The Unwinding of a Transformer

Abstract

The usefulness of passive radio-frequency identification (RFID) and general passive sensor technology is currently limited by relatively low field strengths due to regulations, giving a notoriously short reader-to-tag link range. The energizing and thus communication strength from receiver to detector depends on the amount of dc voltage that a chip can rectify from an interrogating electromagnetic field. Voltage-amplification/rectification methods employ multi-stage charge pumps, using nonlinear circuit components that incur significant losses, with diminishing returns for increasing stages. In RFID/sensor technology, where field strength is limited by regulations, technology, or distance, there is an opportunity for an alternate approach to RF-generated dc voltage. The proposed solution uses the Ferranti effect, where, for certain lengths and distributed parameter values, the voltage amplitude at the receiving end of a transmission line is greater than that of the sending end. As the Ferranti effect is typically an issue in high-voltage, low-frequency, long-distance power transmission, this problem poses significant design puzzles at RFID frequencies, transmission-line lengths, and voltage scales. By extending the formalism of the relevant transmission-line theory to the domain of common microstrip structures, microstrip transmission lines have been investigated and tested to demonstrate the effect. Because significant voltage gains (between two and five times the input amplitude) have been observed at a number of frequencies in the practical or RFID application range, it is concluded that the Ferranti effect is indeed a viable means of voltage amplification for RFID and related passive sensor applications.