Environmental Engineering Reference
In-Depth Information
Chapter 2
Basic Physical Principles
'If we knew what it was we were doing, it would not be
called research, would it?'.
Albert Einstein
Abstract. In this chapter, the basic concepts behind broadband microwave reflec-
tometry (BMR) are recalled. First, a brief overview of the transmission line theory
is provided, and the most common electromagnetic structures are introduced. Sec-
ondly, the major parameters that are used to characterize electrical networks are
introduced, and the related theoretical background is briefly discussed. Finally, a
short overview on dielectric spectroscopy is provided, thus anticipating its connec-
tion with reflectometric measurements.
2.1
Transmission Line Basics
In electric circuits, when the wavelength of the propagating signal is large compared
to the physical dimensions of the system, the electrical characteristics of the system,
at a given time, can be assumed to be the same at all points ( lumped-element model ).
On the other hand, when the the physical dimensions of the system are comparable
to the wavelength of the propagating signal, the dimensions of the cables, connectors
and other components cannot be ignored: in such cases, it is particularly useful to
model the system through transmission line (TL) segments.
TLs are typically used to transfer a signal from the generator to the load, by
guiding the electromagnetic (EM) signal between two conductors [5]. A TL is a
distributed-parameter network and must be described by circuit parameters that are
distributed throughout its length. For the purposes of analysis, a TL can be mod-
eled as a two-port network. The model represents the transmission line as an infinite
series of two-port elementary components, each representing an infinitesimal seg-
ment of the transmission line. The elementary section of a TL can be modeled as
shown in Fig. 2.1. This model includes four parameters, referred to as primary line
constants , which are generally defined 'per unit length'. The primary line constants
are the series resistance ( R ), the series inductance ( L ), the shunt conductance ( G ),
and the shunt capacitance ( C ). For a uniform transmission line, the primary line
constants do not change with distance along the line. These constants are used to
 
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