Instrumentation & Measurement Magazine 25-3 - 47

input reflection coefficient at each operating condition. It is
clearly observed how this parameter is strongly affected by the
oxygen concentration (Fig. 3b). An increase in the oxygen concentration
leads to a variation of the magnitude |S11
| and to a
frequency shift (Δf) of the resonator spectrum. Such changes
were also evaluated over the time in order to estimate the dynamic
performance of the device in oxygen sensing (Fig. 3c
and Fig. 3d), and response time and the recovery time were
evaluated to be 84 s and 160 s, respectively [18]. The change in
the resonant frequency is usually connected with variations of
the real part of the sensing material's permittivity. In this microwave
device, the O2
molecules adsorbed on the sensing
material surface increase its effective permittivity, thus lowering
the resonant frequency by a few MHz. Similarly, a decrease
in the quality factor was recorded. This means that the O2
molecules
adsorbed by the sensing material increase the TiO2
dielectric losses.
Sensor-integrated Antennas for Gas
Detection
A typical wireless gas sensor consists of a transducer able to
convert the variations of the gas concentration into changes
of an electrical parameter (e.g., resistance or capacitance).
The transducer output signal is conditioned and processed
in a circuit that includes a microcontroller, filters, amplifiers
and analog-to-digital converters. Then, the signal is sent to an
RF module like a Wi-Fi, Zigbee or a Bluetooth module able to
communicate with a base station or other sensor nodes using
built-in antennas. An alternative sensor node design that can
avoid the conditioning circuit, the microcontroller unit and/
or the RF module is highly desirable, since it allows to considerably
reduce the power consumption requirements of the
wireless sensor, thereby increasing the battery lifetime and reducing
the implementation costs.
As described in the introduction of this article, microwave
gas sensors are inherently compatible with wireless technology.
As a consequence, the research interest in the integration
of such devices into antennas is rapidly increasing. A sensorintegrated
antenna should be able to detect the gas variations
and, at the same time, provide a wireless signal with an accurate
indication of such changes in the form of a shift in the
resonant frequency or of an alteration of the antenna gain. In
principle, this innovative design allows elimination of the conditioning
circuit, the microcontroller unit and the RF module.
The wireless gas sensor becomes passive and exploits the backscattered
power mechanism: an RF signal is received through
the antenna and reflected back by the gas sensor. This new implementation
is extremely low cost and requires low power
consumption.
As an example of a passive sensor using radio-frequency
identification (RFID) technology, Yang et al. [19] load a bowtie
antenna at its feed location with a single-walled nanotube
(SWNT) film and utilized the change in amplitude of the reflected
signal or scattered power as a measure of gas detection.
Thus, the antenna is loaded by the complex impedance of the
passive RFID tag connected to antenna feed terminals. The
May 2022
Fig. 4. Backscattering sensor signal measurement setup (from [20], ©2017
IEEE).
antenna and the tag together form a wireless sensor node. In
passive RFID systems, the reader antenna sends a radio signal
to the tag. The RFID tag then uses the transmitted signal to
power up and reflect energy back to the reader. This design requires
a very high concentration of gas to get a large variation
in the loading resistance, thus suffering from limited sensitivity
and slow detection response due to mismatch and other
antenna losses. Furthermore, passive RFID operation has a
limited range (less than 10 m), and thus, it cannot be considered
for wide-area wireless sensor networks.
A novel design involving a fully passive wireless sensor
node, comprising a printed RF dipole resonator weakly coupled
to a carbon nanotube-based gas sensing film, is shown in
Fig. 4 [20]. The back-scattered electromagnetic field at the resonant
frequency, measured by a coherent receiver, is used as
a marker for gas detection. The backscattered signal from the
node is limited in range by free space loss and other path losses
such as secondary scattering from other objects in the path. Besides
this path loss, the only power drain in the receiver is the
power required to operate the electronics for data sampling
and communication to wireless networks. The detection element
consists of a thin-film of SWNTs operating in the ambient
environment. The instrumented laboratory set-up shown in
Fig. 4 is used to prove the feasibility of resonant backscattering
as a marker for gas detection [20]. A horn antenna connected
to a VNA transmits a continuous wave signal and receives
the backscattered signal. The reflection coefficient S11
is thus a
measure of the sensor response. The sensor is supported with
its face aligned with the horn antenna about 1.2 m away and
exposed to 25 ppm ammonia through a Teflon tube connected
to the gas cylinder shown.
The S11
time before, during, and after the ammonia exposure. The first
stage (pre-exposure) denotes ambient air with no ammonia,
the second stage (NH3
exposure) shows the response to 25 ppm
ammonia, while the last stage (post-exposure) demonstrates
the sensor reversibility when ammonia is removed. The measured
S11
(Fig. 5) shows a resonance shift of 30 MHz after the
ammonia exposure. An interesting property of SWNT sensors
is that the sensor reverts back to its equilibrium impedance
IEEE Instrumentation & Measurement Magazine
47
parameter is measured by the VNA as a function of
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