A system for determining a density of a non-conducting medium in a tank, wherein a relationship between a dielectric constant and a density of said medium is known, said system comprising: a transceiver configured to generate, transmit and receive a microwave electromagnetic signal comprising frequencies within a predetermined frequency range; a single-line waveguide connected to said transceiver and extending towards and into said medium, arranged to guide a transmitted electromagnetic signal towards and into said medium inside said tank, and to return an electromagnetic signal resulting from a reflection of said transmitted electromagnetic signal; filling level determination circuitry configured to determine a filling level of a product in said tank based on a time-of-flight between said transmitted signal and a received reflected electromagnetic signal reflected at a surface of said medium in said tank; a first microwave resonator having a resonance frequency, and configured to reflect frequencies corresponding to a first bandwidth around said resonance frequency, said first bandwidth being smaller than and within said frequency range, said resonance frequency depending on a dielectric constant of a medium surrounding said resonator according to a known relationship; wherein said first microwave resonator is arranged at a location along said waveguide, and configured to reflect a portion of an electromagnetic signal, in the frequency domain, corresponding to said first bandwidth, when said electromagnetic signal is guided along said waveguide; and processing circuitry connected to said transceiver and configured to: determine said resonance frequency based on a reflected portion of a received signal having been reflected by the first microwave resonator, said processing circuitry being further configured to: determine a density of said medium at the location of said first microwave resonator based on said resonance frequency. a transceiver configured to generate, transmit and receive a microwave electromagnetic signal comprising frequencies within a predetermined frequency range; a single-line waveguide connected to said transceiver and extending towards and into said medium, arranged to guide a transmitted electromagnetic signal towards and into said medium inside said tank, and to return an electromagnetic signal resulting from a reflection of said transmitted electromagnetic signal; filling level determination circuitry configured to determine a filling level of a product in said tank based on a time-of-flight between said transmitted signal and a received reflected electromagnetic signal reflected at a surface of said medium in said tank; a first microwave resonator having a resonance frequency, and configured to reflect frequencies corresponding to a first bandwidth around said resonance frequency, said first bandwidth being smaller than and within said frequency range, said resonance frequency depending on a dielectric constant of a medium surrounding said resonator according to a known relationship; wherein said first microwave resonator is arranged at a location along said waveguide, and configured to reflect a portion of an electromagnetic signal, in the frequency domain, corresponding to said first bandwidth, when said electromagnetic signal is guided along said waveguide; and processing circuitry connected to said transceiver and configured to: determine said resonance frequency based on a reflected portion of a received signal having been reflected by the first microwave resonator, said processing circuitry being further configured to: determine a density of said medium at the location of said first microwave resonator based on said resonance frequency.