The electromagnetic field modes in partial‐height ferrite circulators are classified into two categories: volume modes and surface modes. Their properties are described in detail. The method of expanding the circulator bandwidth at the millimeter wave band is investigated. A set of theoretical results are calculated for a n = 2 Chebyshev response. A Ka‐band circulator is designed. The measured minimum isolation is 15 dB over a 9‐GHz bandwidth. The experimental result and theoretical prediction are similar.