Abstract

A sensor for detecting a substance in a physical space, having an RF energy emitter fed by a signal source on one side of the space and an RF energy receiving element coupled to a detector at an opposite side of the channel, spaced apart by a distance (r) selected so that when the signal wavelength (λ) changes based on the value of the dielectric constant (∈) of the particular substance in the physical space between the emitter and receiver, the mode of field type within the sensor channel changes between near field and intermediate or far field mode.