High-Sensitivity Detection of Solid and Liquid Dielectrics Using a Branch Line Coupler Sensor

This article presents a sensor based on branch line couplers for detection of dielectric samples. The two
$50 \Omega $
branches of the coupler are used as sensing elements, where one of them is considered as a reference and the other one is the test branch. Loading the material under test (MUT) on the test branch modifies its electrical length and characteristic impedance. This breaks the symmetry in the coupler, which, in turn, affects the isolation level. Using this concept, the MUT can be simply identified just by measuring the isolation level at the center frequency of the coupler. The sensor response is analyzed using an analytical circuit model. In addition, the sensing performance has been verified through the fabrication and measurement of two sensor prototypes for measurement of solid and liquid samples.