Environmental Engineering Reference
In-Depth Information
Before proceeding with the individuation of the most appropriate time window, it
is useful to analyze the TDR waveform associated to the antenna. Fig. 6.3(a) shows
the TDR waveform of the RFId antenna for a 100 ns-long time window; Fig. 6.3(b)
zooms the 0-15 ns range.
The first portion of the waveform at approximately 0.25 V corresponds to the
50
-matched cable portion. The measured time between the beginning of the ac-
quisition window and the end of the cable is 2.8 ns, corresponding to the length L
in (6.2). After this portion of cable, there is a brief but abrupt change in the ampli-
tude of the waveform caused by the considerable impedance mismatch introduced
by the 'naked' termination of the feeding cable (feed pin). The following portion of
the waveform carries the antenna 'imprint' and, subsequently, the reflected signal
approaches the steady-state condition (around the value of 0.5 V that corresponds
to the open circuit) through several multiple reflections, whose shape is related to
the resonant behavior of the antenna. The attenuating response of the multiple re-
flections can be clearly distinguished until their peak-to-peak signal excursion is
overwhelmed by noise.
Ω
6.4.1
Practical Guidelines for Retrieving Accurate Measurements
In this subsection, the results achieved through the FD-transformation of TDR data
are reported.
To have a definitive reference to which TDR-based measurements could be com-
pared, the S 11 , ref (
of the AUT was measured through an HP8753C VNA, equipped
with the 85047A S-Parameter Test Set, inside an anechoic chamber (Fig. 6.4). After
the short-open-load (SOL) calibration procedure, VNA measurement was carried
out in the frequency range between 810 MHz and 960 MHz (with a frequency reso-
lution of 1 MHz), since this range corresponds to the operating band of the antenna.
The S 11 , ref
f
)
curves (magnitude and phase) are shown in Fig. 6.5(a) and Fig. 6.5(b).
As for the TD measurements, to avoid undesired spurious reflections, measure-
ments were performed outdoor, making sure that no reflecting object was in the
nearby of the antenna. A first set of measurement was carried out choosing differ-
ent time windows, thus evidencing the consequences in the FD-transformed data.
Therefore, different TDR waveforms of the AUT were acquired, varying the time
windows ( T w ), from 10 ns to 100 ns, and the S 11 (
(
f
)
data were evaluated. Results are
reported in Fig. 6.5(a) and Fig. 6.5(b), for magnitude and phase, respectively, and
they are compared to the S 11 , ref (
f
)
.
The root mean square error ( rmse ) between the magnitude of S 11 , ref (
f
)
f
)
and the
magnitude of each of the S 11 (
evaluated from TDR measurement was calculated
for qualitatively identifying the most appropriate time window duration: results are
summarized in Table 6.1. The rmse analysis anticipates that the most appropriate
windowing seems to be T w =
f
)
32 ns. Starting from this experimental validation, in
the following paragraphs an in-depth analysis is conducted, thus demonstrating how
the optimal time windowing can be predicted, thus avoiding the use of reference
VNA measurements.
 
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