This communication presents a simplified design approach to obtain the turns ratio n(f) which models the coupling between the microstrip feed and slot line of an aperture coupled slot antenna by using the S-11 parameter. Equations are derived using S-11 parameter to determinen(f) and simulations are carried out at 2 GHz using substrates of PCB FR4, Duroid 5870, and Duroid 6010. Results indicate n(f) is proportional to the slot length from 0 to 20 mm increasing rapidly with length. Over 20 mm slot length, the trend reduces and the value of n(f) tends towards 1. In addition, n(f) varies with the height of substrate but is little affected by dielectric constant of substrate which could therefore be ignored to simplify the design procedure. Measured results show good agreement with the theoretical model.