Microwave moisture measurement using two microwave signals of different frequency and phase shift determination

摘要

A microwave measurement apparatus is disclosed for measuring an unknown property of a test material, such as moisture content, by transmitting microwave beams through such material so that a portion of the beam energy is absorbed by the moisture or other property. Two microwave input signals of different frequencies are used to form the microwave beams and the corresponding attenuated received signals are compared in order to compensate for the presence of a component in the test material, such as foundry molding sand, which is not being measured but which also absorbs microwave beam energy. To determine the density of the test material in the event of changes in such density, one of the microwave signals is used to measure the phase shift of the corresponding received signal and produce a phase shift output signal. A single microwave antenna having two microwave signal connections is employed to transmit the two microwave beams of different frequency without interference and another antenna of the same type receives such two beams to produce the two attenuated received signals. The two attenuated received signals of different frequency and the phase shift output signal are combined in a signal processor to produce a percent moisture content signal.