Microwave interrogation

One of the most successful early measurements was the use of microwave interrogation for the determination of moisture content of foods and materials. An issue of the Journal of Microwave Power was dedicated to the subject of microwave aquametry and contains an extensive bibliography on the subject from 1955 through 1979 (JMP 1980). A more recent compilation has been edited by Kraszewski (1994). 

Interrogating the light from stars and the absorption features of atoms and molecules in-between requires some fairly complicated optics in the form of a telescope. However, the telescopes are not restricted to the parts of the electromagnetic spectrum that you can see but use radiation from microwaves to gamma rays to observe the Universe. There is too much to learn about the optics or even adaptive optics of telescopes to be discussed here but there are some properties of telescopes that we must know because they are important for the identification of atoms and molecules. We shall discuss three telescope considerations the atmosphere, the spatial resolution and the spectral resolution. 

Early work using microwaves as a diagnostic tool relied upon measuring a secondary effect of the dielectric properties of the material under interrogation, i.e., reflection, absorption and transmission. The two fundamental microwave parameters, e and e" are related to the food or material composition. These two fundamental parameters also determine the reflection, absorption and transmission of the materials exposed to a microwave signal. Thus by measuring the amplitude and phase of the reflected or transmitted wave, or the characteristics of absorption of a wave through the material, one is able to empirically establish a relationship to the constituency of the product. 

The measurement technique depends upon the determination of the attenuation and phase shift of a microwave signal transmitted through the sample at 22 discrete frequencies. These values are processed via an algorithm to provide an accurate measure of the moisture content. Other types of interrogation schemes are available such as antennas which allow measurements to be made on products on a conveyor system. 

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