We present a method of representing a stepped-radius antenna by a uni-radius loaded antenna. By equalizing the reflection co-efficient of a loaded transmission line for the uni-radius loaded antenna to that of a compound transmission line for the stepped-radius antenna, the lumped impedance of the uni-radius loaded antenna is obtained. To confirm the equivalent relation, both antennas are analyzed numerically based on the electric field integral equation. As a result, current distributions and input impedances agree well. This concept is very useful for the analysis and physical understanding of the stepped-radius antenna, and for applications using the effect of the stepped radius. As a further application, an improvement of the input impedance characteristics of a stepped-radius antenna is also presented.
