be kept in a fixed position, for example no displacement of the antennas is needed. The presented self-calibration methods have in common that the calibration structures consist of a so-called obstacle network which can be partly unknown. The obstacle can either be realized as a transmissive or a reflective network depending on the chosen method. An increase of the frequency bandwidth is achievable with the reflective realization. The theory of the calibration methods and some experimental results will be presented

Novel algorithms for the characterization of n-port networks by using a two-port network analyzer

The measurement of the scattering matrices of n-port networks is an important task. For this purpose two ports of the n-port network are connected with the network analyzer and the remaining ports are connected to reflecting terminations. In order to specify the scattering matrix of a n-port network with the multi-port method (Rolfes et al., 2005), n reflecting terminations are required from which at least one reflection factor needs to be known. There are some cases, in which the multi-port method shows weak convergence properties. For example, a T-junction cannot be identified if the reflecting terminations used are short circuits and if the line length is equivalent to a multiple of a half wavelength. This is due to the fact that the two ports connected to the network analyzer become isolated. Two new algorithms, named the sub-determinant method and the wave-identification method, respectively, which employ a second set of reflection terminations that have to differ from the first set, allow to identify every n-port network without the necessity to distinguish different cases. Both methods are based on least square algorithms and allow to determine all scattering parameters of a n-port-network directly and uniquely