Instrumentation & Measurement Magazine 25-3 - 49

Fig. 8. Slot antenna with an IDC loading for enhanced bandwidth (not to
scale), from [23], used with permission. The slot loop with mean radius of 7.5
mm is etched in the bottom ground plane of an FR-4 dielectric substrate. The
design parameters for the IDC, symmetrically placed between two microstrip
lines on the other (top) side of the substrate, are given by N = 4, G = 0.1 mm, Ge
= 0.1 mm, W = 0.1 mm, Wc
of IDC finger pairs. The microstrip feed line width Wf
impedance.
= 0.25 mm, and L = 4 mm, where N is the number
= 1.45 mm for 50 Ω of
Fig. 9. Illustration of an ACP antenna.
We recently investigated the aperture coupled patch (ACP)
antenna design as one of the possible solutions to reduce the
impact of antenna losses and improve the quality factor [25],
[26]. Following the slot ring sensor design in [22], an IDC is inserted
into the microstrip feed line of the ACP antenna and
coated with a BaTiO3
based nanostructured sensing material.
IDC serves the dual purpose of wideband impedance matching
as well as the host for a thin sensing film superposed on
top of the IDC surface. Fundamentally, the slot ring produces
multiple resonances corresponding to the modes of the ring
resonator. By adjusting the impedance loading of the IDC, it is
possible to couple the two dominant resonant modes, resulting
in a wide antenna bandwidth. Experimental results involving
a graphene film loading produced bandwidths in the range
100% to 128% [22]. This is the first successful design and implementation
of an IDC as the sensing element integrated with an
antenna. The IDC is weakly coupled to the antenna such that
the antenna gain is not affected. Full-wave electromagnetic
(EM) simulations predict a realized gain of 2.5 dBi in the 2.45
GHz ISM band used in wireless sensor networks. A single antenna
with wide bandwidth potentially allows several sensors
to be frequency multiplexed.
In other research efforts, Wu et. al. in [24] deposit reduced
graphene oxide (rGO) and nano-silver ink (Ag-ink) on a microstrip
patch antenna, thus realizing a wireless ammonia
sensor. They integrate interdigital electrodes covered by rGO
and Ag-ink into a patch antenna and were able to detect ammonia
in the range from 100 ppm to 200 ppm. Two identical patch
antennas are used during the test: the measurement antenna is
connected to a VNA, while the other antenna is placed inside
the test chamber in contact with the target gas. The measurement
antenna is used to excite the sensor antenna that reflects
backward the excitation signal. Therefore, the backscattered
power can be used as a tracker for ammonia detection. As the
sensor antenna is mismatched with the excitation antenna because
of the gas exposure, only part of the induced power is
reflected back, thereby limiting the sensitivity drastically.
May 2022
The ACP antenna design consists of two microstrip substrates:
in the first layer (or antenna substrate) the patch radiator is designed,
and in the second layer (or feed substrate) the feed line
is placed. The two substrates are joined together by a common
ground plane, and the patch antenna is coupled to the feed line
through a small rectangular aperture in the ground plane (Fig.
9). The IDC is placed at the end of the antenna feed line. On the
antenna substrate a rectangular patch antenna is designed for
ISM band (i.e., 2.45 GHz). The position of the IDC is chosen so
that weak coupling is established between the sensor and the
antenna. This reduces antenna detuning during the gas detection,
thereby preserving the overall antenna performance. The
device is tested as a humidity detector with response times of
the order of one minute [26].
Conclusions and Discussion
The scientific research on microwave transducers is growing
rapidly, especially after the advent of connected devices
that are widely used for industrial applications and in everyday
life. In this article, different topologies of microwave
transducers for gas sensing applications are discussed, and
their performance in terms of sensitivity and power consumption
are highlighted. However, microwave gas sensors
are still in their infancy and a significant effort is needed to
make them competitive against the more conventional conductometric
gas sensors. Selectivity, long-term stability, and
reproducibility of microwave sensors are rarely reported in
the literature. Moreover, there are still open issues that need
to be addressed. Often, the sensing material deposition increases
the microwave resonator losses reducing the quality
factor, thus leading to considerable degradation of the sensor
IEEE Instrumentation & Measurement Magazine
49
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