Directional coupler sensor

A directional coupler sensor is provided for measuring the moisture content of a substrate, such as hair. The sensor incorporates a high frequency directional coupler having a pair of generally parallel plates defining a coupling gap therebetween. A high frequency signal generator generates an electromagnetic field across the gap with the substrate placed across the coupling gap. The coupled power relates to the moisture content of the substrate. A pressure sensor is provided to ensure that the desired compactness of the substrate across the coupling gap is achieved to obtain accurate, reliable and consistent results.