Background: Ethanol is an important fuel for the automotive industry in different countries
and an alternative to fossil non-renewable fuels. However, it is usual the malpractice of ethanol
adulteration in many countries The illegal adulteration of ethanol fuel results in reduced engine lifetime
and increases the production of toxic gases. The conventional ethanol analyses require expensive
laboratory methodologies and equipment. This paper proposes a microwave sensor to analyze
the complex permittivity of ethanol fuel by applying the cavity perturbation technique.
Methods: The sensor is a microstrip patch antenna coated with Single-Walled Carbon Nanotubes
(SWCNTs). The SWCNTs deposited over the microstrip antenna allows characterizing the sample
vapor-phase due to the change of the effective permittivity. When the ethanol-water vapor is injected,
part of the Material Under Test (MUT) is adsorbed by the SWCNTs, increasing the charge
transfer and changing the resonant frequency and Q factor. After the injection, the vapor is desorbed,
returning to the initial values of frequency and Q factor of the device.
Results: The sensor presented a repetitive response, with no evidence of poisoning or drift. The
measurements allowed to determine a relation between the resonant frequency for each fraction of
ethanol-water sample and the real part of the complex permittivity. The same procedure was used
with Q factor, which is related to the imaginary part of the complex permittivity. The sensor showed
good sensitivity to detect small fractions of ethanol and water based on the complex permittivity of
Conclusion: The sensor was able to discriminate small fractions of ethanol-water with a fast response
and good sensitivity. The proposed methodology allows in situ and real-time analysis of the
ethanol fuel, towards the development of a compact, integrable, and portable electronic system.
Therefore, the proposed microwave sensor presents consistent results related to the qualification of
ethanol fuel, making it a promising tool for developing material characterization sensors and devices.