Coupled-Line Sensor With Marchand Balun as RF System for Dielectric Sample Detection

A novel nondestructive method for effective permittivity measurement and detection of sample's permittivity change in microwave frequency range has been proposed, utilizing a Marchand balun and a coupled-line sensor. The out-of-phase excited coupled-line section is used to obtain high sensitivity on the covering sample, which can be as narrow as the spacing between coupled strips. Formulas describing balanced to unbalanced impedance transformation of a Marchand balun have been provided, hence the proposed method requires only one-port measurement to determine effective permittivity in a wide frequency range. The proposed setup has been theoretically and experimentally investigated. The obtained measurement results proved the usefulness of the proposed approach for dielectric sample detection.