A new design of a compact wideband ring mixer is introduced in this article. The proposed mixer utilizes a compact ring structure comprising a pair of ultra‐wideband planar in‐phase and inverted‐phase transitions as a mixer balun. The implemented mixer has a number of desirable features such as wide bandwidth (∼120%), compact size, excellent port‐to‐port isolations, and return loss. The size of the proposed mixer is 33.2% of the conventional single‐balanced ring mixers. The measured results show flat conversion loss of ∼8.5 dB and greater than 20 dB port‐to‐port isolation from 3 to 12 GHz. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:1919–1922, 2014