This study reports an evaluation of the maximum axial ratio (AR) bandwidth which is obtainable from a transmission type linear (LP) to circular polarization (CP) convertor working at normal incidence. The anisotropic bandpass FSS structure is composed of a thin metal surface perforated with an array of unequal length orthogonal slots. Full wave simulations are used to study the effect of changing the physical dimensions of the individual slot structural parameters by computing the amplitude and phase of the TE and TM transmitted waves at normal incidence over the frequency range 8 - 12 GHz. The results are used to establish design guidelines which are employed to obtain the optimum topology for a single layer LP to CP convertor working at a center frequency of 10 GHz. The numerical results which are shown to be in close agreement with measurements, demonstrate that this class of periodic structure can be designed to give a 3 dB AR fractional bandwidth of 5.0%.