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

Microwave measurement system for determining properties of test material, comprising:

• microwave transmitting means including microwave signal source means for applying two microwave input signals of two different frequencies to a first microwave antenna means for transmitting at least one microwave beam corresponding to said input signals, and reference means for supplying two microwave reference signals corresponding to and with a constant phase relationship to said input signals and each reference signal being of the same frequency as its corresponding input signal;
• testing means for providing test material in the path of the microwave beam so that said beam is transmitted through the test material to cause the phase of the beam to be changed and a portion of the energy of said beam to be absorbed by said test material;
• microwave receiving means including a second microwave antenna means for receiving the microwave beam after said beam is transmitted through the test material to produce a received microwave signal at the output of said second antenna means for each input signal which is attenuated and shifted in phase relative to but of the same frequency as its corresponding input signal, and for producing two output signals corresponding to the attenuation of the two received signals by the measured properties of the test material;
• phase adjustment means connected to the outputs of said receiving means and said reference means for varying the relative phase angle of at least one received signal and the reference signal of the same frequency as said one received signal relative to each other during a phase sweep to cause the phase shifted received signal and reference signal to be approximately 180 degrees out of phase at some point during the phase sweep;
• combiner means connected to the outputs of said phase adjustment means and said reference means for combining said phase shifted received signal and its corresponding reference signal to produce a combined signal whose amplitude is a minimum at the point where the phase shift angle between said phase shifted output signal and reference signal is about 180 degrees;
• detector means for detecting said combined signal to produce a phase shift indicator signal whose voltage level corresponds to the amplitude of said combined signal;
• electronic switch means for switching to produce a pulse when the phase shift indicator signal reaches a minimum voltage level corresponding to when the phase angle between said phase shifted output signal and reference signal is about 180 degrees;
• and
• output means for producing a phase difference output signal in response to said pulse corresponding to the time from the start of the phase sweep to the point where said phase angle is about 180 degrees and corresponding to the phase shift of said one received signal by the test material.