A Scanning Nearfield Optical Microscope (SNOM) tip with partial metallic cladding is presented. For its design, a very demanding 2D eigenvalue analysis of an optical waveguide with material and radiation losses is carried out by the Multiple Multipole Program (MMP) and by the Finite Element Method (FEM). These simulations require some special tricks that are outlined. The computed 2D MMP and FEM results are compared and discussed. This 2D analysis is followed by a full 3D FEM analysis of the SNOM tip. The obtained 3D results confirm the corresponding 2D predictions. Important conclusions regarding the guiding capabilities of the chosen structure and the efficiency of the applied numerical methods are presented.