In this paper, a robust knowledge‐based neural‐network model (RKBNN), whose neurons in the hidden layer are all knowledge‐based neurons instead of conventional neurons, is proposed for the microstrip T‐junction structure. The RKBNN model is electromagnetically developed with a set of data that are produced using the method of moments (MoM) method. Through numerical experiments, many advantages have been shown by the RKBNN model over the conventional multilayer perceptron model. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 42: 257–260, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20270