Instrumentation & Measurement Magazine 23-9 - 17

Fig. 4. Transmission coefficient obtained for the SRR in presence of the concrete block with and without the crack: (a) CST simulations; (b) Measurements.

In future developments of the work, it is planned not only to
extend the number of SRRs as required for the application,
but also to carry out experimental validations in a realistic
environment.
The second SRR, that is a scaled version of the one represented in Fig. 1, has a resonance frequency in air equal to 2.81
GHz. To obtain the results for both SRRs with a single measurement, a Wilkinson power divider is designed [31] using
the AWR Microwave Office software [32]. The measurement
scheme is shown in Fig. 5, where the layout of the system is also
depicted. As can be noted from the figure, the same Wilkinson
power divider can be used for the receiving unit of the system. In this way, from the received signal it can be detected if
there is a crack in the structure or not and, if it is present, its location can be identified (based on which of the two resonance
frequencies has shifted). The network can be easily extended
adding an adequate number of sensors.
The transmitting and the receiving properties of the Wilkinson power divider are shown in Fig. 6a. Since the two split ring

Fig. 5. Scheme of the proposed monitoring system.
December 2020	

resonators, when embedded in the concrete, work at about fc1
= 1.8 GHz and fc2 = 2.2 GHz, the power divider is designed to
have an operational frequency band centered around 2 GHz.
The transmitting coefficient is equal to -3 dB while the isolation
between the port 2 and 3 is lower than -30 dB at fc1 and fc2. These
properties can be easily improved if the number of the SRRs
increases, using an ultra-wideband Wilkinson power divider
using binomial multi-section matching transformers [31].
The whole system depicted in Fig. 5 has been simulated
within Microwave Office to analyze the transmission properties (S61) of the complete network. Results are shown in Fig.
6b. In particular, the simulation considers the two SRRs in the
presence of the concrete block without the crack; as expected,
the two resonance frequencies are clearly discernible, pointing out the possibility to detect the crack and even to localize it.

Conclusions
In this paper, a structural health monitoring system based on
a split ring resonator network is proposed, with the aim of
achieving a very sensitive
EM sensor to detect cracks
in concrete structures.
The network is composed
of sensors with different
resonance frequencies to
both monitor and localize
the crack. Simulation results show that this kind
of sensor is able to detect
a crack in the concrete by
analyzing its resonance frequency. If a frequency shift

IEEE Instrumentation & Measurement Magazine	17



Instrumentation & Measurement Magazine 23-9

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