We report on the use of Gaussian beam mode analysis (GBMA) to study the focusing properties of binary counterparts of lenses, axicons and Gabor zone plates. Optical components at millimetre wavelengths with short focal ratios are inevitably rather thick. It is therefore advantageous to devise a method that can maximize space in a linear imaging system. This can be achieved by using a binary form of refractive and diffractive focusing elements to localize power from beams subject to a large degree of diffraction as is common in quasi-optics at these wavelengths. In this paper, we present numerical analysis of the aforementioned elements using GBMA as a computationally efficient alternative to Fresnel-Kirchoff diffraction integrals.