Design and Analysis of a UHF Transmission Line Coupler for Telemetry and Power Harvesting in a Passive Sensor System

The intra-coaxial transmission line coupler TXC is a four port device designed to harvest power from a host coaxial transmission line and to provide telemetry for remote sensing applications via the same host transmission line. Seven different TXC structures were designed and fabricated to study the effects of varying specific geometric features on the electrical characteristics of the TXC. Each TXC structure was tested using a RF vector network analyzer and analyzed using a commercially available finite-element numerical simulator. Two main observations resulted from the study: 1) the resonant frequency depends mainly on the value of the tuning capacitor and the loop inductance of the TXC, and 2) the electrical characteristics are relatively insensitive to variations in all geometric features except the TXC loop surface area and, to a lesser extent, the separation distance between the coax center conductor and the TXC loop. It was found that the output impedance, Q-factor, voltage transfer characteristic, and power transfer characteristic computed from simulations were systematically greater than those computed from experiment. It was shown that by using experimental results to "fit" the simulations, a much better agreement could be obtained between experimental and simulation results.