Design and validation of liquid permittivity sensor based on RCRR microstrip metamaterial

•The sensor takes advantage of the coupling between two square rings connected by interdigital capacitor.•Relatively low frequency is achieved with compact dimension.•The sensor realizes relatively high sensitivity and the bandwidth is less than 100 MHz at working frequency of 900 MHz.•Equivalent circuit is given and in well agreement with simulations.•The experimental results are well in agreement with simulated results, too.