A new UTD formulation for multiple cylindrical diffraction
This paper describes a new formulation of the uniform theory of diffraction (UTD) with application to multiple cylindrical structures. The new formulas present some modified UTD diffraction coefficients in cooperation with a slope diffraction term for higher accuracy in the electromagnetic modeling. The above are then combined with a simple ray tracing algorithm that takes into account multipath propagation between cylindrical structures. Comparisons with existing analytical models that employ other electromagnetic theories have shown that the new UTD solution can give very accurate results while it seems to be superior in terms of computation time and complexity. The solution can be applicable to radiowave paths which are characterized by convex structures and multipath propagation such as mountainous areas or urban areas with special rounded objects
Cascaded cylinder model for predicting terrain diffraction loss at microwave frequencies
The problem of determining the diffraction loss between a transmitter and receiver in clear air conditions, when the intervening terrain is given, is treated for propagation at microwave frequencies. A model is developed which simulates the terrain by means of a cascaded arrangement of cylinders. This model, which remains valid at grazing angles, extends Vogler's result for cascaded half planes to treat cylinders. Experiments are conducted on two cylinders which share the same shadow boundary, for both vertical and horizontal polarisation. Measurements are performed on cylinders which present both zero and non zero surface impedance, and these are found to be in good agreement with predictions.
Time and Heritage
As a historian studying what can be considered a form of intellectual history, I have slowly come to adopt Michel de Certeau’s observation made at the end of the 1980s that ‘objectifying the past, for the last three centuries, has undoubtedly left unconsidered time within a discipline that has continued to use it as a taxonomic instrument’. To a certain degree, time has become commonplace for the historian who has preserved or instrumentalized it. It is not considered because it is inconceivable, but because we do not think of it or, more simply, we do not think about it. As a historian attempting to pay attention to the time I’m living in, I have thus, like many others, noticed the swift development of the category of the present until it has become obvious that the present is omnipresent. This is what I refer to here as ‘presentism’.