The normalized impedance of a transverse slot on the image plate of an image NRD that is a modification of the original NRD is formulated by means of the method of moments. In terms of the modes that can be supported within the image NRD guide, the electromagnetic field inside the guide is modally expanded and the dyadic Green's function (DGF) is obtained. In addition, by applying the concept of the virtual cavity method, the DGF is modified. The numerical results are compared with the experimental data obtained by the standing wave measurement method. The difference in the resonant frequency is 1.6% and a shift of the peak value is observed. Nevertheless, the tendencies of the frequency locus are identical.