A cylindrical model of the earth-ionosphere cavity at ELF with an angularly dependent surface-impedance boundary condition is analyzed. The variation of the ionospheric surface impedance is chosen to simulate the presence of a dipolar, radial, geomagnetic field. By neglecting TE-TM mode coupling, the coupling between TM modes is analyzed. Cavity resonant frequencies and quality factors as functions of ionospheric electron concentration and collision frequency are compared between constant and dipolar field configurations. Parameters that yield resonances close to experimental values are found for both configurations, and it is shown that the constant field model with an induction equal to 2/π times the polar induction produces resonances very, close to those of the corresponding dipolar model.