Microwave Sensor for Nitrate and Phosphate Concentration Sensing

This paper concerns the analysis and potential applications of a coplanar waveguide with interdigital capacitor-loaded electric-LC resonators for microwave sensors. Two variants for the coplanar waveguide with interdigital capacitor-loaded electric-LC resonators were analyzed: 1) with spiral interdigitated capacitor and 2) fingered interdigital capacitor, and each structure was arranged horizontally and vertically with the coplanar waveguide transmission line. The proposed devices were designed, fabricated, and used to experimentally measure the magnitude of transmission coefficient and resonance frequency parameters to detect nitrate and phosphate concentration. The spectra of the transmission coefficient response behavior of the proposed sensor were analyzed in the range of 1-5 GHz using circuit theory methods and full-wave electromagnetic simulation. Experimental results show that the quality factor of the studied device ranged from 2.66 to 374.50 for deionized water and that the highest value was recorded using coplanar waveguide with vertical spiral interdigitated capacitor structure. The studies clearly showed that the sample concentrations of nitrate and phosphate in the range of 0%-0.1% (WV) were correlated with the magnitude of transmission coefficient and the ratio between resonance frequency and F-H-F-L (frequency bandwidth at the magnitude of 10 dB) which has a great potential for detection by using the proposed sensor.