A new spatial images procedure has been applied to the analysis of microstrip to circular waveguide transitions. The numerical technique is based on the utilization of stratified image rings, where the weights and orientations of either discrete charges or dipole images are determined. This is accomplished by enforcing the scalar and vector potential boundary conditions on the cavity walls. The results obtained with the new Green's functions have been compared to those given by a standard integral equation method based on the free space Green's functions, and a finite elements implementation. Good agreement is shown between the different techniques, thus demonstrating the usefulness of the new approach.