This paper gives a brief summary of the design concept of the so-called “dual-mode” synthesized transmission lines with their applications to novel antenna arrays. The dual-mode synthesized line, by definition, is a miniaturized, nonperiodic basic building block capable of providing distinct circuit responses in different frequency bands of interest. The design methodology will be introduced briefly, followed by two application examples including the dual-mode retrodirective array and dual-band decoupling network. Prospective applications are discussed at the end of the paper.