Microwave resonator sensor for detection of dielectric objects in metal pipes

We present a microwave sensor for detection of dielectric objects in material mixtures that are transported in metal pipes. The sensor exploits several resonant modes that operate below the cut-off frequency of the dominant waveguide mode in the pipe. This non-invasive measurement technique is particularly attractive for detection of undesirable objects in certain industries. Initial measurements on a prototype sensor show good agreement with electromagnetic simulations and indicate favorable detection properties. A detection algorithm based on the likelihood-ratio test is evaluated based on synthetically generated data from a finite-element model that simulates inhomogeneous background materials. We conclude that the multi-mode feature of the sensor is advantageous for the ability to distinguish between undesirable objects and ordinary fluctuations in the background material.