Patent ID: 8049517

Claim:
A sensor for detecting a substance within a physical space comprising: an RF energy signal source producing radio frequency energy having a frequency; an RF energy emitter on a first side of the physical space, coupled to the RF energy signal source; an RF energy receiving element on a second side of the physical space, spaced a determined distance from the RF energy emitter for reception of the radio frequency energy from the RF energy emitter in a near field, intermediate field or far field mode, and an RF energy detector providing a signal corresponding to an amount of RF energy received from the RF energy emitter; wherein the distance between the RF energy emitter and the RF energy receiving element is determined relative to the wavelength of the radio frequency energy in a substance such that: energy from the RF energy emitter is coupled to the RF energy receiving element in a selected one of near field mode, intermediate field mode or far field mode when the substance is not present in the physical space between the RF energy emitter and the RF energy receiving element, and energy coupled from the RF energy emitter to the RF energy receiving element changes to a different one of near field mode, intermediate field mode or far field mode when the substance is present in the physical space between the RF energy emitter and the RF energy receiving element.