Designing a Leaky Wave Antenna is a demanding problem. This paper considers some of the issues involved, referring to an example based on a rectangular waveguide with one sidewall replaced by a resonant insert, as shown in Figure 1 [1]. Figure 1 shows a schematic of a rectangular waveguide with a dual square FSS insert in one broadwall. This paper considers a simple dipole array as described in [1]. Solving for the modes in the waveguide is problematic, but essential to speed up the design process. Ideally, one mode is required to give optimum control over the radiation pattern. A new tool is presented here to facilitate this process [2].