A time-domain Multiconductor Transmission Line (MTL/sup 2/) model of the stub-supported helix circuit used in the AEGIS CFA has been developed. The stub-supported helix circuit is also commonly employed by emitting sole CFAs operating in S, C, and X frequency bands-so the model has broad applicability. Currently, the circuit model used in MASK is a lumped element /spl pi/ network-first used for CFA simulation by Dombrowski. The /spl pi/ circuit model can match the phase shift per section and the slope of the phase curve at one frequency but can not mimic the circuit dispersion over the entire pass band of the anode circuit. The new MTL circuit model for the stub-supported helix is constructed using elements that have one-to-one correspondence with the physical anode circuit. Consequently, the pass-band characteristic of the MTL model can match the measured circuit dispersion quite closely. In addition, the impedance of the MTL model is also more accurate since the characteristic values used by the model are calculated from the actual circuit cross section geometries. Because of these improvements, it is expected that use of this model in MASK will enhance our ability to predict the /spl pi/ mode transition current and to estimate spurious frequency generation. The time-domain algorithm is advanced using a time-centered leap frog scheme which is consistent with the advancement algorithm used in MASK. The new circuit has been implemented in MASK and some results are presented.