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The linear transmission line (LTL) model had been reported for calculating the admittance of a linearly polarized arbitrary shaped patch antenna with symmetrical geometry and constant thickness. In this method, the patch is divided into very small sections along the feed axis length and the width for each section is calculated. However, when this model was simulated and compared for various reported results on rectangular microstrip antennas (RMSA) and circular microstrip antennas (CMSA), an error of around 10% was observed. We propose the following modifications for the reported LTL model. (1) The effective dimensions of the antennas are used instead of physical dimensions to account for the fringing fields. (2) The effective permittivity expression is used to account for the dispersion effect. (3) The characteristic impedance equation was modified to account for the dispersion effect. (4) The effect of probe inductance is taken into account. After incorporating these modifications, the analysis was carried out for the RMSA, CMSA and the equilateral triangular microstrip antenna.