A portable microwave measurement instrument isdisclosed for measuring an unknown property of a testmaterial, such as moisture content, by generating andtransmitting two microwave beams of different frequencythrough such material so that a portion of the beamenergy is absorbed by the moisture or other property.The test material is provided in test containers ofdifferent sizes and an electrical sensor is employed tosense the size of the selected container and to producea sensor output signal corresponding thereto. Thecorresponding attenuated received signals are comparedin order to compensate for the presence of a componentin the test material, which is not being measured butwhich also absorbs microwave beam energy. A temperaturecompensation circuit using two thermistors is connectedto the PIN diode attenuators to adjust their attenuationcharacteristics in compensation for changes intemperature. To determine the density of the testmaterial in the event of changes in such density, one ofthe microwave signals is used to measure the phase shiftof the corresponding received signal and produce a phaseshift output signal. Temperature compensation includinganother thermistor is also use to maintain thefrequency of such one source substantially constant bychanging the source voltage. Microwave signalconnectors are employed in the measurement circuit toisolate the outer conductors of interconnected microwavesignal waveguides, such as coaxial cable connectors,from DC voltage coupling and low modulation frequencycoupling in 10 to 20 kHz range but to allow lowimpedance AC voltage coupling of the outer conductors atmicrowave frequencies. The two attenuated receivedsignals of different frequency and the phase shiftoutput signal are combined in a signal processor toproduce a percent moisture content signal.
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