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F. Costa, S. Genovesi, S. Terranova, G. Manara, “Passive Wireless Sensors and Chipless RFID Sensors”, The 41st PIERS in Rome, Italy, 17-20 June 2019

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Low cost wireless sensors are extremely appealing in pervasive sensors networks [1], [2]. The typical
configuration of commercial sensors consists of an electronic circuit in which a component induces a certain
variation of an observable quantity (frequency shift, phase delay) as a result of the interaction with an external
variable entity such as humidity, temperature, gas concentration. The sensing phenomenon is frequently observed
at low frequencies or in DC and sensors are usually wired and are interrogated by a direct access to the circuit.
Wireless sensors are usually designed by attaching a transceiver module to the sensor. These sensors are versatile
since, in their last releases, they can communicate up to 200 meters and can interface with smartphones [3]. The
main limitations are the finite lifetime of the battery and the cost that, although modest, is not lower than 20-30
Another class of sensors cheaper than abovementioned ones is emerging on the market: the RFID- based sensors.
The advantage of RFID-based sensors is the absence of any battery, which can be a huge benefit in terms of
maintenance and cost. RFID sensors are based on an RFID tag loaded with an additional circuit where the sensing
component is installed [2].
The most challenging approach to sense the environment wirelessly is to use an entirely passive device without
the aid of an electronic circuit. In this case the information is embedded in the electromagnetic footprint of a
resonator and the sensing is carried out by detecting the changes of the electromagnetic response of the device.
Indeed, as every electromagnetic device, the radio frequency response of these tags is dependent on the electric or
magnetic changes of nearby substrates or particles and on the variation of external environment. If these variations
are opportunely controlled and isolated, an indirect measurement of several environmental quantities, or
mechanical changes, can be extracted from the measured backscattering spectrum [4], [5]. This category of sensors
takes the name of chipless RFID sensors [6], [7] or metamaterial sensors [8], [9] This technology could be very
appealing for designing very low cost, green and embedded sensors. The absence of a chip and a battery gives the
opportunity to dramatically decrease the cost of the sensor and to achieve infinite lifetime. Given the absence of
any electronic circuit, chipless RFID sensors are potentially suitable for hazardous environments [4], [6], [10].
Clearly, one of the main limitations is that the reliable reading of the sensor can be achieved under specific
conditions. The interesting aspects of this sensors is that they can be directly printed on paper by using inkjet
printing technology and this fact makes them a promising solution for low-cost sensors. This paper aims to present
some novel low-cost inkjet-printed sensors.