A reflection type gas sensor using conducting polymer as a variable impedance at microwave frquencies

In this paper, a gas sensor using a reflection type variable attenuator structure with conducting polymer (CP) is represented. A common gas sensing mechanism is to measure the conductivity or resistance of the CPs when it is exposed to gas. Because conductivity of the CPs varies due to the gas. A novelty in this paper is that the proposed sensor has employed the CPs as impedance-variable transmission lines that cause the impedance mismatching with conventional hybrid coupler. This structure can be thought as a variable attenuator or phase shifter. Experiment is conducted with 100 ppm ethanol gas at a temperature of 28°C and relative humidity of 80% (humidity condition). As a result the amplitude of S₁₁ and the frequency satisfying ∠S₂₁ = -360° is shifted about 2.875 MHz. It is that this reflection type sensor represented.