AbstractTwo‐ and three‐dimensional electromagnetic resonance calculations have been performed on rising‐sun magnetron structures. These calculations have been made using the transmission‐line matrix method. Cylindrical coordinates have been used in order to facilitate modelling of the essentially cylindrical device. Recent improvements to the TLM technique, such as the development of the symmetric condensed node for electromagnetics, have been fully exploited. The numerical procedure is described, the modelling process is discussed, and the results obtained are evaluated in comparisons with corresponding empiric