Compensating on-chip transmission line losses for a high-sensitivity microwave sensor

The sensitivity of the microwave sensor proposed in [C. Song, P. Wang, A radio frequency device for measurement of minute dielectric property changes in microfluidic channels, Appl. Phys. Lett. 94 (2009) 023901] is determined by the level of parasitic signal cancellation and the coupling between microwave field and the material under test (MUT). In this work, we demonstrate that the cancellation level can be greatly improved through adjusting the thickness of the λ/4 section of the hybrid. As a result, the sensor sensitivity can be significantly improved. We also show that the sensor sensitivity can be further improved by using transmission lines that have stronger couplings. Furthermore, the fabrication and testing processes of the on-chip sensor are described.