This work examines the use of thin ferrite layers to form circuit components in a waveguide. The planar layers are positioned perpendicular to the waveguide axis. A unique lumped-element circuit model for a thin layer is developed, and practical limitations on operating frequency ranges are outlined. The circuit model is used to design bandpass filters in a waveguide with prescribed frequency responses. These waveguide filters are useful in applications where capacitive posts or inductive diaphragms are infeasible. EEsof© simulations of a filter are presented, and directions for further investigation are discussed. © 1998 John Wiley & Sons, Inc.