A Microwave Gas Sensor Using Interdigital Resonator Coated with Conducting Polymer for Agricultural Applications

In this paper, a microwave gas sensor using interdigital resonator with coating of poly (3,4-ethylenedioxythiophene) polystyrene sulfonate or PDOT:PSS is presented. The proposed sensor uses the change of conductivity properties of PDOT:PSS when the ammonia gas is adsorbed on PDOT:PSS layer. The interdigital resonator was designed at the center of ungrounded coplanar waveguide transmission-line by creating the rejection characteristic at 4.08 GHz. The proposed sensor was fabricated on DiClad 880 substrate with the dielectric constant of 2.2 and loss factor of 0.008. To improve the sensitivity of the sensor, the PDOT:PSS was coated on top of the interdigital resonator, resulting in the operating frequency was shifted to 3.584 GHz. The measured results for six samples of ammonia hydroxide concentration (50, 100,200,500, 1000, and 2000 ppm) under a static system in the closed chamber at room temperature were recorded for five minutes after injecting the ammonia sample. When the ammonia gas concentration increases, the resonant frequency is changed 38 MHz from 3.584 GHz to higher frequency of 3.622 GHz by injecting ammonia concentration from 50 ppm to 2000 ppm. The proposed microwave gas sensor using interdigital resonator coated with PDOT:PSS has the potential to clearly observe gas concentration in various agricultural applications.