Techniques for Measuring the Electromagnetic Properties of Materials

The electromagnetic properties (permittivity and permeability) of a material determine how the material interacts with an electromagnetic field. The knowledge of these properties and their frequency and temperature dependence is of great importance in various areas of science and engineering in both basic and applied research. It has always been an important quantity to electrical engineers and physicists involved in the design and application of circuit components. Over the past several decades the knowledge of the electromagnetic properties has become an important property to scientists and engineers involved in the design of stealth vehicles. These applications are most often associated with the defense industry. Besides these traditional applications, the knowledge of the electromagnetic properties has become increasingly important to agricultural engineers, biological engineers and food scientists. The most obvious application of this knowledge is in microwave and RF heating of food products. Here the knowledge of the electromagnetic properties is important in determining how long a food item needs to be exposed to the RF or microwave energy for proper cooking. For prepackaged food items, the knowledge of the electromagnetic properties of the packaging materials is also important. The interaction with the packaging material also determines the cooking time. Besides these obvious applications there are also numerous not-so-obvious applications. Electromagnetic properties can often be related to a physical parameter of interest. A change in the molecular structure or composition of material results in a change in its electromagnetic properties. It has been demonstrated that material properties such as moisture content, fruit ripeness, bacterial content, mechanical stress, tissue health and other seemingly unrelated parameters are related to the dielectric properties or permittivity of the material. Many key parameters of colloids such as structure, consistency and concentration are directly related to the electromagnetic properties. Yeast concentration in a fermentation process, bacterial count in milk, and the detection and monitoring of microorganisms are a few examples on which research has been performed. Diseased tissue has different electromagnetic properties than healthy tissue.

Accurate measurements of these properties can provide scientists and engineers with valuable information that allows them to properly use the material in its intended application or to monitor a process for improved quality control. Measurement techniques typically involve placing the material in an appropriate sample holder and determining the permittivity from measurements made on the sample holder. The sample holder can be a parallel plate or coaxial capacitor, a resonant cavity or a transmission line. These structures are used because the relationship between the electromagnetic properties and measurements are fundamental and well understood. One disadvantage of these types of sample holders is that many materials cannot be easily placed in them. Sample preparation is almost always required. This limits their use in real-time monitoring of processes. Another disadvantage is that several of these sample holders are usable only over a narrow frequency range. Extracting physical properties from electromagnetic property measurements often requires measurements made over a wide frequency range. Techniques for which this relationship, between electromagnetic properties and measurements, is not as straightforward have also been employed. One of these techniques is the open-ended coaxial-line probe. This technique has attracted much attention because of its applicability to nondestructive testing over a relatively broad frequency range. It can be used to measure a wide variety of materials including liquids, solids and semisolids. These attributes make it a very attractive technique for measuring biological, agriculture and food materials. In its simplest form, it consists of a coaxial cable without a connector attached to one end. This end is inserted into the material being measured. All of these measurement techniques will be reviewed. These techniques cover the frequency range from DC to 1 THz.