In this paper we introduce a waveguide antenna applicator design intended to be placed on the surface or in close proximity to a human breast for imaging purposes. Hence, the antenna needs to be compact for easy placement. The design process is carefully carried out dividing the antenna applicator into separate parts, allowing closer analysis towards improved synthesis. A mode applicator antenna was concluded to be necessary, employing a TE10 mode type with minimized near-field and surface (Zennek) wave excitation. Numerical simulations have been used throughout and show that the proposed ridged waveguide antenna is capable of fulfilling the design requirements and the performance goals. Modelling has been carried out using a scenario with a simple breast model and confirms the applicator's capability.