BMR Characterization of Antennas through the Combined TD/FD Approach - Broadband Reflectometry for Enhanced Diagnostics and Monitoring Applications - page 134

time window ( T w ), the number of averages, and the sampling period ( T c ). More-
over, when reference standards are not readily available, de-embedding the effect
of fixtures is easier when using TDR rather than when using a VNA (for example,
through a suitable time domain gating) [16].
It is worth mentioning that many existing works deal with the possibility of com-
pensating errors in time domain antenna measurements through time gating tech-
niques [2, 7, 8, 9, 10, 11, 13, 15]. Nevertheless, no work describes a simple pro-
cedure to be practically implemented even when basic TD instrumentation (i.e.,
without any specific additional tool) is used. On such basis, an in-depth analysis
that pinpoints the crucial steps for an accurate TDR-based antenna characterization
is hereby provided.
Similarly to the applications discussed previously, the overall procedure includes
two main steps, namely the TDR waveforms acquisition and the correspondent FD
signal evaluation. In the following subsections, both steps are discussed in detail.
Starting from theoretical considerations, it is demonstrated that the optimal choice
of a specific time window can suitably balance all the effects that limit the final
measurement accuracy. The optimal trade-off is reached by
1. optimizing the frequency resolution;
2. maximizing the signal to noise ratio (SNR);
3. including as many TDR multiple reflections so as to obtain a complete spectral
picture; and
4. excluding the spurious reflections coming from the surrounding objects.
To assess the proposed method from an experimental point of view, the S 11 (

)
of two antennas was evaluated from TDR measurements performed under differ-
ent experimental conditions (i.e., choosing different acquisition windows, applying
digital filters on TD data, placing reflecting objects near the antenna). The crucial
parameters related to the achievement of a measurement accuracy comparable to
that provided by the use of a VNA in an anechoic chamber, were individuated and
optimized. The obtained data were compared to reference-VNA measurements per-
formed in an anechoic chamber ( S 11 , ref

f

( )

f ), thus definitely validating the accuracy

enhancement [3], [5].
On the basis of the aforementioned discussion, the ultimate goal is to demonstrate
that TDR-based measurements, in conjunction with a specific data processing, can
be regarded as a robust and cost-effective procedure for accurate characterization of
antennas.
This paves the way for the use of inexpensive TDR devices in practical appli-
cations (without compromising measurement accuracy and reliability), and for the
adoption of such a method as the next best thing to VNA measurements in anechoic
chamber.

6.2 Measurement Setup for the Validation of the Method

The first considered device is Alien ALR-8610-AC antenna: a patch antenna in-
tended for a Radio Frequency Identification (RFId) reader (Fig. 6.1(a)). The second