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The equivalent circuits representing the effects from certain step-type discontinuities are obtained for coaxial transmission lines. It is shown that for the cases studied the effects of the local waves in the vicinity of the discontinuity can be accounted for exactly for calculation of relation between input- and output-end quantities by placing an admittance at the plane of the discontinuity in a transmission-line or principal-wave equivalent circuit. If frequency is low enough so that transverse dimensions are a small fraction of a wavelength, the discontinuity admittance is found to be a pure capacitance for these cases. Curves are presented giving the discontinuity capacitances in terms of the coaxial-line radii. Curves of a frequency factor F are calculated showing the amount the discontinuity capacitance must be corrected as transverse dimensions become comparable to wavelength. Curves of a proximity factor P are also given showing corrections to be applied to the discontinuity capacitance if a short-circuiting disk termination is placed near the plane of the discontinuity. The analysis of the case of a simple-step discontinuity on the inner cylinder is carried through in detail, and analyses of other cases are outlined.