This article presents a microwave sensor for liquid permittivity measurement, using substrate integrated waveguide (SIW) resonators. The proposed design consists of two SIW cavities linked via an iris window, with two microstrip ports connected to the upper SIW cavity through a grounded coplanar waveguide (GCPW) transition. To enable sensitivity to permittivity variation, seven rectangular holes are etched into the iris between the cavities. The sensor exhibits a dual-band response: the first band is sensitive to changes in hole permittivity, making it suitable for liquid permittivity detection, while the second band remains fixed and serves as a reference to compensate for environmental variations. The self-calibration mechanism is achieved by comparing the two bands, which ensures that any environmental changes are compensated. The sensor was designed for the ultrahigh-frequency (UHF) industrial, scientific, and medical (ISM) band and validated using ethanol-water mixtures as test liquids. The measured results show that the first resonant frequency shifts from 903.3 to 910.5 MHz as the ethanol volume fraction increases from 0% to 100%. The proposed sensor is highly promising for direct radio frequency (RF) sensing applications because of its environmental interference resistance, ease of integration with other planar circuits, simple design, low-cost fabrication, and enclosed structure, making it ideal for direct liquid contact applications.
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