Elements on Time Domain Reflectometry

Advances in electronic instrumentation have made it possible to extend reflectometry testing to ever wider and more diverse fields of application. In this context, however, we can distinguish two relatively separate fields: one for obstacle detection, the other for measuring the velocity of mechanically unrelated objects. In each case, reflectometry involves the propagation of acoustic or electromagnetic waves, defined in the broadest sense of the term. Depending on the application, waves may be guided by a support in the form of electrical signal transmission lines, or by materials that channel a variety of acoustic waves. In other circumstances, the waves will be propagated in free space in the configuration of spherical waves involving the three dimensions of space. Examples include acoustic waves transmitted directly through the air or, more generally, electromagnetic waves propagated in space from compact antennas fed by ultra-high-frequency signal sources.

This introductory article, however, will be restricted to two fields in which the properties of wave propagation reported in articles [D 1 322] and [D 1 324] of this treatise can be easily transposed to reflectometry. The first article [D 1 322] addresses the general theory of transmission lines, while the second [D 1 324] deals more specifically with the electromagnetic and acoustic equivalents of continuous propagation media, as well as the in-depth properties of a line's input impedance.

This article, divided into two distinct parts, will first examine the detection of obstacles introduced into a transmission line....