Broadband Microstrip Impedance Transformer

Abstract

A broadband impedance transformer is proposed for microstrip realization and to release the tight gap size (30 mu m) required for coupled-stripline. Design curves of the transformer are obtained by solving the analytical equations derived from the transmission line theory. Two transformers are fabricated and measured. One is designed for a 1: 2 ratio and return loss better than 30 dB over a fractional bandwidth Delta=68%, and the other is a 1: 2.8 transformer with vertical bar S-11 vertical bar < -25 dB over a bandwidth of Delta = 71%. In realization, the metal traces are folded so that the circuit sizes are less than lambda/8x lambda/8. Good agreement between measured and simulation is obtained.