# Substrate-Integrated Waveguide Microwave Sensor for Water-in-Diesel Fuel Applications
## 1 Introduction  
The monitoring of hydrocarbon quality is an important goal, not only for modern automotive, industrial and aerospace applications, but also in view of the next uses of biological fuel generation [1, 2]. There are many different contaminants that can compromise hydrocarbon quality, such as alcohol, water, ethanol, and oils [3, 4, 5]. Monitoring should be fast, accurate and real time, because these contaminants could alter the lubrification properties of fuel and reduce motor performance. In particular, they could alter the viscosity, density, and boiling point of the hydrocarbons [6]. In general, traditional laboratory techniques, such as an evaporation test, distillation, gas chromatography, etc., useful to identify the quality of petroleum liquid and its derivative products, are more expensive and introduce a delay time for analysis. Microwave sensor techniques provide an interesting alternative approach [5, 6, 7, 8, 9, 10, 11].  
Water in fuel also causes the corrosion of metallic parts [1]. Therefore, conformity and quality standards must be respected for suitable fuel employment. For example, with reference to water in fuel concentrations, EN590 is the European rule defining the standard for automotive and ASTM D1655 for aerospace applications [12, 13, 14]. In particular, regarding automotive applications, the water content in fuel must not be over 500 ppm [15]. Water contamination can be detected by using different electromagnetic technologies, including optical fiber gratings and microwave reflection techniques [14, 15, 16, 17, 18, 19, 20]. Microwave (MW) sensors can be very efficient devices for real-time water detection, since the dielectric constant of water, \(\varepsilon_{r,H2O}=70\), is about 40 times the dielectric constant of diesel fuel, \(\varepsilon_{r,fuel}=1.75\), at the frequency \(f=10\) GHz. Many different kinds of devices have been proposed, including coaxial antennas and microstrip sensors, stacked multi ring resonators (SMRR) or vertically stacked ring resonators (VSRR), and microwave resonant cavities [5, 6, 7, 8, 9, 10, 11, 14, 15, 20, 21, 22, 23, 24].  
Microstrip technology, in addition to having a number of practical advantages, allows to design compact and integrated devices, but it does not guarantee a complete confinement of the electromagnetic (EM) field and suffers radiating losses. On the other hand, substrate integrated waveguide (SIW) technology allows to obtain compact and low-cost solutions, good performances, integration of active and passive devices on a single substrate, good EM field guidance, a reduction in insertion and radiation losses, and the use of higher power [25; 26; 27; 28; 29]. SIW technology is employed in many fields, including aerospace, medical therapy, industrial monitoring, and free-space communications, thanks to the easiness of its construction process [30; 31; 32; 33; 34; 35; 36; 37; 38; 39].  
In this work, a microwave SIW applicator for water-in-fuel detection is designed and constructed. To the best of our knowledge, the microwave frequency range \(f=9-11\) GHz is investigated to this aim for the first time, allowing the creation of a very compact device. The sensor characterization, using a vector network analyzer (VNA), confirms the simulation results with a quite good agreement. These performances are intriguing when compared to the literature results [14; 15; 22].