Principles of Microwave Sensing (Cont'd)
Through-Transmission, Noncontact System (MMA-2000)
The MMA-2000 is a highly sensitive, non-contacting, two-parameter instrument in which a beam of low level microwave power is transmitted through a thickness of the test material. Three configurations are depicted below. The two measured parameters are the changes in signal absorption or attenuation (dA in dB), and the time or phase delay (df in deg) caused by passing the signal through the material.
Because free water molecules are highly polar at microwave frequencies, water has an exceptionally large dielectric constant (e') and loss factor (e"). Consequently, even very small moisture contents have substantial effects on dA and df. But material basis weight (density X thickness) also effects dA and df. Through proprietary algorythms, the measured dA and df are used to determine the bulk moisture content and basis weight, simultaneously and independently.
 In the upper sketch the microwave signal passes through the test material once and is then detected. In the lower sketch, the signal passes through the material once, whereupon it is received and electronically modulated, and then retransmitted back through the material and detected. Because the received modulated signal must travel the material twice in the lower figure, the measured dA and df are double those in the upper figure, so the sensitivity to the material properties is also doubled. Otherwise, there is little difference in operational principles between the upper and lower sketches. The lower system has another important advantage in that no microwave cable is needed for the lower antenna. This simplifies system installation and significantly enhances stability.
Carrying these multiple pass techniques even further, a four-way ("double bounce") system is also available. This very popular system, as shown on the right/bottom, is useful for thick, very low density materials having low moisture content, such as fiberglass wool insulation, cotton batting, wood fibers and strands, etc. See Products - MMA for examples.
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