In recent decades, the fever issue in radar research is how to reduce radar cross section (RCS) of specific targets like fighters. In regard to the other aspect, to increase the detection probability of air targets is also a highlighting feature for military surveillance radars. In this work, RCS of a typical aircraft target for a monostatic radar system in very high frequency (VHF: 30-300 MHz) band and for a bistatic one in X band (8-12 GHz) are investigated. According to the electromagnetic theory, since the RCS of the aircraft in VHF band is within Mie region; the resonant phenomenon of the RCS is observed so the RCS is enhanced and the polarization effect is also described. For the bistatic radar operating in X band, there is also an apparent enlargement of the bistatic RCS at certain incident angles. Two numerical electromagnetism techniques - integrated equation (IE) solver based on moment of method (MoM) and shooting and bouncing ray (SBR) solver based on ray model, are respectively used to calculate RCSs in VHF and X bands for efficacy and accuracy. A guideline to choose an appropriate simulation method in a certain frequency regime is proposed to increase the calculation efficiency and maintain the fidelity of evaluation. The simulated results reveal that these two radar systems indeed have the advantage of detecting stealthy or small targets.