When a TEM wave is incident upon a narrow slit located in the wall of parallel-plate waveguide, fields scattered from the slit may be calculated from an equivalent magnetic surface current in the slit which may be obtained as Miller's perturbation series of small slit width.
The equivalent transmission-line circuit for TEM mode is derived from the expression of scattered fields to have series resistance and capacitance, where the resistance accounts for the radiation through the slit and capacitance for the stored energy near the slit.
Mutual coupling effects between two neighboring narrow slits are also discussed.
Obtained values of capacitance for one slit discontinuity shows close agreement with those of Oliner and Cohn, where they did not obtain values of resistance.