Fabry-Perot cavities are perhaps the best known of the optical transmission resonators, with cavity field enhancement accomplished by two parallel and partially reflecting planes. Recently it has been shown that arrays of narrow slits cut into a metal substrate are similarly able to exhibit resonant transmission modes. Here, the transmission of normally incident plane wave microwaves through a single stepped sub-wavelength slit in a thick metal plate is explored. The presence of the step substantially increases the radiation wavelength, which may be resonantly transmitted to well beyond twice the plate thickness. Insight into the resonant behaviour of the stepped slit is provided through the analysis of the field solutions produced by a finite element model. This model also predicts resonant transmission which is in excellent agreement with the experimental results.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.