We use simulation software to simulate the plasma sheath flow field of RAM-II at four different altitudes, and get the distribution information of electron density, temperature, and Mach number of plasma sheath at four typical flight altitudes. On this basis, the paper analyzes the cause of the distribution at each altitude. Secondly, Z transform FDTD formula is used to solve the electromagnetic problem in the dispersed medium. According to RAM-II aircraft plasma sheath data, we calculate the power transmission coefficient and power reflectance coefficient of L, S-band electromagnetic waves by Z transform FDTD method at each altitude. Because of the strength of magnetic field used in actual aircraft cannot be uniform, in this calculation the plasma sheath and the magnetic induction are both non-uniform. The transmission characteristics are also different between left-hand circularly polarized waves and right-hand circularly polarized wave in the magnetized plasma. Finally, the paper gives some theoretical design advise about communication antenna for supersonic speed aircraft.