Frequency Division Multiplexing and Random Phasing for Improved Uniformity in Microwave Heating Applications

Microwave heating has been recently used as an active thermal excitation method in thermography. Within the nondestructive testing and evaluation (NDT&E) realm, this method is referred to as active microwave thermography, or AMT. In AMT, unlike other thermal excitation methods utilized in thermography, microwave heating inherently provides a nonuniform heating pattern. This is a direct result of the radiating antenna used to deliver the energy. This nonuniform pattern may result in defect detection errors (i.e., false positives and/or negatives). To reduce this potential for detection error and improve the overall robustness of the technique, a new electromagnetic system, consisting of an array of antennas which are excited by different frequencies or time-varying random phases is proposed. The experimental results presented here show that improved uniformity of the heating pattern can be obtained by the new radiating system, with results showing that a 4-element array provides a uniform thermal excitation over an area ~5.5 times greater than that of a single antenna.