An aperture coupled antenna consists of feed, slot, and a radiation patch. The feed is normally a microstrip line with the slot etched in the ground plane and the radiating patch is located above the ground plane. To obtain maximum power transfer the microstrip line is orientated at right angle to the slot to obtain tight coupling. It is shown by Pozar that the patch and the slot modelled as two transmission lines connected in parallel produces a series impedance on the microstrip line. In the equivalent circuit of this antenna, the coupling between the microstrip line and the slot line is modelled as a transformer by a turn ratio nf. The coupling between the slot and the patch is modelled as a transformer by a turn ratio np. Initially derived equations for nf for finite lengths of the slot obtained by different researchers are reviewed and shown that they do not agree. To investigate this turn ratio the microstrip line and slot structure is modelled using CST software while the equivalent circuit of this structure is modelled using AWR software. For different lengths of the slots the series impedance Z on the feed line is obtained from the S21 parameter. The turn ratio in the equivalent circuit is tuned for the lengths of slots to obtain the value of the S21 parameter. Then the turn ratio of the transformer modelling the coupling between the slot line and the patch is also investigated. The antenna for both models has several interdependent design parameters so that an analytical design process is very difficult to implement. From the investigation carried out a design procedure for the antenna is proposed which is used to design the antenna for a frequency 2 GHz. The designed antenna is modelled using CST and AWR software and a good agreement has been obtained for the return loss of both models.