High sensitivity microwave sensors using Fresnel zone break point as a design parameter

This paper investigates microwave propagation through an open ended cylindrical metallic tube. It is shown that there exists a Fresnel zone break point for which the direct and the reflected paths are exactly 180/spl deg/ out of phase. Microwave signals within this distance are in-phase and do not fully cancel each other out. This principle is applied to develop a microwave spectrometer for efficient detection of unknowns in liquid and airborne toxins in real-time. The instrument can be installed on cell phone towers and integrated within the cellular communications network to develop a nationwide sensor network for supporting homeland security and services.