Impedance matching devices are often used in high-frequency circuits, typically to match the impedance of a device or component to the characteristic impedance of a circuit or system. In some circuits, it is desirable for the impedance matching to achieve multiple-octave frequency coverage accompanied by low insertion loss. To help designers working with impedance transformations, this article explores the design of unbalanced-to-unbalanced (unun) wideband impedance transformers with an impedance ratio of 1:4. Such transformers are useful in radio communications systems, typically in hybrid circuits, signal combiners and dividers, and for interstage coupling of amplifier chains.
Such wideband unun impedance transformers are also useful for test circuits, optical receiver systems,1 microwave circuits with wideband impedance matching,2 and antenna coupling.3 Modern computational programs usable for high-frequency circuit design and simulation include this device in their tool boxes.4 A wideband unun impedance transformer consists of a toroidal ferrite core wrapped with a twisted bifilar transmission line, with the wires isolated by means of an enamel film. By combining design elements of conventional and transmission-line impedance transformers, it is possible to create a true wideband component. The design offers high efficiency for an impedance transformation ratio of 1:4 (Fig. 1).5