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

摘要

A portable microwave measurement instrument is disclosed for measuring an unknown property of a test material, such as moisture content, by generating and transmitting two microwave beams of different frequency through such material so a portion of the beam energy is absorbed by the moisture. The corresponding attenuated received signals are compared in order to compensate for the presence of a component in the test material, which is not being measured but which also absorbs microwave beam energy. A temperature compensation circuit using two thermistors is connected to the PIN diode attenuators to adjust their attenuation characteristics in compensation for changes in temperature. To determine the density of the test material, one of the microwave signals is used to measure the phase shift of the corresponding received signal and produce a phase shift output signal. Temperature compensation including another thermistor is also used to maintain the frequency of such one source substantially constant by changing the source voltage. Microwave signal connectors with high-pass characteristics are employed. 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.