Patent ID: 7928739

Claim:
A sensor for measuring moisture content of a substrate, comprising: (a) a signal generator operating in the VHF, UHF or microwave range, from which a signal at a chosen operating frequency is generated; (b) a directional coupler comprising two strips, a first strip and a second strip, wherein the first strip is generally parallel to the second strip such that a coupling gap is defined therebetween, wherein the substrate to be measured is placed across the coupling gap, the first strip being electrically coupled to the signal generator; and the first strip generating an electromagnetic field across the coupling gap to the second strip, (c) the first strip is electrically coupled to a first port of a mixer; (d) the second strip is electrically coupled to a section of coplanar waveguide, wherein the section of coplanar waveguide is designed to give a 90 degree phase shift to the signal at the chosen operating frequency, and the coplanar waveguide is electrically coupled to a second port of the mixer; (e) the output of the mixer is electrically coupled to a computer with a visual display capable of a user-perceptible indication of the measured moisture content of the substrate; wherein the signal generator, the coplanar waveguide, and the mixer are integrated into a printed circuit board.