Simulated scalar feed horn antennas are found to be exhibiting identical radiation characteristics of metallic corrugated feed horn antennas of same dimensions. Hence, in this paper, the E-plane radiation characteristics of a simulated scalar feed horn antenna is analysed theoretically by extending the theory of metallic corrugated horn antennas. The theoretical E-plane radiation patterns at different frequencies in X- band are compared with that of experimentally obtained E-plane radiation patterns. Even though the nulls of of the radiation patterns are not fully realised by the theory, the main beam characteristics are well explained. Losses that may occur in the dielectric substrate of the E-plane walls are not considered in the theoretical explanation. This may be the reason for slight discrepancies observed in the experimental and theoretical 3-dB and 10 dB beam widths.