AbstractRecently, compact wideband BPFs for a UWB system have been studied actively. In this paper we propose a compact diplexer built into an LTCC substrate for a UWB system and 2.4‐GHz wireless systems. First, a wideband BPF for a UWB system and an LPF with multiple attenuation poles for 2.4‐GHz wireless systems are described. Second, we design matching circuits of a common port to retain the basic performance of both the BPF and the LPF. Third, in accordance with the results of the study, we design a compact diplexer built into an LTCC substrate. Finally, we verify the effectiveness of the proposed method by experiments. © 2010 Wiley Periodicals, Inc. Electron Comm Jpn, 93(5): 24–32, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10210