Loss Angle and Permittivity Determination of a Dielectric Rod Using a Three-Wire Cage-Shape System

A simple innovative measurement system is proposed for the simultaneous determination of the loss angle and permittivity of a cylindrical dielectric rod at high-frequency regimes (megahertz band). The measurement method, based on multiconductor transmission-line (MTL) theory, explores the frequency-domain resonant features of the new proposed structure, which consists of three equally spaced wires where the cylindrical rod is tightly caged inside. The wires are bonded together at the far end of the MTL structure, while at the sending end, only two are bonded. The input impedance of the transmission line is measured against the frequency, with sharp peaks being observed when the length of the structure comes close to an odd multiple of a quarter wavelength. A theory is developed showing how the propagation parameters of the MTL device depend on the dielectric rod properties, and based on that, a measuring technique is devised for retrieving the loss angle and permittivity of the rod from the critical resonance frequencies and input impedance maximum values. Experimental results are presented to show the effectiveness of the method.