Design of Dual-Frequency Transformers Using Three Equal-Length Transmission Lines

The Monzon transformer uses two equal-length transmission lines in cascade to match a real load impedance at two frequencies. The two-section transformer, however, has two issues. In this paper, we study the dual-frequency transformers using three equal-length transmission lines in cascade. In contrast to the two-section transformer that has only a unique solution to a given dual-frequency matching problem, the three-section transformer has infinitely many solutions. Thus, we can generally choose an appropriate one from the solutions to deal with the two issues. For experimental verification, three dual-frequency transformers that match 100-$\Omega$ loads to 50 $\Omega$ for frequency ratios 7.75, 13.6, and 2 are implemented in microstrip on a 20-mil RO4003C substrate. The measured results agree with the theoretical performance.