Development of compact wideband balun using multilayer substrate-integrated coaxial line

In this paper, we have designed and fabricated a broadband balun for wireless communications. Wireless communication devices primarily consist of an antenna and an IC. In recent years, different signal transmission methods have been adopted between the antenna and the IC to enhance performance. The circuits used for this signal conversion are known as balanced-unbalanced converters, or baluns. In this study, a compact wideband balun was designed on a multilayer substrate by applying the design theory of substrate-integrated coaxial lines. The operating frequency is designed to cover 1 to 10 GHz, suitable for devices utilizing microwave bands. The balun features a flat substrate for easy connection to ICs, ensuring compatibility with existing ICs. Additionally, by inserting open stubs parallel to the input-side line, it was found that specific frequency bands could be attenuated as desired.