Design of dual/tri-frequency impedance transformer with ultra-high transforming ratio

In this article, the design of a new dual-band impedance transformer with ultra-high transforming ratio (UHTR) is presented. The closed-form expressions are derived analytically using a lossless transmission-line theory. To practically validate the proposed design concept, two examples of dual-band transformer working at smaller and larger frequency ratios are designed for different load impedance of 500, 1000, and 1500 Omega. Finally, three prototypes of dual-band transformer with UHTR are designed, fabricated, and tested. For all these prototypes, different frequency ratios (two operating frequencies can be chosen arbitrarily) are considered. The measured return loss of the prototypes is better than 15 dB at all the operating frequencies. The measured results are matched very closely with the simulated results. This design is then extended to match a complex load at two different frequencies. The multi-band characteristic is obtained by decomposing a shorted stub into a stepped impedance section of the proposed structure. With the necessary derivation and analysis, a tri-frequency matching network is designed for a transforming ratio of 10.