Environmental Engineering Reference
In-Depth Information
to establish long-term trends in systems under observation;
to estimate changes with respect to a reference status .
There are four major features that can summarize the desirable requirements that
monitoring systems should satisfy: i) time saving; ii) energy saving; iii) economi-
cal feasibility (or profitability); and iv) accuracy and reliability. As a matter of fact,
the Scientific Community is in constant struggle to individuate innovative monitor-
ing solutions that can help to simultaneously satisfy such requirements or, at least,
achieve an optimal trade-off among them.
Monitoring implies measuring, either directly or indirectly, a quantity that repre-
sents the value of the parameter of interest. Most measurement techniques rely on
applying a stimulus to the system under test (SUT) and on analyzing how the system
responds; one of the most popular approaches consists in pulsing the SUT. A broad
classification of measurement techniques based on pulsing includes transmission
measurements and reflectometric measurements: the former rely on the analysis of
the signal that is transmitted through the SUT; whereas the latter are based on the
analysis of the signal reflected by the SUT. Reflectometric measurements typically
require a simpler measurement setup; hence, they are generally more adaptable to a
wider range of conditions. Depending on the specific application field, the stimulus
signal may be acoustic, electromagnetic, or optic.
The present topic focuses on the use of electromagnetic (EM) signals as stimula:
this approach is herein referred to as broadband microwave reflectometry (BMR).
This expression emphasizes the fact that the analysis is performed over a wide fre-
quency range, that theoretically goes from 0 Hz up to the microwave region of the
frequency spectrum (this is contrast, for example, to resonance methods which rely
on analysis at a specific value of frequency).
In such a context, this topic is intended to provide a comprehensive overview
on the capabilities offered by BMR: in fact, this technique arguably encompasses
the aforementioned needs, thus allowing to achieve, simultaneously, low implemen-
tation costs, accuracy of results, and in situ implementation in numerous practical
applications.
As will be detailed in the following chapters, BMR-based measurements can be
performed either in the time domain (time domain reflectometry - TDR) or, equiva-
lently, in the frequency domain (frequency domain reflectometry - FDR). Depending
on the specific application, one approach may be more suitable than the other. Typi-
cally, instrumentation operating in time domain (TD) is usually less expensive than
instruments operating in frequency domain (FD). In fact, portable low-cost units are
readily available on the market, thus making the TDR technique appealing for in situ
applications. Commonly, step-like voltage function and impulse are used as stimula
for TD measurements.
On the other hand, reflectometric measurements performed directly in FD are
typically carried out through vector network analyzers (VNAs). These instruments
use sinusoidal signals as stimula, and they usually provide high measurement accu-
racy, thanks to the possibility of directly performing calibration procedures, which
are crucial for reducing the effect of systematic error sources.
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