This article presents optimization results for practical manufacturable resonant tunnel diode (RTD) THz oscillators with symmetrical slot antennas. A series-parallel model is constructed to account for antenna losses and RTD parasitics that include the top contact and the spreading resistances, the frequency dependence and nonlinearity of RTD characteristics are also taken into account; the model is then transformed to a parallel model for simulation. The results of the model simulation are in good agreement with experimental data and lead to a clear understanding of the impact of antenna geometry and ohmic losses, the shape of the RTD I-V curve, and the RTD barrier thickness on the frequency roll-off of the output power, and of the impact of top-RTD contact resistance on output power at the high frequency end of the operating range approaching the cut-off frequency.